This is libc.info, produced by makeinfo version 4.8 from libc.texinfo.

INFO-DIR-SECTION GNU libraries
START-INFO-DIR-ENTRY
* Libc: (libc).                 C library.
END-INFO-DIR-ENTRY

INFO-DIR-SECTION GNU C library functions and macros
START-INFO-DIR-ENTRY
* ALTWERASE: (libc)Local Modes.
* ARGP_ERR_UNKNOWN: (libc)Argp Parser Functions.
* ARG_MAX: (libc)General Limits.
* BC_BASE_MAX: (libc)Utility Limits.
* BC_DIM_MAX: (libc)Utility Limits.
* BC_SCALE_MAX: (libc)Utility Limits.
* BC_STRING_MAX: (libc)Utility Limits.
* BRKINT: (libc)Input Modes.
* BUFSIZ: (libc)Controlling Buffering.
* CCTS_OFLOW: (libc)Control Modes.
* CHILD_MAX: (libc)General Limits.
* CIGNORE: (libc)Control Modes.
* CLK_TCK: (libc)CPU Time.
* CLOCAL: (libc)Control Modes.
* CLOCKS_PER_SEC: (libc)CPU Time.
* COLL_WEIGHTS_MAX: (libc)Utility Limits.
* CPU_CLR: (libc)CPU Affinity.
* CPU_ISSET: (libc)CPU Affinity.
* CPU_SET: (libc)CPU Affinity.
* CPU_SETSIZE: (libc)CPU Affinity.
* CPU_ZERO: (libc)CPU Affinity.
* CREAD: (libc)Control Modes.
* CRTS_IFLOW: (libc)Control Modes.
* CS5: (libc)Control Modes.
* CS6: (libc)Control Modes.
* CS7: (libc)Control Modes.
* CS8: (libc)Control Modes.
* CSIZE: (libc)Control Modes.
* CSTOPB: (libc)Control Modes.
* DES_FAILED: (libc)DES Encryption.
* DTTOIF: (libc)Directory Entries.
* E2BIG: (libc)Error Codes.
* EACCES: (libc)Error Codes.
* EADDRINUSE: (libc)Error Codes.
* EADDRNOTAVAIL: (libc)Error Codes.
* EADV: (libc)Error Codes.
* EAFNOSUPPORT: (libc)Error Codes.
* EAGAIN: (libc)Error Codes.
* EALREADY: (libc)Error Codes.
* EAUTH: (libc)Error Codes.
* EBACKGROUND: (libc)Error Codes.
* EBADE: (libc)Error Codes.
* EBADF: (libc)Error Codes.
* EBADFD: (libc)Error Codes.
* EBADMSG: (libc)Error Codes.
* EBADR: (libc)Error Codes.
* EBADRPC: (libc)Error Codes.
* EBADRQC: (libc)Error Codes.
* EBADSLT: (libc)Error Codes.
* EBFONT: (libc)Error Codes.
* EBUSY: (libc)Error Codes.
* ECANCELED: (libc)Error Codes.
* ECHILD: (libc)Error Codes.
* ECHO: (libc)Local Modes.
* ECHOCTL: (libc)Local Modes.
* ECHOE: (libc)Local Modes.
* ECHOK: (libc)Local Modes.
* ECHOKE: (libc)Local Modes.
* ECHONL: (libc)Local Modes.
* ECHOPRT: (libc)Local Modes.
* ECHRNG: (libc)Error Codes.
* ECOMM: (libc)Error Codes.
* ECONNABORTED: (libc)Error Codes.
* ECONNREFUSED: (libc)Error Codes.
* ECONNRESET: (libc)Error Codes.
* ED: (libc)Error Codes.
* EDEADLK: (libc)Error Codes.
* EDEADLOCK: (libc)Error Codes.
* EDESTADDRREQ: (libc)Error Codes.
* EDIED: (libc)Error Codes.
* EDOM: (libc)Error Codes.
* EDOTDOT: (libc)Error Codes.
* EDQUOT: (libc)Error Codes.
* EEXIST: (libc)Error Codes.
* EFAULT: (libc)Error Codes.
* EFBIG: (libc)Error Codes.
* EFTYPE: (libc)Error Codes.
* EGRATUITOUS: (libc)Error Codes.
* EGREGIOUS: (libc)Error Codes.
* EHOSTDOWN: (libc)Error Codes.
* EHOSTUNREACH: (libc)Error Codes.
* EIDRM: (libc)Error Codes.
* EIEIO: (libc)Error Codes.
* EILSEQ: (libc)Error Codes.
* EINPROGRESS: (libc)Error Codes.
* EINTR: (libc)Error Codes.
* EINVAL: (libc)Error Codes.
* EIO: (libc)Error Codes.
* EISCONN: (libc)Error Codes.
* EISDIR: (libc)Error Codes.
* EISNAM: (libc)Error Codes.
* EL2HLT: (libc)Error Codes.
* EL2NSYNC: (libc)Error Codes.
* EL3HLT: (libc)Error Codes.
* EL3RST: (libc)Error Codes.
* ELIBACC: (libc)Error Codes.
* ELIBBAD: (libc)Error Codes.
* ELIBEXEC: (libc)Error Codes.
* ELIBMAX: (libc)Error Codes.
* ELIBSCN: (libc)Error Codes.
* ELNRNG: (libc)Error Codes.
* ELOOP: (libc)Error Codes.
* EMEDIUMTYPE: (libc)Error Codes.
* EMFILE: (libc)Error Codes.
* EMLINK: (libc)Error Codes.
* EMSGSIZE: (libc)Error Codes.
* EMULTIHOP: (libc)Error Codes.
* ENAMETOOLONG: (libc)Error Codes.
* ENAVAIL: (libc)Error Codes.
* ENEEDAUTH: (libc)Error Codes.
* ENETDOWN: (libc)Error Codes.
* ENETRESET: (libc)Error Codes.
* ENETUNREACH: (libc)Error Codes.
* ENFILE: (libc)Error Codes.
* ENOANO: (libc)Error Codes.
* ENOBUFS: (libc)Error Codes.
* ENOCSI: (libc)Error Codes.
* ENODATA: (libc)Error Codes.
* ENODEV: (libc)Error Codes.
* ENOENT: (libc)Error Codes.
* ENOEXEC: (libc)Error Codes.
* ENOLCK: (libc)Error Codes.
* ENOLINK: (libc)Error Codes.
* ENOMEDIUM: (libc)Error Codes.
* ENOMEM: (libc)Error Codes.
* ENOMSG: (libc)Error Codes.
* ENONET: (libc)Error Codes.
* ENOPKG: (libc)Error Codes.
* ENOPROTOOPT: (libc)Error Codes.
* ENOSPC: (libc)Error Codes.
* ENOSR: (libc)Error Codes.
* ENOSTR: (libc)Error Codes.
* ENOSYS: (libc)Error Codes.
* ENOTBLK: (libc)Error Codes.
* ENOTCONN: (libc)Error Codes.
* ENOTDIR: (libc)Error Codes.
* ENOTEMPTY: (libc)Error Codes.
* ENOTNAM: (libc)Error Codes.
* ENOTSOCK: (libc)Error Codes.
* ENOTSUP: (libc)Error Codes.
* ENOTTY: (libc)Error Codes.
* ENOTUNIQ: (libc)Error Codes.
* ENXIO: (libc)Error Codes.
* EOF: (libc)EOF and Errors.
* EOPNOTSUPP: (libc)Error Codes.
* EOVERFLOW: (libc)Error Codes.
* EPERM: (libc)Error Codes.
* EPFNOSUPPORT: (libc)Error Codes.
* EPIPE: (libc)Error Codes.
* EPROCLIM: (libc)Error Codes.
* EPROCUNAVAIL: (libc)Error Codes.
* EPROGMISMATCH: (libc)Error Codes.
* EPROGUNAVAIL: (libc)Error Codes.
* EPROTO: (libc)Error Codes.
* EPROTONOSUPPORT: (libc)Error Codes.
* EPROTOTYPE: (libc)Error Codes.
* EQUIV_CLASS_MAX: (libc)Utility Limits.
* ERANGE: (libc)Error Codes.
* EREMCHG: (libc)Error Codes.
* EREMOTE: (libc)Error Codes.
* EREMOTEIO: (libc)Error Codes.
* ERESTART: (libc)Error Codes.
* EROFS: (libc)Error Codes.
* ERPCMISMATCH: (libc)Error Codes.
* ESHUTDOWN: (libc)Error Codes.
* ESOCKTNOSUPPORT: (libc)Error Codes.
* ESPIPE: (libc)Error Codes.
* ESRCH: (libc)Error Codes.
* ESRMNT: (libc)Error Codes.
* ESTALE: (libc)Error Codes.
* ESTRPIPE: (libc)Error Codes.
* ETIME: (libc)Error Codes.
* ETIMEDOUT: (libc)Error Codes.
* ETOOMANYREFS: (libc)Error Codes.
* ETXTBSY: (libc)Error Codes.
* EUCLEAN: (libc)Error Codes.
* EUNATCH: (libc)Error Codes.
* EUSERS: (libc)Error Codes.
* EWOULDBLOCK: (libc)Error Codes.
* EXDEV: (libc)Error Codes.
* EXFULL: (libc)Error Codes.
* EXIT_FAILURE: (libc)Exit Status.
* EXIT_SUCCESS: (libc)Exit Status.
* EXPR_NEST_MAX: (libc)Utility Limits.
* FD_CLOEXEC: (libc)Descriptor Flags.
* FD_CLR: (libc)Waiting for I/O.
* FD_ISSET: (libc)Waiting for I/O.
* FD_SET: (libc)Waiting for I/O.
* FD_SETSIZE: (libc)Waiting for I/O.
* FD_ZERO: (libc)Waiting for I/O.
* FILENAME_MAX: (libc)Limits for Files.
* FLUSHO: (libc)Local Modes.
* FOPEN_MAX: (libc)Opening Streams.
* FP_ILOGB0: (libc)Exponents and Logarithms.
* FP_ILOGBNAN: (libc)Exponents and Logarithms.
* F_DUPFD: (libc)Duplicating Descriptors.
* F_GETFD: (libc)Descriptor Flags.
* F_GETFL: (libc)Getting File Status Flags.
* F_GETLK: (libc)File Locks.
* F_GETOWN: (libc)Interrupt Input.
* F_OK: (libc)Testing File Access.
* F_SETFD: (libc)Descriptor Flags.
* F_SETFL: (libc)Getting File Status Flags.
* F_SETLK: (libc)File Locks.
* F_SETLKW: (libc)File Locks.
* F_SETOWN: (libc)Interrupt Input.
* HUGE_VAL: (libc)Math Error Reporting.
* HUGE_VALF: (libc)Math Error Reporting.
* HUGE_VALL: (libc)Math Error Reporting.
* HUPCL: (libc)Control Modes.
* I: (libc)Complex Numbers.
* ICANON: (libc)Local Modes.
* ICRNL: (libc)Input Modes.
* IEXTEN: (libc)Local Modes.
* IFNAMSIZ: (libc)Interface Naming.
* IFTODT: (libc)Directory Entries.
* IGNBRK: (libc)Input Modes.
* IGNCR: (libc)Input Modes.
* IGNPAR: (libc)Input Modes.
* IMAXBEL: (libc)Input Modes.
* INADDR_ANY: (libc)Host Address Data Type.
* INADDR_BROADCAST: (libc)Host Address Data Type.
* INADDR_LOOPBACK: (libc)Host Address Data Type.
* INADDR_NONE: (libc)Host Address Data Type.
* INFINITY: (libc)Infinity and NaN.
* INLCR: (libc)Input Modes.
* INPCK: (libc)Input Modes.
* IPPORT_RESERVED: (libc)Ports.
* IPPORT_USERRESERVED: (libc)Ports.
* ISIG: (libc)Local Modes.
* ISTRIP: (libc)Input Modes.
* IXANY: (libc)Input Modes.
* IXOFF: (libc)Input Modes.
* IXON: (libc)Input Modes.
* LINE_MAX: (libc)Utility Limits.
* LINK_MAX: (libc)Limits for Files.
* L_ctermid: (libc)Identifying the Terminal.
* L_cuserid: (libc)Who Logged In.
* L_tmpnam: (libc)Temporary Files.
* MAXNAMLEN: (libc)Limits for Files.
* MAXSYMLINKS: (libc)Symbolic Links.
* MAX_CANON: (libc)Limits for Files.
* MAX_INPUT: (libc)Limits for Files.
* MB_CUR_MAX: (libc)Selecting the Conversion.
* MB_LEN_MAX: (libc)Selecting the Conversion.
* MDMBUF: (libc)Control Modes.
* MSG_DONTROUTE: (libc)Socket Data Options.
* MSG_OOB: (libc)Socket Data Options.
* MSG_PEEK: (libc)Socket Data Options.
* NAME_MAX: (libc)Limits for Files.
* NAN: (libc)Infinity and NaN.
* NCCS: (libc)Mode Data Types.
* NGROUPS_MAX: (libc)General Limits.
* NOFLSH: (libc)Local Modes.
* NOKERNINFO: (libc)Local Modes.
* NSIG: (libc)Standard Signals.
* NULL: (libc)Null Pointer Constant.
* ONLCR: (libc)Output Modes.
* ONOEOT: (libc)Output Modes.
* OPEN_MAX: (libc)General Limits.
* OPOST: (libc)Output Modes.
* OXTABS: (libc)Output Modes.
* O_ACCMODE: (libc)Access Modes.
* O_APPEND: (libc)Operating Modes.
* O_ASYNC: (libc)Operating Modes.
* O_CREAT: (libc)Open-time Flags.
* O_EXCL: (libc)Open-time Flags.
* O_EXEC: (libc)Access Modes.
* O_EXLOCK: (libc)Open-time Flags.
* O_FSYNC: (libc)Operating Modes.
* O_IGNORE_CTTY: (libc)Open-time Flags.
* O_NDELAY: (libc)Operating Modes.
* O_NOATIME: (libc)Operating Modes.
* O_NOCTTY: (libc)Open-time Flags.
* O_NOLINK: (libc)Open-time Flags.
* O_NONBLOCK: (libc)Open-time Flags.
* O_NONBLOCK: (libc)Operating Modes.
* O_NOTRANS: (libc)Open-time Flags.
* O_RDONLY: (libc)Access Modes.
* O_RDWR: (libc)Access Modes.
* O_READ: (libc)Access Modes.
* O_SHLOCK: (libc)Open-time Flags.
* O_SYNC: (libc)Operating Modes.
* O_TRUNC: (libc)Open-time Flags.
* O_WRITE: (libc)Access Modes.
* O_WRONLY: (libc)Access Modes.
* PARENB: (libc)Control Modes.
* PARMRK: (libc)Input Modes.
* PARODD: (libc)Control Modes.
* PATH_MAX: (libc)Limits for Files.
* PA_FLAG_MASK: (libc)Parsing a Template String.
* PENDIN: (libc)Local Modes.
* PF_FILE: (libc)Local Namespace Details.
* PF_INET6: (libc)Internet Namespace.
* PF_INET: (libc)Internet Namespace.
* PF_LOCAL: (libc)Local Namespace Details.
* PF_UNIX: (libc)Local Namespace Details.
* PIPE_BUF: (libc)Limits for Files.
* P_tmpdir: (libc)Temporary Files.
* RAND_MAX: (libc)ISO Random.
* RE_DUP_MAX: (libc)General Limits.
* RLIM_INFINITY: (libc)Limits on Resources.
* R_OK: (libc)Testing File Access.
* SA_NOCLDSTOP: (libc)Flags for Sigaction.
* SA_ONSTACK: (libc)Flags for Sigaction.
* SA_RESTART: (libc)Flags for Sigaction.
* SEEK_CUR: (libc)File Positioning.
* SEEK_END: (libc)File Positioning.
* SEEK_SET: (libc)File Positioning.
* SIGABRT: (libc)Program Error Signals.
* SIGALRM: (libc)Alarm Signals.
* SIGBUS: (libc)Program Error Signals.
* SIGCHLD: (libc)Job Control Signals.
* SIGCLD: (libc)Job Control Signals.
* SIGCONT: (libc)Job Control Signals.
* SIGEMT: (libc)Program Error Signals.
* SIGFPE: (libc)Program Error Signals.
* SIGHUP: (libc)Termination Signals.
* SIGILL: (libc)Program Error Signals.
* SIGINFO: (libc)Miscellaneous Signals.
* SIGINT: (libc)Termination Signals.
* SIGIO: (libc)Asynchronous I/O Signals.
* SIGIOT: (libc)Program Error Signals.
* SIGKILL: (libc)Termination Signals.
* SIGLOST: (libc)Operation Error Signals.
* SIGPIPE: (libc)Operation Error Signals.
* SIGPOLL: (libc)Asynchronous I/O Signals.
* SIGPROF: (libc)Alarm Signals.
* SIGQUIT: (libc)Termination Signals.
* SIGSEGV: (libc)Program Error Signals.
* SIGSTOP: (libc)Job Control Signals.
* SIGSYS: (libc)Program Error Signals.
* SIGTERM: (libc)Termination Signals.
* SIGTRAP: (libc)Program Error Signals.
* SIGTSTP: (libc)Job Control Signals.
* SIGTTIN: (libc)Job Control Signals.
* SIGTTOU: (libc)Job Control Signals.
* SIGURG: (libc)Asynchronous I/O Signals.
* SIGUSR1: (libc)Miscellaneous Signals.
* SIGUSR2: (libc)Miscellaneous Signals.
* SIGVTALRM: (libc)Alarm Signals.
* SIGWINCH: (libc)Miscellaneous Signals.
* SIGXCPU: (libc)Operation Error Signals.
* SIGXFSZ: (libc)Operation Error Signals.
* SIG_ERR: (libc)Basic Signal Handling.
* SOCK_DGRAM: (libc)Communication Styles.
* SOCK_RAW: (libc)Communication Styles.
* SOCK_RDM: (libc)Communication Styles.
* SOCK_SEQPACKET: (libc)Communication Styles.
* SOCK_STREAM: (libc)Communication Styles.
* SOL_SOCKET: (libc)Socket-Level Options.
* SSIZE_MAX: (libc)General Limits.
* STREAM_MAX: (libc)General Limits.
* SUN_LEN: (libc)Local Namespace Details.
* SV_INTERRUPT: (libc)BSD Handler.
* SV_ONSTACK: (libc)BSD Handler.
* SV_RESETHAND: (libc)BSD Handler.
* S_IFMT: (libc)Testing File Type.
* S_ISBLK: (libc)Testing File Type.
* S_ISCHR: (libc)Testing File Type.
* S_ISDIR: (libc)Testing File Type.
* S_ISFIFO: (libc)Testing File Type.
* S_ISLNK: (libc)Testing File Type.
* S_ISREG: (libc)Testing File Type.
* S_ISSOCK: (libc)Testing File Type.
* S_TYPEISMQ: (libc)Testing File Type.
* S_TYPEISSEM: (libc)Testing File Type.
* S_TYPEISSHM: (libc)Testing File Type.
* TMP_MAX: (libc)Temporary Files.
* TOSTOP: (libc)Local Modes.
* TZNAME_MAX: (libc)General Limits.
* VDISCARD: (libc)Other Special.
* VDSUSP: (libc)Signal Characters.
* VEOF: (libc)Editing Characters.
* VEOL2: (libc)Editing Characters.
* VEOL: (libc)Editing Characters.
* VERASE: (libc)Editing Characters.
* VINTR: (libc)Signal Characters.
* VKILL: (libc)Editing Characters.
* VLNEXT: (libc)Other Special.
* VMIN: (libc)Noncanonical Input.
* VQUIT: (libc)Signal Characters.
* VREPRINT: (libc)Editing Characters.
* VSTART: (libc)Start/Stop Characters.
* VSTATUS: (libc)Other Special.
* VSTOP: (libc)Start/Stop Characters.
* VSUSP: (libc)Signal Characters.
* VTIME: (libc)Noncanonical Input.
* VWERASE: (libc)Editing Characters.
* WCHAR_MAX: (libc)Extended Char Intro.
* WCHAR_MIN: (libc)Extended Char Intro.
* WCOREDUMP: (libc)Process Completion Status.
* WEOF: (libc)EOF and Errors.
* WEOF: (libc)Extended Char Intro.
* WEXITSTATUS: (libc)Process Completion Status.
* WIFEXITED: (libc)Process Completion Status.
* WIFSIGNALED: (libc)Process Completion Status.
* WIFSTOPPED: (libc)Process Completion Status.
* WSTOPSIG: (libc)Process Completion Status.
* WTERMSIG: (libc)Process Completion Status.
* W_OK: (libc)Testing File Access.
* X_OK: (libc)Testing File Access.
* _Complex_I: (libc)Complex Numbers.
* _Exit: (libc)Termination Internals.
* _IOFBF: (libc)Controlling Buffering.
* _IOLBF: (libc)Controlling Buffering.
* _IONBF: (libc)Controlling Buffering.
* _Imaginary_I: (libc)Complex Numbers.
* _PATH_UTMP: (libc)Manipulating the Database.
* _PATH_WTMP: (libc)Manipulating the Database.
* _POSIX2_C_DEV: (libc)System Options.
* _POSIX2_C_VERSION: (libc)Version Supported.
* _POSIX2_FORT_DEV: (libc)System Options.
* _POSIX2_FORT_RUN: (libc)System Options.
* _POSIX2_LOCALEDEF: (libc)System Options.
* _POSIX2_SW_DEV: (libc)System Options.
* _POSIX_CHOWN_RESTRICTED: (libc)Options for Files.
* _POSIX_JOB_CONTROL: (libc)System Options.
* _POSIX_NO_TRUNC: (libc)Options for Files.
* _POSIX_SAVED_IDS: (libc)System Options.
* _POSIX_VDISABLE: (libc)Options for Files.
* _POSIX_VERSION: (libc)Version Supported.
* __fbufsize: (libc)Controlling Buffering.
* __flbf: (libc)Controlling Buffering.
* __fpending: (libc)Controlling Buffering.
* __fpurge: (libc)Flushing Buffers.
* __freadable: (libc)Opening Streams.
* __freading: (libc)Opening Streams.
* __fsetlocking: (libc)Streams and Threads.
* __fwritable: (libc)Opening Streams.
* __fwriting: (libc)Opening Streams.
* __gconv_end_fct: (libc)glibc iconv Implementation.
* __gconv_fct: (libc)glibc iconv Implementation.
* __gconv_init_fct: (libc)glibc iconv Implementation.
* __va_copy: (libc)Argument Macros.
* _exit: (libc)Termination Internals.
* _flushlbf: (libc)Flushing Buffers.
* _tolower: (libc)Case Conversion.
* _toupper: (libc)Case Conversion.
* a64l: (libc)Encode Binary Data.
* abort: (libc)Aborting a Program.
* abs: (libc)Absolute Value.
* accept: (libc)Accepting Connections.
* access: (libc)Testing File Access.
* acos: (libc)Inverse Trig Functions.
* acosf: (libc)Inverse Trig Functions.
* acosh: (libc)Hyperbolic Functions.
* acoshf: (libc)Hyperbolic Functions.
* acoshl: (libc)Hyperbolic Functions.
* acosl: (libc)Inverse Trig Functions.
* addmntent: (libc)mtab.
* addseverity: (libc)Adding Severity Classes.
* adjtime: (libc)High-Resolution Calendar.
* adjtimex: (libc)High-Resolution Calendar.
* aio_cancel64: (libc)Cancel AIO Operations.
* aio_cancel: (libc)Cancel AIO Operations.
* aio_error64: (libc)Status of AIO Operations.
* aio_error: (libc)Status of AIO Operations.
* aio_fsync64: (libc)Synchronizing AIO Operations.
* aio_fsync: (libc)Synchronizing AIO Operations.
* aio_init: (libc)Configuration of AIO.
* aio_read64: (libc)Asynchronous Reads/Writes.
* aio_read: (libc)Asynchronous Reads/Writes.
* aio_return64: (libc)Status of AIO Operations.
* aio_return: (libc)Status of AIO Operations.
* aio_suspend64: (libc)Synchronizing AIO Operations.
* aio_suspend: (libc)Synchronizing AIO Operations.
* aio_write64: (libc)Asynchronous Reads/Writes.
* aio_write: (libc)Asynchronous Reads/Writes.
* alarm: (libc)Setting an Alarm.
* alloca: (libc)Variable Size Automatic.
* alphasort64: (libc)Scanning Directory Content.
* alphasort: (libc)Scanning Directory Content.
* argp_error: (libc)Argp Helper Functions.
* argp_failure: (libc)Argp Helper Functions.
* argp_help: (libc)Argp Help.
* argp_parse: (libc)Argp.
* argp_state_help: (libc)Argp Helper Functions.
* argp_usage: (libc)Argp Helper Functions.
* argz_add: (libc)Argz Functions.
* argz_add_sep: (libc)Argz Functions.
* argz_append: (libc)Argz Functions.
* argz_count: (libc)Argz Functions.
* argz_create: (libc)Argz Functions.
* argz_create_sep: (libc)Argz Functions.
* argz_delete: (libc)Argz Functions.
* argz_extract: (libc)Argz Functions.
* argz_insert: (libc)Argz Functions.
* argz_next: (libc)Argz Functions.
* argz_replace: (libc)Argz Functions.
* argz_stringify: (libc)Argz Functions.
* asctime: (libc)Formatting Calendar Time.
* asctime_r: (libc)Formatting Calendar Time.
* asin: (libc)Inverse Trig Functions.
* asinf: (libc)Inverse Trig Functions.
* asinh: (libc)Hyperbolic Functions.
* asinhf: (libc)Hyperbolic Functions.
* asinhl: (libc)Hyperbolic Functions.
* asinl: (libc)Inverse Trig Functions.
* asprintf: (libc)Dynamic Output.
* assert: (libc)Consistency Checking.
* assert_perror: (libc)Consistency Checking.
* atan2: (libc)Inverse Trig Functions.
* atan2f: (libc)Inverse Trig Functions.
* atan2l: (libc)Inverse Trig Functions.
* atan: (libc)Inverse Trig Functions.
* atanf: (libc)Inverse Trig Functions.
* atanh: (libc)Hyperbolic Functions.
* atanhf: (libc)Hyperbolic Functions.
* atanhl: (libc)Hyperbolic Functions.
* atanl: (libc)Inverse Trig Functions.
* atexit: (libc)Cleanups on Exit.
* atof: (libc)Parsing of Floats.
* atoi: (libc)Parsing of Integers.
* atol: (libc)Parsing of Integers.
* atoll: (libc)Parsing of Integers.
* backtrace: (libc)Backtraces.
* backtrace_symbols: (libc)Backtraces.
* backtrace_symbols_fd: (libc)Backtraces.
* basename: (libc)Finding Tokens in a String.
* basename: (libc)Finding Tokens in a String.
* bcmp: (libc)String/Array Comparison.
* bcopy: (libc)Copying and Concatenation.
* bind: (libc)Setting Address.
* bind_textdomain_codeset: (libc)Charset conversion in gettext.
* bindtextdomain: (libc)Locating gettext catalog.
* brk: (libc)Resizing the Data Segment.
* bsearch: (libc)Array Search Function.
* btowc: (libc)Converting a Character.
* bzero: (libc)Copying and Concatenation.
* cabs: (libc)Absolute Value.
* cabsf: (libc)Absolute Value.
* cabsl: (libc)Absolute Value.
* cacos: (libc)Inverse Trig Functions.
* cacosf: (libc)Inverse Trig Functions.
* cacosh: (libc)Hyperbolic Functions.
* cacoshf: (libc)Hyperbolic Functions.
* cacoshl: (libc)Hyperbolic Functions.
* cacosl: (libc)Inverse Trig Functions.
* calloc: (libc)Allocating Cleared Space.
* canonicalize_file_name: (libc)Symbolic Links.
* carg: (libc)Operations on Complex.
* cargf: (libc)Operations on Complex.
* cargl: (libc)Operations on Complex.
* casin: (libc)Inverse Trig Functions.
* casinf: (libc)Inverse Trig Functions.
* casinh: (libc)Hyperbolic Functions.
* casinhf: (libc)Hyperbolic Functions.
* casinhl: (libc)Hyperbolic Functions.
* casinl: (libc)Inverse Trig Functions.
* catan: (libc)Inverse Trig Functions.
* catanf: (libc)Inverse Trig Functions.
* catanh: (libc)Hyperbolic Functions.
* catanhf: (libc)Hyperbolic Functions.
* catanhl: (libc)Hyperbolic Functions.
* catanl: (libc)Inverse Trig Functions.
* catclose: (libc)The catgets Functions.
* catgets: (libc)The catgets Functions.
* catopen: (libc)The catgets Functions.
* cbc_crypt: (libc)DES Encryption.
* cbrt: (libc)Exponents and Logarithms.
* cbrtf: (libc)Exponents and Logarithms.
* cbrtl: (libc)Exponents and Logarithms.
* ccos: (libc)Trig Functions.
* ccosf: (libc)Trig Functions.
* ccosh: (libc)Hyperbolic Functions.
* ccoshf: (libc)Hyperbolic Functions.
* ccoshl: (libc)Hyperbolic Functions.
* ccosl: (libc)Trig Functions.
* ceil: (libc)Rounding Functions.
* ceilf: (libc)Rounding Functions.
* ceill: (libc)Rounding Functions.
* cexp: (libc)Exponents and Logarithms.
* cexpf: (libc)Exponents and Logarithms.
* cexpl: (libc)Exponents and Logarithms.
* cfgetispeed: (libc)Line Speed.
* cfgetospeed: (libc)Line Speed.
* cfmakeraw: (libc)Noncanonical Input.
* cfree: (libc)Freeing after Malloc.
* cfsetispeed: (libc)Line Speed.
* cfsetospeed: (libc)Line Speed.
* cfsetspeed: (libc)Line Speed.
* chdir: (libc)Working Directory.
* chmod: (libc)Setting Permissions.
* chown: (libc)File Owner.
* cimag: (libc)Operations on Complex.
* cimagf: (libc)Operations on Complex.
* cimagl: (libc)Operations on Complex.
* clearenv: (libc)Environment Access.
* clearerr: (libc)Error Recovery.
* clearerr_unlocked: (libc)Error Recovery.
* clock: (libc)CPU Time.
* clog10: (libc)Exponents and Logarithms.
* clog10f: (libc)Exponents and Logarithms.
* clog10l: (libc)Exponents and Logarithms.
* clog: (libc)Exponents and Logarithms.
* clogf: (libc)Exponents and Logarithms.
* clogl: (libc)Exponents and Logarithms.
* close: (libc)Opening and Closing Files.
* closedir: (libc)Reading/Closing Directory.
* closelog: (libc)closelog.
* confstr: (libc)String Parameters.
* conj: (libc)Operations on Complex.
* conjf: (libc)Operations on Complex.
* conjl: (libc)Operations on Complex.
* connect: (libc)Connecting.
* copysign: (libc)FP Bit Twiddling.
* copysignf: (libc)FP Bit Twiddling.
* copysignl: (libc)FP Bit Twiddling.
* cos: (libc)Trig Functions.
* cosf: (libc)Trig Functions.
* cosh: (libc)Hyperbolic Functions.
* coshf: (libc)Hyperbolic Functions.
* coshl: (libc)Hyperbolic Functions.
* cosl: (libc)Trig Functions.
* cpow: (libc)Exponents and Logarithms.
* cpowf: (libc)Exponents and Logarithms.
* cpowl: (libc)Exponents and Logarithms.
* cproj: (libc)Operations on Complex.
* cprojf: (libc)Operations on Complex.
* cprojl: (libc)Operations on Complex.
* creal: (libc)Operations on Complex.
* crealf: (libc)Operations on Complex.
* creall: (libc)Operations on Complex.
* creat64: (libc)Opening and Closing Files.
* creat: (libc)Opening and Closing Files.
* crypt: (libc)crypt.
* crypt_r: (libc)crypt.
* csin: (libc)Trig Functions.
* csinf: (libc)Trig Functions.
* csinh: (libc)Hyperbolic Functions.
* csinhf: (libc)Hyperbolic Functions.
* csinhl: (libc)Hyperbolic Functions.
* csinl: (libc)Trig Functions.
* csqrt: (libc)Exponents and Logarithms.
* csqrtf: (libc)Exponents and Logarithms.
* csqrtl: (libc)Exponents and Logarithms.
* ctan: (libc)Trig Functions.
* ctanf: (libc)Trig Functions.
* ctanh: (libc)Hyperbolic Functions.
* ctanhf: (libc)Hyperbolic Functions.
* ctanhl: (libc)Hyperbolic Functions.
* ctanl: (libc)Trig Functions.
* ctermid: (libc)Identifying the Terminal.
* ctime: (libc)Formatting Calendar Time.
* ctime_r: (libc)Formatting Calendar Time.
* cuserid: (libc)Who Logged In.
* dcgettext: (libc)Translation with gettext.
* dcngettext: (libc)Advanced gettext functions.
* des_setparity: (libc)DES Encryption.
* dgettext: (libc)Translation with gettext.
* difftime: (libc)Elapsed Time.
* dirfd: (libc)Opening a Directory.
* dirname: (libc)Finding Tokens in a String.
* div: (libc)Integer Division.
* dngettext: (libc)Advanced gettext functions.
* drand48: (libc)SVID Random.
* drand48_r: (libc)SVID Random.
* drem: (libc)Remainder Functions.
* dremf: (libc)Remainder Functions.
* dreml: (libc)Remainder Functions.
* dup2: (libc)Duplicating Descriptors.
* dup: (libc)Duplicating Descriptors.
* ecb_crypt: (libc)DES Encryption.
* ecvt: (libc)System V Number Conversion.
* ecvt_r: (libc)System V Number Conversion.
* encrypt: (libc)DES Encryption.
* encrypt_r: (libc)DES Encryption.
* endfsent: (libc)fstab.
* endgrent: (libc)Scanning All Groups.
* endhostent: (libc)Host Names.
* endmntent: (libc)mtab.
* endnetent: (libc)Networks Database.
* endnetgrent: (libc)Lookup Netgroup.
* endprotoent: (libc)Protocols Database.
* endpwent: (libc)Scanning All Users.
* endservent: (libc)Services Database.
* endutent: (libc)Manipulating the Database.
* endutxent: (libc)XPG Functions.
* envz_add: (libc)Envz Functions.
* envz_entry: (libc)Envz Functions.
* envz_get: (libc)Envz Functions.
* envz_merge: (libc)Envz Functions.
* envz_strip: (libc)Envz Functions.
* erand48: (libc)SVID Random.
* erand48_r: (libc)SVID Random.
* erf: (libc)Special Functions.
* erfc: (libc)Special Functions.
* erfcf: (libc)Special Functions.
* erfcl: (libc)Special Functions.
* erff: (libc)Special Functions.
* erfl: (libc)Special Functions.
* err: (libc)Error Messages.
* errno: (libc)Checking for Errors.
* error: (libc)Error Messages.
* error_at_line: (libc)Error Messages.
* errx: (libc)Error Messages.
* execl: (libc)Executing a File.
* execle: (libc)Executing a File.
* execlp: (libc)Executing a File.
* execv: (libc)Executing a File.
* execve: (libc)Executing a File.
* execvp: (libc)Executing a File.
* exit: (libc)Normal Termination.
* exp10: (libc)Exponents and Logarithms.
* exp10f: (libc)Exponents and Logarithms.
* exp10l: (libc)Exponents and Logarithms.
* exp2: (libc)Exponents and Logarithms.
* exp2f: (libc)Exponents and Logarithms.
* exp2l: (libc)Exponents and Logarithms.
* exp: (libc)Exponents and Logarithms.
* expf: (libc)Exponents and Logarithms.
* expl: (libc)Exponents and Logarithms.
* expm1: (libc)Exponents and Logarithms.
* expm1f: (libc)Exponents and Logarithms.
* expm1l: (libc)Exponents and Logarithms.
* fabs: (libc)Absolute Value.
* fabsf: (libc)Absolute Value.
* fabsl: (libc)Absolute Value.
* fchdir: (libc)Working Directory.
* fchmod: (libc)Setting Permissions.
* fchown: (libc)File Owner.
* fclean: (libc)Cleaning Streams.
* fclose: (libc)Closing Streams.
* fcloseall: (libc)Closing Streams.
* fcntl: (libc)Control Operations.
* fcvt: (libc)System V Number Conversion.
* fcvt_r: (libc)System V Number Conversion.
* fdatasync: (libc)Synchronizing I/O.
* fdim: (libc)Misc FP Arithmetic.
* fdimf: (libc)Misc FP Arithmetic.
* fdiml: (libc)Misc FP Arithmetic.
* fdopen: (libc)Descriptors and Streams.
* feclearexcept: (libc)Status bit operations.
* fedisableexcept: (libc)Control Functions.
* feenableexcept: (libc)Control Functions.
* fegetenv: (libc)Control Functions.
* fegetexcept: (libc)Control Functions.
* fegetexceptflag: (libc)Status bit operations.
* fegetround: (libc)Rounding.
* feholdexcept: (libc)Control Functions.
* feof: (libc)EOF and Errors.
* feof_unlocked: (libc)EOF and Errors.
* feraiseexcept: (libc)Status bit operations.
* ferror: (libc)EOF and Errors.
* ferror_unlocked: (libc)EOF and Errors.
* fesetenv: (libc)Control Functions.
* fesetexceptflag: (libc)Status bit operations.
* fesetround: (libc)Rounding.
* fetestexcept: (libc)Status bit operations.
* feupdateenv: (libc)Control Functions.
* fflush: (libc)Flushing Buffers.
* fflush_unlocked: (libc)Flushing Buffers.
* fgetc: (libc)Character Input.
* fgetc_unlocked: (libc)Character Input.
* fgetgrent: (libc)Scanning All Groups.
* fgetgrent_r: (libc)Scanning All Groups.
* fgetpos64: (libc)Portable Positioning.
* fgetpos: (libc)Portable Positioning.
* fgetpwent: (libc)Scanning All Users.
* fgetpwent_r: (libc)Scanning All Users.
* fgets: (libc)Line Input.
* fgets_unlocked: (libc)Line Input.
* fgetwc: (libc)Character Input.
* fgetwc_unlocked: (libc)Character Input.
* fgetws: (libc)Line Input.
* fgetws_unlocked: (libc)Line Input.
* fileno: (libc)Descriptors and Streams.
* fileno_unlocked: (libc)Descriptors and Streams.
* finite: (libc)Floating Point Classes.
* finitef: (libc)Floating Point Classes.
* finitel: (libc)Floating Point Classes.
* flockfile: (libc)Streams and Threads.
* floor: (libc)Rounding Functions.
* floorf: (libc)Rounding Functions.
* floorl: (libc)Rounding Functions.
* fma: (libc)Misc FP Arithmetic.
* fmaf: (libc)Misc FP Arithmetic.
* fmal: (libc)Misc FP Arithmetic.
* fmax: (libc)Misc FP Arithmetic.
* fmaxf: (libc)Misc FP Arithmetic.
* fmaxl: (libc)Misc FP Arithmetic.
* fmemopen: (libc)String Streams.
* fmin: (libc)Misc FP Arithmetic.
* fminf: (libc)Misc FP Arithmetic.
* fminl: (libc)Misc FP Arithmetic.
* fmod: (libc)Remainder Functions.
* fmodf: (libc)Remainder Functions.
* fmodl: (libc)Remainder Functions.
* fmtmsg: (libc)Printing Formatted Messages.
* fnmatch: (libc)Wildcard Matching.
* fopen64: (libc)Opening Streams.
* fopen: (libc)Opening Streams.
* fopencookie: (libc)Streams and Cookies.
* fork: (libc)Creating a Process.
* forkpty: (libc)Pseudo-Terminal Pairs.
* fpathconf: (libc)Pathconf.
* fpclassify: (libc)Floating Point Classes.
* fprintf: (libc)Formatted Output Functions.
* fputc: (libc)Simple Output.
* fputc_unlocked: (libc)Simple Output.
* fputs: (libc)Simple Output.
* fputs_unlocked: (libc)Simple Output.
* fputwc: (libc)Simple Output.
* fputwc_unlocked: (libc)Simple Output.
* fputws: (libc)Simple Output.
* fputws_unlocked: (libc)Simple Output.
* fread: (libc)Block Input/Output.
* fread_unlocked: (libc)Block Input/Output.
* free: (libc)Freeing after Malloc.
* freopen64: (libc)Opening Streams.
* freopen: (libc)Opening Streams.
* frexp: (libc)Normalization Functions.
* frexpf: (libc)Normalization Functions.
* frexpl: (libc)Normalization Functions.
* fscanf: (libc)Formatted Input Functions.
* fseek: (libc)File Positioning.
* fseeko64: (libc)File Positioning.
* fseeko: (libc)File Positioning.
* fsetpos64: (libc)Portable Positioning.
* fsetpos: (libc)Portable Positioning.
* fstat64: (libc)Reading Attributes.
* fstat: (libc)Reading Attributes.
* fsync: (libc)Synchronizing I/O.
* ftell: (libc)File Positioning.
* ftello64: (libc)File Positioning.
* ftello: (libc)File Positioning.
* ftruncate64: (libc)File Size.
* ftruncate: (libc)File Size.
* ftrylockfile: (libc)Streams and Threads.
* ftw64: (libc)Working with Directory Trees.
* ftw: (libc)Working with Directory Trees.
* funlockfile: (libc)Streams and Threads.
* futimes: (libc)File Times.
* fwide: (libc)Streams and I18N.
* fwprintf: (libc)Formatted Output Functions.
* fwrite: (libc)Block Input/Output.
* fwrite_unlocked: (libc)Block Input/Output.
* fwscanf: (libc)Formatted Input Functions.
* gamma: (libc)Special Functions.
* gammaf: (libc)Special Functions.
* gammal: (libc)Special Functions.
* gcvt: (libc)System V Number Conversion.
* get_avphys_pages: (libc)Query Memory Parameters.
* get_current_dir_name: (libc)Working Directory.
* get_nprocs: (libc)Processor Resources.
* get_nprocs_conf: (libc)Processor Resources.
* get_phys_pages: (libc)Query Memory Parameters.
* getc: (libc)Character Input.
* getc_unlocked: (libc)Character Input.
* getchar: (libc)Character Input.
* getchar_unlocked: (libc)Character Input.
* getcontext: (libc)System V contexts.
* getcwd: (libc)Working Directory.
* getdate: (libc)General Time String Parsing.
* getdate_r: (libc)General Time String Parsing.
* getdelim: (libc)Line Input.
* getdomainnname: (libc)Host Identification.
* getegid: (libc)Reading Persona.
* getenv: (libc)Environment Access.
* geteuid: (libc)Reading Persona.
* getfsent: (libc)fstab.
* getfsfile: (libc)fstab.
* getfsspec: (libc)fstab.
* getgid: (libc)Reading Persona.
* getgrent: (libc)Scanning All Groups.
* getgrent_r: (libc)Scanning All Groups.
* getgrgid: (libc)Lookup Group.
* getgrgid_r: (libc)Lookup Group.
* getgrnam: (libc)Lookup Group.
* getgrnam_r: (libc)Lookup Group.
* getgrouplist: (libc)Setting Groups.
* getgroups: (libc)Reading Persona.
* gethostbyaddr: (libc)Host Names.
* gethostbyaddr_r: (libc)Host Names.
* gethostbyname2: (libc)Host Names.
* gethostbyname2_r: (libc)Host Names.
* gethostbyname: (libc)Host Names.
* gethostbyname_r: (libc)Host Names.
* gethostent: (libc)Host Names.
* gethostid: (libc)Host Identification.
* gethostname: (libc)Host Identification.
* getitimer: (libc)Setting an Alarm.
* getline: (libc)Line Input.
* getloadavg: (libc)Processor Resources.
* getlogin: (libc)Who Logged In.
* getmntent: (libc)mtab.
* getmntent_r: (libc)mtab.
* getnetbyaddr: (libc)Networks Database.
* getnetbyname: (libc)Networks Database.
* getnetent: (libc)Networks Database.
* getnetgrent: (libc)Lookup Netgroup.
* getnetgrent_r: (libc)Lookup Netgroup.
* getopt: (libc)Using Getopt.
* getopt_long: (libc)Getopt Long Options.
* getopt_long_only: (libc)Getopt Long Options.
* getpagesize: (libc)Query Memory Parameters.
* getpass: (libc)getpass.
* getpeername: (libc)Who is Connected.
* getpgid: (libc)Process Group Functions.
* getpgrp: (libc)Process Group Functions.
* getpgrp: (libc)Process Group Functions.
* getpid: (libc)Process Identification.
* getppid: (libc)Process Identification.
* getpriority: (libc)Traditional Scheduling Functions.
* getprotobyname: (libc)Protocols Database.
* getprotobynumber: (libc)Protocols Database.
* getprotoent: (libc)Protocols Database.
* getpt: (libc)Allocation.
* getpwent: (libc)Scanning All Users.
* getpwent_r: (libc)Scanning All Users.
* getpwnam: (libc)Lookup User.
* getpwnam_r: (libc)Lookup User.
* getpwuid: (libc)Lookup User.
* getpwuid_r: (libc)Lookup User.
* getrlimit64: (libc)Limits on Resources.
* getrlimit: (libc)Limits on Resources.
* getrusage: (libc)Resource Usage.
* gets: (libc)Line Input.
* getservbyname: (libc)Services Database.
* getservbyport: (libc)Services Database.
* getservent: (libc)Services Database.
* getsid: (libc)Process Group Functions.
* getsockname: (libc)Reading Address.
* getsockopt: (libc)Socket Option Functions.
* getsubopt: (libc)Suboptions.
* gettext: (libc)Translation with gettext.
* gettimeofday: (libc)High-Resolution Calendar.
* getuid: (libc)Reading Persona.
* getumask: (libc)Setting Permissions.
* getutent: (libc)Manipulating the Database.
* getutent_r: (libc)Manipulating the Database.
* getutid: (libc)Manipulating the Database.
* getutid_r: (libc)Manipulating the Database.
* getutline: (libc)Manipulating the Database.
* getutline_r: (libc)Manipulating the Database.
* getutmp: (libc)XPG Functions.
* getutmpx: (libc)XPG Functions.
* getutxent: (libc)XPG Functions.
* getutxid: (libc)XPG Functions.
* getutxline: (libc)XPG Functions.
* getw: (libc)Character Input.
* getwc: (libc)Character Input.
* getwc_unlocked: (libc)Character Input.
* getwchar: (libc)Character Input.
* getwchar_unlocked: (libc)Character Input.
* getwd: (libc)Working Directory.
* glob64: (libc)Calling Glob.
* glob: (libc)Calling Glob.
* globfree64: (libc)More Flags for Globbing.
* globfree: (libc)More Flags for Globbing.
* gmtime: (libc)Broken-down Time.
* gmtime_r: (libc)Broken-down Time.
* grantpt: (libc)Allocation.
* gsignal: (libc)Signaling Yourself.
* gtty: (libc)BSD Terminal Modes.
* hasmntopt: (libc)mtab.
* hcreate: (libc)Hash Search Function.
* hcreate_r: (libc)Hash Search Function.
* hdestroy: (libc)Hash Search Function.
* hdestroy_r: (libc)Hash Search Function.
* hsearch: (libc)Hash Search Function.
* hsearch_r: (libc)Hash Search Function.
* htonl: (libc)Byte Order.
* htons: (libc)Byte Order.
* hypot: (libc)Exponents and Logarithms.
* hypotf: (libc)Exponents and Logarithms.
* hypotl: (libc)Exponents and Logarithms.
* iconv: (libc)Generic Conversion Interface.
* iconv_close: (libc)Generic Conversion Interface.
* iconv_open: (libc)Generic Conversion Interface.
* if_freenameindex: (libc)Interface Naming.
* if_indextoname: (libc)Interface Naming.
* if_nameindex: (libc)Interface Naming.
* if_nametoindex: (libc)Interface Naming.
* ilogb: (libc)Exponents and Logarithms.
* ilogbf: (libc)Exponents and Logarithms.
* ilogbl: (libc)Exponents and Logarithms.
* imaxabs: (libc)Absolute Value.
* imaxdiv: (libc)Integer Division.
* in6addr_any: (libc)Host Address Data Type.
* in6addr_loopback: (libc)Host Address Data Type.
* index: (libc)Search Functions.
* inet_addr: (libc)Host Address Functions.
* inet_aton: (libc)Host Address Functions.
* inet_lnaof: (libc)Host Address Functions.
* inet_makeaddr: (libc)Host Address Functions.
* inet_netof: (libc)Host Address Functions.
* inet_network: (libc)Host Address Functions.
* inet_ntoa: (libc)Host Address Functions.
* inet_ntop: (libc)Host Address Functions.
* inet_pton: (libc)Host Address Functions.
* initgroups: (libc)Setting Groups.
* initstate: (libc)BSD Random.
* initstate_r: (libc)BSD Random.
* innetgr: (libc)Netgroup Membership.
* int: (libc)Random Access Directory.
* ioctl: (libc)IOCTLs.
* isalnum: (libc)Classification of Characters.
* isalpha: (libc)Classification of Characters.
* isascii: (libc)Classification of Characters.
* isatty: (libc)Is It a Terminal.
* isblank: (libc)Classification of Characters.
* iscntrl: (libc)Classification of Characters.
* isdigit: (libc)Classification of Characters.
* isfinite: (libc)Floating Point Classes.
* isgraph: (libc)Classification of Characters.
* isgreater: (libc)FP Comparison Functions.
* isgreaterequal: (libc)FP Comparison Functions.
* isinf: (libc)Floating Point Classes.
* isinff: (libc)Floating Point Classes.
* isinfl: (libc)Floating Point Classes.
* isless: (libc)FP Comparison Functions.
* islessequal: (libc)FP Comparison Functions.
* islessgreater: (libc)FP Comparison Functions.
* islower: (libc)Classification of Characters.
* isnan: (libc)Floating Point Classes.
* isnan: (libc)Floating Point Classes.
* isnanf: (libc)Floating Point Classes.
* isnanl: (libc)Floating Point Classes.
* isnormal: (libc)Floating Point Classes.
* isprint: (libc)Classification of Characters.
* ispunct: (libc)Classification of Characters.
* isspace: (libc)Classification of Characters.
* isunordered: (libc)FP Comparison Functions.
* isupper: (libc)Classification of Characters.
* iswalnum: (libc)Classification of Wide Characters.
* iswalpha: (libc)Classification of Wide Characters.
* iswblank: (libc)Classification of Wide Characters.
* iswcntrl: (libc)Classification of Wide Characters.
* iswctype: (libc)Classification of Wide Characters.
* iswdigit: (libc)Classification of Wide Characters.
* iswgraph: (libc)Classification of Wide Characters.
* iswlower: (libc)Classification of Wide Characters.
* iswprint: (libc)Classification of Wide Characters.
* iswpunct: (libc)Classification of Wide Characters.
* iswspace: (libc)Classification of Wide Characters.
* iswupper: (libc)Classification of Wide Characters.
* iswxdigit: (libc)Classification of Wide Characters.
* isxdigit: (libc)Classification of Characters.
* j0: (libc)Special Functions.
* j0f: (libc)Special Functions.
* j0l: (libc)Special Functions.
* j1: (libc)Special Functions.
* j1f: (libc)Special Functions.
* j1l: (libc)Special Functions.
* jn: (libc)Special Functions.
* jnf: (libc)Special Functions.
* jnl: (libc)Special Functions.
* jrand48: (libc)SVID Random.
* jrand48_r: (libc)SVID Random.
* kill: (libc)Signaling Another Process.
* killpg: (libc)Signaling Another Process.
* l64a: (libc)Encode Binary Data.
* labs: (libc)Absolute Value.
* lcong48: (libc)SVID Random.
* lcong48_r: (libc)SVID Random.
* ldexp: (libc)Normalization Functions.
* ldexpf: (libc)Normalization Functions.
* ldexpl: (libc)Normalization Functions.
* ldiv: (libc)Integer Division.
* lfind: (libc)Array Search Function.
* lgamma: (libc)Special Functions.
* lgamma_r: (libc)Special Functions.
* lgammaf: (libc)Special Functions.
* lgammaf_r: (libc)Special Functions.
* lgammal: (libc)Special Functions.
* lgammal_r: (libc)Special Functions.
* link: (libc)Hard Links.
* lio_listio64: (libc)Asynchronous Reads/Writes.
* lio_listio: (libc)Asynchronous Reads/Writes.
* listen: (libc)Listening.
* llabs: (libc)Absolute Value.
* lldiv: (libc)Integer Division.
* llrint: (libc)Rounding Functions.
* llrintf: (libc)Rounding Functions.
* llrintl: (libc)Rounding Functions.
* llround: (libc)Rounding Functions.
* llroundf: (libc)Rounding Functions.
* llroundl: (libc)Rounding Functions.
* localeconv: (libc)The Lame Way to Locale Data.
* localtime: (libc)Broken-down Time.
* localtime_r: (libc)Broken-down Time.
* log10: (libc)Exponents and Logarithms.
* log10f: (libc)Exponents and Logarithms.
* log10l: (libc)Exponents and Logarithms.
* log1p: (libc)Exponents and Logarithms.
* log1pf: (libc)Exponents and Logarithms.
* log1pl: (libc)Exponents and Logarithms.
* log2: (libc)Exponents and Logarithms.
* log2f: (libc)Exponents and Logarithms.
* log2l: (libc)Exponents and Logarithms.
* log: (libc)Exponents and Logarithms.
* logb: (libc)Exponents and Logarithms.
* logbf: (libc)Exponents and Logarithms.
* logbl: (libc)Exponents and Logarithms.
* logf: (libc)Exponents and Logarithms.
* login: (libc)Logging In and Out.
* login_tty: (libc)Logging In and Out.
* logl: (libc)Exponents and Logarithms.
* logout: (libc)Logging In and Out.
* logwtmp: (libc)Logging In and Out.
* longjmp: (libc)Non-Local Details.
* lrand48: (libc)SVID Random.
* lrand48_r: (libc)SVID Random.
* lrint: (libc)Rounding Functions.
* lrintf: (libc)Rounding Functions.
* lrintl: (libc)Rounding Functions.
* lround: (libc)Rounding Functions.
* lroundf: (libc)Rounding Functions.
* lroundl: (libc)Rounding Functions.
* lsearch: (libc)Array Search Function.
* lseek64: (libc)File Position Primitive.
* lseek: (libc)File Position Primitive.
* lstat64: (libc)Reading Attributes.
* lstat: (libc)Reading Attributes.
* lutimes: (libc)File Times.
* madvise: (libc)Memory-mapped I/O.
* makecontext: (libc)System V contexts.
* mallinfo: (libc)Statistics of Malloc.
* malloc: (libc)Basic Allocation.
* mallopt: (libc)Malloc Tunable Parameters.
* mblen: (libc)Non-reentrant Character Conversion.
* mbrlen: (libc)Converting a Character.
* mbrtowc: (libc)Converting a Character.
* mbsinit: (libc)Keeping the state.
* mbsnrtowcs: (libc)Converting Strings.
* mbsrtowcs: (libc)Converting Strings.
* mbstowcs: (libc)Non-reentrant String Conversion.
* mbtowc: (libc)Non-reentrant Character Conversion.
* mcheck: (libc)Heap Consistency Checking.
* memalign: (libc)Aligned Memory Blocks.
* memccpy: (libc)Copying and Concatenation.
* memchr: (libc)Search Functions.
* memcmp: (libc)String/Array Comparison.
* memcpy: (libc)Copying and Concatenation.
* memfrob: (libc)Trivial Encryption.
* memmem: (libc)Search Functions.
* memmove: (libc)Copying and Concatenation.
* mempcpy: (libc)Copying and Concatenation.
* memrchr: (libc)Search Functions.
* memset: (libc)Copying and Concatenation.
* mkdir: (libc)Creating Directories.
* mkdtemp: (libc)Temporary Files.
* mkfifo: (libc)FIFO Special Files.
* mknod: (libc)Making Special Files.
* mkstemp: (libc)Temporary Files.
* mktemp: (libc)Temporary Files.
* mktime: (libc)Broken-down Time.
* mlock: (libc)Page Lock Functions.
* mlockall: (libc)Page Lock Functions.
* mmap64: (libc)Memory-mapped I/O.
* mmap: (libc)Memory-mapped I/O.
* modf: (libc)Rounding Functions.
* modff: (libc)Rounding Functions.
* modfl: (libc)Rounding Functions.
* mount: (libc)Mount-Unmount-Remount.
* mprobe: (libc)Heap Consistency Checking.
* mrand48: (libc)SVID Random.
* mrand48_r: (libc)SVID Random.
* mremap: (libc)Memory-mapped I/O.
* msync: (libc)Memory-mapped I/O.
* mtrace: (libc)Tracing malloc.
* munlock: (libc)Page Lock Functions.
* munlockall: (libc)Page Lock Functions.
* munmap: (libc)Memory-mapped I/O.
* muntrace: (libc)Tracing malloc.
* nan: (libc)FP Bit Twiddling.
* nanf: (libc)FP Bit Twiddling.
* nanl: (libc)FP Bit Twiddling.
* nanosleep: (libc)Sleeping.
* nearbyint: (libc)Rounding Functions.
* nearbyintf: (libc)Rounding Functions.
* nearbyintl: (libc)Rounding Functions.
* nextafter: (libc)FP Bit Twiddling.
* nextafterf: (libc)FP Bit Twiddling.
* nextafterl: (libc)FP Bit Twiddling.
* nexttoward: (libc)FP Bit Twiddling.
* nexttowardf: (libc)FP Bit Twiddling.
* nexttowardl: (libc)FP Bit Twiddling.
* nftw64: (libc)Working with Directory Trees.
* nftw: (libc)Working with Directory Trees.
* ngettext: (libc)Advanced gettext functions.
* nice: (libc)Traditional Scheduling Functions.
* nl_langinfo: (libc)The Elegant and Fast Way.
* nrand48: (libc)SVID Random.
* nrand48_r: (libc)SVID Random.
* ntohl: (libc)Byte Order.
* ntohs: (libc)Byte Order.
* ntp_adjtime: (libc)High Accuracy Clock.
* ntp_gettime: (libc)High Accuracy Clock.
* obstack_1grow: (libc)Growing Objects.
* obstack_1grow_fast: (libc)Extra Fast Growing.
* obstack_alignment_mask: (libc)Obstacks Data Alignment.
* obstack_alloc: (libc)Allocation in an Obstack.
* obstack_base: (libc)Status of an Obstack.
* obstack_blank: (libc)Growing Objects.
* obstack_blank_fast: (libc)Extra Fast Growing.
* obstack_chunk_size: (libc)Obstack Chunks.
* obstack_copy0: (libc)Allocation in an Obstack.
* obstack_copy: (libc)Allocation in an Obstack.
* obstack_finish: (libc)Growing Objects.
* obstack_free: (libc)Freeing Obstack Objects.
* obstack_grow0: (libc)Growing Objects.
* obstack_grow: (libc)Growing Objects.
* obstack_init: (libc)Preparing for Obstacks.
* obstack_int_grow: (libc)Growing Objects.
* obstack_int_grow_fast: (libc)Extra Fast Growing.
* obstack_next_free: (libc)Status of an Obstack.
* obstack_object_size: (libc)Growing Objects.
* obstack_object_size: (libc)Status of an Obstack.
* obstack_printf: (libc)Dynamic Output.
* obstack_ptr_grow: (libc)Growing Objects.
* obstack_ptr_grow_fast: (libc)Extra Fast Growing.
* obstack_room: (libc)Extra Fast Growing.
* obstack_vprintf: (libc)Variable Arguments Output.
* offsetof: (libc)Structure Measurement.
* on_exit: (libc)Cleanups on Exit.
* open64: (libc)Opening and Closing Files.
* open: (libc)Opening and Closing Files.
* open_memstream: (libc)String Streams.
* open_obstack_stream: (libc)Obstack Streams.
* opendir: (libc)Opening a Directory.
* openlog: (libc)openlog.
* openpty: (libc)Pseudo-Terminal Pairs.
* parse_printf_format: (libc)Parsing a Template String.
* pathconf: (libc)Pathconf.
* pause: (libc)Using Pause.
* pclose: (libc)Pipe to a Subprocess.
* perror: (libc)Error Messages.
* pipe: (libc)Creating a Pipe.
* popen: (libc)Pipe to a Subprocess.
* posix_memalign: (libc)Aligned Memory Blocks.
* pow10: (libc)Exponents and Logarithms.
* pow10f: (libc)Exponents and Logarithms.
* pow10l: (libc)Exponents and Logarithms.
* pow: (libc)Exponents and Logarithms.
* powf: (libc)Exponents and Logarithms.
* powl: (libc)Exponents and Logarithms.
* pread64: (libc)I/O Primitives.
* pread: (libc)I/O Primitives.
* printf: (libc)Formatted Output Functions.
* printf_size: (libc)Predefined Printf Handlers.
* printf_size_info: (libc)Predefined Printf Handlers.
* psignal: (libc)Signal Messages.
* ptsname: (libc)Allocation.
* ptsname_r: (libc)Allocation.
* putc: (libc)Simple Output.
* putc_unlocked: (libc)Simple Output.
* putchar: (libc)Simple Output.
* putchar_unlocked: (libc)Simple Output.
* putenv: (libc)Environment Access.
* putpwent: (libc)Writing a User Entry.
* puts: (libc)Simple Output.
* pututline: (libc)Manipulating the Database.
* pututxline: (libc)XPG Functions.
* putw: (libc)Simple Output.
* putwc: (libc)Simple Output.
* putwc_unlocked: (libc)Simple Output.
* putwchar: (libc)Simple Output.
* putwchar_unlocked: (libc)Simple Output.
* pwrite64: (libc)I/O Primitives.
* pwrite: (libc)I/O Primitives.
* qecvt: (libc)System V Number Conversion.
* qecvt_r: (libc)System V Number Conversion.
* qfcvt: (libc)System V Number Conversion.
* qfcvt_r: (libc)System V Number Conversion.
* qgcvt: (libc)System V Number Conversion.
* qsort: (libc)Array Sort Function.
* raise: (libc)Signaling Yourself.
* rand: (libc)ISO Random.
* rand_r: (libc)ISO Random.
* random: (libc)BSD Random.
* random_r: (libc)BSD Random.
* rawmemchr: (libc)Search Functions.
* read: (libc)I/O Primitives.
* readdir64: (libc)Reading/Closing Directory.
* readdir64_r: (libc)Reading/Closing Directory.
* readdir: (libc)Reading/Closing Directory.
* readdir_r: (libc)Reading/Closing Directory.
* readlink: (libc)Symbolic Links.
* readv: (libc)Scatter-Gather.
* realloc: (libc)Changing Block Size.
* realpath: (libc)Symbolic Links.
* recv: (libc)Receiving Data.
* recvfrom: (libc)Receiving Datagrams.
* recvmsg: (libc)Receiving Datagrams.
* regcomp: (libc)POSIX Regexp Compilation.
* regerror: (libc)Regexp Cleanup.
* regexec: (libc)Matching POSIX Regexps.
* regfree: (libc)Regexp Cleanup.
* register_printf_function: (libc)Registering New Conversions.
* remainder: (libc)Remainder Functions.
* remainderf: (libc)Remainder Functions.
* remainderl: (libc)Remainder Functions.
* remove: (libc)Deleting Files.
* rename: (libc)Renaming Files.
* rewind: (libc)File Positioning.
* rewinddir: (libc)Random Access Directory.
* rindex: (libc)Search Functions.
* rint: (libc)Rounding Functions.
* rintf: (libc)Rounding Functions.
* rintl: (libc)Rounding Functions.
* rmdir: (libc)Deleting Files.
* round: (libc)Rounding Functions.
* roundf: (libc)Rounding Functions.
* roundl: (libc)Rounding Functions.
* rpmatch: (libc)Yes-or-No Questions.
* sbrk: (libc)Resizing the Data Segment.
* scalb: (libc)Normalization Functions.
* scalbf: (libc)Normalization Functions.
* scalbl: (libc)Normalization Functions.
* scalbln: (libc)Normalization Functions.
* scalblnf: (libc)Normalization Functions.
* scalblnl: (libc)Normalization Functions.
* scalbn: (libc)Normalization Functions.
* scalbnf: (libc)Normalization Functions.
* scalbnl: (libc)Normalization Functions.
* scandir64: (libc)Scanning Directory Content.
* scandir: (libc)Scanning Directory Content.
* scanf: (libc)Formatted Input Functions.
* sched_get_priority_max: (libc)Basic Scheduling Functions.
* sched_get_priority_min: (libc)Basic Scheduling Functions.
* sched_getaffinity: (libc)CPU Affinity.
* sched_getparam: (libc)Basic Scheduling Functions.
* sched_getscheduler: (libc)Basic Scheduling Functions.
* sched_rr_get_interval: (libc)Basic Scheduling Functions.
* sched_setaffinity: (libc)CPU Affinity.
* sched_setparam: (libc)Basic Scheduling Functions.
* sched_setscheduler: (libc)Basic Scheduling Functions.
* sched_yield: (libc)Basic Scheduling Functions.
* seed48: (libc)SVID Random.
* seed48_r: (libc)SVID Random.
* seekdir: (libc)Random Access Directory.
* select: (libc)Waiting for I/O.
* send: (libc)Sending Data.
* sendmsg: (libc)Receiving Datagrams.
* sendto: (libc)Sending Datagrams.
* setbuf: (libc)Controlling Buffering.
* setbuffer: (libc)Controlling Buffering.
* setcontext: (libc)System V contexts.
* setdomainname: (libc)Host Identification.
* setegid: (libc)Setting Groups.
* setenv: (libc)Environment Access.
* seteuid: (libc)Setting User ID.
* setfsent: (libc)fstab.
* setgid: (libc)Setting Groups.
* setgrent: (libc)Scanning All Groups.
* setgroups: (libc)Setting Groups.
* sethostent: (libc)Host Names.
* sethostid: (libc)Host Identification.
* sethostname: (libc)Host Identification.
* setitimer: (libc)Setting an Alarm.
* setjmp: (libc)Non-Local Details.
* setkey: (libc)DES Encryption.
* setkey_r: (libc)DES Encryption.
* setlinebuf: (libc)Controlling Buffering.
* setlocale: (libc)Setting the Locale.
* setlogmask: (libc)setlogmask.
* setmntent: (libc)mtab.
* setnetent: (libc)Networks Database.
* setnetgrent: (libc)Lookup Netgroup.
* setpgid: (libc)Process Group Functions.
* setpgrp: (libc)Process Group Functions.
* setpriority: (libc)Traditional Scheduling Functions.
* setprotoent: (libc)Protocols Database.
* setpwent: (libc)Scanning All Users.
* setregid: (libc)Setting Groups.
* setreuid: (libc)Setting User ID.
* setrlimit64: (libc)Limits on Resources.
* setrlimit: (libc)Limits on Resources.
* setservent: (libc)Services Database.
* setsid: (libc)Process Group Functions.
* setsockopt: (libc)Socket Option Functions.
* setstate: (libc)BSD Random.
* setstate_r: (libc)BSD Random.
* settimeofday: (libc)High-Resolution Calendar.
* setuid: (libc)Setting User ID.
* setutent: (libc)Manipulating the Database.
* setutxent: (libc)XPG Functions.
* setvbuf: (libc)Controlling Buffering.
* shutdown: (libc)Closing a Socket.
* sigaction: (libc)Advanced Signal Handling.
* sigaddset: (libc)Signal Sets.
* sigaltstack: (libc)Signal Stack.
* sigblock: (libc)Blocking in BSD.
* sigdelset: (libc)Signal Sets.
* sigemptyset: (libc)Signal Sets.
* sigfillset: (libc)Signal Sets.
* siginterrupt: (libc)BSD Handler.
* sigismember: (libc)Signal Sets.
* siglongjmp: (libc)Non-Local Exits and Signals.
* sigmask: (libc)Blocking in BSD.
* signal: (libc)Basic Signal Handling.
* signbit: (libc)FP Bit Twiddling.
* significand: (libc)Normalization Functions.
* significandf: (libc)Normalization Functions.
* significandl: (libc)Normalization Functions.
* sigpause: (libc)Blocking in BSD.
* sigpending: (libc)Checking for Pending Signals.
* sigprocmask: (libc)Process Signal Mask.
* sigsetjmp: (libc)Non-Local Exits and Signals.
* sigsetmask: (libc)Blocking in BSD.
* sigstack: (libc)Signal Stack.
* sigsuspend: (libc)Sigsuspend.
* sigvec: (libc)BSD Handler.
* sin: (libc)Trig Functions.
* sincos: (libc)Trig Functions.
* sincosf: (libc)Trig Functions.
* sincosl: (libc)Trig Functions.
* sinf: (libc)Trig Functions.
* sinh: (libc)Hyperbolic Functions.
* sinhf: (libc)Hyperbolic Functions.
* sinhl: (libc)Hyperbolic Functions.
* sinl: (libc)Trig Functions.
* sleep: (libc)Sleeping.
* snprintf: (libc)Formatted Output Functions.
* socket: (libc)Creating a Socket.
* socketpair: (libc)Socket Pairs.
* sprintf: (libc)Formatted Output Functions.
* sqrt: (libc)Exponents and Logarithms.
* sqrtf: (libc)Exponents and Logarithms.
* sqrtl: (libc)Exponents and Logarithms.
* srand48: (libc)SVID Random.
* srand48_r: (libc)SVID Random.
* srand: (libc)ISO Random.
* srandom: (libc)BSD Random.
* srandom_r: (libc)BSD Random.
* sscanf: (libc)Formatted Input Functions.
* ssignal: (libc)Basic Signal Handling.
* stat64: (libc)Reading Attributes.
* stat: (libc)Reading Attributes.
* stime: (libc)Simple Calendar Time.
* stpcpy: (libc)Copying and Concatenation.
* stpncpy: (libc)Copying and Concatenation.
* strcasecmp: (libc)String/Array Comparison.
* strcasestr: (libc)Search Functions.
* strcat: (libc)Copying and Concatenation.
* strchr: (libc)Search Functions.
* strchrnul: (libc)Search Functions.
* strcmp: (libc)String/Array Comparison.
* strcoll: (libc)Collation Functions.
* strcpy: (libc)Copying and Concatenation.
* strcspn: (libc)Search Functions.
* strdup: (libc)Copying and Concatenation.
* strdupa: (libc)Copying and Concatenation.
* strerror: (libc)Error Messages.
* strerror_r: (libc)Error Messages.
* strfmon: (libc)Formatting Numbers.
* strfry: (libc)strfry.
* strftime: (libc)Formatting Calendar Time.
* strlen: (libc)String Length.
* strncasecmp: (libc)String/Array Comparison.
* strncat: (libc)Copying and Concatenation.
* strncmp: (libc)String/Array Comparison.
* strncpy: (libc)Copying and Concatenation.
* strndup: (libc)Copying and Concatenation.
* strndupa: (libc)Copying and Concatenation.
* strnlen: (libc)String Length.
* strpbrk: (libc)Search Functions.
* strptime: (libc)Low-Level Time String Parsing.
* strrchr: (libc)Search Functions.
* strsep: (libc)Finding Tokens in a String.
* strsignal: (libc)Signal Messages.
* strspn: (libc)Search Functions.
* strstr: (libc)Search Functions.
* strtod: (libc)Parsing of Floats.
* strtof: (libc)Parsing of Floats.
* strtoimax: (libc)Parsing of Integers.
* strtok: (libc)Finding Tokens in a String.
* strtok_r: (libc)Finding Tokens in a String.
* strtol: (libc)Parsing of Integers.
* strtold: (libc)Parsing of Floats.
* strtoll: (libc)Parsing of Integers.
* strtoq: (libc)Parsing of Integers.
* strtoul: (libc)Parsing of Integers.
* strtoull: (libc)Parsing of Integers.
* strtoumax: (libc)Parsing of Integers.
* strtouq: (libc)Parsing of Integers.
* strverscmp: (libc)String/Array Comparison.
* strxfrm: (libc)Collation Functions.
* stty: (libc)BSD Terminal Modes.
* swapcontext: (libc)System V contexts.
* swprintf: (libc)Formatted Output Functions.
* swscanf: (libc)Formatted Input Functions.
* symlink: (libc)Symbolic Links.
* sync: (libc)Synchronizing I/O.
* syscall: (libc)System Calls.
* sysconf: (libc)Sysconf Definition.
* sysctl: (libc)System Parameters.
* syslog: (libc)syslog; vsyslog.
* system: (libc)Running a Command.
* sysv_signal: (libc)Basic Signal Handling.
* tan: (libc)Trig Functions.
* tanf: (libc)Trig Functions.
* tanh: (libc)Hyperbolic Functions.
* tanhf: (libc)Hyperbolic Functions.
* tanhl: (libc)Hyperbolic Functions.
* tanl: (libc)Trig Functions.
* tcdrain: (libc)Line Control.
* tcflow: (libc)Line Control.
* tcflush: (libc)Line Control.
* tcgetattr: (libc)Mode Functions.
* tcgetpgrp: (libc)Terminal Access Functions.
* tcgetsid: (libc)Terminal Access Functions.
* tcsendbreak: (libc)Line Control.
* tcsetattr: (libc)Mode Functions.
* tcsetpgrp: (libc)Terminal Access Functions.
* tdelete: (libc)Tree Search Function.
* tdestroy: (libc)Tree Search Function.
* tempnam: (libc)Temporary Files.
* textdomain: (libc)Locating gettext catalog.
* tfind: (libc)Tree Search Function.
* tgamma: (libc)Special Functions.
* tgammaf: (libc)Special Functions.
* tgammal: (libc)Special Functions.
* time: (libc)Simple Calendar Time.
* timegm: (libc)Broken-down Time.
* timelocal: (libc)Broken-down Time.
* times: (libc)Processor Time.
* tmpfile64: (libc)Temporary Files.
* tmpfile: (libc)Temporary Files.
* tmpnam: (libc)Temporary Files.
* tmpnam_r: (libc)Temporary Files.
* toascii: (libc)Case Conversion.
* tolower: (libc)Case Conversion.
* toupper: (libc)Case Conversion.
* towctrans: (libc)Wide Character Case Conversion.
* towlower: (libc)Wide Character Case Conversion.
* towupper: (libc)Wide Character Case Conversion.
* trunc: (libc)Rounding Functions.
* truncate64: (libc)File Size.
* truncate: (libc)File Size.
* truncf: (libc)Rounding Functions.
* truncl: (libc)Rounding Functions.
* tsearch: (libc)Tree Search Function.
* ttyname: (libc)Is It a Terminal.
* ttyname_r: (libc)Is It a Terminal.
* twalk: (libc)Tree Search Function.
* tzset: (libc)Time Zone Functions.
* ulimit: (libc)Limits on Resources.
* umask: (libc)Setting Permissions.
* umount2: (libc)Mount-Unmount-Remount.
* umount: (libc)Mount-Unmount-Remount.
* uname: (libc)Platform Type.
* ungetc: (libc)How Unread.
* ungetwc: (libc)How Unread.
* unlink: (libc)Deleting Files.
* unlockpt: (libc)Allocation.
* unsetenv: (libc)Environment Access.
* updwtmp: (libc)Manipulating the Database.
* utime: (libc)File Times.
* utimes: (libc)File Times.
* utmpname: (libc)Manipulating the Database.
* utmpxname: (libc)XPG Functions.
* va_arg: (libc)Argument Macros.
* va_end: (libc)Argument Macros.
* va_start: (libc)Argument Macros.
* va_start: (libc)Old Varargs.
* valloc: (libc)Aligned Memory Blocks.
* vasprintf: (libc)Variable Arguments Output.
* verr: (libc)Error Messages.
* verrx: (libc)Error Messages.
* versionsort64: (libc)Scanning Directory Content.
* versionsort: (libc)Scanning Directory Content.
* vfork: (libc)Creating a Process.
* vfprintf: (libc)Variable Arguments Output.
* vfscanf: (libc)Variable Arguments Input.
* vfwprintf: (libc)Variable Arguments Output.
* vfwscanf: (libc)Variable Arguments Input.
* vlimit: (libc)Limits on Resources.
* vprintf: (libc)Variable Arguments Output.
* vscanf: (libc)Variable Arguments Input.
* vsnprintf: (libc)Variable Arguments Output.
* vsprintf: (libc)Variable Arguments Output.
* vsscanf: (libc)Variable Arguments Input.
* vswprintf: (libc)Variable Arguments Output.
* vswscanf: (libc)Variable Arguments Input.
* vsyslog: (libc)syslog; vsyslog.
* vtimes: (libc)Resource Usage.
* vwarn: (libc)Error Messages.
* vwarnx: (libc)Error Messages.
* vwprintf: (libc)Variable Arguments Output.
* vwscanf: (libc)Variable Arguments Input.
* wait3: (libc)BSD Wait Functions.
* wait4: (libc)Process Completion.
* wait: (libc)Process Completion.
* waitpid: (libc)Process Completion.
* warn: (libc)Error Messages.
* warnx: (libc)Error Messages.
* wcpcpy: (libc)Copying and Concatenation.
* wcpncpy: (libc)Copying and Concatenation.
* wcrtomb: (libc)Converting a Character.
* wcscasecmp: (libc)String/Array Comparison.
* wcscat: (libc)Copying and Concatenation.
* wcschr: (libc)Search Functions.
* wcschrnul: (libc)Search Functions.
* wcscmp: (libc)String/Array Comparison.
* wcscoll: (libc)Collation Functions.
* wcscpy: (libc)Copying and Concatenation.
* wcscspn: (libc)Search Functions.
* wcsdup: (libc)Copying and Concatenation.
* wcsftime: (libc)Formatting Calendar Time.
* wcslen: (libc)String Length.
* wcsncasecmp: (libc)String/Array Comparison.
* wcsncat: (libc)Copying and Concatenation.
* wcsncmp: (libc)String/Array Comparison.
* wcsncpy: (libc)Copying and Concatenation.
* wcsnlen: (libc)String Length.
* wcsnrtombs: (libc)Converting Strings.
* wcspbrk: (libc)Search Functions.
* wcsrchr: (libc)Search Functions.
* wcsrtombs: (libc)Converting Strings.
* wcsspn: (libc)Search Functions.
* wcsstr: (libc)Search Functions.
* wcstod: (libc)Parsing of Floats.
* wcstof: (libc)Parsing of Floats.
* wcstoimax: (libc)Parsing of Integers.
* wcstok: (libc)Finding Tokens in a String.
* wcstol: (libc)Parsing of Integers.
* wcstold: (libc)Parsing of Floats.
* wcstoll: (libc)Parsing of Integers.
* wcstombs: (libc)Non-reentrant String Conversion.
* wcstoq: (libc)Parsing of Integers.
* wcstoul: (libc)Parsing of Integers.
* wcstoull: (libc)Parsing of Integers.
* wcstoumax: (libc)Parsing of Integers.
* wcstouq: (libc)Parsing of Integers.
* wcswcs: (libc)Search Functions.
* wcsxfrm: (libc)Collation Functions.
* wctob: (libc)Converting a Character.
* wctomb: (libc)Non-reentrant Character Conversion.
* wctrans: (libc)Wide Character Case Conversion.
* wctype: (libc)Classification of Wide Characters.
* wmemchr: (libc)Search Functions.
* wmemcmp: (libc)String/Array Comparison.
* wmemcpy: (libc)Copying and Concatenation.
* wmemmove: (libc)Copying and Concatenation.
* wmempcpy: (libc)Copying and Concatenation.
* wmemset: (libc)Copying and Concatenation.
* wordexp: (libc)Calling Wordexp.
* wordfree: (libc)Calling Wordexp.
* wprintf: (libc)Formatted Output Functions.
* write: (libc)I/O Primitives.
* writev: (libc)Scatter-Gather.
* wscanf: (libc)Formatted Input Functions.
* y0: (libc)Special Functions.
* y0f: (libc)Special Functions.
* y0l: (libc)Special Functions.
* y1: (libc)Special Functions.
* y1f: (libc)Special Functions.
* y1l: (libc)Special Functions.
* yn: (libc)Special Functions.
* ynf: (libc)Special Functions.
* ynl: (libc)Special Functions.
END-INFO-DIR-ENTRY

   This file documents the GNU C library.

   This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library
Reference Manual', for Version 2.3.x.

   Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
2003 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software Needs Free Documentation" and
"GNU Lesser General Public License", the Front-Cover texts being (a)
(see below), and with the Back-Cover Texts being (b) (see below).  A
copy of the license is included in the section entitled "GNU Free
Documentation License".

   (a) The FSF's Front-Cover Text is:

   A GNU Manual

   (b) The FSF's Back-Cover Text is:

   You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.


File: libc.info,  Node: Basic Scheduling Functions,  Next: Traditional Scheduling,  Prev: Realtime Scheduling,  Up: Priority

22.3.3 Basic Scheduling Functions
---------------------------------

This section describes functions in the GNU C library for setting the
absolute priority and scheduling policy of a process.

   *Portability Note:*  On systems that have the functions in this
section, the macro _POSIX_PRIORITY_SCHEDULING is defined in
`<unistd.h>'.

   For the case that the scheduling policy is traditional scheduling,
more functions to fine tune the scheduling are in *Note Traditional
Scheduling::.

   Don't try to make too much out of the naming and structure of these
functions.  They don't match the concepts described in this manual
because the functions are as defined by POSIX.1b, but the implementation
on systems that use the GNU C library is the inverse of what the POSIX
structure contemplates.  The POSIX scheme assumes that the primary
scheduling parameter is the scheduling policy and that the priority
value, if any, is a parameter of the scheduling policy.  In the
implementation, though, the priority value is king and the scheduling
policy, if anything, only fine tunes the effect of that priority.

   The symbols in this section are declared by including file `sched.h'.

 -- Data Type: struct sched_param
     This structure describes an absolute priority.
    `int sched_priority'
          absolute priority value

 -- Function: int sched_setscheduler (pid_t PID, int POLICY, const
          struct sched_param *PARAM)
     This function sets both the absolute priority and the scheduling
     policy for a process.

     It assigns the absolute priority value given by PARAM and the
     scheduling policy POLICY to the process with Process ID PID, or
     the calling process if PID is zero.  If POLICY is negative,
     `sched_setscheduler' keeps the existing scheduling policy.

     The following macros represent the valid values for POLICY:

    `SCHED_OTHER'
          Traditional Scheduling

    `SCHED_FIFO'
          First In First Out

    `SCHED_RR'
          Round Robin

     On success, the return value is `0'.  Otherwise, it is `-1' and
     `ERRNO' is set accordingly.  The `errno' values specific to this
     function are:

    `EPERM'
             * The calling process does not have `CAP_SYS_NICE'
               permission and POLICY is not `SCHED_OTHER' (or it's
               negative and the existing policy is not `SCHED_OTHER'.

             * The calling process does not have `CAP_SYS_NICE'
               permission and its owner is not the target process'
               owner.  I.e.  the effective uid of the calling process
               is neither the effective nor the real uid of process PID.

    `ESRCH'
          There is no process with pid PID and PID is not zero.

    `EINVAL'
             * POLICY does not identify an existing scheduling policy.

             * The absolute priority value identified by *PARAM is
               outside the valid range for the scheduling policy POLICY
               (or the existing scheduling policy if POLICY is
               negative) or PARAM is null.  `sched_get_priority_max'
               and `sched_get_priority_min' tell you what the valid
               range is.

             * PID is negative.


 -- Function: int sched_getscheduler (pid_t PID)
     This function returns the scheduling policy assigned to the
     process with Process ID (pid) PID, or the calling process if PID
     is zero.

     The return value is the scheduling policy.  See
     `sched_setscheduler' for the possible values.

     If the function fails, the return value is instead `-1' and
     `errno' is set accordingly.

     The `errno' values specific to this function are:

    `ESRCH'
          There is no process with pid PID and it is not zero.

    `EINVAL'
          PID is negative.


     Note that this function is not an exact mate to
     `sched_setscheduler' because while that function sets the
     scheduling policy and the absolute priority, this function gets
     only the scheduling policy.  To get the absolute priority, use
     `sched_getparam'.


 -- Function: int sched_setparam (pid_t PID, const struct sched_param
          *PARAM)
     This function sets a process' absolute priority.

     It is functionally identical to `sched_setscheduler' with POLICY =
     `-1'.


 -- Function: int sched_getparam (pid_t PID, const struct sched_param
          *PARAM)
     This function returns a process' absolute priority.

     PID is the Process ID (pid) of the process whose absolute priority
     you want to know.

     PARAM is a pointer to a structure in which the function stores the
     absolute priority of the process.

     On success, the return value is `0'.  Otherwise, it is `-1' and
     `ERRNO' is set accordingly.  The `errno' values specific to this
     function are:

    `ESRCH'
          There is no process with pid PID and it is not zero.

    `EINVAL'
          PID is negative.



 -- Function: int sched_get_priority_min (int *POLICY);
     This function returns the lowest absolute priority value that is
     allowable for a process with scheduling policy POLICY.

     On Linux, it is 0 for SCHED_OTHER and 1 for everything else.

     On success, the return value is `0'.  Otherwise, it is `-1' and
     `ERRNO' is set accordingly.  The `errno' values specific to this
     function are:

    `EINVAL'
          POLICY does not identify an existing scheduling policy.


 -- Function: int sched_get_priority_max (int *POLICY);
     This function returns the highest absolute priority value that is
     allowable for a process that with scheduling policy POLICY.

     On Linux, it is 0 for SCHED_OTHER and 99 for everything else.

     On success, the return value is `0'.  Otherwise, it is `-1' and
     `ERRNO' is set accordingly.  The `errno' values specific to this
     function are:

    `EINVAL'
          POLICY does not identify an existing scheduling policy.


 -- Function: int sched_rr_get_interval (pid_t PID, struct timespec
          *INTERVAL)
     This function returns the length of the quantum (time slice) used
     with the Round Robin scheduling policy, if it is used, for the
     process with Process ID PID.

     It returns the length of time as INTERVAL.

     With a Linux kernel, the round robin time slice is always 150
     microseconds, and PID need not even be a real pid.

     The return value is `0' on success and in the pathological case
     that it fails, the return value is `-1' and `errno' is set
     accordingly.  There is nothing specific that can go wrong with this
     function, so there are no specific `errno' values.


 -- Function: int sched_yield (void)
     This function voluntarily gives up the process' claim on the CPU.

     Technically, `sched_yield' causes the calling process to be made
     immediately ready to run (as opposed to running, which is what it
     was before).  This means that if it has absolute priority higher
     than 0, it gets pushed onto the tail of the queue of processes
     that share its absolute priority and are ready to run, and it will
     run again when its turn next arrives.  If its absolute priority is
     0, it is more complicated, but still has the effect of yielding
     the CPU to other processes.

     If there are no other processes that share the calling process'
     absolute priority, this function doesn't have any effect.

     To the extent that the containing program is oblivious to what
     other processes in the system are doing and how fast it executes,
     this function appears as a no-op.

     The return value is `0' on success and in the pathological case
     that it fails, the return value is `-1' and `errno' is set
     accordingly.  There is nothing specific that can go wrong with this
     function, so there are no specific `errno' values.



File: libc.info,  Node: Traditional Scheduling,  Next: CPU Affinity,  Prev: Basic Scheduling Functions,  Up: Priority

22.3.4 Traditional Scheduling
-----------------------------

This section is about the scheduling among processes whose absolute
priority is 0.  When the system hands out the scraps of CPU time that
are left over after the processes with higher absolute priority have
taken all they want, the scheduling described herein determines who
among the great unwashed processes gets them.

* Menu:

* Traditional Scheduling Intro::
* Traditional Scheduling Functions::


File: libc.info,  Node: Traditional Scheduling Intro,  Next: Traditional Scheduling Functions,  Up: Traditional Scheduling

22.3.4.1 Introduction To Traditional Scheduling
...............................................

Long before there was absolute priority (See *Note Absolute Priority::),
Unix systems were scheduling the CPU using this system.  When Posix came
in like the Romans and imposed absolute priorities to accommodate the
needs of realtime processing, it left the indigenous Absolute Priority
Zero processes to govern themselves by their own familiar scheduling
policy.

   Indeed, absolute priorities higher than zero are not available on
many systems today and are not typically used when they are, being
intended mainly for computers that do realtime processing.  So this
section describes the only scheduling many programmers need to be
concerned about.

   But just to be clear about the scope of this scheduling: Any time a
process with a absolute priority of 0 and a process with an absolute
priority higher than 0 are ready to run at the same time, the one with
absolute priority 0 does not run.  If it's already running when the
higher priority ready-to-run process comes into existence, it stops
immediately.

   In addition to its absolute priority of zero, every process has
another priority, which we will refer to as "dynamic priority" because
it changes over time.  The dynamic priority is meaningless for
processes with an absolute priority higher than zero.

   The dynamic priority sometimes determines who gets the next turn on
the CPU.  Sometimes it determines how long turns last.  Sometimes it
determines whether a process can kick another off the CPU.

   In Linux, the value is a combination of these things, but mostly it
is just determines the length of the time slice.  The higher a process'
dynamic priority, the longer a shot it gets on the CPU when it gets one.
If it doesn't use up its time slice before giving up the CPU to do
something like wait for I/O, it is favored for getting the CPU back when
it's ready for it, to finish out its time slice.  Other than that,
selection of processes for new time slices is basically round robin.
But the scheduler does throw a bone to the low priority processes: A
process' dynamic priority rises every time it is snubbed in the
scheduling process.  In Linux, even the fat kid gets to play.

   The fluctuation of a process' dynamic priority is regulated by
another value: The "nice" value.  The nice value is an integer, usually
in the range -20 to 20, and represents an upper limit on a process'
dynamic priority.  The higher the nice number, the lower that limit.

   On a typical Linux system, for example, a process with a nice value
of 20 can get only 10 milliseconds on the CPU at a time, whereas a
process with a nice value of -20 can achieve a high enough priority to
get 400 milliseconds.

   The idea of the nice value is deferential courtesy.  In the
beginning, in the Unix garden of Eden, all processes shared equally in
the bounty of the computer system.  But not all processes really need
the same share of CPU time, so the nice value gave a courteous process
the ability to refuse its equal share of CPU time that others might
prosper.  Hence, the higher a process' nice value, the nicer the
process is.  (Then a snake came along and offered some process a
negative nice value and the system became the crass resource allocation
system we know today).

   Dynamic priorities tend upward and downward with an objective of
smoothing out allocation of CPU time and giving quick response time to
infrequent requests.  But they never exceed their nice limits, so on a
heavily loaded CPU, the nice value effectively determines how fast a
process runs.

   In keeping with the socialistic heritage of Unix process priority, a
process begins life with the same nice value as its parent process and
can raise it at will.  A process can also raise the nice value of any
other process owned by the same user (or effective user).  But only a
privileged process can lower its nice value.  A privileged process can
also raise or lower another process' nice value.

   GNU C Library functions for getting and setting nice values are
described in *Note Traditional Scheduling Functions::.


File: libc.info,  Node: Traditional Scheduling Functions,  Prev: Traditional Scheduling Intro,  Up: Traditional Scheduling

22.3.4.2 Functions For Traditional Scheduling
.............................................

This section describes how you can read and set the nice value of a
process.  All these symbols are declared in `sys/resource.h'.

   The function and macro names are defined by POSIX, and refer to
"priority," but the functions actually have to do with nice values, as
the terms are used both in the manual and POSIX.

   The range of valid nice values depends on the kernel, but typically
it runs from `-20' to `20'.  A lower nice value corresponds to higher
priority for the process.  These constants describe the range of
priority values:

`PRIO_MIN'
     The lowest valid nice value.

`PRIO_MAX'
     The highest valid nice value.

 -- Function: int getpriority (int CLASS, int ID)
     Return the nice value of a set of processes; CLASS and ID specify
     which ones (see below).  If the processes specified do not all
     have the same nice value, this returns the lowest value that any
     of them has.

     On success, the return value is `0'.  Otherwise, it is `-1' and
     `ERRNO' is set accordingly.  The `errno' values specific to this
     function are:

    `ESRCH'
          The combination of CLASS and ID does not match any existing
          process.

    `EINVAL'
          The value of CLASS is not valid.

     If the return value is `-1', it could indicate failure, or it could
     be the nice value.  The only way to make certain is to set `errno =
     0' before calling `getpriority', then use `errno != 0' afterward
     as the criterion for failure.

 -- Function: int setpriority (int CLASS, int ID, int NICEVAL)
     Set the nice value of a set of processes to NICEVAL; CLASS and ID
     specify which ones (see below).

     The return value is `0' on success, and `-1' on failure.  The
     following `errno' error condition are possible for this function:

    `ESRCH'
          The combination of CLASS and ID does not match any existing
          process.

    `EINVAL'
          The value of CLASS is not valid.

    `EPERM'
          The call would set the nice value of a process which is owned
          by a different user than the calling process (i.e. the target
          process' real or effective uid does not match the calling
          process' effective uid) and the calling process does not have
          `CAP_SYS_NICE' permission.

    `EACCES'
          The call would lower the process' nice value and the process
          does not have `CAP_SYS_NICE' permission.


   The arguments CLASS and ID together specify a set of processes in
which you are interested.  These are the possible values of CLASS:

`PRIO_PROCESS'
     One particular process.  The argument ID is a process ID (pid).

`PRIO_PGRP'
     All the processes in a particular process group.  The argument ID
     is a process group ID (pgid).

`PRIO_USER'
     All the processes owned by a particular user (i.e. whose real uid
     indicates the user).  The argument ID is a user ID (uid).

   If the argument ID is 0, it stands for the calling process, its
process group, or its owner (real uid), according to CLASS.

 -- Function: int nice (int INCREMENT)
     Increment the nice value of the calling process by INCREMENT.  The
     return value is the new nice value on success, and `-1' on
     failure.  In the case of failure, `errno' will be set to the same
     values as for `setpriority'.

     Here is an equivalent definition of `nice':

          int
          nice (int increment)
          {
            int result, old = getpriority (PRIO_PROCESS, 0);
            result = setpriority (PRIO_PROCESS, 0, old + increment);
            if (result != -1)
                return old + increment;
            else
                return -1;
          }


File: libc.info,  Node: CPU Affinity,  Prev: Traditional Scheduling,  Up: Priority

22.3.5 Limiting execution to certain CPUs
-----------------------------------------

On a multi-processor system the operating system usually distributes
the different processes which are runnable on all available CPUs in a
way which allows the system to work most efficiently.  Which processes
and threads run can be to some extend be control with the scheduling
functionality described in the last sections.  But which CPU finally
executes which process or thread is not covered.

   There are a number of reasons why a program might want to have
control over this aspect of the system as well:

   * One thread or process is responsible for absolutely critical work
     which under no circumstances must be interrupted or hindered from
     making process by other process or threads using CPU resources.  In
     this case the special process would be confined to a CPU which no
     other process or thread is allowed to use.

   * The access to certain resources (RAM, I/O ports) has different
     costs from different CPUs.  This is the case in NUMA (Non-Uniform
     Memory Architecture) machines.  Preferrably memory should be
     accessed locally but this requirement is usually not visible to
     the scheduler.  Therefore forcing a process or thread to the CPUs
     which have local access to the mostly used memory helps to
     significantly boost the performance.

   * In controlled runtimes resource allocation and book-keeping work
     (for instance garbage collection) is performance local to
     processors.  This can help to reduce locking costs if the
     resources do not have to be protected from concurrent accesses
     from different processors.

   The POSIX standard up to this date is of not much help to solve this
problem.  The Linux kernel provides a set of interfaces to allow
specifying _affinity sets_ for a process.  The scheduler will schedule
the thread or process on on CPUs specified by the affinity masks.  The
interfaces which the GNU C library define follow to some extend the
Linux kernel interface.

 -- Data Type: cpu_set_t
     This data set is a bitset where each bit represents a CPU.  How the
     system's CPUs are mapped to bits in the bitset is system dependent.
     The data type has a fixed size; in the unlikely case that the
     number of bits are not sufficient to describe the CPUs of the
     system a different interface has to be used.

     This type is a GNU extension and is defined in `sched.h'.

   To manipulate the bitset, to set and reset bits, a number of macros
is defined.  Some of the macros take a CPU number as a parameter.  Here
it is important to never exceed the size of the bitset.  The following
macro specifies the number of bits in the `cpu_set_t' bitset.

 -- Macro: int CPU_SETSIZE
     The value of this macro is the maximum number of CPUs which can be
     handled with a `cpu_set_t' object.

   The type `cpu_set_t' should be considered opaque; all manipulation
should happen via the next four macros.

 -- Macro: void CPU_ZERO (cpu_set_t *SET)
     This macro initializes the CPU set SET to be the empty set.

     This macro is a GNU extension and is defined in `sched.h'.

 -- Macro: void CPU_SET (int CPU, cpu_set_t *SET)
     This macro adds CPU to the CPU set SET.

     The CPU parameter must not have side effects since it is evaluated
     more than once.

     This macro is a GNU extension and is defined in `sched.h'.

 -- Macro: void CPU_CLR (int CPU, cpu_set_t *SET)
     This macro removes CPU from the CPU set SET.

     The CPU parameter must not have side effects since it is evaluated
     more than once.

     This macro is a GNU extension and is defined in `sched.h'.

 -- Macro: int CPU_ISSET (int CPU, const cpu_set_t *SET)
     This macro returns a nonzero value (true) if CPU is a member of
     the CPU set SET, and zero (false) otherwise.

     The CPU parameter must not have side effects since it is evaluated
     more than once.

     This macro is a GNU extension and is defined in `sched.h'.

   CPU bitsets can be constructed from scratch or the currently
installed affinity mask can be retrieved from the system.

 -- Function: int sched_getaffinity (pid_t PID, size_t CPUSETSIZE,
          cpu_set_t *CPUSET)
     This functions stores the CPU affinity mask for the process or
     thread with the ID PID in the CPUSETSIZE bytes long bitmap pointed
     to by CPUSET.  If successful, the function always initializes all
     bits in the `cpu_set_t' object and returns zero.

     If PID does not correspond to a process or thread on the system
     the or the function fails for some other reason, it returns `-1'
     and `errno' is set to represent the error condition.

    `ESRCH'
          No process or thread with the given ID found.

    `EFAULT'
          The pointer CPUSET is does not point to a valid object.

     This function is a GNU extension and is declared in `sched.h'.

   Note that it is not portably possible to use this information to
retrieve the information for different POSIX threads.  A separate
interface must be provided for that.

 -- Function: int sched_setaffinity (pid_t PID, size_t CPUSETSIZE,
          const cpu_set_t *CPUSET)
     This function installs the CPUSETSIZE bytes long affinity mask
     pointed to by CPUSET for the process or thread with the ID PID.
     If successful the function returns zero and the scheduler will in
     future take the affinity information into account.

     If the function fails it will return `-1' and `errno' is set to
     the error code:

    `ESRCH'
          No process or thread with the given ID found.

    `EFAULT'
          The pointer CPUSET is does not point to a valid object.

    `EINVAL'
          The bitset is not valid.  This might mean that the affinity
          set might not leave a processor for the process or thread to
          run on.

     This function is a GNU extension and is declared in `sched.h'.


File: libc.info,  Node: Memory Resources,  Next: Processor Resources,  Prev: Priority,  Up: Resource Usage And Limitation

22.4 Querying memory available resources
========================================

The amount of memory available in the system and the way it is organized
determines oftentimes the way programs can and have to work.  For
functions like `mmap' it is necessary to know about the size of
individual memory pages and knowing how much memory is available enables
a program to select appropriate sizes for, say, caches.  Before we get
into these details a few words about memory subsystems in traditional
Unix systems will be given.

* Menu:

* Memory Subsystem::           Overview about traditional Unix memory handling.
* Query Memory Parameters::    How to get information about the memory
                                subsystem?


File: libc.info,  Node: Memory Subsystem,  Next: Query Memory Parameters,  Up: Memory Resources

22.4.1 Overview about traditional Unix memory handling
------------------------------------------------------

Unix systems normally provide processes virtual address spaces.  This
means that the addresses of the memory regions do not have to correspond
directly to the addresses of the actual physical memory which stores the
data.  An extra level of indirection is introduced which translates
virtual addresses into physical addresses.  This is normally done by the
hardware of the processor.

   Using a virtual address space has several advantage.  The most
important is process isolation.  The different processes running on the
system cannot interfere directly with each other.  No process can write
into the address space of another process (except when shared memory is
used but then it is wanted and controlled).

   Another advantage of virtual memory is that the address space the
processes see can actually be larger than the physical memory available.
The physical memory can be extended by storage on an external media
where the content of currently unused memory regions is stored.  The
address translation can then intercept accesses to these memory regions
and make memory content available again by loading the data back into
memory.  This concept makes it necessary that programs which have to use
lots of memory know the difference between available virtual address
space and available physical memory.  If the working set of virtual
memory of all the processes is larger than the available physical memory
the system will slow down dramatically due to constant swapping of
memory content from the memory to the storage media and back.  This is
called "thrashing".  

   A final aspect of virtual memory which is important and follows from
what is said in the last paragraph is the granularity of the virtual
address space handling.  When we said that the virtual address handling
stores memory content externally it cannot do this on a byte-by-byte
basis.  The administrative overhead does not allow this (leaving alone
the processor hardware).  Instead several thousand bytes are handled
together and form a "page".  The size of each page is always a power of
two byte.  The smallest page size in use today is 4096, with 8192,
16384, and 65536 being other popular sizes.


File: libc.info,  Node: Query Memory Parameters,  Prev: Memory Subsystem,  Up: Memory Resources

22.4.2 How to get information about the memory subsystem?
---------------------------------------------------------

The page size of the virtual memory the process sees is essential to
know in several situations.  Some programming interface (e.g., `mmap',
*note Memory-mapped I/O::) require the user to provide information
adjusted to the page size.  In the case of `mmap' is it necessary to
provide a length argument which is a multiple of the page size.
Another place where the knowledge about the page size is useful is in
memory allocation.  If one allocates pieces of memory in larger chunks
which are then subdivided by the application code it is useful to
adjust the size of the larger blocks to the page size.  If the total
memory requirement for the block is close (but not larger) to a multiple
of the page size the kernel's memory handling can work more effectively
since it only has to allocate memory pages which are fully used.  (To do
this optimization it is necessary to know a bit about the memory
allocator which will require a bit of memory itself for each block and
this overhead must not push the total size over the page size multiple.

   The page size traditionally was a compile time constant.  But recent
development of processors changed this.  Processors now support
different page sizes and they can possibly even vary among different
processes on the same system.  Therefore the system should be queried at
runtime about the current page size and no assumptions (except about it
being a power of two) should be made.

   The correct interface to query about the page size is `sysconf'
(*note Sysconf Definition::) with the parameter `_SC_PAGESIZE'.  There
is a much older interface available, too.

 -- Function: int getpagesize (void)
     The `getpagesize' function returns the page size of the process.
     This value is fixed for the runtime of the process but can vary in
     different runs of the application.

     The function is declared in `unistd.h'.

   Widely available on System V derived systems is a method to get
information about the physical memory the system has.  The call

       sysconf (_SC_PHYS_PAGES)

returns the total number of pages of physical the system has.  This
does not mean all this memory is available.  This information can be
found using

       sysconf (_SC_AVPHYS_PAGES)

   These two values help to optimize applications.  The value returned
for `_SC_AVPHYS_PAGES' is the amount of memory the application can use
without hindering any other process (given that no other process
increases its memory usage).  The value returned for `_SC_PHYS_PAGES'
is more or less a hard limit for the working set.  If all applications
together constantly use more than that amount of memory the system is
in trouble.

   The GNU C library provides in addition to these already described
way to get this information two functions.  They are declared in the
file `sys/sysinfo.h'.  Programmers should prefer to use the `sysconf'
method described above.

 -- Function: long int get_phys_pages (void)
     The `get_phys_pages' function returns the total number of pages of
     physical the system has.  To get the amount of memory this number
     has to be multiplied by the page size.

     This function is a GNU extension.

 -- Function: long int get_avphys_pages (void)
     The `get_phys_pages' function returns the number of available
     pages of physical the system has.  To get the amount of memory
     this number has to be multiplied by the page size.

     This function is a GNU extension.


File: libc.info,  Node: Processor Resources,  Prev: Memory Resources,  Up: Resource Usage And Limitation

22.5 Learn about the processors available
=========================================

The use of threads or processes with shared memory allows an application
to take advantage of all the processing power a system can provide.  If
the task can be parallelized the optimal way to write an application is
to have at any time as many processes running as there are processors.
To determine the number of processors available to the system one can
run

       sysconf (_SC_NPROCESSORS_CONF)

which returns the number of processors the operating system configured.
But it might be possible for the operating system to disable individual
processors and so the call

       sysconf (_SC_NPROCESSORS_ONLN)

returns the number of processors which are currently inline (i.e.,
available).

   For these two pieces of information the GNU C library also provides
functions to get the information directly.  The functions are declared
in `sys/sysinfo.h'.

 -- Function: int get_nprocs_conf (void)
     The `get_nprocs_conf' function returns the number of processors the
     operating system configured.

     This function is a GNU extension.

 -- Function: int get_nprocs (void)
     The `get_nprocs' function returns the number of available
     processors.

     This function is a GNU extension.

   Before starting more threads it should be checked whether the
processors are not already overused.  Unix systems calculate something
called the "load average".  This is a number indicating how many
processes were running.  This number is average over different periods
of times (normally 1, 5, and 15 minutes).

 -- Function: int getloadavg (double LOADAVG[], int NELEM)
     This function gets the 1, 5 and 15 minute load averages of the
     system. The values are placed in LOADAVG.  `getloadavg' will place
     at most NELEM elements into the array but never more than three
     elements.  The return value is the number of elements written to
     LOADAVG, or -1 on error.

     This function is declared in `stdlib.h'.


File: libc.info,  Node: Non-Local Exits,  Next: Signal Handling,  Prev: Resource Usage And Limitation,  Up: Top

23 Non-Local Exits
******************

Sometimes when your program detects an unusual situation inside a deeply
nested set of function calls, you would like to be able to immediately
return to an outer level of control.  This section describes how you can
do such "non-local exits" using the `setjmp' and `longjmp' functions.

* Menu:

* Intro: Non-Local Intro.        When and how to use these facilities.
* Details: Non-Local Details.    Functions for non-local exits.
* Non-Local Exits and Signals::  Portability issues.
* System V contexts::            Complete context control a la System V.


File: libc.info,  Node: Non-Local Intro,  Next: Non-Local Details,  Up: Non-Local Exits

23.1 Introduction to Non-Local Exits
====================================

As an example of a situation where a non-local exit can be useful,
suppose you have an interactive program that has a "main loop" that
prompts for and executes commands.  Suppose the "read" command reads
input from a file, doing some lexical analysis and parsing of the input
while processing it.  If a low-level input error is detected, it would
be useful to be able to return immediately to the "main loop" instead
of having to make each of the lexical analysis, parsing, and processing
phases all have to explicitly deal with error situations initially
detected by nested calls.

   (On the other hand, if each of these phases has to do a substantial
amount of cleanup when it exits--such as closing files, deallocating
buffers or other data structures, and the like--then it can be more
appropriate to do a normal return and have each phase do its own
cleanup, because a non-local exit would bypass the intervening phases
and their associated cleanup code entirely.  Alternatively, you could
use a non-local exit but do the cleanup explicitly either before or
after returning to the "main loop".)

   In some ways, a non-local exit is similar to using the `return'
statement to return from a function.  But while `return' abandons only
a single function call, transferring control back to the point at which
it was called, a non-local exit can potentially abandon many levels of
nested function calls.

   You identify return points for non-local exits by calling the
function `setjmp'.  This function saves information about the execution
environment in which the call to `setjmp' appears in an object of type
`jmp_buf'.  Execution of the program continues normally after the call
to `setjmp', but if an exit is later made to this return point by
calling `longjmp' with the corresponding `jmp_buf' object, control is
transferred back to the point where `setjmp' was called.  The return
value from `setjmp' is used to distinguish between an ordinary return
and a return made by a call to `longjmp', so calls to `setjmp' usually
appear in an `if' statement.

   Here is how the example program described above might be set up:

     #include <setjmp.h>
     #include <stdlib.h>
     #include <stdio.h>

     jmp_buf main_loop;

     void
     abort_to_main_loop (int status)
     {
       longjmp (main_loop, status);
     }

     int
     main (void)
     {
       while (1)
         if (setjmp (main_loop))
           puts ("Back at main loop....");
         else
           do_command ();
     }


     void
     do_command (void)
     {
       char buffer[128];
       if (fgets (buffer, 128, stdin) == NULL)
         abort_to_main_loop (-1);
       else
         exit (EXIT_SUCCESS);
     }

   The function `abort_to_main_loop' causes an immediate transfer of
control back to the main loop of the program, no matter where it is
called from.

   The flow of control inside the `main' function may appear a little
mysterious at first, but it is actually a common idiom with `setjmp'.
A normal call to `setjmp' returns zero, so the "else" clause of the
conditional is executed.  If `abort_to_main_loop' is called somewhere
within the execution of `do_command', then it actually appears as if
the _same_ call to `setjmp' in `main' were returning a second time with
a value of `-1'.

   So, the general pattern for using `setjmp' looks something like:

     if (setjmp (BUFFER))
       /* Code to clean up after premature return. */
       ...
     else
       /* Code to be executed normally after setting up the return point. */
       ...


File: libc.info,  Node: Non-Local Details,  Next: Non-Local Exits and Signals,  Prev: Non-Local Intro,  Up: Non-Local Exits

23.2 Details of Non-Local Exits
===============================

Here are the details on the functions and data structures used for
performing non-local exits.  These facilities are declared in
`setjmp.h'.  

 -- Data Type: jmp_buf
     Objects of type `jmp_buf' hold the state information to be
     restored by a non-local exit.  The contents of a `jmp_buf'
     identify a specific place to return to.

 -- Macro: int setjmp (jmp_buf STATE)
     When called normally, `setjmp' stores information about the
     execution state of the program in STATE and returns zero.  If
     `longjmp' is later used to perform a non-local exit to this STATE,
     `setjmp' returns a nonzero value.

 -- Function: void longjmp (jmp_buf STATE, int VALUE)
     This function restores current execution to the state saved in
     STATE, and continues execution from the call to `setjmp' that
     established that return point.  Returning from `setjmp' by means of
     `longjmp' returns the VALUE argument that was passed to `longjmp',
     rather than `0'.  (But if VALUE is given as `0', `setjmp' returns
     `1').

   There are a lot of obscure but important restrictions on the use of
`setjmp' and `longjmp'.  Most of these restrictions are present because
non-local exits require a fair amount of magic on the part of the C
compiler and can interact with other parts of the language in strange
ways.

   The `setjmp' function is actually a macro without an actual function
definition, so you shouldn't try to `#undef' it or take its address.
In addition, calls to `setjmp' are safe in only the following contexts:

   * As the test expression of a selection or iteration statement (such
     as `if', `switch', or `while').

   * As one operand of a equality or comparison operator that appears
     as the test expression of a selection or iteration statement.  The
     other operand must be an integer constant expression.

   * As the operand of a unary `!' operator, that appears as the test
     expression of a selection or iteration statement.

   * By itself as an expression statement.

   Return points are valid only during the dynamic extent of the
function that called `setjmp' to establish them.  If you `longjmp' to a
return point that was established in a function that has already
returned, unpredictable and disastrous things are likely to happen.

   You should use a nonzero VALUE argument to `longjmp'.  While
`longjmp' refuses to pass back a zero argument as the return value from
`setjmp', this is intended as a safety net against accidental misuse
and is not really good programming style.

   When you perform a non-local exit, accessible objects generally
retain whatever values they had at the time `longjmp' was called.  The
exception is that the values of automatic variables local to the
function containing the `setjmp' call that have been changed since the
call to `setjmp' are indeterminate, unless you have declared them
`volatile'.


File: libc.info,  Node: Non-Local Exits and Signals,  Next: System V contexts,  Prev: Non-Local Details,  Up: Non-Local Exits

23.3 Non-Local Exits and Signals
================================

In BSD Unix systems, `setjmp' and `longjmp' also save and restore the
set of blocked signals; see *Note Blocking Signals::.  However, the
POSIX.1 standard requires `setjmp' and `longjmp' not to change the set
of blocked signals, and provides an additional pair of functions
(`sigsetjmp' and `siglongjmp') to get the BSD behavior.

   The behavior of `setjmp' and `longjmp' in the GNU library is
controlled by feature test macros; see *Note Feature Test Macros::.  The
default in the GNU system is the POSIX.1 behavior rather than the BSD
behavior.

   The facilities in this section are declared in the header file
`setjmp.h'.  

 -- Data Type: sigjmp_buf
     This is similar to `jmp_buf', except that it can also store state
     information about the set of blocked signals.

 -- Function: int sigsetjmp (sigjmp_buf STATE, int SAVESIGS)
     This is similar to `setjmp'.  If SAVESIGS is nonzero, the set of
     blocked signals is saved in STATE and will be restored if a
     `siglongjmp' is later performed with this STATE.

 -- Function: void siglongjmp (sigjmp_buf STATE, int VALUE)
     This is similar to `longjmp' except for the type of its STATE
     argument.  If the `sigsetjmp' call that set this STATE used a
     nonzero SAVESIGS flag, `siglongjmp' also restores the set of
     blocked signals.


File: libc.info,  Node: System V contexts,  Prev: Non-Local Exits and Signals,  Up: Non-Local Exits

23.4 Complete Context Control
=============================

The Unix standard one more set of function to control the execution path
and these functions are more powerful than those discussed in this
chapter so far.  These function were part of the original System V API
and by this route were added to the Unix API.  Beside on branded Unix
implementations these interfaces are not widely available.  Not all
platforms and/or architectures the GNU C Library is available on provide
this interface.  Use `configure' to detect the availability.

   Similar to the `jmp_buf' and `sigjmp_buf' types used for the
variables to contain the state of the `longjmp' functions the
interfaces of interest here have an appropriate type as well.  Objects
of this type are normally much larger since more information is
contained.  The type is also used in a few more places as we will see.
The types and functions described in this section are all defined and
declared respectively in the `ucontext.h' header file.

 -- Data Type: ucontext_t
     The `ucontext_t' type is defined as a structure with as least the
     following elements:

    `ucontext_t *uc_link'
          This is a pointer to the next context structure which is used
          if the context described in the current structure returns.

    `sigset_t uc_sigmask'
          Set of signals which are blocked when this context is used.

    `stack_t uc_stack'
          Stack used for this context.  The value need not be (and
          normally is not) the stack pointer.  *Note Signal Stack::.

    `mcontext_t uc_mcontext'
          This element contains the actual state of the process.  The
          `mcontext_t' type is also defined in this header but the
          definition should be treated as opaque.  Any use of knowledge
          of the type makes applications less portable.


   Objects of this type have to be created by the user.  The
initialization and modification happens through one of the following
functions:

 -- Function: int getcontext (ucontext_t *UCP)
     The `getcontext' function initializes the variable pointed to by
     UCP with the context of the calling thread.  The context contains
     the content of the registers, the signal mask, and the current
     stack.  Executing the contents would start at the point where the
     `getcontext' call just returned.

     The function returns `0' if successful.  Otherwise it returns `-1'
     and sets ERRNO accordingly.

   The `getcontext' function is similar to `setjmp' but it does not
provide an indication of whether the function returns for the first
time or whether the initialized context was used and the execution is
resumed at just that point.  If this is necessary the user has to take
determine this herself.  This must be done carefully since the context
contains registers which might contain register variables.  This is a
good situation to define variables with `volatile'.

   Once the context variable is initialized it can be used as is or it
can be modified.  The latter is normally done to implement co-routines
or similar constructs.  The `makecontext' function is what has to be
used to do that.

 -- Function: void makecontext (ucontext_t *UCP, void (*FUNC) (void),
          int ARGC, ...)
     The UCP parameter passed to the `makecontext' shall be initialized
     by a call to `getcontext'.  The context will be modified to in a
     way so that if the context is resumed it will start by calling the
     function `func' which gets ARGC integer arguments passed.  The
     integer arguments which are to be passed should follow the ARGC
     parameter in the call to `makecontext'.

     Before the call to this function the `uc_stack' and `uc_link'
     element of the UCP structure should be initialized.  The
     `uc_stack' element describes the stack which is used for this
     context.  No two contexts which are used at the same time should
     use the same memory region for a stack.

     The `uc_link' element of the object pointed to by UCP should be a
     pointer to the context to be executed when the function FUNC
     returns or it should be a null pointer.  See `setcontext' for more
     information about the exact use.

   While allocating the memory for the stack one has to be careful.
Most modern processors keep track of whether a certain memory region is
allowed to contain code which is executed or not.  Data segments and
heap memory is normally not tagged to allow this.  The result is that
programs would fail.  Examples for such code include the calling
sequences the GNU C compiler generates for calls to nested functions.
Safe ways to allocate stacks correctly include using memory on the
original threads stack or explicitly allocate memory tagged for
execution using (*note Memory-mapped I/O::).

   *Compatibility note*: The current Unix standard is very imprecise
about the way the stack is allocated.  All implementations seem to agree
that the `uc_stack' element must be used but the values stored in the
elements of the `stack_t' value are unclear.  The GNU C library and
most other Unix implementations require the `ss_sp' value of the
`uc_stack' element to point to the base of the memory region allocated
for the stack and the size of the memory region is stored in `ss_size'.
There are implements out there which require `ss_sp' to be set to the
value the stack pointer will have (which can depending on the direction
the stack grows be different).  This difference makes the `makecontext'
function hard to use and it requires detection of the platform at
compile time.

 -- Function: int setcontext (const ucontext_t *UCP)
     The `setcontext' function restores the context described by UCP.
     The context is not modified and can be reused as often as wanted.

     If the context was created by `getcontext' execution resumes with
     the registers filled with the same values and the same stack as if
     the `getcontext' call just returned.

     If the context was modified with a call to `makecontext' execution
     continues with the function passed to `makecontext' which gets the
     specified parameters passed.  If this function returns execution is
     resumed in the context which was referenced by the `uc_link'
     element of the context structure passed to `makecontext' at the
     time of the call.  If `uc_link' was a null pointer the application
     terminates in this case.

     Since the context contains information about the stack no two
     threads should use the same context at the same time.  The result
     in most cases would be disastrous.

     The `setcontext' function does not return unless an error occurred
     in which case it returns `-1'.

   The `setcontext' function simply replaces the current context with
the one described by the UCP parameter.  This is often useful but there
are situations where the current context has to be preserved.

 -- Function: int swapcontext (ucontext_t *restrict OUCP, const
          ucontext_t *restrict UCP)
     The `swapcontext' function is similar to `setcontext' but instead
     of just replacing the current context the latter is first saved in
     the object pointed to by OUCP as if this was a call to
     `getcontext'.  The saved context would resume after the call to
     `swapcontext'.

     Once the current context is saved the context described in UCP is
     installed and execution continues as described in this context.

     If `swapcontext' succeeds the function does not return unless the
     context OUCP is used without prior modification by `makecontext'.
     The return value in this case is `0'.  If the function fails it
     returns `-1' and set ERRNO accordingly.

Example for SVID Context Handling
=================================

The easiest way to use the context handling functions is as a
replacement for `setjmp' and `longjmp'.  The context contains on most
platforms more information which might lead to less surprises but this
also means using these functions is more expensive (beside being less
portable).

     int
     random_search (int n, int (*fp) (int, ucontext_t *))
     {
       volatile int cnt = 0;
       ucontext_t uc;

       /* Safe current context.  */
       if (getcontext (&uc) < 0)
         return -1;

       /* If we have not tried N times try again.  */
       if (cnt++ < n)
         /* Call the function with a new random number
            and the context.  */
         if (fp (rand (), &uc) != 0)
           /* We found what we were looking for.  */
           return 1;

       /* Not found.  */
       return 0;
     }

   Using contexts in such a way enables emulating exception handling.
The search functions passed in the FP parameter could be very large,
nested, and complex which would make it complicated (or at least would
require a lot of code) to leave the function with an error value which
has to be passed down to the caller.  By using the context it is
possible to leave the search function in one step and allow restarting
the search which also has the nice side effect that it can be
significantly faster.

   Something which is harder to implement with `setjmp' and `longjmp'
is to switch temporarily to a different execution path and then resume
where execution was stopped.

     #include <signal.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <ucontext.h>
     #include <sys/time.h>

     /* Set by the signal handler. */
     static volatile int expired;

     /* The contexts. */
     static ucontext_t uc[3];

     /* We do only a certain number of switches. */
     static int switches;


     /* This is the function doing the work.  It is just a
        skeleton, real code has to be filled in. */
     static void
     f (int n)
     {
       int m = 0;
       while (1)
         {
           /* This is where the work would be done. */
           if (++m % 100 == 0)
             {
               putchar ('.');
               fflush (stdout);
             }

           /* Regularly the EXPIRE variable must be checked. */
           if (expired)
             {
               /* We do not want the program to run forever. */
               if (++switches == 20)
                 return;

               printf ("\nswitching from %d to %d\n", n, 3 - n);
               expired = 0;
               /* Switch to the other context, saving the current one. */
               swapcontext (&uc[n], &uc[3 - n]);
             }
         }
     }

     /* This is the signal handler which simply set the variable. */
     void
     handler (int signal)
     {
       expired = 1;
     }


     int
     main (void)
     {
       struct sigaction sa;
       struct itimerval it;
       char st1[8192];
       char st2[8192];

       /* Initialize the data structures for the interval timer. */
       sa.sa_flags = SA_RESTART;
       sigfillset (&sa.sa_mask);
       sa.sa_handler = handler;
       it.it_interval.tv_sec = 0;
       it.it_interval.tv_usec = 1;
       it.it_value = it.it_interval;

       /* Install the timer and get the context we can manipulate. */
       if (sigaction (SIGPROF, &sa, NULL) < 0
           || setitimer (ITIMER_PROF, &it, NULL) < 0
           || getcontext (&uc[1]) == -1
           || getcontext (&uc[2]) == -1)
         abort ();

       /* Create a context with a separate stack which causes the
          function `f' to be call with the parameter `1'.
          Note that the `uc_link' points to the main context
          which will cause the program to terminate once the function
          return. */
       uc[1].uc_link = &uc[0];
       uc[1].uc_stack.ss_sp = st1;
       uc[1].uc_stack.ss_size = sizeof st1;
       makecontext (&uc[1], (void (*) (void)) f, 1, 1);

       /* Similarly, but `2' is passed as the parameter to `f'. */
       uc[2].uc_link = &uc[0];
       uc[2].uc_stack.ss_sp = st2;
       uc[2].uc_stack.ss_size = sizeof st2;
       makecontext (&uc[2], (void (*) (void)) f, 1, 2);

       /* Start running. */
       swapcontext (&uc[0], &uc[1]);
       putchar ('\n');

       return 0;
     }

   This an example how the context functions can be used to implement
co-routines or cooperative multi-threading.  All that has to be done is
to call every once in a while `swapcontext' to continue running a
different context.  It is not allowed to do the context switching from
the signal handler directly since neither `setcontext' nor
`swapcontext' are functions which can be called from a signal handler.
But setting a variable in the signal handler and checking it in the
body of the functions which are executed.  Since `swapcontext' is
saving the current context it is possible to have multiple different
scheduling points in the code.  Execution will always resume where it
was left.


File: libc.info,  Node: Signal Handling,  Next: Program Basics,  Prev: Non-Local Exits,  Up: Top

24 Signal Handling
******************

A "signal" is a software interrupt delivered to a process.  The
operating system uses signals to report exceptional situations to an
executing program.  Some signals report errors such as references to
invalid memory addresses; others report asynchronous events, such as
disconnection of a phone line.

   The GNU C library defines a variety of signal types, each for a
particular kind of event.  Some kinds of events make it inadvisable or
impossible for the program to proceed as usual, and the corresponding
signals normally abort the program.  Other kinds of signals that report
harmless events are ignored by default.

   If you anticipate an event that causes signals, you can define a
handler function and tell the operating system to run it when that
particular type of signal arrives.

   Finally, one process can send a signal to another process; this
allows a parent process to abort a child, or two related processes to
communicate and synchronize.

* Menu:

* Concepts of Signals::         Introduction to the signal facilities.
* Standard Signals::            Particular kinds of signals with
                                 standard names and meanings.
* Signal Actions::              Specifying what happens when a
                                 particular signal is delivered.
* Defining Handlers::           How to write a signal handler function.
* Interrupted Primitives::	Signal handlers affect use of `open',
				 `read', `write' and other functions.
* Generating Signals::          How to send a signal to a process.
* Blocking Signals::            Making the system hold signals temporarily.
* Waiting for a Signal::        Suspending your program until a signal
                                 arrives.
* Signal Stack::                Using a Separate Signal Stack.
* BSD Signal Handling::         Additional functions for backward
			         compatibility with BSD.


File: libc.info,  Node: Concepts of Signals,  Next: Standard Signals,  Up: Signal Handling

24.1 Basic Concepts of Signals
==============================

This section explains basic concepts of how signals are generated, what
happens after a signal is delivered, and how programs can handle
signals.

* Menu:

* Kinds of Signals::            Some examples of what can cause a signal.
* Signal Generation::           Concepts of why and how signals occur.
* Delivery of Signal::          Concepts of what a signal does to the
                                 process.


File: libc.info,  Node: Kinds of Signals,  Next: Signal Generation,  Up: Concepts of Signals

24.1.1 Some Kinds of Signals
----------------------------

A signal reports the occurrence of an exceptional event.  These are some
of the events that can cause (or "generate", or "raise") a signal:

   * A program error such as dividing by zero or issuing an address
     outside the valid range.

   * A user request to interrupt or terminate the program.  Most
     environments are set up to let a user suspend the program by
     typing `C-z', or terminate it with `C-c'.  Whatever key sequence
     is used, the operating system sends the proper signal to interrupt
     the process.

   * The termination of a child process.

   * Expiration of a timer or alarm.

   * A call to `kill' or `raise' by the same process.

   * A call to `kill' from another process.  Signals are a limited but
     useful form of interprocess communication.

   * An attempt to perform an I/O operation that cannot be done.
     Examples are reading from a pipe that has no writer (*note Pipes
     and FIFOs::), and reading or writing to a terminal in certain
     situations (*note Job Control::).

   Each of these kinds of events (excepting explicit calls to `kill'
and `raise') generates its own particular kind of signal.  The various
kinds of signals are listed and described in detail in *Note Standard
Signals::.


File: libc.info,  Node: Signal Generation,  Next: Delivery of Signal,  Prev: Kinds of Signals,  Up: Concepts of Signals

24.1.2 Concepts of Signal Generation
------------------------------------

In general, the events that generate signals fall into three major
categories: errors, external events, and explicit requests.

   An error means that a program has done something invalid and cannot
continue execution.  But not all kinds of errors generate signals--in
fact, most do not.  For example, opening a nonexistent file is an error,
but it does not raise a signal; instead, `open' returns `-1'.  In
general, errors that are necessarily associated with certain library
functions are reported by returning a value that indicates an error.
The errors which raise signals are those which can happen anywhere in
the program, not just in library calls.  These include division by zero
and invalid memory addresses.

   An external event generally has to do with I/O or other processes.
These include the arrival of input, the expiration of a timer, and the
termination of a child process.

   An explicit request means the use of a library function such as
`kill' whose purpose is specifically to generate a signal.

   Signals may be generated "synchronously" or "asynchronously".  A
synchronous signal pertains to a specific action in the program, and is
delivered (unless blocked) during that action.  Most errors generate
signals synchronously, and so do explicit requests by a process to
generate a signal for that same process.  On some machines, certain
kinds of hardware errors (usually floating-point exceptions) are not
reported completely synchronously, but may arrive a few instructions
later.

   Asynchronous signals are generated by events outside the control of
the process that receives them.  These signals arrive at unpredictable
times during execution.  External events generate signals
asynchronously, and so do explicit requests that apply to some other
process.

   A given type of signal is either typically synchronous or typically
asynchronous.  For example, signals for errors are typically synchronous
because errors generate signals synchronously.  But any type of signal
can be generated synchronously or asynchronously with an explicit
request.


File: libc.info,  Node: Delivery of Signal,  Prev: Signal Generation,  Up: Concepts of Signals

24.1.3 How Signals Are Delivered
--------------------------------

When a signal is generated, it becomes "pending".  Normally it remains
pending for just a short period of time and then is "delivered" to the
process that was signaled.  However, if that kind of signal is
currently "blocked", it may remain pending indefinitely--until signals
of that kind are "unblocked".  Once unblocked, it will be delivered
immediately.  *Note Blocking Signals::.

   When the signal is delivered, whether right away or after a long
delay, the "specified action" for that signal is taken.  For certain
signals, such as `SIGKILL' and `SIGSTOP', the action is fixed, but for
most signals, the program has a choice: ignore the signal, specify a
"handler function", or accept the "default action" for that kind of
signal.  The program specifies its choice using functions such as
`signal' or `sigaction' (*note Signal Actions::).  We sometimes say
that a handler "catches" the signal.  While the handler is running,
that particular signal is normally blocked.

   If the specified action for a kind of signal is to ignore it, then
any such signal which is generated is discarded immediately.  This
happens even if the signal is also blocked at the time.  A signal
discarded in this way will never be delivered, not even if the program
subsequently specifies a different action for that kind of signal and
then unblocks it.

   If a signal arrives which the program has neither handled nor
ignored, its "default action" takes place.  Each kind of signal has its
own default action, documented below (*note Standard Signals::).  For
most kinds of signals, the default action is to terminate the process.
For certain kinds of signals that represent "harmless" events, the
default action is to do nothing.

   When a signal terminates a process, its parent process can determine
the cause of termination by examining the termination status code
reported by the `wait' or `waitpid' functions.  (This is discussed in
more detail in *Note Process Completion::.)  The information it can get
includes the fact that termination was due to a signal and the kind of
signal involved.  If a program you run from a shell is terminated by a
signal, the shell typically prints some kind of error message.

   The signals that normally represent program errors have a special
property: when one of these signals terminates the process, it also
writes a "core dump file" which records the state of the process at the
time of termination.  You can examine the core dump with a debugger to
investigate what caused the error.

   If you raise a "program error" signal by explicit request, and this
terminates the process, it makes a core dump file just as if the signal
had been due directly to an error.


File: libc.info,  Node: Standard Signals,  Next: Signal Actions,  Prev: Concepts of Signals,  Up: Signal Handling

24.2 Standard Signals
=====================

This section lists the names for various standard kinds of signals and
describes what kind of event they mean.  Each signal name is a macro
which stands for a positive integer--the "signal number" for that kind
of signal.  Your programs should never make assumptions about the
numeric code for a particular kind of signal, but rather refer to them
always by the names defined here.  This is because the number for a
given kind of signal can vary from system to system, but the meanings of
the names are standardized and fairly uniform.

   The signal names are defined in the header file `signal.h'.

 -- Macro: int NSIG
     The value of this symbolic constant is the total number of signals
     defined.  Since the signal numbers are allocated consecutively,
     `NSIG' is also one greater than the largest defined signal number.

* Menu:

* Program Error Signals::       Used to report serious program errors.
* Termination Signals::         Used to interrupt and/or terminate the
                                 program.
* Alarm Signals::               Used to indicate expiration of timers.
* Asynchronous I/O Signals::    Used to indicate input is available.
* Job Control Signals::         Signals used to support job control.
* Operation Error Signals::     Used to report operational system errors.
* Miscellaneous Signals::       Miscellaneous Signals.
* Signal Messages::             Printing a message describing a signal.


File: libc.info,  Node: Program Error Signals,  Next: Termination Signals,  Up: Standard Signals

24.2.1 Program Error Signals
----------------------------

The following signals are generated when a serious program error is
detected by the operating system or the computer itself.  In general,
all of these signals are indications that your program is seriously
broken in some way, and there's usually no way to continue the
computation which encountered the error.

   Some programs handle program error signals in order to tidy up before
terminating; for example, programs that turn off echoing of terminal
input should handle program error signals in order to turn echoing back
on.  The handler should end by specifying the default action for the
signal that happened and then reraising it; this will cause the program
to terminate with that signal, as if it had not had a handler.  (*Note
Termination in Handler::.)

   Termination is the sensible ultimate outcome from a program error in
most programs.  However, programming systems such as Lisp that can load
compiled user programs might need to keep executing even if a user
program incurs an error.  These programs have handlers which use
`longjmp' to return control to the command level.

   The default action for all of these signals is to cause the process
to terminate.  If you block or ignore these signals or establish
handlers for them that return normally, your program will probably
break horribly when such signals happen, unless they are generated by
`raise' or `kill' instead of a real error.

   When one of these program error signals terminates a process, it also
writes a "core dump file" which records the state of the process at the
time of termination.  The core dump file is named `core' and is written
in whichever directory is current in the process at the time.  (On the
GNU system, you can specify the file name for core dumps with the
environment variable `COREFILE'.)  The purpose of core dump files is so
that you can examine them with a debugger to investigate what caused
the error.

 -- Macro: int SIGFPE
     The `SIGFPE' signal reports a fatal arithmetic error.  Although the
     name is derived from "floating-point exception", this signal
     actually covers all arithmetic errors, including division by zero
     and overflow.  If a program stores integer data in a location
     which is then used in a floating-point operation, this often
     causes an "invalid operation" exception, because the processor
     cannot recognize the data as a floating-point number.  

     Actual floating-point exceptions are a complicated subject because
     there are many types of exceptions with subtly different meanings,
     and the `SIGFPE' signal doesn't distinguish between them.  The
     `IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std
     754-1985 and ANSI/IEEE Std 854-1987)' defines various
     floating-point exceptions and requires conforming computer systems
     to report their occurrences.  However, this standard does not
     specify how the exceptions are reported, or what kinds of handling
     and control the operating system can offer to the programmer.

   BSD systems provide the `SIGFPE' handler with an extra argument that
distinguishes various causes of the exception.  In order to access this
argument, you must define the handler to accept two arguments, which
means you must cast it to a one-argument function type in order to
establish the handler.  The GNU library does provide this extra
argument, but the value is meaningful only on operating systems that
provide the information (BSD systems and GNU systems).

`FPE_INTOVF_TRAP'
     Integer overflow (impossible in a C program unless you enable
     overflow trapping in a hardware-specific fashion).

`FPE_INTDIV_TRAP'
     Integer division by zero.

`FPE_SUBRNG_TRAP'
     Subscript-range (something that C programs never check for).

`FPE_FLTOVF_TRAP'
     Floating overflow trap.

`FPE_FLTDIV_TRAP'
     Floating/decimal division by zero.

`FPE_FLTUND_TRAP'
     Floating underflow trap.  (Trapping on floating underflow is not
     normally enabled.)

`FPE_DECOVF_TRAP'
     Decimal overflow trap.  (Only a few machines have decimal
     arithmetic and C never uses it.)

 -- Macro: int SIGILL
     The name of this signal is derived from "illegal instruction"; it
     usually means your program is trying to execute garbage or a
     privileged instruction.  Since the C compiler generates only valid
     instructions, `SIGILL' typically indicates that the executable
     file is corrupted, or that you are trying to execute data.  Some
     common ways of getting into the latter situation are by passing an
     invalid object where a pointer to a function was expected, or by
     writing past the end of an automatic array (or similar problems
     with pointers to automatic variables) and corrupting other data on
     the stack such as the return address of a stack frame.

     `SIGILL' can also be generated when the stack overflows, or when
     the system has trouble running the handler for a signal.
   
 -- Macro: int SIGSEGV
     This signal is generated when a program tries to read or write
     outside the memory that is allocated for it, or to write memory
     that can only be read.  (Actually, the signals only occur when the
     program goes far enough outside to be detected by the system's
     memory protection mechanism.)  The name is an abbreviation for
     "segmentation violation".

     Common ways of getting a `SIGSEGV' condition include dereferencing
     a null or uninitialized pointer, or when you use a pointer to step
     through an array, but fail to check for the end of the array.  It
     varies among systems whether dereferencing a null pointer generates
     `SIGSEGV' or `SIGBUS'.

 -- Macro: int SIGBUS
     This signal is generated when an invalid pointer is dereferenced.
     Like `SIGSEGV', this signal is typically the result of
     dereferencing an uninitialized pointer.  The difference between
     the two is that `SIGSEGV' indicates an invalid access to valid
     memory, while `SIGBUS' indicates an access to an invalid address.
     In particular, `SIGBUS' signals often result from dereferencing a
     misaligned pointer, such as referring to a four-word integer at an
     address not divisible by four.  (Each kind of computer has its own
     requirements for address alignment.)

     The name of this signal is an abbreviation for "bus error".
   
 -- Macro: int SIGABRT
     This signal indicates an error detected by the program itself and
     reported by calling `abort'.  *Note Aborting a Program::.

 -- Macro: int SIGIOT
     Generated by the PDP-11 "iot" instruction.  On most machines, this
     is just another name for `SIGABRT'.

 -- Macro: int SIGTRAP
     Generated by the machine's breakpoint instruction, and possibly
     other trap instructions.  This signal is used by debuggers.  Your
     program will probably only see `SIGTRAP' if it is somehow
     executing bad instructions.

 -- Macro: int SIGEMT
     Emulator trap; this results from certain unimplemented instructions
     which might be emulated in software, or the operating system's
     failure to properly emulate them.

 -- Macro: int SIGSYS
     Bad system call; that is to say, the instruction to trap to the
     operating system was executed, but the code number for the system
     call to perform was invalid.


File: libc.info,  Node: Termination Signals,  Next: Alarm Signals,  Prev: Program Error Signals,  Up: Standard Signals

24.2.2 Termination Signals
--------------------------

These signals are all used to tell a process to terminate, in one way
or another.  They have different names because they're used for slightly
different purposes, and programs might want to handle them differently.

   The reason for handling these signals is usually so your program can
tidy up as appropriate before actually terminating.  For example, you
might want to save state information, delete temporary files, or restore
the previous terminal modes.  Such a handler should end by specifying
the default action for the signal that happened and then reraising it;
this will cause the program to terminate with that signal, as if it had
not had a handler.  (*Note Termination in Handler::.)

   The (obvious) default action for all of these signals is to cause the
process to terminate.

 -- Macro: int SIGTERM
     The `SIGTERM' signal is a generic signal used to cause program
     termination.  Unlike `SIGKILL', this signal can be blocked,
     handled, and ignored.  It is the normal way to politely ask a
     program to terminate.

     The shell command `kill' generates `SIGTERM' by default.  

 -- Macro: int SIGINT
     The `SIGINT' ("program interrupt") signal is sent when the user
     types the INTR character (normally `C-c').  *Note Special
     Characters::, for information about terminal driver support for
     `C-c'.

 -- Macro: int SIGQUIT
     The `SIGQUIT' signal is similar to `SIGINT', except that it's
     controlled by a different key--the QUIT character, usually
     `C-\'--and produces a core dump when it terminates the process,
     just like a program error signal.  You can think of this as a
     program error condition "detected" by the user.

     *Note Program Error Signals::, for information about core dumps.
     *Note Special Characters::, for information about terminal driver
     support.

     Certain kinds of cleanups are best omitted in handling `SIGQUIT'.
     For example, if the program creates temporary files, it should
     handle the other termination requests by deleting the temporary
     files.  But it is better for `SIGQUIT' not to delete them, so that
     the user can examine them in conjunction with the core dump.

 -- Macro: int SIGKILL
     The `SIGKILL' signal is used to cause immediate program
     termination.  It cannot be handled or ignored, and is therefore
     always fatal.  It is also not possible to block this signal.

     This signal is usually generated only by explicit request.  Since
     it cannot be handled, you should generate it only as a last
     resort, after first trying a less drastic method such as `C-c' or
     `SIGTERM'.  If a process does not respond to any other termination
     signals, sending it a `SIGKILL' signal will almost always cause it
     to go away.

     In fact, if `SIGKILL' fails to terminate a process, that by itself
     constitutes an operating system bug which you should report.

     The system will generate `SIGKILL' for a process itself under some
     unusual conditions where the program cannot possibly continue to
     run (even to run a signal handler).
   
 -- Macro: int SIGHUP
     The `SIGHUP' ("hang-up") signal is used to report that the user's
     terminal is disconnected, perhaps because a network or telephone
     connection was broken.  For more information about this, see *Note
     Control Modes::.

     This signal is also used to report the termination of the
     controlling process on a terminal to jobs associated with that
     session; this termination effectively disconnects all processes in
     the session from the controlling terminal.  For more information,
     see *Note Termination Internals::.


File: libc.info,  Node: Alarm Signals,  Next: Asynchronous I/O Signals,  Prev: Termination Signals,  Up: Standard Signals

24.2.3 Alarm Signals
--------------------

These signals are used to indicate the expiration of timers.  *Note
Setting an Alarm::, for information about functions that cause these
signals to be sent.

   The default behavior for these signals is to cause program
termination.  This default is rarely useful, but no other default would
be useful; most of the ways of using these signals would require
handler functions in any case.

 -- Macro: int SIGALRM
     This signal typically indicates expiration of a timer that
     measures real or clock time.  It is used by the `alarm' function,
     for example.
   
 -- Macro: int SIGVTALRM
     This signal typically indicates expiration of a timer that
     measures CPU time used by the current process.  The name is an
     abbreviation for "virtual time alarm".
   
 -- Macro: int SIGPROF
     This signal typically indicates expiration of a timer that measures
     both CPU time used by the current process, and CPU time expended on
     behalf of the process by the system.  Such a timer is used to
     implement code profiling facilities, hence the name of this signal.
   

File: libc.info,  Node: Asynchronous I/O Signals,  Next: Job Control Signals,  Prev: Alarm Signals,  Up: Standard Signals

24.2.4 Asynchronous I/O Signals
-------------------------------

The signals listed in this section are used in conjunction with
asynchronous I/O facilities.  You have to take explicit action by
calling `fcntl' to enable a particular file descriptor to generate
these signals (*note Interrupt Input::).  The default action for these
signals is to ignore them.

 -- Macro: int SIGIO
     This signal is sent when a file descriptor is ready to perform
     input or output.

     On most operating systems, terminals and sockets are the only
     kinds of files that can generate `SIGIO'; other kinds, including
     ordinary files, never generate `SIGIO' even if you ask them to.

     In the GNU system `SIGIO' will always be generated properly if you
     successfully set asynchronous mode with `fcntl'.

 -- Macro: int SIGURG
     This signal is sent when "urgent" or out-of-band data arrives on a
     socket.  *Note Out-of-Band Data::.

 -- Macro: int SIGPOLL
     This is a System V signal name, more or less similar to `SIGIO'.
     It is defined only for compatibility.


File: libc.info,  Node: Job Control Signals,  Next: Operation Error Signals,  Prev: Asynchronous I/O Signals,  Up: Standard Signals

24.2.5 Job Control Signals
--------------------------

These signals are used to support job control.  If your system doesn't
support job control, then these macros are defined but the signals
themselves can't be raised or handled.

   You should generally leave these signals alone unless you really
understand how job control works.  *Note Job Control::.

 -- Macro: int SIGCHLD
     This signal is sent to a parent process whenever one of its child
     processes terminates or stops.

     The default action for this signal is to ignore it.  If you
     establish a handler for this signal while there are child
     processes that have terminated but not reported their status via
     `wait' or `waitpid' (*note Process Completion::), whether your new
     handler applies to those processes or not depends on the
     particular operating system.

 -- Macro: int SIGCLD
     This is an obsolete name for `SIGCHLD'.

 -- Macro: int SIGCONT
     You can send a `SIGCONT' signal to a process to make it continue.
     This signal is special--it always makes the process continue if it
     is stopped, before the signal is delivered.  The default behavior
     is to do nothing else.  You cannot block this signal.  You can set
     a handler, but `SIGCONT' always makes the process continue
     regardless.

     Most programs have no reason to handle `SIGCONT'; they simply
     resume execution without realizing they were ever stopped.  You
     can use a handler for `SIGCONT' to make a program do something
     special when it is stopped and continued--for example, to reprint
     a prompt when it is suspended while waiting for input.

 -- Macro: int SIGSTOP
     The `SIGSTOP' signal stops the process.  It cannot be handled,
     ignored, or blocked.
   
 -- Macro: int SIGTSTP
     The `SIGTSTP' signal is an interactive stop signal.  Unlike
     `SIGSTOP', this signal can be handled and ignored.

     Your program should handle this signal if you have a special need
     to leave files or system tables in a secure state when a process is
     stopped.  For example, programs that turn off echoing should handle
     `SIGTSTP' so they can turn echoing back on before stopping.

     This signal is generated when the user types the SUSP character
     (normally `C-z').  For more information about terminal driver
     support, see *Note Special Characters::.
   
 -- Macro: int SIGTTIN
     A process cannot read from the user's terminal while it is running
     as a background job.  When any process in a background job tries to
     read from the terminal, all of the processes in the job are sent a
     `SIGTTIN' signal.  The default action for this signal is to stop
     the process.  For more information about how this interacts with
     the terminal driver, see *Note Access to the Terminal::.
   
 -- Macro: int SIGTTOU
     This is similar to `SIGTTIN', but is generated when a process in a
     background job attempts to write to the terminal or set its modes.
     Again, the default action is to stop the process.  `SIGTTOU' is
     only generated for an attempt to write to the terminal if the
     `TOSTOP' output mode is set; *note Output Modes::.
   
   While a process is stopped, no more signals can be delivered to it
until it is continued, except `SIGKILL' signals and (obviously)
`SIGCONT' signals.  The signals are marked as pending, but not
delivered until the process is continued.  The `SIGKILL' signal always
causes termination of the process and can't be blocked, handled or
ignored.  You can ignore `SIGCONT', but it always causes the process to
be continued anyway if it is stopped.  Sending a `SIGCONT' signal to a
process causes any pending stop signals for that process to be
discarded.  Likewise, any pending `SIGCONT' signals for a process are
discarded when it receives a stop signal.

   When a process in an orphaned process group (*note Orphaned Process
Groups::) receives a `SIGTSTP', `SIGTTIN', or `SIGTTOU' signal and does
not handle it, the process does not stop.  Stopping the process would
probably not be very useful, since there is no shell program that will
notice it stop and allow the user to continue it.  What happens instead
depends on the operating system you are using.  Some systems may do
nothing; others may deliver another signal instead, such as `SIGKILL'
or `SIGHUP'.  In the GNU system, the process dies with `SIGKILL'; this
avoids the problem of many stopped, orphaned processes lying around the
system.


File: libc.info,  Node: Operation Error Signals,  Next: Miscellaneous Signals,  Prev: Job Control Signals,  Up: Standard Signals

24.2.6 Operation Error Signals
------------------------------

These signals are used to report various errors generated by an
operation done by the program.  They do not necessarily indicate a
programming error in the program, but an error that prevents an
operating system call from completing.  The default action for all of
them is to cause the process to terminate.

 -- Macro: int SIGPIPE
     Broken pipe.  If you use pipes or FIFOs, you have to design your
     application so that one process opens the pipe for reading before
     another starts writing.  If the reading process never starts, or
     terminates unexpectedly, writing to the pipe or FIFO raises a
     `SIGPIPE' signal.  If `SIGPIPE' is blocked, handled or ignored,
     the offending call fails with `EPIPE' instead.

     Pipes and FIFO special files are discussed in more detail in *Note
     Pipes and FIFOs::.

     Another cause of `SIGPIPE' is when you try to output to a socket
     that isn't connected.  *Note Sending Data::.

 -- Macro: int SIGLOST
     Resource lost.  This signal is generated when you have an advisory
     lock on an NFS file, and the NFS server reboots and forgets about
     your lock.

     In the GNU system, `SIGLOST' is generated when any server program
     dies unexpectedly.  It is usually fine to ignore the signal;
     whatever call was made to the server that died just returns an
     error.

 -- Macro: int SIGXCPU
     CPU time limit exceeded.  This signal is generated when the process
     exceeds its soft resource limit on CPU time.  *Note Limits on
     Resources::.

 -- Macro: int SIGXFSZ
     File size limit exceeded.  This signal is generated when the
     process attempts to extend a file so it exceeds the process's soft
     resource limit on file size.  *Note Limits on Resources::.


File: libc.info,  Node: Miscellaneous Signals,  Next: Signal Messages,  Prev: Operation Error Signals,  Up: Standard Signals

24.2.7 Miscellaneous Signals
----------------------------

These signals are used for various other purposes.  In general, they
will not affect your program unless it explicitly uses them for
something.

 -- Macro: int SIGUSR1
 -- Macro: int SIGUSR2
     The `SIGUSR1' and `SIGUSR2' signals are set aside for you to use
     any way you want.  They're useful for simple interprocess
     communication, if you write a signal handler for them in the
     program that receives the signal.

     There is an example showing the use of `SIGUSR1' and `SIGUSR2' in
     *Note Signaling Another Process::.

     The default action is to terminate the process.

 -- Macro: int SIGWINCH
     Window size change.  This is generated on some systems (including
     GNU) when the terminal driver's record of the number of rows and
     columns on the screen is changed.  The default action is to ignore
     it.

     If a program does full-screen display, it should handle `SIGWINCH'.
     When the signal arrives, it should fetch the new screen size and
     reformat its display accordingly.

 -- Macro: int SIGINFO
     Information request.  In 4.4 BSD and the GNU system, this signal
     is sent to all the processes in the foreground process group of
     the controlling terminal when the user types the STATUS character
     in canonical mode; *note Signal Characters::.

     If the process is the leader of the process group, the default
     action is to print some status information about the system and
     what the process is doing.  Otherwise the default is to do nothing.


File: libc.info,  Node: Signal Messages,  Prev: Miscellaneous Signals,  Up: Standard Signals

24.2.8 Signal Messages
----------------------

We mentioned above that the shell prints a message describing the signal
that terminated a child process.  The clean way to print a message
describing a signal is to use the functions `strsignal' and `psignal'.
These functions use a signal number to specify which kind of signal to
describe.  The signal number may come from the termination status of a
child process (*note Process Completion::) or it may come from a signal
handler in the same process.

 -- Function: char * strsignal (int SIGNUM)
     This function returns a pointer to a statically-allocated string
     containing a message describing the signal SIGNUM.  You should not
     modify the contents of this string; and, since it can be rewritten
     on subsequent calls, you should save a copy of it if you need to
     reference it later.

     This function is a GNU extension, declared in the header file
     `string.h'.

 -- Function: void psignal (int SIGNUM, const char *MESSAGE)
     This function prints a message describing the signal SIGNUM to the
     standard error output stream `stderr'; see *Note Standard
     Streams::.

     If you call `psignal' with a MESSAGE that is either a null pointer
     or an empty string, `psignal' just prints the message
     corresponding to SIGNUM, adding a trailing newline.

     If you supply a non-null MESSAGE argument, then `psignal' prefixes
     its output with this string.  It adds a colon and a space
     character to separate the MESSAGE from the string corresponding to
     SIGNUM.

     This function is a BSD feature, declared in the header file
     `signal.h'.

   There is also an array `sys_siglist' which contains the messages for
the various signal codes.  This array exists on BSD systems, unlike
`strsignal'.


File: libc.info,  Node: Signal Actions,  Next: Defining Handlers,  Prev: Standard Signals,  Up: Signal Handling

24.3 Specifying Signal Actions
==============================

The simplest way to change the action for a signal is to use the
`signal' function.  You can specify a built-in action (such as to
ignore the signal), or you can "establish a handler".

   The GNU library also implements the more versatile `sigaction'
facility.  This section describes both facilities and gives suggestions
on which to use when.

* Menu:

* Basic Signal Handling::       The simple `signal' function.
* Advanced Signal Handling::    The more powerful `sigaction' function.
* Signal and Sigaction::        How those two functions interact.
* Sigaction Function Example::  An example of using the sigaction function.
* Flags for Sigaction::         Specifying options for signal handling.
* Initial Signal Actions::      How programs inherit signal actions.


File: libc.info,  Node: Basic Signal Handling,  Next: Advanced Signal Handling,  Up: Signal Actions

24.3.1 Basic Signal Handling
----------------------------

The `signal' function provides a simple interface for establishing an
action for a particular signal.  The function and associated macros are
declared in the header file `signal.h'.  

 -- Data Type: sighandler_t
     This is the type of signal handler functions.  Signal handlers
     take one integer argument specifying the signal number, and have
     return type `void'.  So, you should define handler functions like
     this:

          void HANDLER (int `signum') { ... }

     The name `sighandler_t' for this data type is a GNU extension.

 -- Function: sighandler_t signal (int SIGNUM, sighandler_t ACTION)
     The `signal' function establishes ACTION as the action for the
     signal SIGNUM.

     The first argument, SIGNUM, identifies the signal whose behavior
     you want to control, and should be a signal number.  The proper
     way to specify a signal number is with one of the symbolic signal
     names (*note Standard Signals::)--don't use an explicit number,
     because the numerical code for a given kind of signal may vary
     from operating system to operating system.

     The second argument, ACTION, specifies the action to use for the
     signal SIGNUM.  This can be one of the following:

    `SIG_DFL'
          `SIG_DFL' specifies the default action for the particular
          signal.  The default actions for various kinds of signals are
          stated in *Note Standard Signals::.

    `SIG_IGN'
          `SIG_IGN' specifies that the signal should be ignored.

          Your program generally should not ignore signals that
          represent serious events or that are normally used to request
          termination.  You cannot ignore the `SIGKILL' or `SIGSTOP'
          signals at all.  You can ignore program error signals like
          `SIGSEGV', but ignoring the error won't enable the program to
          continue executing meaningfully.  Ignoring user requests such
          as `SIGINT', `SIGQUIT', and `SIGTSTP' is unfriendly.

          When you do not wish signals to be delivered during a certain
          part of the program, the thing to do is to block them, not
          ignore them.  *Note Blocking Signals::.

    `HANDLER'
          Supply the address of a handler function in your program, to
          specify running this handler as the way to deliver the signal.

          For more information about defining signal handler functions,
          see *Note Defining Handlers::.

     If you set the action for a signal to `SIG_IGN', or if you set it
     to `SIG_DFL' and the default action is to ignore that signal, then
     any pending signals of that type are discarded (even if they are
     blocked).  Discarding the pending signals means that they will
     never be delivered, not even if you subsequently specify another
     action and unblock this kind of signal.

     The `signal' function returns the action that was previously in
     effect for the specified SIGNUM.  You can save this value and
     restore it later by calling `signal' again.

     If `signal' can't honor the request, it returns `SIG_ERR' instead.
     The following `errno' error conditions are defined for this
     function:

    `EINVAL'
          You specified an invalid SIGNUM; or you tried to ignore or
          provide a handler for `SIGKILL' or `SIGSTOP'.

   *Compatibility Note:* A problem encountered when working with the
`signal' function is that it has different semantics on BSD and SVID
systems.  The difference is that on SVID systems the signal handler is
deinstalled after signal delivery.  On BSD systems the handler must be
explicitly deinstalled.  In the GNU C Library we use the BSD version by
default.  To use the SVID version you can either use the function
`sysv_signal' (see below) or use the `_XOPEN_SOURCE' feature select
macro (*note Feature Test Macros::).  In general, use of these
functions should be avoided because of compatibility problems.  It is
better to use `sigaction' if it is available since the results are much
more reliable.

   Here is a simple example of setting up a handler to delete temporary
files when certain fatal signals happen:

     #include <signal.h>

     void
     termination_handler (int signum)
     {
       struct temp_file *p;

       for (p = temp_file_list; p; p = p->next)
         unlink (p->name);
     }

     int
     main (void)
     {
       ...
       if (signal (SIGINT, termination_handler) == SIG_IGN)
         signal (SIGINT, SIG_IGN);
       if (signal (SIGHUP, termination_handler) == SIG_IGN)
         signal (SIGHUP, SIG_IGN);
       if (signal (SIGTERM, termination_handler) == SIG_IGN)
         signal (SIGTERM, SIG_IGN);
       ...
     }

Note that if a given signal was previously set to be ignored, this code
avoids altering that setting.  This is because non-job-control shells
often ignore certain signals when starting children, and it is important
for the children to respect this.

   We do not handle `SIGQUIT' or the program error signals in this
example because these are designed to provide information for debugging
(a core dump), and the temporary files may give useful information.

 -- Function: sighandler_t sysv_signal (int SIGNUM, sighandler_t ACTION)
     The `sysv_signal' implements the behavior of the standard `signal'
     function as found on SVID systems.  The difference to BSD systems
     is that the handler is deinstalled after a delivery of a signal.

     *Compatibility Note:* As said above for `signal', this function
     should be avoided when possible.  `sigaction' is the preferred
     method.

 -- Function: sighandler_t ssignal (int SIGNUM, sighandler_t ACTION)
     The `ssignal' function does the same thing as `signal'; it is
     provided only for compatibility with SVID.

 -- Macro: sighandler_t SIG_ERR
     The value of this macro is used as the return value from `signal'
     to indicate an error.


File: libc.info,  Node: Advanced Signal Handling,  Next: Signal and Sigaction,  Prev: Basic Signal Handling,  Up: Signal Actions

24.3.2 Advanced Signal Handling
-------------------------------

The `sigaction' function has the same basic effect as `signal': to
specify how a signal should be handled by the process.  However,
`sigaction' offers more control, at the expense of more complexity.  In
particular, `sigaction' allows you to specify additional flags to
control when the signal is generated and how the handler is invoked.

   The `sigaction' function is declared in `signal.h'.  

 -- Data Type: struct sigaction
     Structures of type `struct sigaction' are used in the `sigaction'
     function to specify all the information about how to handle a
     particular signal.  This structure contains at least the following
     members:

    `sighandler_t sa_handler'
          This is used in the same way as the ACTION argument to the
          `signal' function.  The value can be `SIG_DFL', `SIG_IGN', or
          a function pointer.  *Note Basic Signal Handling::.

    `sigset_t sa_mask'
          This specifies a set of signals to be blocked while the
          handler runs.  Blocking is explained in *Note Blocking for
          Handler::.  Note that the signal that was delivered is
          automatically blocked by default before its handler is
          started; this is true regardless of the value in `sa_mask'.
          If you want that signal not to be blocked within its handler,
          you must write code in the handler to unblock it.

    `int sa_flags'
          This specifies various flags which can affect the behavior of
          the signal.  These are described in more detail in *Note
          Flags for Sigaction::.

 -- Function: int sigaction (int SIGNUM, const struct sigaction
          *restrict ACTION, struct sigaction *restrict OLD-ACTION)
     The ACTION argument is used to set up a new action for the signal
     SIGNUM, while the OLD-ACTION argument is used to return
     information about the action previously associated with this
     symbol.  (In other words, OLD-ACTION has the same purpose as the
     `signal' function's return value--you can check to see what the
     old action in effect for the signal was, and restore it later if
     you want.)

     Either ACTION or OLD-ACTION can be a null pointer.  If OLD-ACTION
     is a null pointer, this simply suppresses the return of
     information about the old action.  If ACTION is a null pointer,
     the action associated with the signal SIGNUM is unchanged; this
     allows you to inquire about how a signal is being handled without
     changing that handling.

     The return value from `sigaction' is zero if it succeeds, and `-1'
     on failure.  The following `errno' error conditions are defined
     for this function:

    `EINVAL'
          The SIGNUM argument is not valid, or you are trying to trap
          or ignore `SIGKILL' or `SIGSTOP'.


File: libc.info,  Node: Signal and Sigaction,  Next: Sigaction Function Example,  Prev: Advanced Signal Handling,  Up: Signal Actions

24.3.3 Interaction of `signal' and `sigaction'
----------------------------------------------

It's possible to use both the `signal' and `sigaction' functions within
a single program, but you have to be careful because they can interact
in slightly strange ways.

   The `sigaction' function specifies more information than the
`signal' function, so the return value from `signal' cannot express the
full range of `sigaction' possibilities.  Therefore, if you use
`signal' to save and later reestablish an action, it may not be able to
reestablish properly a handler that was established with `sigaction'.

   To avoid having problems as a result, always use `sigaction' to save
and restore a handler if your program uses `sigaction' at all.  Since
`sigaction' is more general, it can properly save and reestablish any
action, regardless of whether it was established originally with
`signal' or `sigaction'.

   On some systems if you establish an action with `signal' and then
examine it with `sigaction', the handler address that you get may not
be the same as what you specified with `signal'.  It may not even be
suitable for use as an action argument with `signal'.  But you can rely
on using it as an argument to `sigaction'.  This problem never happens
on the GNU system.

   So, you're better off using one or the other of the mechanisms
consistently within a single program.

   *Portability Note:* The basic `signal' function is a feature of
ISO C, while `sigaction' is part of the POSIX.1 standard.  If you are
concerned about portability to non-POSIX systems, then you should use
the `signal' function instead.


File: libc.info,  Node: Sigaction Function Example,  Next: Flags for Sigaction,  Prev: Signal and Sigaction,  Up: Signal Actions

24.3.4 `sigaction' Function Example
-----------------------------------

In *Note Basic Signal Handling::, we gave an example of establishing a
simple handler for termination signals using `signal'.  Here is an
equivalent example using `sigaction':

     #include <signal.h>

     void
     termination_handler (int signum)
     {
       struct temp_file *p;

       for (p = temp_file_list; p; p = p->next)
         unlink (p->name);
     }

     int
     main (void)
     {
       ...
       struct sigaction new_action, old_action;

       /* Set up the structure to specify the new action. */
       new_action.sa_handler = termination_handler;
       sigemptyset (&new_action.sa_mask);
       new_action.sa_flags = 0;

       sigaction (SIGINT, NULL, &old_action);
       if (old_action.sa_handler != SIG_IGN)
         sigaction (SIGINT, &new_action, NULL);
       sigaction (SIGHUP, NULL, &old_action);
       if (old_action.sa_handler != SIG_IGN)
         sigaction (SIGHUP, &new_action, NULL);
       sigaction (SIGTERM, NULL, &old_action);
       if (old_action.sa_handler != SIG_IGN)
         sigaction (SIGTERM, &new_action, NULL);
       ...
     }

   The program just loads the `new_action' structure with the desired
parameters and passes it in the `sigaction' call.  The usage of
`sigemptyset' is described later; see *Note Blocking Signals::.

   As in the example using `signal', we avoid handling signals
previously set to be ignored.  Here we can avoid altering the signal
handler even momentarily, by using the feature of `sigaction' that lets
us examine the current action without specifying a new one.

   Here is another example.  It retrieves information about the current
action for `SIGINT' without changing that action.

     struct sigaction query_action;

     if (sigaction (SIGINT, NULL, &query_action) < 0)
       /* `sigaction' returns -1 in case of error. */
     else if (query_action.sa_handler == SIG_DFL)
       /* `SIGINT' is handled in the default, fatal manner. */
     else if (query_action.sa_handler == SIG_IGN)
       /* `SIGINT' is ignored. */
     else
       /* A programmer-defined signal handler is in effect. */


File: libc.info,  Node: Flags for Sigaction,  Next: Initial Signal Actions,  Prev: Sigaction Function Example,  Up: Signal Actions

24.3.5 Flags for `sigaction'
----------------------------

The `sa_flags' member of the `sigaction' structure is a catch-all for
special features.  Most of the time, `SA_RESTART' is a good value to
use for this field.

   The value of `sa_flags' is interpreted as a bit mask.  Thus, you
should choose the flags you want to set, OR those flags together, and
store the result in the `sa_flags' member of your `sigaction' structure.

   Each signal number has its own set of flags.  Each call to
`sigaction' affects one particular signal number, and the flags that
you specify apply only to that particular signal.

   In the GNU C library, establishing a handler with `signal' sets all
the flags to zero except for `SA_RESTART', whose value depends on the
settings you have made with `siginterrupt'.  *Note Interrupted
Primitives::, to see what this is about.

   These macros are defined in the header file `signal.h'.

 -- Macro: int SA_NOCLDSTOP
     This flag is meaningful only for the `SIGCHLD' signal.  When the
     flag is set, the system delivers the signal for a terminated child
     process but not for one that is stopped.  By default, `SIGCHLD' is
     delivered for both terminated children and stopped children.

     Setting this flag for a signal other than `SIGCHLD' has no effect.

 -- Macro: int SA_ONSTACK
     If this flag is set for a particular signal number, the system
     uses the signal stack when delivering that kind of signal.  *Note
     Signal Stack::.  If a signal with this flag arrives and you have
     not set a signal stack, the system terminates the program with
     `SIGILL'.

 -- Macro: int SA_RESTART
     This flag controls what happens when a signal is delivered during
     certain primitives (such as `open', `read' or `write'), and the
     signal handler returns normally.  There are two alternatives: the
     library function can resume, or it can return failure with error
     code `EINTR'.

     The choice is controlled by the `SA_RESTART' flag for the
     particular kind of signal that was delivered.  If the flag is set,
     returning from a handler resumes the library function.  If the
     flag is clear, returning from a handler makes the function fail.
     *Note Interrupted Primitives::.


File: libc.info,  Node: Initial Signal Actions,  Prev: Flags for Sigaction,  Up: Signal Actions

24.3.6 Initial Signal Actions
-----------------------------

When a new process is created (*note Creating a Process::), it inherits
handling of signals from its parent process.  However, when you load a
new process image using the `exec' function (*note Executing a File::),
any signals that you've defined your own handlers for revert to their
`SIG_DFL' handling.  (If you think about it a little, this makes sense;
the handler functions from the old program are specific to that
program, and aren't even present in the address space of the new
program image.)  Of course, the new program can establish its own
handlers.

   When a program is run by a shell, the shell normally sets the initial
actions for the child process to `SIG_DFL' or `SIG_IGN', as
appropriate.  It's a good idea to check to make sure that the shell has
not set up an initial action of `SIG_IGN' before you establish your own
signal handlers.

   Here is an example of how to establish a handler for `SIGHUP', but
not if `SIGHUP' is currently ignored:

     ...
     struct sigaction temp;

     sigaction (SIGHUP, NULL, &temp);

     if (temp.sa_handler != SIG_IGN)
       {
         temp.sa_handler = handle_sighup;
         sigemptyset (&temp.sa_mask);
         sigaction (SIGHUP, &temp, NULL);
       }


File: libc.info,  Node: Defining Handlers,  Next: Interrupted Primitives,  Prev: Signal Actions,  Up: Signal Handling

24.4 Defining Signal Handlers
=============================

This section describes how to write a signal handler function that can
be established with the `signal' or `sigaction' functions.

   A signal handler is just a function that you compile together with
the rest of the program.  Instead of directly invoking the function,
you use `signal' or `sigaction' to tell the operating system to call it
when a signal arrives.  This is known as "establishing" the handler.
*Note Signal Actions::.

   There are two basic strategies you can use in signal handler
functions:

   * You can have the handler function note that the signal arrived by
     tweaking some global data structures, and then return normally.

   * You can have the handler function terminate the program or transfer
     control to a point where it can recover from the situation that
     caused the signal.

   You need to take special care in writing handler functions because
they can be called asynchronously.  That is, a handler might be called
at any point in the program, unpredictably.  If two signals arrive
during a very short interval, one handler can run within another.  This
section describes what your handler should do, and what you should
avoid.

* Menu:

* Handler Returns::             Handlers that return normally, and what
                                 this means.
* Termination in Handler::      How handler functions terminate a program.
* Longjmp in Handler::          Nonlocal transfer of control out of a
                                 signal handler.
* Signals in Handler::          What happens when signals arrive while
                                 the handler is already occupied.
* Merged Signals::		When a second signal arrives before the
				 first is handled.
* Nonreentrancy::               Do not call any functions unless you know they
                                 are reentrant with respect to signals.
* Atomic Data Access::          A single handler can run in the middle of
                                 reading or writing a single object.


File: libc.info,  Node: Handler Returns,  Next: Termination in Handler,  Up: Defining Handlers

24.4.1 Signal Handlers that Return
----------------------------------

Handlers which return normally are usually used for signals such as
`SIGALRM' and the I/O and interprocess communication signals.  But a
handler for `SIGINT' might also return normally after setting a flag
that tells the program to exit at a convenient time.

   It is not safe to return normally from the handler for a program
error signal, because the behavior of the program when the handler
function returns is not defined after a program error.  *Note Program
Error Signals::.

   Handlers that return normally must modify some global variable in
order to have any effect.  Typically, the variable is one that is
examined periodically by the program during normal operation.  Its data
type should be `sig_atomic_t' for reasons described in *Note Atomic
Data Access::.

   Here is a simple example of such a program.  It executes the body of
the loop until it has noticed that a `SIGALRM' signal has arrived.
This technique is useful because it allows the iteration in progress
when the signal arrives to complete before the loop exits.

     #include <signal.h>
     #include <stdio.h>
     #include <stdlib.h>

     /* This flag controls termination of the main loop. */
     volatile sig_atomic_t keep_going = 1;

     /* The signal handler just clears the flag and re-enables itself. */
     void
     catch_alarm (int sig)
     {
       keep_going = 0;
       signal (sig, catch_alarm);
     }

     void
     do_stuff (void)
     {
       puts ("Doing stuff while waiting for alarm....");
     }

     int
     main (void)
     {
       /* Establish a handler for SIGALRM signals. */
       signal (SIGALRM, catch_alarm);

       /* Set an alarm to go off in a little while. */
       alarm (2);

       /* Check the flag once in a while to see when to quit. */
       while (keep_going)
         do_stuff ();

       return EXIT_SUCCESS;
     }


File: libc.info,  Node: Termination in Handler,  Next: Longjmp in Handler,  Prev: Handler Returns,  Up: Defining Handlers

24.4.2 Handlers That Terminate the Process
------------------------------------------

Handler functions that terminate the program are typically used to cause
orderly cleanup or recovery from program error signals and interactive
interrupts.

   The cleanest way for a handler to terminate the process is to raise
the same signal that ran the handler in the first place.  Here is how
to do this:

     volatile sig_atomic_t fatal_error_in_progress = 0;

     void
     fatal_error_signal (int sig)
     {
       /* Since this handler is established for more than one kind of signal,
          it might still get invoked recursively by delivery of some other kind
          of signal.  Use a static variable to keep track of that. */
       if (fatal_error_in_progress)
         raise (sig);
       fatal_error_in_progress = 1;

       /* Now do the clean up actions:
          - reset terminal modes
          - kill child processes
          - remove lock files */
       ...

       /* Now reraise the signal.  We reactivate the signal's
          default handling, which is to terminate the process.
          We could just call `exit' or `abort',
          but reraising the signal sets the return status
          from the process correctly. */
       signal (sig, SIG_DFL);
       raise (sig);
     }


File: libc.info,  Node: Longjmp in Handler,  Next: Signals in Handler,  Prev: Termination in Handler,  Up: Defining Handlers

24.4.3 Nonlocal Control Transfer in Handlers
--------------------------------------------

You can do a nonlocal transfer of control out of a signal handler using
the `setjmp' and `longjmp' facilities (*note Non-Local Exits::).

   When the handler does a nonlocal control transfer, the part of the
program that was running will not continue.  If this part of the program
was in the middle of updating an important data structure, the data
structure will remain inconsistent.  Since the program does not
terminate, the inconsistency is likely to be noticed later on.

   There are two ways to avoid this problem.  One is to block the signal
for the parts of the program that update important data structures.
Blocking the signal delays its delivery until it is unblocked, once the
critical updating is finished.  *Note Blocking Signals::.

   The other way to re-initialize the crucial data structures in the
signal handler, or make their values consistent.

   Here is a rather schematic example showing the reinitialization of
one global variable.

     #include <signal.h>
     #include <setjmp.h>

     jmp_buf return_to_top_level;

     volatile sig_atomic_t waiting_for_input;

     void
     handle_sigint (int signum)
     {
       /* We may have been waiting for input when the signal arrived,
          but we are no longer waiting once we transfer control. */
       waiting_for_input = 0;
       longjmp (return_to_top_level, 1);
     }

     int
     main (void)
     {
       ...
       signal (SIGINT, sigint_handler);
       ...
       while (1) {
         prepare_for_command ();
         if (setjmp (return_to_top_level) == 0)
           read_and_execute_command ();
       }
     }

     /* Imagine this is a subroutine used by various commands. */
     char *
     read_data ()
     {
       if (input_from_terminal) {
         waiting_for_input = 1;
         ...
         waiting_for_input = 0;
       } else {
         ...
       }
     }


File: libc.info,  Node: Signals in Handler,  Next: Merged Signals,  Prev: Longjmp in Handler,  Up: Defining Handlers

24.4.4 Signals Arriving While a Handler Runs
--------------------------------------------

What happens if another signal arrives while your signal handler
function is running?

   When the handler for a particular signal is invoked, that signal is
automatically blocked until the handler returns.  That means that if two
signals of the same kind arrive close together, the second one will be
held until the first has been handled.  (The handler can explicitly
unblock the signal using `sigprocmask', if you want to allow more
signals of this type to arrive; see *Note Process Signal Mask::.)

   However, your handler can still be interrupted by delivery of another
kind of signal.  To avoid this, you can use the `sa_mask' member of the
action structure passed to `sigaction' to explicitly specify which
signals should be blocked while the signal handler runs.  These signals
are in addition to the signal for which the handler was invoked, and
any other signals that are normally blocked by the process.  *Note
Blocking for Handler::.

   When the handler returns, the set of blocked signals is restored to
the value it had before the handler ran.  So using `sigprocmask' inside
the handler only affects what signals can arrive during the execution of
the handler itself, not what signals can arrive once the handler
returns.

   *Portability Note:* Always use `sigaction' to establish a handler
for a signal that you expect to receive asynchronously, if you want
your program to work properly on System V Unix.  On this system, the
handling of a signal whose handler was established with `signal'
automatically sets the signal's action back to `SIG_DFL', and the
handler must re-establish itself each time it runs.  This practice,
while inconvenient, does work when signals cannot arrive in succession.
However, if another signal can arrive right away, it may arrive before
the handler can re-establish itself.  Then the second signal would
receive the default handling, which could terminate the process.


File: libc.info,  Node: Merged Signals,  Next: Nonreentrancy,  Prev: Signals in Handler,  Up: Defining Handlers

24.4.5 Signals Close Together Merge into One
--------------------------------------------

If multiple signals of the same type are delivered to your process
before your signal handler has a chance to be invoked at all, the
handler may only be invoked once, as if only a single signal had
arrived.  In effect, the signals merge into one.  This situation can
arise when the signal is blocked, or in a multiprocessing environment
where the system is busy running some other processes while the signals
are delivered.  This means, for example, that you cannot reliably use a
signal handler to count signals.  The only distinction you can reliably
make is whether at least one signal has arrived since a given time in
the past.

   Here is an example of a handler for `SIGCHLD' that compensates for
the fact that the number of signals received may not equal the number of
child processes that generate them.  It assumes that the program keeps
track of all the child processes with a chain of structures as follows:

     struct process
     {
       struct process *next;
       /* The process ID of this child.  */
       int pid;
       /* The descriptor of the pipe or pseudo terminal
          on which output comes from this child.  */
       int input_descriptor;
       /* Nonzero if this process has stopped or terminated.  */
       sig_atomic_t have_status;
       /* The status of this child; 0 if running,
          otherwise a status value from `waitpid'.  */
       int status;
     };

     struct process *process_list;

   This example also uses a flag to indicate whether signals have
arrived since some time in the past--whenever the program last cleared
it to zero.

     /* Nonzero means some child's status has changed
        so look at `process_list' for the details.  */
     int process_status_change;

   Here is the handler itself:

     void
     sigchld_handler (int signo)
     {
       int old_errno = errno;

       while (1) {
         register int pid;
         int w;
         struct process *p;

         /* Keep asking for a status until we get a definitive result.  */
         do
           {
             errno = 0;
             pid = waitpid (WAIT_ANY, &w, WNOHANG | WUNTRACED);
           }
         while (pid <= 0 && errno == EINTR);

         if (pid <= 0) {
           /* A real failure means there are no more
              stopped or terminated child processes, so return.  */
           errno = old_errno;
           return;
         }

         /* Find the process that signaled us, and record its status.  */

         for (p = process_list; p; p = p->next)
           if (p->pid == pid) {
             p->status = w;
             /* Indicate that the `status' field
                has data to look at.  We do this only after storing it.  */
             p->have_status = 1;

             /* If process has terminated, stop waiting for its output.  */
             if (WIFSIGNALED (w) || WIFEXITED (w))
               if (p->input_descriptor)
                 FD_CLR (p->input_descriptor, &input_wait_mask);

             /* The program should check this flag from time to time
                to see if there is any news in `process_list'.  */
             ++process_status_change;
           }

         /* Loop around to handle all the processes
            that have something to tell us.  */
       }
     }

   Here is the proper way to check the flag `process_status_change':

     if (process_status_change) {
       struct process *p;
       process_status_change = 0;
       for (p = process_list; p; p = p->next)
         if (p->have_status) {
           ... Examine `p->status' ...
         }
     }

It is vital to clear the flag before examining the list; otherwise, if a
signal were delivered just before the clearing of the flag, and after
the appropriate element of the process list had been checked, the status
change would go unnoticed until the next signal arrived to set the flag
again.  You could, of course, avoid this problem by blocking the signal
while scanning the list, but it is much more elegant to guarantee
correctness by doing things in the right order.

   The loop which checks process status avoids examining `p->status'
until it sees that status has been validly stored.  This is to make sure
that the status cannot change in the middle of accessing it.  Once
`p->have_status' is set, it means that the child process is stopped or
terminated, and in either case, it cannot stop or terminate again until
the program has taken notice.  *Note Atomic Usage::, for more
information about coping with interruptions during accesses of a
variable.

   Here is another way you can test whether the handler has run since
the last time you checked.  This technique uses a counter which is never
changed outside the handler.  Instead of clearing the count, the program
remembers the previous value and sees whether it has changed since the
previous check.  The advantage of this method is that different parts of
the program can check independently, each part checking whether there
has been a signal since that part last checked.

     sig_atomic_t process_status_change;

     sig_atomic_t last_process_status_change;

     ...
     {
       sig_atomic_t prev = last_process_status_change;
       last_process_status_change = process_status_change;
       if (last_process_status_change != prev) {
         struct process *p;
         for (p = process_list; p; p = p->next)
           if (p->have_status) {
             ... Examine `p->status' ...
           }
       }
     }


File: libc.info,  Node: Nonreentrancy,  Next: Atomic Data Access,  Prev: Merged Signals,  Up: Defining Handlers

24.4.6 Signal Handling and Nonreentrant Functions
-------------------------------------------------

Handler functions usually don't do very much.  The best practice is to
write a handler that does nothing but set an external variable that the
program checks regularly, and leave all serious work to the program.
This is best because the handler can be called asynchronously, at
unpredictable times--perhaps in the middle of a primitive function, or
even between the beginning and the end of a C operator that requires
multiple instructions.  The data structures being manipulated might
therefore be in an inconsistent state when the handler function is
invoked.  Even copying one `int' variable into another can take two
instructions on most machines.

   This means you have to be very careful about what you do in a signal
handler.

   * If your handler needs to access any global variables from your
     program, declare those variables `volatile'.  This tells the
     compiler that the value of the variable might change
     asynchronously, and inhibits certain optimizations that would be
     invalidated by such modifications.

   * If you call a function in the handler, make sure it is "reentrant"
     with respect to signals, or else make sure that the signal cannot
     interrupt a call to a related function.

   A function can be non-reentrant if it uses memory that is not on the
stack.

   * If a function uses a static variable or a global variable, or a
     dynamically-allocated object that it finds for itself, then it is
     non-reentrant and any two calls to the function can interfere.

     For example, suppose that the signal handler uses `gethostbyname'.
     This function returns its value in a static object, reusing the
     same object each time.  If the signal happens to arrive during a
     call to `gethostbyname', or even after one (while the program is
     still using the value), it will clobber the value that the program
     asked for.

     However, if the program does not use `gethostbyname' or any other
     function that returns information in the same object, or if it
     always blocks signals around each use, then you are safe.

     There are a large number of library functions that return values
     in a fixed object, always reusing the same object in this fashion,
     and all of them cause the same problem.  Function descriptions in
     this manual always mention this behavior.

   * If a function uses and modifies an object that you supply, then it
     is potentially non-reentrant; two calls can interfere if they use
     the same object.

     This case arises when you do I/O using streams.  Suppose that the
     signal handler prints a message with `fprintf'.  Suppose that the
     program was in the middle of an `fprintf' call using the same
     stream when the signal was delivered.  Both the signal handler's
     message and the program's data could be corrupted, because both
     calls operate on the same data structure--the stream itself.

     However, if you know that the stream that the handler uses cannot
     possibly be used by the program at a time when signals can arrive,
     then you are safe.  It is no problem if the program uses some
     other stream.

   * On most systems, `malloc' and `free' are not reentrant, because
     they use a static data structure which records what memory blocks
     are free.  As a result, no library functions that allocate or free
     memory are reentrant.  This includes functions that allocate space
     to store a result.

     The best way to avoid the need to allocate memory in a handler is
     to allocate in advance space for signal handlers to use.

     The best way to avoid freeing memory in a handler is to flag or
     record the objects to be freed, and have the program check from
     time to time whether anything is waiting to be freed.  But this
     must be done with care, because placing an object on a chain is
     not atomic, and if it is interrupted by another signal handler
     that does the same thing, you could "lose" one of the objects.

   * Any function that modifies `errno' is non-reentrant, but you can
     correct for this: in the handler, save the original value of
     `errno' and restore it before returning normally.  This prevents
     errors that occur within the signal handler from being confused
     with errors from system calls at the point the program is
     interrupted to run the handler.

     This technique is generally applicable; if you want to call in a
     handler a function that modifies a particular object in memory,
     you can make this safe by saving and restoring that object.

   * Merely reading from a memory object is safe provided that you can
     deal with any of the values that might appear in the object at a
     time when the signal can be delivered.  Keep in mind that
     assignment to some data types requires more than one instruction,
     which means that the handler could run "in the middle of" an
     assignment to the variable if its type is not atomic.  *Note
     Atomic Data Access::.

   * Merely writing into a memory object is safe as long as a sudden
     change in the value, at any time when the handler might run, will
     not disturb anything.


File: libc.info,  Node: Atomic Data Access,  Prev: Nonreentrancy,  Up: Defining Handlers

24.4.7 Atomic Data Access and Signal Handling
---------------------------------------------

Whether the data in your application concerns atoms, or mere text, you
have to be careful about the fact that access to a single datum is not
necessarily "atomic".  This means that it can take more than one
instruction to read or write a single object.  In such cases, a signal
handler might be invoked in the middle of reading or writing the object.

   There are three ways you can cope with this problem.  You can use
data types that are always accessed atomically; you can carefully
arrange that nothing untoward happens if an access is interrupted, or
you can block all signals around any access that had better not be
interrupted (*note Blocking Signals::).

* Menu:

* Non-atomic Example::		A program illustrating interrupted access.
* Types: Atomic Types.		Data types that guarantee no interruption.
* Usage: Atomic Usage.		Proving that interruption is harmless.


File: libc.info,  Node: Non-atomic Example,  Next: Atomic Types,  Up: Atomic Data Access

24.4.7.1 Problems with Non-Atomic Access
........................................

Here is an example which shows what can happen if a signal handler runs
in the middle of modifying a variable.  (Interrupting the reading of a
variable can also lead to paradoxical results, but here we only show
writing.)

     #include <signal.h>
     #include <stdio.h>

     struct two_words { int a, b; } memory;

     void
     handler(int signum)
     {
        printf ("%d,%d\n", memory.a, memory.b);
        alarm (1);
     }

     int
     main (void)
     {
        static struct two_words zeros = { 0, 0 }, ones = { 1, 1 };
        signal (SIGALRM, handler);
        memory = zeros;
        alarm (1);
        while (1)
          {
            memory = zeros;
            memory = ones;
          }
     }

   This program fills `memory' with zeros, ones, zeros, ones,
alternating forever; meanwhile, once per second, the alarm signal
handler prints the current contents.  (Calling `printf' in the handler
is safe in this program because it is certainly not being called outside
the handler when the signal happens.)

   Clearly, this program can print a pair of zeros or a pair of ones.
But that's not all it can do!  On most machines, it takes several
instructions to store a new value in `memory', and the value is stored
one word at a time.  If the signal is delivered in between these
instructions, the handler might find that `memory.a' is zero and
`memory.b' is one (or vice versa).

   On some machines it may be possible to store a new value in `memory'
with just one instruction that cannot be interrupted.  On these
machines, the handler will always print two zeros or two ones.


File: libc.info,  Node: Atomic Types,  Next: Atomic Usage,  Prev: Non-atomic Example,  Up: Atomic Data Access

24.4.7.2 Atomic Types
.....................

To avoid uncertainty about interrupting access to a variable, you can
use a particular data type for which access is always atomic:
`sig_atomic_t'.  Reading and writing this data type is guaranteed to
happen in a single instruction, so there's no way for a handler to run
"in the middle" of an access.

   The type `sig_atomic_t' is always an integer data type, but which
one it is, and how many bits it contains, may vary from machine to
machine.

 -- Data Type: sig_atomic_t
     This is an integer data type.  Objects of this type are always
     accessed atomically.

   In practice, you can assume that `int' and other integer types no
longer than `int' are atomic.  You can also assume that pointer types
are atomic; that is very convenient.  Both of these assumptions are
true on all of the machines that the GNU C library supports and on all
POSIX systems we know of.


File: libc.info,  Node: Atomic Usage,  Prev: Atomic Types,  Up: Atomic Data Access

24.4.7.3 Atomic Usage Patterns
..............................

Certain patterns of access avoid any problem even if an access is
interrupted.  For example, a flag which is set by the handler, and
tested and cleared by the main program from time to time, is always safe
even if access actually requires two instructions.  To show that this is
so, we must consider each access that could be interrupted, and show
that there is no problem if it is interrupted.

   An interrupt in the middle of testing the flag is safe because
either it's recognized to be nonzero, in which case the precise value
doesn't matter, or it will be seen to be nonzero the next time it's
tested.

   An interrupt in the middle of clearing the flag is no problem because
either the value ends up zero, which is what happens if a signal comes
in just before the flag is cleared, or the value ends up nonzero, and
subsequent events occur as if the signal had come in just after the flag
was cleared.  As long as the code handles both of these cases properly,
it can also handle a signal in the middle of clearing the flag.  (This
is an example of the sort of reasoning you need to do to figure out
whether non-atomic usage is safe.)

   Sometimes you can insure uninterrupted access to one object by
protecting its use with another object, perhaps one whose type
guarantees atomicity.  *Note Merged Signals::, for an example.


File: libc.info,  Node: Interrupted Primitives,  Next: Generating Signals,  Prev: Defining Handlers,  Up: Signal Handling

24.5 Primitives Interrupted by Signals
======================================

A signal can arrive and be handled while an I/O primitive such as
`open' or `read' is waiting for an I/O device.  If the signal handler
returns, the system faces the question: what should happen next?

   POSIX specifies one approach: make the primitive fail right away.
The error code for this kind of failure is `EINTR'.  This is flexible,
but usually inconvenient.  Typically, POSIX applications that use signal
handlers must check for `EINTR' after each library function that can
return it, in order to try the call again.  Often programmers forget to
check, which is a common source of error.

   The GNU library provides a convenient way to retry a call after a
temporary failure, with the macro `TEMP_FAILURE_RETRY':

 -- Macro: TEMP_FAILURE_RETRY (EXPRESSION)
     This macro evaluates EXPRESSION once, and examines its value as
     type `long int'.  If the value equals `-1', that indicates a
     failure and `errno' should be set to show what kind of failure.
     If it fails and reports error code `EINTR', `TEMP_FAILURE_RETRY'
     evaluates it again, and over and over until the result is not a
     temporary failure.

     The value returned by `TEMP_FAILURE_RETRY' is whatever value
     EXPRESSION produced.

   BSD avoids `EINTR' entirely and provides a more convenient approach:
to restart the interrupted primitive, instead of making it fail.  If
you choose this approach, you need not be concerned with `EINTR'.

   You can choose either approach with the GNU library.  If you use
`sigaction' to establish a signal handler, you can specify how that
handler should behave.  If you specify the `SA_RESTART' flag, return
from that handler will resume a primitive; otherwise, return from that
handler will cause `EINTR'.  *Note Flags for Sigaction::.

   Another way to specify the choice is with the `siginterrupt'
function.  *Note BSD Handler::.

   When you don't specify with `sigaction' or `siginterrupt' what a
particular handler should do, it uses a default choice.  The default
choice in the GNU library depends on the feature test macros you have
defined.  If you define `_BSD_SOURCE' or `_GNU_SOURCE' before calling
`signal', the default is to resume primitives; otherwise, the default
is to make them fail with `EINTR'.  (The library contains alternate
versions of the `signal' function, and the feature test macros
determine which one you really call.)  *Note Feature Test Macros::.  

   The description of each primitive affected by this issue lists
`EINTR' among the error codes it can return.

   There is one situation where resumption never happens no matter which
choice you make: when a data-transfer function such as `read' or
`write' is interrupted by a signal after transferring part of the data.
In this case, the function returns the number of bytes already
transferred, indicating partial success.

   This might at first appear to cause unreliable behavior on
record-oriented devices (including datagram sockets; *note Datagrams::),
where splitting one `read' or `write' into two would read or write two
records.  Actually, there is no problem, because interruption after a
partial transfer cannot happen on such devices; they always transfer an
entire record in one burst, with no waiting once data transfer has
started.


File: libc.info,  Node: Generating Signals,  Next: Blocking Signals,  Prev: Interrupted Primitives,  Up: Signal Handling

24.6 Generating Signals
=======================

Besides signals that are generated as a result of a hardware trap or
interrupt, your program can explicitly send signals to itself or to
another process.

* Menu:

* Signaling Yourself::          A process can send a signal to itself.
* Signaling Another Process::   Send a signal to another process.
* Permission for kill::         Permission for using `kill'.
* Kill Example::                Using `kill' for Communication.


File: libc.info,  Node: Signaling Yourself,  Next: Signaling Another Process,  Up: Generating Signals

24.6.1 Signaling Yourself
-------------------------

A process can send itself a signal with the `raise' function.  This
function is declared in `signal.h'.  

 -- Function: int raise (int SIGNUM)
     The `raise' function sends the signal SIGNUM to the calling
     process.  It returns zero if successful and a nonzero value if it
     fails.  About the only reason for failure would be if the value of
     SIGNUM is invalid.

 -- Function: int gsignal (int SIGNUM)
     The `gsignal' function does the same thing as `raise'; it is
     provided only for compatibility with SVID.

   One convenient use for `raise' is to reproduce the default behavior
of a signal that you have trapped.  For instance, suppose a user of your
program types the SUSP character (usually `C-z'; *note Special
Characters::) to send it an interactive stop signal (`SIGTSTP'), and
you want to clean up some internal data buffers before stopping.  You
might set this up like this:

     #include <signal.h>

     /* When a stop signal arrives, set the action back to the default
        and then resend the signal after doing cleanup actions. */

     void
     tstp_handler (int sig)
     {
       signal (SIGTSTP, SIG_DFL);
       /* Do cleanup actions here. */
       ...
       raise (SIGTSTP);
     }

     /* When the process is continued again, restore the signal handler. */

     void
     cont_handler (int sig)
     {
       signal (SIGCONT, cont_handler);
       signal (SIGTSTP, tstp_handler);
     }

     /* Enable both handlers during program initialization. */

     int
     main (void)
     {
       signal (SIGCONT, cont_handler);
       signal (SIGTSTP, tstp_handler);
       ...
     }

   *Portability note:* `raise' was invented by the ISO C committee.
Older systems may not support it, so using `kill' may be more portable.
*Note Signaling Another Process::.


File: libc.info,  Node: Signaling Another Process,  Next: Permission for kill,  Prev: Signaling Yourself,  Up: Generating Signals

24.6.2 Signaling Another Process
--------------------------------

The `kill' function can be used to send a signal to another process.
In spite of its name, it can be used for a lot of things other than
causing a process to terminate.  Some examples of situations where you
might want to send signals between processes are:

   * A parent process starts a child to perform a task--perhaps having
     the child running an infinite loop--and then terminates the child
     when the task is no longer needed.

   * A process executes as part of a group, and needs to terminate or
     notify the other processes in the group when an error or other
     event occurs.

   * Two processes need to synchronize while working together.

   This section assumes that you know a little bit about how processes
work.  For more information on this subject, see *Note Processes::.

   The `kill' function is declared in `signal.h'.  

 -- Function: int kill (pid_t PID, int SIGNUM)
     The `kill' function sends the signal SIGNUM to the process or
     process group specified by PID.  Besides the signals listed in
     *Note Standard Signals::, SIGNUM can also have a value of zero to
     check the validity of the PID.

     The PID specifies the process or process group to receive the
     signal:

    `PID > 0'
          The process whose identifier is PID.

    `PID == 0'
          All processes in the same process group as the sender.

    `PID < -1'
          The process group whose identifier is -PID.

    `PID == -1'
          If the process is privileged, send the signal to all
          processes except for some special system processes.
          Otherwise, send the signal to all processes with the same
          effective user ID.

     A process can send a signal to itself with a call like
     `kill (getpid(), SIGNUM)'.  If `kill' is used by a process to send
     a signal to itself, and the signal is not blocked, then `kill'
     delivers at least one signal (which might be some other pending
     unblocked signal instead of the signal SIGNUM) to that process
     before it returns.

     The return value from `kill' is zero if the signal can be sent
     successfully.  Otherwise, no signal is sent, and a value of `-1' is
     returned.  If PID specifies sending a signal to several processes,
     `kill' succeeds if it can send the signal to at least one of them.
     There's no way you can tell which of the processes got the signal
     or whether all of them did.

     The following `errno' error conditions are defined for this
     function:

    `EINVAL'
          The SIGNUM argument is an invalid or unsupported number.

    `EPERM'
          You do not have the privilege to send a signal to the process
          or any of the processes in the process group named by PID.

    `ESCRH'
          The PID argument does not refer to an existing process or
          group.

 -- Function: int killpg (int PGID, int SIGNUM)
     This is similar to `kill', but sends signal SIGNUM to the process
     group PGID.  This function is provided for compatibility with BSD;
     using `kill' to do this is more portable.

   As a simple example of `kill', the call `kill (getpid (), SIG)' has
the same effect as `raise (SIG)'.


File: libc.info,  Node: Permission for kill,  Next: Kill Example,  Prev: Signaling Another Process,  Up: Generating Signals

24.6.3 Permission for using `kill'
----------------------------------

There are restrictions that prevent you from using `kill' to send
signals to any random process.  These are intended to prevent antisocial
behavior such as arbitrarily killing off processes belonging to another
user.  In typical use, `kill' is used to pass signals between parent,
child, and sibling processes, and in these situations you normally do
have permission to send signals.  The only common exception is when you
run a setuid program in a child process; if the program changes its
real UID as well as its effective UID, you may not have permission to
send a signal.  The `su' program does this.

   Whether a process has permission to send a signal to another process
is determined by the user IDs of the two processes.  This concept is
discussed in detail in *Note Process Persona::.

   Generally, for a process to be able to send a signal to another
process, either the sending process must belong to a privileged user
(like `root'), or the real or effective user ID of the sending process
must match the real or effective user ID of the receiving process.  If
the receiving process has changed its effective user ID from the
set-user-ID mode bit on its process image file, then the owner of the
process image file is used in place of its current effective user ID.
In some implementations, a parent process might be able to send signals
to a child process even if the user ID's don't match, and other
implementations might enforce other restrictions.

   The `SIGCONT' signal is a special case.  It can be sent if the
sender is part of the same session as the receiver, regardless of user
IDs.


File: libc.info,  Node: Kill Example,  Prev: Permission for kill,  Up: Generating Signals

24.6.4 Using `kill' for Communication
-------------------------------------

Here is a longer example showing how signals can be used for
interprocess communication.  This is what the `SIGUSR1' and `SIGUSR2'
signals are provided for.  Since these signals are fatal by default,
the process that is supposed to receive them must trap them through
`signal' or `sigaction'.

   In this example, a parent process forks a child process and then
waits for the child to complete its initialization.  The child process
tells the parent when it is ready by sending it a `SIGUSR1' signal,
using the `kill' function.

     #include <signal.h>
     #include <stdio.h>
     #include <sys/types.h>
     #include <unistd.h>

     /* When a `SIGUSR1' signal arrives, set this variable. */
     volatile sig_atomic_t usr_interrupt = 0;

     void
     synch_signal (int sig)
     {
       usr_interrupt = 1;
     }

     /* The child process executes this function. */
     void
     child_function (void)
     {
       /* Perform initialization. */
       printf ("I'm here!!!  My pid is %d.\n", (int) getpid ());

       /* Let parent know you're done. */
       kill (getppid (), SIGUSR1);

       /* Continue with execution. */
       puts ("Bye, now....");
       exit (0);
     }

     int
     main (void)
     {
       struct sigaction usr_action;
       sigset_t block_mask;
       pid_t child_id;

       /* Establish the signal handler. */
       sigfillset (&block_mask);
       usr_action.sa_handler = synch_signal;
       usr_action.sa_mask = block_mask;
       usr_action.sa_flags = 0;
       sigaction (SIGUSR1, &usr_action, NULL);

       /* Create the child process. */
       child_id = fork ();
       if (child_id == 0)
         child_function ();          /* Does not return. */

       /* Busy wait for the child to send a signal. */
       while (!usr_interrupt)
         ;

       /* Now continue execution. */
       puts ("That's all, folks!");

       return 0;
     }

   This example uses a busy wait, which is bad, because it wastes CPU
cycles that other programs could otherwise use.  It is better to ask the
system to wait until the signal arrives.  See the example in *Note
Waiting for a Signal::.


File: libc.info,  Node: Blocking Signals,  Next: Waiting for a Signal,  Prev: Generating Signals,  Up: Signal Handling

24.7 Blocking Signals
=====================

Blocking a signal means telling the operating system to hold it and
deliver it later.  Generally, a program does not block signals
indefinitely--it might as well ignore them by setting their actions to
`SIG_IGN'.  But it is useful to block signals briefly, to prevent them
from interrupting sensitive operations.  For instance:

   * You can use the `sigprocmask' function to block signals while you
     modify global variables that are also modified by the handlers for
     these signals.

   * You can set `sa_mask' in your `sigaction' call to block certain
     signals while a particular signal handler runs.  This way, the
     signal handler can run without being interrupted itself by signals.

* Menu:

* Why Block::                           The purpose of blocking signals.
* Signal Sets::                         How to specify which signals to
                                         block.
* Process Signal Mask::                 Blocking delivery of signals to your
				         process during normal execution.
* Testing for Delivery::                Blocking to Test for Delivery of
                                         a Signal.
* Blocking for Handler::                Blocking additional signals while a
				         handler is being run.
* Checking for Pending Signals::        Checking for Pending Signals
* Remembering a Signal::                How you can get almost the same
                                         effect as blocking a signal, by
                                         handling it and setting a flag
                                         to be tested later.


File: libc.info,  Node: Why Block,  Next: Signal Sets,  Up: Blocking Signals

24.7.1 Why Blocking Signals is Useful
-------------------------------------

Temporary blocking of signals with `sigprocmask' gives you a way to
prevent interrupts during critical parts of your code.  If signals
arrive in that part of the program, they are delivered later, after you
unblock them.

   One example where this is useful is for sharing data between a signal
handler and the rest of the program.  If the type of the data is not
`sig_atomic_t' (*note Atomic Data Access::), then the signal handler
could run when the rest of the program has only half finished reading
or writing the data.  This would lead to confusing consequences.

   To make the program reliable, you can prevent the signal handler from
running while the rest of the program is examining or modifying that
data--by blocking the appropriate signal around the parts of the
program that touch the data.

   Blocking signals is also necessary when you want to perform a certain
action only if a signal has not arrived.  Suppose that the handler for
the signal sets a flag of type `sig_atomic_t'; you would like to test
the flag and perform the action if the flag is not set.  This is
unreliable.  Suppose the signal is delivered immediately after you test
the flag, but before the consequent action: then the program will
perform the action even though the signal has arrived.

   The only way to test reliably for whether a signal has yet arrived
is to test while the signal is blocked.


File: libc.info,  Node: Signal Sets,  Next: Process Signal Mask,  Prev: Why Block,  Up: Blocking Signals

24.7.2 Signal Sets
------------------

All of the signal blocking functions use a data structure called a
"signal set" to specify what signals are affected.  Thus, every
activity involves two stages: creating the signal set, and then passing
it as an argument to a library function.  

   These facilities are declared in the header file `signal.h'.  

 -- Data Type: sigset_t
     The `sigset_t' data type is used to represent a signal set.
     Internally, it may be implemented as either an integer or structure
     type.

     For portability, use only the functions described in this section
     to initialize, change, and retrieve information from `sigset_t'
     objects--don't try to manipulate them directly.

   There are two ways to initialize a signal set.  You can initially
specify it to be empty with `sigemptyset' and then add specified
signals individually.  Or you can specify it to be full with
`sigfillset' and then delete specified signals individually.

   You must always initialize the signal set with one of these two
functions before using it in any other way.  Don't try to set all the
signals explicitly because the `sigset_t' object might include some
other information (like a version field) that needs to be initialized as
well.  (In addition, it's not wise to put into your program an
assumption that the system has no signals aside from the ones you know
about.)

 -- Function: int sigemptyset (sigset_t *SET)
     This function initializes the signal set SET to exclude all of the
     defined signals.  It always returns `0'.

 -- Function: int sigfillset (sigset_t *SET)
     This function initializes the signal set SET to include all of the
     defined signals.  Again, the return value is `0'.

 -- Function: int sigaddset (sigset_t *SET, int SIGNUM)
     This function adds the signal SIGNUM to the signal set SET.  All
     `sigaddset' does is modify SET; it does not block or unblock any
     signals.

     The return value is `0' on success and `-1' on failure.  The
     following `errno' error condition is defined for this function:

    `EINVAL'
          The SIGNUM argument doesn't specify a valid signal.

 -- Function: int sigdelset (sigset_t *SET, int SIGNUM)
     This function removes the signal SIGNUM from the signal set SET.
     All `sigdelset' does is modify SET; it does not block or unblock
     any signals.  The return value and error conditions are the same
     as for `sigaddset'.

   Finally, there is a function to test what signals are in a signal
set:

 -- Function: int sigismember (const sigset_t *SET, int SIGNUM)
     The `sigismember' function tests whether the signal SIGNUM is a
     member of the signal set SET.  It returns `1' if the signal is in
     the set, `0' if not, and `-1' if there is an error.

     The following `errno' error condition is defined for this function:

    `EINVAL'
          The SIGNUM argument doesn't specify a valid signal.


File: libc.info,  Node: Process Signal Mask,  Next: Testing for Delivery,  Prev: Signal Sets,  Up: Blocking Signals

24.7.3 Process Signal Mask
--------------------------

The collection of signals that are currently blocked is called the
"signal mask".  Each process has its own signal mask.  When you create
a new process (*note Creating a Process::), it inherits its parent's
mask.  You can block or unblock signals with total flexibility by
modifying the signal mask.

   The prototype for the `sigprocmask' function is in `signal.h'.  

   Note that you must not use `sigprocmask' in multi-threaded processes,
because each thread has its own signal mask and there is no single
process signal mask. According to POSIX, the behavior of `sigprocmask'
in a multi-threaded process is "unspeficied".  Instead, use
`pthread_sigmask'.

 -- Function: int sigprocmask (int HOW, const sigset_t *restrict SET,
          sigset_t *restrict OLDSET)
     The `sigprocmask' function is used to examine or change the calling
     process's signal mask.  The HOW argument determines how the signal
     mask is changed, and must be one of the following values:

    `SIG_BLOCK'
          Block the signals in `set'--add them to the existing mask.  In
          other words, the new mask is the union of the existing mask
          and SET.

    `SIG_UNBLOCK'
          Unblock the signals in SET--remove them from the existing
          mask.

    `SIG_SETMASK'
          Use SET for the mask; ignore the previous value of the mask.

     The last argument, OLDSET, is used to return information about the
     old process signal mask.  If you just want to change the mask
     without looking at it, pass a null pointer as the OLDSET argument.
     Similarly, if you want to know what's in the mask without changing
     it, pass a null pointer for SET (in this case the HOW argument is
     not significant).  The OLDSET argument is often used to remember
     the previous signal mask in order to restore it later.  (Since the
     signal mask is inherited over `fork' and `exec' calls, you can't
     predict what its contents are when your program starts running.)

     If invoking `sigprocmask' causes any pending signals to be
     unblocked, at least one of those signals is delivered to the
     process before `sigprocmask' returns.  The order in which pending
     signals are delivered is not specified, but you can control the
     order explicitly by making multiple `sigprocmask' calls to unblock
     various signals one at a time.

     The `sigprocmask' function returns `0' if successful, and `-1' to
     indicate an error.  The following `errno' error conditions are
     defined for this function:

    `EINVAL'
          The HOW argument is invalid.

     You can't block the `SIGKILL' and `SIGSTOP' signals, but if the
     signal set includes these, `sigprocmask' just ignores them instead
     of returning an error status.

     Remember, too, that blocking program error signals such as `SIGFPE'
     leads to undesirable results for signals generated by an actual
     program error (as opposed to signals sent with `raise' or `kill').
     This is because your program may be too broken to be able to
     continue executing to a point where the signal is unblocked again.
     *Note Program Error Signals::.


File: libc.info,  Node: Testing for Delivery,  Next: Blocking for Handler,  Prev: Process Signal Mask,  Up: Blocking Signals

24.7.4 Blocking to Test for Delivery of a Signal
------------------------------------------------

Now for a simple example.  Suppose you establish a handler for
`SIGALRM' signals that sets a flag whenever a signal arrives, and your
main program checks this flag from time to time and then resets it.
You can prevent additional `SIGALRM' signals from arriving in the
meantime by wrapping the critical part of the code with calls to
`sigprocmask', like this:

     /* This variable is set by the SIGALRM signal handler. */
     volatile sig_atomic_t flag = 0;

     int
     main (void)
     {
       sigset_t block_alarm;

       ...

       /* Initialize the signal mask. */
       sigemptyset (&block_alarm);
       sigaddset (&block_alarm, SIGALRM);

       while (1)
         {
           /* Check if a signal has arrived; if so, reset the flag. */
           sigprocmask (SIG_BLOCK, &block_alarm, NULL);
           if (flag)
             {
               ACTIONS-IF-NOT-ARRIVED
               flag = 0;
             }
           sigprocmask (SIG_UNBLOCK, &block_alarm, NULL);

           ...
         }
     }


File: libc.info,  Node: Blocking for Handler,  Next: Checking for Pending Signals,  Prev: Testing for Delivery,  Up: Blocking Signals

24.7.5 Blocking Signals for a Handler
-------------------------------------

When a signal handler is invoked, you usually want it to be able to
finish without being interrupted by another signal.  From the moment the
handler starts until the moment it finishes, you must block signals that
might confuse it or corrupt its data.

   When a handler function is invoked on a signal, that signal is
automatically blocked (in addition to any other signals that are already
in the process's signal mask) during the time the handler is running.
If you set up a handler for `SIGTSTP', for instance, then the arrival
of that signal forces further `SIGTSTP' signals to wait during the
execution of the handler.

   However, by default, other kinds of signals are not blocked; they can
arrive during handler execution.

   The reliable way to block other kinds of signals during the
execution of the handler is to use the `sa_mask' member of the
`sigaction' structure.

   Here is an example:

     #include <signal.h>
     #include <stddef.h>

     void catch_stop ();

     void
     install_handler (void)
     {
       struct sigaction setup_action;
       sigset_t block_mask;

       sigemptyset (&block_mask);
       /* Block other terminal-generated signals while handler runs. */
       sigaddset (&block_mask, SIGINT);
       sigaddset (&block_mask, SIGQUIT);
       setup_action.sa_handler = catch_stop;
       setup_action.sa_mask = block_mask;
       setup_action.sa_flags = 0;
       sigaction (SIGTSTP, &setup_action, NULL);
     }

   This is more reliable than blocking the other signals explicitly in
the code for the handler.  If you block signals explicitly in the
handler, you can't avoid at least a short interval at the beginning of
the handler where they are not yet blocked.

   You cannot remove signals from the process's current mask using this
mechanism.  However, you can make calls to `sigprocmask' within your
handler to block or unblock signals as you wish.

   In any case, when the handler returns, the system restores the mask
that was in place before the handler was entered.  If any signals that
become unblocked by this restoration are pending, the process will
receive those signals immediately, before returning to the code that was
interrupted.


File: libc.info,  Node: Checking for Pending Signals,  Next: Remembering a Signal,  Prev: Blocking for Handler,  Up: Blocking Signals

24.7.6 Checking for Pending Signals
-----------------------------------

You can find out which signals are pending at any time by calling
`sigpending'.  This function is declared in `signal.h'.  

 -- Function: int sigpending (sigset_t *SET)
     The `sigpending' function stores information about pending signals
     in SET.  If there is a pending signal that is blocked from
     delivery, then that signal is a member of the returned set.  (You
     can test whether a particular signal is a member of this set using
     `sigismember'; see *Note Signal Sets::.)

     The return value is `0' if successful, and `-1' on failure.

   Testing whether a signal is pending is not often useful.  Testing
when that signal is not blocked is almost certainly bad design.

   Here is an example.

     #include <signal.h>
     #include <stddef.h>

     sigset_t base_mask, waiting_mask;

     sigemptyset (&base_mask);
     sigaddset (&base_mask, SIGINT);
     sigaddset (&base_mask, SIGTSTP);

     /* Block user interrupts while doing other processing. */
     sigprocmask (SIG_SETMASK, &base_mask, NULL);
     ...

     /* After a while, check to see whether any signals are pending. */
     sigpending (&waiting_mask);
     if (sigismember (&waiting_mask, SIGINT)) {
       /* User has tried to kill the process. */
     }
     else if (sigismember (&waiting_mask, SIGTSTP)) {
       /* User has tried to stop the process. */
     }

   Remember that if there is a particular signal pending for your
process, additional signals of that same type that arrive in the
meantime might be discarded.  For example, if a `SIGINT' signal is
pending when another `SIGINT' signal arrives, your program will
probably only see one of them when you unblock this signal.

   *Portability Note:* The `sigpending' function is new in POSIX.1.
Older systems have no equivalent facility.


File: libc.info,  Node: Remembering a Signal,  Prev: Checking for Pending Signals,  Up: Blocking Signals

24.7.7 Remembering a Signal to Act On Later
-------------------------------------------

Instead of blocking a signal using the library facilities, you can get
almost the same results by making the handler set a flag to be tested
later, when you "unblock".  Here is an example:

     /* If this flag is nonzero, don't handle the signal right away. */
     volatile sig_atomic_t signal_pending;

     /* This is nonzero if a signal arrived and was not handled. */
     volatile sig_atomic_t defer_signal;

     void
     handler (int signum)
     {
       if (defer_signal)
         signal_pending = signum;
       else
         ... /* "Really" handle the signal. */
     }

     ...

     void
     update_mumble (int frob)
     {
       /* Prevent signals from having immediate effect. */
       defer_signal++;
       /* Now update `mumble', without worrying about interruption. */
       mumble.a = 1;
       mumble.b = hack ();
       mumble.c = frob;
       /* We have updated `mumble'.  Handle any signal that came in. */
       defer_signal--;
       if (defer_signal == 0 && signal_pending != 0)
         raise (signal_pending);
     }

   Note how the particular signal that arrives is stored in
`signal_pending'.  That way, we can handle several types of
inconvenient signals with the same mechanism.

   We increment and decrement `defer_signal' so that nested critical
sections will work properly; thus, if `update_mumble' were called with
`signal_pending' already nonzero, signals would be deferred not only
within `update_mumble', but also within the caller.  This is also why
we do not check `signal_pending' if `defer_signal' is still nonzero.

   The incrementing and decrementing of `defer_signal' each require more
than one instruction; it is possible for a signal to happen in the
middle.  But that does not cause any problem.  If the signal happens
early enough to see the value from before the increment or decrement,
that is equivalent to a signal which came before the beginning of the
increment or decrement, which is a case that works properly.

   It is absolutely vital to decrement `defer_signal' before testing
`signal_pending', because this avoids a subtle bug.  If we did these
things in the other order, like this,

       if (defer_signal == 1 && signal_pending != 0)
         raise (signal_pending);
       defer_signal--;

then a signal arriving in between the `if' statement and the decrement
would be effectively "lost" for an indefinite amount of time.  The
handler would merely set `defer_signal', but the program having already
tested this variable, it would not test the variable again.

   Bugs like these are called "timing errors".  They are especially bad
because they happen only rarely and are nearly impossible to reproduce.
You can't expect to find them with a debugger as you would find a
reproducible bug.  So it is worth being especially careful to avoid
them.

   (You would not be tempted to write the code in this order, given the
use of `defer_signal' as a counter which must be tested along with
`signal_pending'.  After all, testing for zero is cleaner than testing
for one.  But if you did not use `defer_signal' as a counter, and gave
it values of zero and one only, then either order might seem equally
simple.  This is a further advantage of using a counter for
`defer_signal': it will reduce the chance you will write the code in
the wrong order and create a subtle bug.)


File: libc.info,  Node: Waiting for a Signal,  Next: Signal Stack,  Prev: Blocking Signals,  Up: Signal Handling

24.8 Waiting for a Signal
=========================

If your program is driven by external events, or uses signals for
synchronization, then when it has nothing to do it should probably wait
until a signal arrives.

* Menu:

* Using Pause::                 The simple way, using `pause'.
* Pause Problems::              Why the simple way is often not very good.
* Sigsuspend::                  Reliably waiting for a specific signal.


File: libc.info,  Node: Using Pause,  Next: Pause Problems,  Up: Waiting for a Signal

24.8.1 Using `pause'
--------------------

The simple way to wait until a signal arrives is to call `pause'.
Please read about its disadvantages, in the following section, before
you use it.

 -- Function: int pause ()
     The `pause' function suspends program execution until a signal
     arrives whose action is either to execute a handler function, or to
     terminate the process.

     If the signal causes a handler function to be executed, then
     `pause' returns.  This is considered an unsuccessful return (since
     "successful" behavior would be to suspend the program forever), so
     the return value is `-1'.  Even if you specify that other
     primitives should resume when a system handler returns (*note
     Interrupted Primitives::), this has no effect on `pause'; it
     always fails when a signal is handled.

     The following `errno' error conditions are defined for this
     function:

    `EINTR'
          The function was interrupted by delivery of a signal.

     If the signal causes program termination, `pause' doesn't return
     (obviously).

     This function is a cancellation point in multithreaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `pause' is called.  If the thread gets cancelled these resources
     stay allocated until the program ends.  To avoid this calls to
     `pause' should be protected using cancellation handlers.

     The `pause' function is declared in  `unistd.h'.


File: libc.info,  Node: Pause Problems,  Next: Sigsuspend,  Prev: Using Pause,  Up: Waiting for a Signal

24.8.2 Problems with `pause'
----------------------------

The simplicity of `pause' can conceal serious timing errors that can
make a program hang mysteriously.

   It is safe to use `pause' if the real work of your program is done
by the signal handlers themselves, and the "main program" does nothing
but call `pause'.  Each time a signal is delivered, the handler will do
the next batch of work that is to be done, and then return, so that the
main loop of the program can call `pause' again.

   You can't safely use `pause' to wait until one more signal arrives,
and then resume real work.  Even if you arrange for the signal handler
to cooperate by setting a flag, you still can't use `pause' reliably.
Here is an example of this problem:

     /* `usr_interrupt' is set by the signal handler.  */
     if (!usr_interrupt)
       pause ();

     /* Do work once the signal arrives.  */
     ...

This has a bug: the signal could arrive after the variable
`usr_interrupt' is checked, but before the call to `pause'.  If no
further signals arrive, the process would never wake up again.

   You can put an upper limit on the excess waiting by using `sleep' in
a loop, instead of using `pause'.  (*Note Sleeping::, for more about
`sleep'.)  Here is what this looks like:

     /* `usr_interrupt' is set by the signal handler.
     while (!usr_interrupt)
       sleep (1);

     /* Do work once the signal arrives.  */
     ...

   For some purposes, that is good enough.  But with a little more
complexity, you can wait reliably until a particular signal handler is
run, using `sigsuspend'.  *Note Sigsuspend::.


File: libc.info,  Node: Sigsuspend,  Prev: Pause Problems,  Up: Waiting for a Signal

24.8.3 Using `sigsuspend'
-------------------------

The clean and reliable way to wait for a signal to arrive is to block it
and then use `sigsuspend'.  By using `sigsuspend' in a loop, you can
wait for certain kinds of signals, while letting other kinds of signals
be handled by their handlers.

 -- Function: int sigsuspend (const sigset_t *SET)
     This function replaces the process's signal mask with SET and then
     suspends the process until a signal is delivered whose action is
     either to terminate the process or invoke a signal handling
     function.  In other words, the program is effectively suspended
     until one of the signals that is not a member of SET arrives.

     If the process is woken up by delivery of a signal that invokes a
     handler function, and the handler function returns, then
     `sigsuspend' also returns.

     The mask remains SET only as long as `sigsuspend' is waiting.  The
     function `sigsuspend' always restores the previous signal mask
     when it returns.

     The return value and error conditions are the same as for `pause'.

   With `sigsuspend', you can replace the `pause' or `sleep' loop in
the previous section with something completely reliable:

     sigset_t mask, oldmask;

     ...

     /* Set up the mask of signals to temporarily block. */
     sigemptyset (&mask);
     sigaddset (&mask, SIGUSR1);

     ...

     /* Wait for a signal to arrive. */
     sigprocmask (SIG_BLOCK, &mask, &oldmask);
     while (!usr_interrupt)
       sigsuspend (&oldmask);
     sigprocmask (SIG_UNBLOCK, &mask, NULL);

   This last piece of code is a little tricky.  The key point to
remember here is that when `sigsuspend' returns, it resets the process's
signal mask to the original value, the value from before the call to
`sigsuspend'--in this case, the `SIGUSR1' signal is once again blocked.
The second call to `sigprocmask' is necessary to explicitly unblock
this signal.

   One other point: you may be wondering why the `while' loop is
necessary at all, since the program is apparently only waiting for one
`SIGUSR1' signal.  The answer is that the mask passed to `sigsuspend'
permits the process to be woken up by the delivery of other kinds of
signals, as well--for example, job control signals.  If the process is
woken up by a signal that doesn't set `usr_interrupt', it just suspends
itself again until the "right" kind of signal eventually arrives.

   This technique takes a few more lines of preparation, but that is
needed just once for each kind of wait criterion you want to use.  The
code that actually waits is just four lines.


File: libc.info,  Node: Signal Stack,  Next: BSD Signal Handling,  Prev: Waiting for a Signal,  Up: Signal Handling

24.9 Using a Separate Signal Stack
==================================

A signal stack is a special area of memory to be used as the execution
stack during signal handlers.  It should be fairly large, to avoid any
danger that it will overflow in turn; the macro `SIGSTKSZ' is defined
to a canonical size for signal stacks.  You can use `malloc' to
allocate the space for the stack.  Then call `sigaltstack' or
`sigstack' to tell the system to use that space for the signal stack.

   You don't need to write signal handlers differently in order to use a
signal stack.  Switching from one stack to the other happens
automatically.  (Some non-GNU debuggers on some machines may get
confused if you examine a stack trace while a handler that uses the
signal stack is running.)

   There are two interfaces for telling the system to use a separate
signal stack.  `sigstack' is the older interface, which comes from 4.2
BSD.  `sigaltstack' is the newer interface, and comes from 4.4 BSD.
The `sigaltstack' interface has the advantage that it does not require
your program to know which direction the stack grows, which depends on
the specific machine and operating system.

 -- Data Type: stack_t
     This structure describes a signal stack.  It contains the
     following members:

    `void *ss_sp'
          This points to the base of the signal stack.

    `size_t ss_size'
          This is the size (in bytes) of the signal stack which `ss_sp'
          points to.  You should set this to however much space you
          allocated for the stack.

          There are two macros defined in `signal.h' that you should
          use in calculating this size:

         `SIGSTKSZ'
               This is the canonical size for a signal stack.  It is
               judged to be sufficient for normal uses.

         `MINSIGSTKSZ'
               This is the amount of signal stack space the operating
               system needs just to implement signal delivery.  The
               size of a signal stack *must* be greater than this.

               For most cases, just using `SIGSTKSZ' for `ss_size' is
               sufficient.  But if you know how much stack space your
               program's signal handlers will need, you may want to use
               a different size.  In this case, you should allocate
               `MINSIGSTKSZ' additional bytes for the signal stack and
               increase `ss_size' accordingly.

    `int ss_flags'
          This field contains the bitwise OR of these flags:

         `SS_DISABLE'
               This tells the system that it should not use the signal
               stack.

         `SS_ONSTACK'
               This is set by the system, and indicates that the signal
               stack is currently in use.  If this bit is not set, then
               signals will be delivered on the normal user stack.

 -- Function: int sigaltstack (const stack_t *restrict STACK, stack_t
          *restrict OLDSTACK)
     The `sigaltstack' function specifies an alternate stack for use
     during signal handling.  When a signal is received by the process
     and its action indicates that the signal stack is used, the system
     arranges a switch to the currently installed signal stack while
     the handler for that signal is executed.

     If OLDSTACK is not a null pointer, information about the currently
     installed signal stack is returned in the location it points to.
     If STACK is not a null pointer, then this is installed as the new
     stack for use by signal handlers.

     The return value is `0' on success and `-1' on failure.  If
     `sigaltstack' fails, it sets `errno' to one of these values:

    `EINVAL'
          You tried to disable a stack that was in fact currently in
          use.

    `ENOMEM'
          The size of the alternate stack was too small.  It must be
          greater than `MINSIGSTKSZ'.

   Here is the older `sigstack' interface.  You should use
`sigaltstack' instead on systems that have it.

 -- Data Type: struct sigstack
     This structure describes a signal stack.  It contains the
     following members:

    `void *ss_sp'
          This is the stack pointer.  If the stack grows downwards on
          your machine, this should point to the top of the area you
          allocated.  If the stack grows upwards, it should point to
          the bottom.

    `int ss_onstack'
          This field is true if the process is currently using this
          stack.

 -- Function: int sigstack (const struct sigstack *STACK, struct
          sigstack *OLDSTACK)
     The `sigstack' function specifies an alternate stack for use during
     signal handling.  When a signal is received by the process and its
     action indicates that the signal stack is used, the system
     arranges a switch to the currently installed signal stack while
     the handler for that signal is executed.

     If OLDSTACK is not a null pointer, information about the currently
     installed signal stack is returned in the location it points to.
     If STACK is not a null pointer, then this is installed as the new
     stack for use by signal handlers.

     The return value is `0' on success and `-1' on failure.


File: libc.info,  Node: BSD Signal Handling,  Prev: Signal Stack,  Up: Signal Handling

24.10 BSD Signal Handling
=========================

This section describes alternative signal handling functions derived
from BSD Unix.  These facilities were an advance, in their time; today,
they are mostly obsolete, and supported mainly for compatibility with
BSD Unix.

   There are many similarities between the BSD and POSIX signal handling
facilities, because the POSIX facilities were inspired by the BSD
facilities.  Besides having different names for all the functions to
avoid conflicts, the main differences between the two are:

   * BSD Unix represents signal masks as an `int' bit mask, rather than
     as a `sigset_t' object.

   * The BSD facilities use a different default for whether an
     interrupted primitive should fail or resume.  The POSIX facilities
     make system calls fail unless you specify that they should resume.
     With the BSD facility, the default is to make system calls resume
     unless you say they should fail.  *Note Interrupted Primitives::.

   The BSD facilities are declared in `signal.h'.  

* Menu:

* BSD Handler::                 BSD Function to Establish a Handler.
* Blocking in BSD::             BSD Functions for Blocking Signals.


File: libc.info,  Node: BSD Handler,  Next: Blocking in BSD,  Up: BSD Signal Handling

24.10.1 BSD Function to Establish a Handler
-------------------------------------------

 -- Data Type: struct sigvec
     This data type is the BSD equivalent of `struct sigaction' (*note
     Advanced Signal Handling::); it is used to specify signal actions
     to the `sigvec' function.  It contains the following members:

    `sighandler_t sv_handler'
          This is the handler function.

    `int sv_mask'
          This is the mask of additional signals to be blocked while
          the handler function is being called.

    `int sv_flags'
          This is a bit mask used to specify various flags which affect
          the behavior of the signal.  You can also refer to this field
          as `sv_onstack'.

These symbolic constants can be used to provide values for the
`sv_flags' field of a `sigvec' structure.  This field is a bit mask
value, so you bitwise-OR the flags of interest to you together.

 -- Macro: int SV_ONSTACK
     If this bit is set in the `sv_flags' field of a `sigvec'
     structure, it means to use the signal stack when delivering the
     signal.

 -- Macro: int SV_INTERRUPT
     If this bit is set in the `sv_flags' field of a `sigvec'
     structure, it means that system calls interrupted by this kind of
     signal should not be restarted if the handler returns; instead,
     the system calls should return with a `EINTR' error status.  *Note
     Interrupted Primitives::.

 -- Macro: int SV_RESETHAND
     If this bit is set in the `sv_flags' field of a `sigvec'
     structure, it means to reset the action for the signal back to
     `SIG_DFL' when the signal is received.

 -- Function: int sigvec (int SIGNUM, const struct sigvec
          *ACTION,struct sigvec *OLD-ACTION)
     This function is the equivalent of `sigaction' (*note Advanced
     Signal Handling::); it installs the action ACTION for the signal
     SIGNUM, returning information about the previous action in effect
     for that signal in OLD-ACTION.

 -- Function: int siginterrupt (int SIGNUM, int FAILFLAG)
     This function specifies which approach to use when certain
     primitives are interrupted by handling signal SIGNUM.  If FAILFLAG
     is false, signal SIGNUM restarts primitives.  If FAILFLAG is true,
     handling SIGNUM causes these primitives to fail with error code
     `EINTR'.  *Note Interrupted Primitives::.


File: libc.info,  Node: Blocking in BSD,  Prev: BSD Handler,  Up: BSD Signal Handling

24.10.2 BSD Functions for Blocking Signals
------------------------------------------

 -- Macro: int sigmask (int SIGNUM)
     This macro returns a signal mask that has the bit for signal SIGNUM
     set.  You can bitwise-OR the results of several calls to `sigmask'
     together to specify more than one signal.  For example,

          (sigmask (SIGTSTP) | sigmask (SIGSTOP)
           | sigmask (SIGTTIN) | sigmask (SIGTTOU))

     specifies a mask that includes all the job-control stop signals.

 -- Function: int sigblock (int MASK)
     This function is equivalent to `sigprocmask' (*note Process Signal
     Mask::) with a HOW argument of `SIG_BLOCK': it adds the signals
     specified by MASK to the calling process's set of blocked signals.
     The return value is the previous set of blocked signals.

 -- Function: int sigsetmask (int MASK)
     This function equivalent to `sigprocmask' (*note Process Signal
     Mask::) with a HOW argument of `SIG_SETMASK': it sets the calling
     process's signal mask to MASK.  The return value is the previous
     set of blocked signals.

 -- Function: int sigpause (int MASK)
     This function is the equivalent of `sigsuspend' (*note Waiting for
     a Signal::):  it sets the calling process's signal mask to MASK,
     and waits for a signal to arrive.  On return the previous set of
     blocked signals is restored.


File: libc.info,  Node: Program Basics,  Next: Processes,  Prev: Signal Handling,  Up: Top

25 The Basic Program/System Interface
*************************************

"Processes" are the primitive units for allocation of system resources.
Each process has its own address space and (usually) one thread of
control.  A process executes a program; you can have multiple processes
executing the same program, but each process has its own copy of the
program within its own address space and executes it independently of
the other copies.  Though it may have multiple threads of control
within the same program and a program may be composed of multiple
logically separate modules, a process always executes exactly one
program.

   Note that we are using a specific definition of "program" for the
purposes of this manual, which corresponds to a common definition in the
context of Unix system.  In popular usage, "program" enjoys a much
broader definition; it can refer for example to a system's kernel, an
editor macro, a complex package of software, or a discrete section of
code executing within a process.

   Writing the program is what this manual is all about.  This chapter
explains the most basic interface between your program and the system
that runs, or calls, it.  This includes passing of parameters (arguments
and environment) from the system, requesting basic services from the
system, and telling the system the program is done.

   A program starts another program with the `exec' family of system
calls.  This chapter looks at program startup from the execee's point
of view.  To see the event from the execor's point of view, *Note
Executing a File::.

* Menu:

* Program Arguments::           Parsing your program's command-line arguments.
* Environment Variables::       Less direct parameters affecting your program
* System Calls::                Requesting service from the system
* Program Termination::         Telling the system you're done; return status


File: libc.info,  Node: Program Arguments,  Next: Environment Variables,  Up: Program Basics

25.1 Program Arguments
======================

The system starts a C program by calling the function `main'.  It is up
to you to write a function named `main'--otherwise, you won't even be
able to link your program without errors.

   In ISO C you can define `main' either to take no arguments, or to
take two arguments that represent the command line arguments to the
program, like this:

     int main (int ARGC, char *ARGV[])

   The command line arguments are the whitespace-separated tokens given
in the shell command used to invoke the program; thus, in `cat foo
bar', the arguments are `foo' and `bar'.  The only way a program can
look at its command line arguments is via the arguments of `main'.  If
`main' doesn't take arguments, then you cannot get at the command line.

   The value of the ARGC argument is the number of command line
arguments.  The ARGV argument is a vector of C strings; its elements
are the individual command line argument strings.  The file name of the
program being run is also included in the vector as the first element;
the value of ARGC counts this element.  A null pointer always follows
the last element: `ARGV[ARGC]' is this null pointer.

   For the command `cat foo bar', ARGC is 3 and ARGV has three
elements, `"cat"', `"foo"' and `"bar"'.

   In Unix systems you can define `main' a third way, using three
arguments:

     int main (int ARGC, char *ARGV[], char *ENVP[])

   The first two arguments are just the same.  The third argument ENVP
gives the program's environment; it is the same as the value of
`environ'.  *Note Environment Variables::.  POSIX.1 does not allow this
three-argument form, so to be portable it is best to write `main' to
take two arguments, and use the value of `environ'.

* Menu:

* Argument Syntax::             By convention, options start with a hyphen.
* Parsing Program Arguments::   Ways to parse program options and arguments.


File: libc.info,  Node: Argument Syntax,  Next: Parsing Program Arguments,  Up: Program Arguments

25.1.1 Program Argument Syntax Conventions
------------------------------------------

POSIX recommends these conventions for command line arguments.
`getopt' (*note Getopt::) and `argp_parse' (*note Argp::) make it easy
to implement them.

   * Arguments are options if they begin with a hyphen delimiter (`-').

   * Multiple options may follow a hyphen delimiter in a single token if
     the options do not take arguments.  Thus, `-abc' is equivalent to
     `-a -b -c'.

   * Option names are single alphanumeric characters (as for `isalnum';
     *note Classification of Characters::).

   * Certain options require an argument.  For example, the `-o' command
     of the `ld' command requires an argument--an output file name.

   * An option and its argument may or may not appear as separate
     tokens.  (In other words, the whitespace separating them is
     optional.)  Thus, `-o foo' and `-ofoo' are equivalent.

   * Options typically precede other non-option arguments.

     The implementations of `getopt' and `argp_parse' in the GNU C
     library normally make it appear as if all the option arguments were
     specified before all the non-option arguments for the purposes of
     parsing, even if the user of your program intermixed option and
     non-option arguments.  They do this by reordering the elements of
     the ARGV array.  This behavior is nonstandard; if you want to
     suppress it, define the `_POSIX_OPTION_ORDER' environment variable.
     *Note Standard Environment::.

   * The argument `--' terminates all options; any following arguments
     are treated as non-option arguments, even if they begin with a
     hyphen.

   * A token consisting of a single hyphen character is interpreted as
     an ordinary non-option argument.  By convention, it is used to
     specify input from or output to the standard input and output
     streams.

   * Options may be supplied in any order, or appear multiple times.
     The interpretation is left up to the particular application
     program.

   GNU adds "long options" to these conventions.  Long options consist
of `--' followed by a name made of alphanumeric characters and dashes.
Option names are typically one to three words long, with hyphens to
separate words.  Users can abbreviate the option names as long as the
abbreviations are unique.

   To specify an argument for a long option, write `--NAME=VALUE'.
This syntax enables a long option to accept an argument that is itself
optional.

   Eventually, the GNU system will provide completion for long option
names in the shell.


File: libc.info,  Node: Parsing Program Arguments,  Prev: Argument Syntax,  Up: Program Arguments

25.1.2 Parsing Program Arguments
--------------------------------

If the syntax for the command line arguments to your program is simple
enough, you can simply pick the arguments off from ARGV by hand.  But
unless your program takes a fixed number of arguments, or all of the
arguments are interpreted in the same way (as file names, for example),
you are usually better off using `getopt' (*note Getopt::) or
`argp_parse' (*note Argp::) to do the parsing.

   `getopt' is more standard (the short-option only version of it is a
part of the POSIX standard), but using `argp_parse' is often easier,
both for very simple and very complex option structures, because it
does more of the dirty work for you.

* Menu:

* Getopt::                      Parsing program options using `getopt'.
* Argp::                        Parsing program options using `argp_parse'.
* Suboptions::                  Some programs need more detailed options.
* Suboptions Example::          This shows how it could be done for `mount'.


File: libc.info,  Node: Getopt,  Next: Argp,  Up: Parsing Program Arguments

25.2 Parsing program options using `getopt'
===========================================

The `getopt' and `getopt_long' functions automate some of the chore
involved in parsing typical unix command line options.

* Menu:

* Using Getopt::                Using the `getopt' function.
* Example of Getopt::           An example of parsing options with `getopt'.
* Getopt Long Options::         GNU suggests utilities accept long-named
                                 options; here is one way to do.
* Getopt Long Option Example::  An example of using `getopt_long'.


File: libc.info,  Node: Using Getopt,  Next: Example of Getopt,  Up: Getopt

25.2.1 Using the `getopt' function
----------------------------------

Here are the details about how to call the `getopt' function.  To use
this facility, your program must include the header file `unistd.h'.  

 -- Variable: int opterr
     If the value of this variable is nonzero, then `getopt' prints an
     error message to the standard error stream if it encounters an
     unknown option character or an option with a missing required
     argument.  This is the default behavior.  If you set this variable
     to zero, `getopt' does not print any messages, but it still
     returns the character `?' to indicate an error.

 -- Variable: int optopt
     When `getopt' encounters an unknown option character or an option
     with a missing required argument, it stores that option character
     in this variable.  You can use this for providing your own
     diagnostic messages.

 -- Variable: int optind
     This variable is set by `getopt' to the index of the next element
     of the ARGV array to be processed.  Once `getopt' has found all of
     the option arguments, you can use this variable to determine where
     the remaining non-option arguments begin.  The initial value of
     this variable is `1'.

 -- Variable: char * optarg
     This variable is set by `getopt' to point at the value of the
     option argument, for those options that accept arguments.

 -- Function: int getopt (int ARGC, char **ARGV, const char *OPTIONS)
     The `getopt' function gets the next option argument from the
     argument list specified by the ARGV and ARGC arguments.  Normally
     these values come directly from the arguments received by `main'.

     The OPTIONS argument is a string that specifies the option
     characters that are valid for this program.  An option character
     in this string can be followed by a colon (`:') to indicate that
     it takes a required argument.  If an option character is followed
     by two colons (`::'), its argument is optional; this is a GNU
     extension.

     `getopt' has three ways to deal with options that follow
     non-options ARGV elements.  The special argument `--' forces in
     all cases the end of option scanning.

        * The default is to permute the contents of ARGV while scanning
          it so that eventually all the non-options are at the end.
          This allows options to be given in any order, even with
          programs that were not written to expect this.

        * If the OPTIONS argument string begins with a hyphen (`-'),
          this is treated specially.  It permits arguments that are not
          options to be returned as if they were associated with option
          character `\1'.

        * POSIX demands the following behavior: The first non-option
          stops option processing.  This mode is selected by either
          setting the environment variable `POSIXLY_CORRECT' or
          beginning the OPTIONS argument string with a plus sign (`+').

     The `getopt' function returns the option character for the next
     command line option.  When no more option arguments are available,
     it returns `-1'.  There may still be more non-option arguments; you
     must compare the external variable `optind' against the ARGC
     parameter to check this.

     If the option has an argument, `getopt' returns the argument by
     storing it in the variable OPTARG.  You don't ordinarily need to
     copy the `optarg' string, since it is a pointer into the original
     ARGV array, not into a static area that might be overwritten.

     If `getopt' finds an option character in ARGV that was not
     included in OPTIONS, or a missing option argument, it returns `?'
     and sets the external variable `optopt' to the actual option
     character.  If the first character of OPTIONS is a colon (`:'),
     then `getopt' returns `:' instead of `?' to indicate a missing
     option argument.  In addition, if the external variable `opterr'
     is nonzero (which is the default), `getopt' prints an error
     message.


File: libc.info,  Node: Example of Getopt,  Next: Getopt Long Options,  Prev: Using Getopt,  Up: Getopt

25.2.2 Example of Parsing Arguments with `getopt'
-------------------------------------------------

Here is an example showing how `getopt' is typically used.  The key
points to notice are:

   * Normally, `getopt' is called in a loop.  When `getopt' returns
     `-1', indicating no more options are present, the loop terminates.

   * A `switch' statement is used to dispatch on the return value from
     `getopt'.  In typical use, each case just sets a variable that is
     used later in the program.

   * A second loop is used to process the remaining non-option
     arguments.

     #include <ctype.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <unistd.h>

     int
     main (int argc, char **argv)
     {
       int aflag = 0;
       int bflag = 0;
       char *cvalue = NULL;
       int index;
       int c;

       opterr = 0;

       while ((c = getopt (argc, argv, "abc:")) != -1)
         switch (c)
           {
           case 'a':
             aflag = 1;
             break;
           case 'b':
             bflag = 1;
             break;
           case 'c':
             cvalue = optarg;
             break;
           case '?':
             if (optopt == 'c')
               fprintf (stderr, "Option -%c requires an argument.\n", optopt);
             else if (isprint (optopt))
               fprintf (stderr, "Unknown option `-%c'.\n", optopt);
             else
               fprintf (stderr,
                        "Unknown option character `\\x%x'.\n",
                        optopt);
             return 1;
           default:
             abort ();
           }

       printf ("aflag = %d, bflag = %d, cvalue = %s\n",
               aflag, bflag, cvalue);

       for (index = optind; index < argc; index++)
         printf ("Non-option argument %s\n", argv[index]);
       return 0;
     }

   Here are some examples showing what this program prints with
different combinations of arguments:

     % testopt
     aflag = 0, bflag = 0, cvalue = (null)

     % testopt -a -b
     aflag = 1, bflag = 1, cvalue = (null)

     % testopt -ab
     aflag = 1, bflag = 1, cvalue = (null)

     % testopt -c foo
     aflag = 0, bflag = 0, cvalue = foo

     % testopt -cfoo
     aflag = 0, bflag = 0, cvalue = foo

     % testopt arg1
     aflag = 0, bflag = 0, cvalue = (null)
     Non-option argument arg1

     % testopt -a arg1
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument arg1

     % testopt -c foo arg1
     aflag = 0, bflag = 0, cvalue = foo
     Non-option argument arg1

     % testopt -a -- -b
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument -b

     % testopt -a -
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument -


File: libc.info,  Node: Getopt Long Options,  Next: Getopt Long Option Example,  Prev: Example of Getopt,  Up: Getopt

25.2.3 Parsing Long Options with `getopt_long'
----------------------------------------------

To accept GNU-style long options as well as single-character options,
use `getopt_long' instead of `getopt'.  This function is declared in
`getopt.h', not `unistd.h'.  You should make every program accept long
options if it uses any options, for this takes little extra work and
helps beginners remember how to use the program.

 -- Data Type: struct option
     This structure describes a single long option name for the sake of
     `getopt_long'.  The argument LONGOPTS must be an array of these
     structures, one for each long option.  Terminate the array with an
     element containing all zeros.

     The `struct option' structure has these fields:

    `const char *name'
          This field is the name of the option.  It is a string.

    `int has_arg'
          This field says whether the option takes an argument.  It is
          an integer, and there are three legitimate values:
          `no_argument', `required_argument' and `optional_argument'.

    `int *flag'
    `int val'
          These fields control how to report or act on the option when
          it occurs.

          If `flag' is a null pointer, then the `val' is a value which
          identifies this option.  Often these values are chosen to
          uniquely identify particular long options.

          If `flag' is not a null pointer, it should be the address of
          an `int' variable which is the flag for this option.  The
          value in `val' is the value to store in the flag to indicate
          that the option was seen.

 -- Function: int getopt_long (int ARGC, char *const *ARGV, const char
          *SHORTOPTS, const struct option *LONGOPTS, int *INDEXPTR)
     Decode options from the vector ARGV (whose length is ARGC).  The
     argument SHORTOPTS describes the short options to accept, just as
     it does in `getopt'.  The argument LONGOPTS describes the long
     options to accept (see above).

     When `getopt_long' encounters a short option, it does the same
     thing that `getopt' would do: it returns the character code for the
     option, and stores the options argument (if it has one) in
     `optarg'.

     When `getopt_long' encounters a long option, it takes actions based
     on the `flag' and `val' fields of the definition of that option.

     If `flag' is a null pointer, then `getopt_long' returns the
     contents of `val' to indicate which option it found.  You should
     arrange distinct values in the `val' field for options with
     different meanings, so you can decode these values after
     `getopt_long' returns.  If the long option is equivalent to a short
     option, you can use the short option's character code in `val'.

     If `flag' is not a null pointer, that means this option should just
     set a flag in the program.  The flag is a variable of type `int'
     that you define.  Put the address of the flag in the `flag' field.
     Put in the `val' field the value you would like this option to
     store in the flag.  In this case, `getopt_long' returns `0'.

     For any long option, `getopt_long' tells you the index in the array
     LONGOPTS of the options definition, by storing it into
     `*INDEXPTR'.  You can get the name of the option with
     `LONGOPTS[*INDEXPTR].name'.  So you can distinguish among long
     options either by the values in their `val' fields or by their
     indices.  You can also distinguish in this way among long options
     that set flags.

     When a long option has an argument, `getopt_long' puts the argument
     value in the variable `optarg' before returning.  When the option
     has no argument, the value in `optarg' is a null pointer.  This is
     how you can tell whether an optional argument was supplied.

     When `getopt_long' has no more options to handle, it returns `-1',
     and leaves in the variable `optind' the index in ARGV of the next
     remaining argument.

   Since long option names were used before before the `getopt_long'
options was invented there are program interfaces which require programs
to recognize options like `-option value' instead of `--option value'.
To enable these programs to use the GNU getopt functionality there is
one more function available.

 -- Function: int getopt_long_only (int ARGC, char *const *ARGV, const
          char *SHORTOPTS, const struct option *LONGOPTS, int *INDEXPTR)
     The `getopt_long_only' function is equivalent to the `getopt_long'
     function but it allows to specify the user of the application to
     pass long options with only `-' instead of `--'.  The `--' prefix
     is still recognized but instead of looking through the short
     options if a `-' is seen it is first tried whether this parameter
     names a long option.  If not, it is parsed as a short option.

     Assuming `getopt_long_only' is used starting an application with

            app -foo

     the `getopt_long_only' will first look for a long option named
     `foo'.  If this is not found, the short options `f', `o', and
     again `o' are recognized.


File: libc.info,  Node: Getopt Long Option Example,  Prev: Getopt Long Options,  Up: Getopt

25.2.4 Example of Parsing Long Options with `getopt_long'
---------------------------------------------------------

     #include <stdio.h>
     #include <stdlib.h>
     #include <getopt.h>

     /* Flag set by `--verbose'. */
     static int verbose_flag;

     int
     main (argc, argv)
          int argc;
          char **argv;
     {
       int c;

       while (1)
         {
           static struct option long_options[] =
             {
               /* These options set a flag. */
               {"verbose", no_argument,       &verbose_flag, 1},
               {"brief",   no_argument,       &verbose_flag, 0},
               /* These options don't set a flag.
                  We distinguish them by their indices. */
               {"add",     no_argument,       0, 'a'},
               {"append",  no_argument,       0, 'b'},
               {"delete",  required_argument, 0, 'd'},
               {"create",  required_argument, 0, 'c'},
               {"file",    required_argument, 0, 'f'},
               {0, 0, 0, 0}
             };
           /* `getopt_long' stores the option index here. */
           int option_index = 0;

           c = getopt_long (argc, argv, "abc:d:f:",
                            long_options, &option_index);

           /* Detect the end of the options. */
           if (c == -1)
             break;

           switch (c)
             {
             case 0:
               /* If this option set a flag, do nothing else now. */
               if (long_options[option_index].flag != 0)
                 break;
               printf ("option %s", long_options[option_index].name);
               if (optarg)
                 printf (" with arg %s", optarg);
               printf ("\n");
               break;

             case 'a':
               puts ("option -a\n");
               break;

             case 'b':
               puts ("option -b\n");
               break;

             case 'c':
               printf ("option -c with value `%s'\n", optarg);
               break;

             case 'd':
               printf ("option -d with value `%s'\n", optarg);
               break;

             case 'f':
               printf ("option -f with value `%s'\n", optarg);
               break;

             case '?':
               /* `getopt_long' already printed an error message. */
               break;

             default:
               abort ();
             }
         }

       /* Instead of reporting `--verbose'
          and `--brief' as they are encountered,
          we report the final status resulting from them. */
       if (verbose_flag)
         puts ("verbose flag is set");

       /* Print any remaining command line arguments (not options). */
       if (optind < argc)
         {
           printf ("non-option ARGV-elements: ");
           while (optind < argc)
             printf ("%s ", argv[optind++]);
           putchar ('\n');
         }

       exit (0);
     }


File: libc.info,  Node: Argp,  Next: Suboptions,  Prev: Getopt,  Up: Parsing Program Arguments

25.3 Parsing Program Options with Argp
======================================

"Argp" is an interface for parsing unix-style argument vectors.  *Note
Program Arguments::.

   Argp provides features unavailable in the more commonly used
`getopt' interface.  These features include automatically producing
output in response to the `--help' and `--version' options, as
described in the GNU coding standards.  Using argp makes it less likely
that programmers will neglect to implement these additional options or
keep them up to date.

   Argp also provides the ability to merge several independently defined
option parsers into one, mediating conflicts between them and making the
result appear seamless.  A library can export an argp option parser that
user programs might employ in conjunction with their own option parsers,
resulting in less work for the user programs.  Some programs may use
only argument parsers exported by libraries, thereby achieving
consistent and efficient option-parsing for abstractions implemented by
the libraries.

   The header file `<argp.h>' should be included to use argp.

25.3.1 The `argp_parse' Function
--------------------------------

The main interface to argp is the `argp_parse' function.  In many
cases, calling `argp_parse' is the only argument-parsing code needed in
`main'.  *Note Program Arguments::.

 -- Function: error_t argp_parse (const struct argp *ARGP, int ARGC,
          char **ARGV, unsigned FLAGS, int *ARG_INDEX, void *INPUT)
     The `argp_parse' function parses the arguments in ARGV, of length
     ARGC, using the argp parser ARGP.  *Note Argp Parsers::.

     A value of zero is the same as a `struct argp'containing all
     zeros.  FLAGS is a set of flag bits that modify the parsing
     behavior.  *Note Argp Flags::.  INPUT is passed through to the argp
     parser ARGP, and has meaning defined by ARGP.  A typical usage is
     to pass a pointer to a structure which is used for specifying
     parameters to the parser and passing back the results.

     Unless the `ARGP_NO_EXIT' or `ARGP_NO_HELP' flags are included in
     FLAGS, calling `argp_parse' may result in the program exiting.
     This behavior is true if an error is detected, or when an unknown
     option is encountered.  *Note Program Termination::.

     If ARG_INDEX is non-null, the index of the first unparsed option
     in ARGV is returned as a value.

     The return value is zero for successful parsing, or an error code
     (*note Error Codes::) if an error is detected.  Different argp
     parsers may return arbitrary error codes, but the standard error
     codes are: `ENOMEM' if a memory allocation error occurred, or
     `EINVAL' if an unknown option or option argument is encountered.

* Menu:

* Globals: Argp Global Variables.  Global argp parameters.
* Parsers: Argp Parsers.        Defining parsers for use with `argp_parse'.
* Flags: Argp Flags.            Flags that modify the behavior of `argp_parse'.
* Help: Argp Help.              Printing help messages when not parsing.
* Examples: Argp Examples.      Simple examples of programs using argp.
* Customization: Argp User Customization.
                                Users may control the `--help' output format.


File: libc.info,  Node: Argp Global Variables,  Next: Argp Parsers,  Up: Argp

25.3.2 Argp Global Variables
----------------------------

These variables make it easy for user programs to implement the
`--version' option and provide a bug-reporting address in the `--help'
output.  These are implemented in argp by default.

 -- Variable: const char * argp_program_version
     If defined or set by the user program to a non-zero value, then a
     `--version' option is added when parsing with `argp_parse', which
     will print the `--version' string followed by a newline and exit.
     The exception to this is if the `ARGP_NO_EXIT' flag is used.

 -- Variable: const char * argp_program_bug_address
     If defined or set by the user program to a non-zero value,
     `argp_program_bug_address' should point to a string that will be
     printed at the end of the standard output for the `--help' option,
     embedded in a sentence that says `Report bugs to ADDRESS.'.

 -- Variable: argp_program_version_hook
     If defined or set by the user program to a non-zero value, a
     `--version' option is added when parsing with `arg_parse', which
     prints the program version and exits with a status of zero.  This
     is not the case if the `ARGP_NO_HELP' flag is used.  If the
     `ARGP_NO_EXIT' flag is set, the exit behavior of the program is
     suppressed or modified, as when the argp parser is going to be
     used by other programs.

     It should point to a function with this type of signature:

          void PRINT-VERSION (FILE *STREAM, struct argp_state *STATE)

     *Note Argp Parsing State::, for an explanation of STATE.

     This variable takes precedence over `argp_program_version', and is
     useful if a program has version information not easily expressed
     in a simple string.

 -- Variable: error_t argp_err_exit_status
     This is the exit status used when argp exits due to a parsing
     error.  If not defined or set by the user program, this defaults
     to: `EX_USAGE' from `<sysexits.h>'.


File: libc.info,  Node: Argp Parsers,  Next: Argp Flags,  Prev: Argp Global Variables,  Up: Argp

25.3.3 Specifying Argp Parsers
------------------------------

The first argument to the `argp_parse' function is a pointer to a
`struct argp', which is known as an "argp parser":

 -- Data Type: struct argp
     This structure specifies how to parse a given set of options and
     arguments, perhaps in conjunction with other argp parsers.  It has
     the following fields:

    `const struct argp_option *options'
          A pointer to a vector of `argp_option' structures specifying
          which options this argp parser understands; it may be zero if
          there are no options at all.  *Note Argp Option Vectors::.

    `argp_parser_t parser'
          A pointer to a function that defines actions for this parser;
          it is called for each option parsed, and at other
          well-defined points in the parsing process.  A value of zero
          is the same as a pointer to a function that always returns
          `ARGP_ERR_UNKNOWN'.  *Note Argp Parser Functions::.

    `const char *args_doc'
          If non-zero, a string describing what non-option arguments
          are called by this parser.  This is only used to print the
          `Usage:' message.  If it contains newlines, the strings
          separated by them are considered alternative usage patterns
          and printed on separate lines.  Lines after the first are
          prefixed by ` or: ' instead of `Usage:'.

    `const char *doc'
          If non-zero, a string containing extra text to be printed
          before and after the options in a long help message, with the
          two sections separated by a vertical tab (`'\v'', `'\013'')
          character.  By convention, the documentation before the
          options is just a short string explaining what the program
          does.  Documentation printed after the options describe
          behavior in more detail.

    `const struct argp_child *children'
          A pointer to a vector of `argp_children' structures.  This
          pointer specifies which additional argp parsers should be
          combined with this one.  *Note Argp Children::.

    `char *(*help_filter)(int KEY, const char *TEXT, void *INPUT)'
          If non-zero, a pointer to a function that filters the output
          of help messages.  *Note Argp Help Filtering::.

    `const char *argp_domain'
          If non-zero, the strings used in the argp library are
          translated using the domain described by this string.  If
          zero, the current default domain is used.


   Of the above group, `options', `parser', `args_doc', and the `doc'
fields are usually all that are needed.  If an argp parser is defined
as an initialized C variable, only the fields used need be specified in
the initializer.  The rest will default to zero due to the way C
structure initialization works.  This design is exploited in most argp
structures; the most-used fields are grouped near the beginning, the
unused fields left unspecified.

* Menu:

* Options: Argp Option Vectors.   Specifying options in an argp parser.
* Argp Parser Functions::         Defining actions for an argp parser.
* Children: Argp Children.        Combining multiple argp parsers.
* Help Filtering: Argp Help Filtering.  Customizing help output for an argp parser.


File: libc.info,  Node: Argp Option Vectors,  Next: Argp Parser Functions,  Prev: Argp Parsers,  Up: Argp Parsers

25.3.4 Specifying Options in an Argp Parser
-------------------------------------------

The `options' field in a `struct argp' points to a vector of `struct
argp_option' structures, each of which specifies an option that the
argp parser supports.  Multiple entries may be used for a single option
provided it has multiple names.  This should be terminated by an entry
with zero in all fields.  Note that when using an initialized C array
for options, writing `{ 0 }' is enough to achieve this.

 -- Data Type: struct argp_option
     This structure specifies a single option that an argp parser
     understands, as well as how to parse and document that option.  It
     has the following fields:

    `const char *name'
          The long name for this option, corresponding to the long
          option `--NAME'; this field may be zero if this option _only_
          has a short name.  To specify multiple names for an option,
          additional entries may follow this one, with the
          `OPTION_ALIAS' flag set.  *Note Argp Option Flags::.

    `int key'
          The integer key provided by the current option to the option
          parser.  If KEY has a value that is a printable ASCII
          character (i.e., `isascii (KEY)' is true), it _also_
          specifies a short option `-CHAR', where CHAR is the ASCII
          character with the code KEY.

    `const char *arg'
          If non-zero, this is the name of an argument associated with
          this option, which must be provided (e.g., with the
          `--NAME=VALUE' or `-CHAR VALUE' syntaxes), unless the
          `OPTION_ARG_OPTIONAL' flag (*note Argp Option Flags::) is
          set, in which case it _may_ be provided.

    `int flags'
          Flags associated with this option, some of which are referred
          to above.  *Note Argp Option Flags::.

    `const char *doc'
          A documentation string for this option, for printing in help
          messages.

          If both the `name' and `key' fields are zero, this string
          will be printed tabbed left from the normal option column,
          making it useful as a group header.  This will be the first
          thing printed in its group.  In this usage, it's conventional
          to end the string with a `:' character.

    `int group'
          Group identity for this option.

          In a long help message, options are sorted alphabetically
          within each group, and the groups presented in the order 0,
          1, 2, ..., N, -M, ..., -2, -1.

          Every entry in an options array with this field 0 will
          inherit the group number of the previous entry, or zero if
          it's the first one.  If it's a group header with `name' and
          `key' fields both zero, the previous entry + 1 is the
          default.  Automagic options such as `--help' are put into
          group -1.

          Note that because of C structure initialization rules, this
          field often need not be specified, because 0 is the correct
          value.

* Menu:

* Flags: Argp Option Flags.     Flags for options.


File: libc.info,  Node: Argp Option Flags,  Up: Argp Option Vectors

25.3.4.1 Flags for Argp Options
...............................

The following flags may be or'd together in the `flags' field of a
`struct argp_option'.  These flags control various aspects of how that
option is parsed or displayed in help messages:

`OPTION_ARG_OPTIONAL'
     The argument associated with this option is optional.

`OPTION_HIDDEN'
     This option isn't displayed in any help messages.

`OPTION_ALIAS'
     This option is an alias for the closest previous non-alias option.
     This means that it will be displayed in the same help entry, and
     will inherit fields other than `name' and `key' from the option
     being aliased.

`OPTION_DOC'
     This option isn't actually an option and should be ignored by the
     actual option parser.  It is an arbitrary section of documentation
     that should be displayed in much the same manner as the options.
     This is known as a "documentation option".

     If this flag is set, then the option `name' field is displayed
     unmodified (e.g., no `--' prefix is added) at the left-margin where
     a _short_ option would normally be displayed, and this
     documentation string is left in it's usual place.  For purposes of
     sorting, any leading whitespace and punctuation is ignored, unless
     the first non-whitespace character is `-'.  This entry is displayed
     after all options, after `OPTION_DOC' entries with a leading `-',
     in the same group.

`OPTION_NO_USAGE'
     This option shouldn't be included in `long' usage messages, but
     should still be included in other help messages.  This is intended
     for options that are completely documented in an argp's `args_doc'
     field.  *Note Argp Parsers::.  Including this option in the
     generic usage list would be redundant, and should be avoided.

     For instance, if `args_doc' is `"FOO BAR\n-x BLAH"', and the `-x'
     option's purpose is to distinguish these two cases, `-x' should
     probably be marked `OPTION_NO_USAGE'.


File: libc.info,  Node: Argp Parser Functions,  Next: Argp Children,  Prev: Argp Option Vectors,  Up: Argp Parsers

25.3.5 Argp Parser Functions
----------------------------

The function pointed to by the `parser' field in a `struct argp' (*note
Argp Parsers::) defines what actions take place in response to each
option or argument parsed.  It is also used as a hook, allowing a
parser to perform tasks at certain other points during parsing.

   Argp parser functions have the following type signature:

     error_t PARSER (int KEY, char *ARG, struct argp_state *STATE)

where the arguments are as follows:

KEY
     For each option that is parsed, PARSER is called with a value of
     KEY from that option's `key' field in the option vector.  *Note
     Argp Option Vectors::.  PARSER is also called at other times with
     special reserved keys, such as `ARGP_KEY_ARG' for non-option
     arguments.  *Note Argp Special Keys::.

ARG
     If KEY is an option, ARG is its given value.  This defaults to
     zero if no value is specified.  Only options that have a non-zero
     `arg' field can ever have a value.  These must _always_ have a
     value unless the `OPTION_ARG_OPTIONAL' flag is specified.  If the
     input being parsed specifies a value for an option that doesn't
     allow one, an error results before PARSER ever gets called.

     If KEY is `ARGP_KEY_ARG', ARG is a non-option argument.  Other
     special keys always have a zero ARG.

STATE
     STATE points to a `struct argp_state', containing useful
     information about the current parsing state for use by PARSER.
     *Note Argp Parsing State::.

   When PARSER is called, it should perform whatever action is
appropriate for KEY, and return `0' for success, `ARGP_ERR_UNKNOWN' if
the value of KEY is not handled by this parser function, or a unix
error code if a real error occurred.  *Note Error Codes::.

 -- Macro: int ARGP_ERR_UNKNOWN
     Argp parser functions should return `ARGP_ERR_UNKNOWN' for any KEY
     value they do not recognize, or for non-option arguments (`KEY ==
     ARGP_KEY_ARG') that they are not equipped to handle.

   A typical parser function uses a switch statement on KEY:

     error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       switch (key)
         {
         case OPTION_KEY:
           ACTION
           break;
         ...
         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }

* Menu:

* Keys: Argp Special Keys.           Special values for the KEY argument.
* State: Argp Parsing State.         What the STATE argument refers to.
* Functions: Argp Helper Functions.  Functions to help during argp parsing.


File: libc.info,  Node: Argp Special Keys,  Next: Argp Parsing State,  Up: Argp Parser Functions

25.3.5.1 Special Keys for Argp Parser Functions
...............................................

In addition to key values corresponding to user options, the KEY
argument to argp parser functions may have a number of other special
values.  In the following example ARG and STATE refer to parser
function arguments.  *Note Argp Parser Functions::.

`ARGP_KEY_ARG'
     This is not an option at all, but rather a command line argument,
     whose value is pointed to by ARG.

     When there are multiple parser functions in play due to argp
     parsers being combined, it's impossible to know which one will
     handle a specific argument.  Each is called until one returns 0 or
     an error other than `ARGP_ERR_UNKNOWN'; if an argument is not
     handled, `argp_parse' immediately returns success, without parsing
     any more arguments.

     Once a parser function returns success for this key, that fact is
     recorded, and the `ARGP_KEY_NO_ARGS' case won't be used.
     _However_, if while processing the argument a parser function
     decrements the `next' field of its STATE argument, the option
     won't be considered processed; this is to allow you to actually
     modify the argument, perhaps into an option, and have it processed
     again.

`ARGP_KEY_ARGS'
     If a parser function returns `ARGP_ERR_UNKNOWN' for
     `ARGP_KEY_ARG', it is immediately called again with the key
     `ARGP_KEY_ARGS', which has a similar meaning, but is slightly more
     convenient for consuming all remaining arguments.  ARG is 0, and
     the tail of the argument vector may be found at `STATE->argv +
     STATE->next'.  If success is returned for this key, and
     `STATE->next' is unchanged, all remaining arguments are considered
     to have been consumed.  Otherwise, the amount by which
     `STATE->next' has been adjusted indicates how many were used.
     Here's an example that uses both, for different args:

          ...
          case ARGP_KEY_ARG:
            if (STATE->arg_num == 0)
              /* First argument */
              first_arg = ARG;
            else
              /* Let the next case parse it.  */
              return ARGP_KEY_UNKNOWN;
            break;
          case ARGP_KEY_ARGS:
            remaining_args = STATE->argv + STATE->next;
            num_remaining_args = STATE->argc - STATE->next;
            break;

`ARGP_KEY_END'
     This indicates that there are no more command line arguments.
     Parser functions are called in a different order, children first.
     This allows each parser to clean up its state for the parent.

`ARGP_KEY_NO_ARGS'
     Because it's common to do some special processing if there aren't
     any non-option args, parser functions are called with this key if
     they didn't successfully process any non-option arguments.  This
     is called just before `ARGP_KEY_END', where more general validity
     checks on previously parsed arguments take place.

`ARGP_KEY_INIT'
     This is passed in before any parsing is done.  Afterwards, the
     values of each element of the `child_input' field of STATE, if
     any, are copied to each child's state to be the initial value of
     the `input' when _their_ parsers are called.

`ARGP_KEY_SUCCESS'
     Passed in when parsing has successfully been completed, even if
     arguments remain.

`ARGP_KEY_ERROR'
     Passed in if an error has occurred and parsing is terminated.  In
     this case a call with a key of `ARGP_KEY_SUCCESS' is never made.

`ARGP_KEY_FINI'
     The final key ever seen by any parser, even after
     `ARGP_KEY_SUCCESS' and `ARGP_KEY_ERROR'.  Any resources allocated
     by `ARGP_KEY_INIT' may be freed here.  At times, certain resources
     allocated are to be returned to the caller after a successful
     parse.  In that case, those particular resources can be freed in
     the `ARGP_KEY_ERROR' case.

   In all cases, `ARGP_KEY_INIT' is the first key seen by parser
functions, and `ARGP_KEY_FINI' the last, unless an error was returned
by the parser for `ARGP_KEY_INIT'.  Other keys can occur in one the
following orders.  OPT refers to an arbitrary option key:

OPT... `ARGP_KEY_NO_ARGS' `ARGP_KEY_END' `ARGP_KEY_SUCCESS'
     The arguments being parsed did not contain any non-option
     arguments.

( OPT | `ARGP_KEY_ARG' )... `ARGP_KEY_END' `ARGP_KEY_SUCCESS'
     All non-option arguments were successfully handled by a parser
     function.  There may be multiple parser functions if multiple argp
     parsers were combined.

( OPT | `ARGP_KEY_ARG' )... `ARGP_KEY_SUCCESS'
     Some non-option argument went unrecognized.

     This occurs when every parser function returns `ARGP_KEY_UNKNOWN'
     for an argument, in which case parsing stops at that argument if
     ARG_INDEX is a null pointer.  Otherwise an error occurs.

   In all cases, if a non-null value for ARG_INDEX gets passed to
`argp_parse', the index of the first unparsed command-line argument is
passed back in that value.

   If an error occurs and is either detected by argp or because a parser
function returned an error value, each parser is called with
`ARGP_KEY_ERROR'.  No further calls are made, except the final call
with `ARGP_KEY_FINI'.


File: libc.info,  Node: Argp Helper Functions,  Prev: Argp Parsing State,  Up: Argp Parser Functions

25.3.5.2 Functions For Use in Argp Parsers
..........................................

Argp provides a number of functions available to the user of argp
(*note Argp Parser Functions::), mostly for producing error messages.
These take as their first argument the STATE argument to the parser
function.  *Note Argp Parsing State::.

 -- Function: void argp_usage (const struct argp_state *STATE)
     Outputs the standard usage message for the argp parser referred to
     by STATE to `STATE->err_stream' and terminate the program with
     `exit (argp_err_exit_status)'.  *Note Argp Global Variables::.

 -- Function: void argp_error (const struct argp_state *STATE, const
          char *FMT, ...)
     Prints the printf format string FMT and following args, preceded
     by the program name and `:', and followed by a `Try ... --help'
     message, and terminates the program with an exit status of
     `argp_err_exit_status'.  *Note Argp Global Variables::.

 -- Function: void argp_failure (const struct argp_state *STATE, int
          STATUS, int ERRNUM, const char *FMT, ...)
     Similar to the standard gnu error-reporting function `error', this
     prints the program name and `:', the printf format string FMT, and
     the appropriate following args.  If it is non-zero, the standard
     unix error text for ERRNUM is printed.  If STATUS is non-zero, it
     terminates the program with that value as its exit status.

     The difference between `argp_failure' and `argp_error' is that
     `argp_error' is for _parsing errors_, whereas `argp_failure' is
     for other problems that occur during parsing but don't reflect a
     syntactic problem with the input, such as illegal values for
     options, bad phase of the moon, etc.

 -- Function: void argp_state_help (const struct argp_state *STATE,
          FILE *STREAM, unsigned FLAGS)
     Outputs a help message for the argp parser referred to by STATE,
     to STREAM.  The FLAGS argument determines what sort of help
     message is produced.  *Note Argp Help Flags::.

   Error output is sent to `STATE->err_stream', and the program name
printed is `STATE->name'.

   The output or program termination behavior of these functions may be
suppressed if the `ARGP_NO_EXIT' or `ARGP_NO_ERRS' flags are passed to
`argp_parse'.  *Note Argp Flags::.

   This behavior is useful if an argp parser is exported for use by
other programs (e.g., by a library), and may be used in a context where
it is not desirable to terminate the program in response to parsing
errors.  In argp parsers intended for such general use, and for the
case where the program _doesn't_ terminate, calls to any of these
functions should be followed by code that returns the appropriate error
code:

     if (BAD ARGUMENT SYNTAX)
       {
          argp_usage (STATE);
          return EINVAL;
       }

If a parser function will _only_ be used when `ARGP_NO_EXIT' is not
set, the return may be omitted.


File: libc.info,  Node: Argp Parsing State,  Next: Argp Helper Functions,  Prev: Argp Special Keys,  Up: Argp Parser Functions

25.3.5.3 Argp Parsing State
...........................

The third argument to argp parser functions (*note Argp Parser
Functions::) is a pointer to a `struct argp_state', which contains
information about the state of the option parsing.

 -- Data Type: struct argp_state
     This structure has the following fields, which may be modified as
     noted:

    `const struct argp *const root_argp'
          The top level argp parser being parsed.  Note that this is
          often _not_ the same `struct argp' passed into `argp_parse' by
          the invoking program.  *Note Argp::.  It is an internal argp
          parser that contains options implemented by `argp_parse'
          itself, such as `--help'.

    `int argc'
    `char **argv'
          The argument vector being parsed.  This may be modified.

    `int next'
          The index in `argv' of the next argument to be parsed.  This
          may be modified.

          One way to consume all remaining arguments in the input is to
          set `STATE->next = STATE->argc', perhaps after recording the
          value of the `next' field to find the consumed arguments.  The
          current option can be re-parsed immediately by decrementing
          this field, then modifying `STATE->argv[STATE->next]' to
          reflect the option that should be reexamined.

    `unsigned flags'
          The flags supplied to `argp_parse'.  These may be modified,
          although some flags may only take effect when `argp_parse' is
          first invoked.  *Note Argp Flags::.

    `unsigned arg_num'
          While calling a parsing function with the KEY argument
          `ARGP_KEY_ARG', this represents the number of the current arg,
          starting at 0.  It is incremented after each `ARGP_KEY_ARG'
          call returns.  At all other times, this is the number of
          `ARGP_KEY_ARG' arguments that have been processed.

    `int quoted'
          If non-zero, the index in `argv' of the first argument
          following a special `--' argument.  This prevents anything
          that follows from being interpreted as an option.  It is only
          set after argument parsing has proceeded past this point.

    `void *input'
          An arbitrary pointer passed in from the caller of
          `argp_parse', in the INPUT argument.

    `void **child_inputs'
          These are values that will be passed to child parsers.  This
          vector will be the same length as the number of children in
          the current parser.  Each child parser will be given the
          value of `STATE->child_inputs[I]' as _its_ `STATE->input'
          field, where I is the index of the child in the this parser's
          `children' field.  *Note Argp Children::.

    `void *hook'
          For the parser function's use.  Initialized to 0, but
          otherwise ignored by argp.

    `char *name'
          The name used when printing messages.  This is initialized to
          `argv[0]', or `program_invocation_name' if `argv[0]' is
          unavailable.

    `FILE *err_stream'
    `FILE *out_stream'
          The stdio streams used when argp prints.  Error messages are
          printed to `err_stream', all other output, such as `--help'
          output) to `out_stream'.  These are initialized to `stderr'
          and `stdout' respectively.  *Note Standard Streams::.

    `void *pstate'
          Private, for use by the argp implementation.


File: libc.info,  Node: Argp Children,  Next: Argp Help Filtering,  Prev: Argp Parser Functions,  Up: Argp Parsers

25.3.6 Combining Multiple Argp Parsers
--------------------------------------

The `children' field in a `struct argp' enables other argp parsers to
be combined with the referencing one for the parsing of a single set of
arguments.  This field should point to a vector of `struct argp_child',
which is terminated by an entry having a value of zero in the `argp'
field.

   Where conflicts between combined parsers arise, as when two specify
an option with the same name, the parser conflicts are resolved in
favor of the parent argp parser(s), or the earlier of the argp parsers
in the list of children.

 -- Data Type: struct argp_child
     An entry in the list of subsidiary argp parsers pointed to by the
     `children' field in a `struct argp'.  The fields are as follows:

    `const struct argp *argp'
          The child argp parser, or zero to end of the list.

    `int flags'
          Flags for this child.

    `const char *header'
          If non-zero, this is an optional header to be printed within
          help output before the child options.  As a side-effect, a
          non-zero value forces the child options to be grouped
          together.  To achieve this effect without actually printing a
          header string, use a value of `""'.  As with header strings
          specified in an option entry, the conventional value of the
          last character is `:'.  *Note Argp Option Vectors::.

    `int group'
          This is where the child options are grouped relative to the
          other `consolidated' options in the parent argp parser.  The
          values are the same as the `group' field in `struct
          argp_option'.  *Note Argp Option Vectors::.  All
          child-groupings follow parent options at a particular group
          level.  If both this field and `header' are zero, then the
          child's options aren't grouped together, they are merged with
          parent options at the parent option group level.



File: libc.info,  Node: Argp Flags,  Next: Argp Help,  Prev: Argp Parsers,  Up: Argp

25.3.7 Flags for `argp_parse'
-----------------------------

The default behavior of `argp_parse' is designed to be convenient for
the most common case of parsing program command line argument.  To
modify these defaults, the following flags may be or'd together in the
FLAGS argument to `argp_parse':

`ARGP_PARSE_ARGV0'
     Don't ignore the first element of the ARGV argument to
     `argp_parse'.  Unless `ARGP_NO_ERRS' is set, the first element of
     the argument vector is skipped for option parsing purposes, as it
     corresponds to the program name in a command line.

`ARGP_NO_ERRS'
     Don't print error messages for unknown options to `stderr'; unless
     this flag is set, `ARGP_PARSE_ARGV0' is ignored, as `argv[0]' is
     used as the program name in the error messages.  This flag implies
     `ARGP_NO_EXIT'.  This is based on the assumption that silent
     exiting upon errors is bad behavior.

`ARGP_NO_ARGS'
     Don't parse any non-option args.  Normally these are parsed by
     calling the parse functions with a key of `ARGP_KEY_ARG', the
     actual argument being the value.  This flag needn't normally be
     set, as the default behavior is to stop parsing as soon as an
     argument fails to be parsed.  *Note Argp Parser Functions::.

`ARGP_IN_ORDER'
     Parse options and arguments in the same order they occur on the
     command line.  Normally they're rearranged so that all options
     come first.

`ARGP_NO_HELP'
     Don't provide the standard long option `--help', which ordinarily
     causes usage and option help information to be output to `stdout'
     and `exit (0)'.

`ARGP_NO_EXIT'
     Don't exit on errors, although they may still result in error
     messages.

`ARGP_LONG_ONLY'
     Use the gnu getopt `long-only' rules for parsing arguments.  This
     allows long-options to be recognized with only a single `-' (i.e.
     `-help').  This results in a less useful interface, and its use is
     discouraged as it conflicts with the way most GNU programs work as
     well as the GNU coding standards.

`ARGP_SILENT'
     Turns off any message-printing/exiting options, specifically
     `ARGP_NO_EXIT', `ARGP_NO_ERRS', and `ARGP_NO_HELP'.


File: libc.info,  Node: Argp Help Filtering,  Prev: Argp Children,  Up: Argp Parsers

25.3.8 Customizing Argp Help Output
-----------------------------------

The `help_filter' field in a `struct argp' is a pointer to a function
that filters the text of help messages before displaying them.  They
have a function signature like:

     char *HELP-FILTER (int KEY, const char *TEXT, void *INPUT)

Where KEY is either a key from an option, in which case TEXT is that
option's help text.  *Note Argp Option Vectors::.  Alternately, one of
the special keys with names beginning with `ARGP_KEY_HELP_' might be
used, describing which other help text TEXT will contain.  *Note Argp
Help Filter Keys::.

   The function should return either TEXT if it remains as-is, or a
replacement string allocated using `malloc'.  This will be either be
freed by argp or zero, which prints nothing.  The value of TEXT is
supplied _after_ any translation has been done, so if any of the
replacement text needs translation, it will be done by the filter
function.  INPUT is either the input supplied to `argp_parse' or it is
zero, if `argp_help' was called directly by the user.

* Menu:

* Keys: Argp Help Filter Keys.  Special KEY values for help filter functions.


File: libc.info,  Node: Argp Help Filter Keys,  Up: Argp Help Filtering

25.3.8.1 Special Keys for Argp Help Filter Functions
....................................................

The following special values may be passed to an argp help filter
function as the first argument in addition to key values for user
options.  They specify which help text the TEXT argument contains:

`ARGP_KEY_HELP_PRE_DOC'
     The help text preceding options.

`ARGP_KEY_HELP_POST_DOC'
     The help text following options.

`ARGP_KEY_HELP_HEADER'
     The option header string.

`ARGP_KEY_HELP_EXTRA'
     This is used after all other documentation; TEXT is zero for this
     key.

`ARGP_KEY_HELP_DUP_ARGS_NOTE'
     The explanatory note printed when duplicate option arguments have
     been suppressed.

`ARGP_KEY_HELP_ARGS_DOC'
     The argument doc string; formally the `args_doc' field from the
     argp parser.  *Note Argp Parsers::.


File: libc.info,  Node: Argp Help,  Next: Argp Examples,  Prev: Argp Flags,  Up: Argp

25.3.9 The `argp_help' Function
-------------------------------

Normally programs using argp need not be written with particular
printing argument-usage-type help messages in mind as the standard
`--help' option is handled automatically by argp.  Typical error cases
can be handled using `argp_usage' and `argp_error'.  *Note Argp Helper
Functions::.  However, if it's desirable to print a help message in
some context other than parsing the program options, argp offers the
`argp_help' interface.

 -- Function: void argp_help (const struct argp *ARGP, FILE *STREAM,
          unsigned FLAGS, char *NAME)
     This outputs a help message for the argp parser ARGP to STREAM.
     The type of messages printed will be determined by FLAGS.

     Any options such as `--help' that are implemented automatically by
     argp itself will _not_ be present in the help output; for this
     reason it is best to use `argp_state_help' if calling from within
     an argp parser function.  *Note Argp Helper Functions::.

* Menu:

* Flags: Argp Help Flags.       Specifying what sort of help message to print.


File: libc.info,  Node: Argp Help Flags,  Up: Argp Help

25.3.10 Flags for the `argp_help' Function
------------------------------------------

When calling `argp_help' (*note Argp Help::) or `argp_state_help'
(*note Argp Helper Functions::) the exact output is determined by the
FLAGS argument.  This should consist of any of the following flags,
or'd together:

`ARGP_HELP_USAGE'
     A unix `Usage:' message that explicitly lists all options.

`ARGP_HELP_SHORT_USAGE'
     A unix `Usage:' message that displays an appropriate placeholder to
     indicate where the options go; useful for showing the non-option
     argument syntax.

`ARGP_HELP_SEE'
     A `Try ... for more help' message; `...' contains the program name
     and `--help'.

`ARGP_HELP_LONG'
     A verbose option help message that gives each option available
     along with its documentation string.

`ARGP_HELP_PRE_DOC'
     The part of the argp parser doc string preceding the verbose
     option help.

`ARGP_HELP_POST_DOC'
     The part of the argp parser doc string that following the verbose
     option help.

`ARGP_HELP_DOC'
     `(ARGP_HELP_PRE_DOC | ARGP_HELP_POST_DOC)'

`ARGP_HELP_BUG_ADDR'
     A message that prints where to report bugs for this program, if the
     `argp_program_bug_address' variable contains this information.

`ARGP_HELP_LONG_ONLY'
     This will modify any output to reflect the `ARGP_LONG_ONLY' mode.

   The following flags are only understood when used with
`argp_state_help'.  They control whether the function returns after
printing its output, or terminates the program:

`ARGP_HELP_EXIT_ERR'
     This will terminate the program with `exit (argp_err_exit_status)'.

`ARGP_HELP_EXIT_OK'
     This will terminate the program with `exit (0)'.

   The following flags are combinations of the basic flags for printing
standard messages:

`ARGP_HELP_STD_ERR'
     Assuming that an error message for a parsing error has printed,
     this prints a message on how to get help, and terminates the
     program with an error.

`ARGP_HELP_STD_USAGE'
     This prints a standard usage message and terminates the program
     with an error.  This is used when no other specific error messages
     are appropriate or available.

`ARGP_HELP_STD_HELP'
     This prints the standard response for a `--help' option, and
     terminates the program successfully.


File: libc.info,  Node: Argp Examples,  Next: Argp User Customization,  Prev: Argp Help,  Up: Argp

25.3.11 Argp Examples
---------------------

These example programs demonstrate the basic usage of argp.

* Menu:

* 1: Argp Example 1.            A minimal program using argp.
* 2: Argp Example 2.            A program using only default options.
* 3: Argp Example 3.            A simple program with user options.
* 4: Argp Example 4.            Combining multiple argp parsers.


File: libc.info,  Node: Argp Example 1,  Next: Argp Example 2,  Up: Argp Examples

25.3.11.1 A Minimal Program Using Argp
......................................

This is perhaps the smallest program possible that uses argp.  It won't
do much except give an error messages and exit when there are any
arguments, and prints a rather pointless message for `--help'.

     /* Argp example #1 - a minimal program using argp */

     /* This is (probably) the smallest possible program that
        uses argp.  It won't do much except give an error
        messages and exit when there are any arguments, and print
        a (rather pointless) messages for -help. */

     #include <argp.h>

     int main (int argc, char **argv)
     {
       argp_parse (0, argc, argv, 0, 0, 0);
       exit (0);
     }


File: libc.info,  Node: Argp Example 2,  Next: Argp Example 3,  Prev: Argp Example 1,  Up: Argp Examples

25.3.11.2 A Program Using Argp with Only Default Options
........................................................

This program doesn't use any options or arguments, it uses argp to be
compliant with the GNU standard command line format.

   In addition to giving no arguments and implementing a `--help'
option, this example has a `--version' option, which will put the given
documentation string and bug address in the `--help' output, as per GNU
standards.

   The variable `argp' contains the argument parser specification.
Adding fields to this structure is the way most parameters are passed
to `argp_parse'.  The first three fields are normally used, but they
are not in this small program.  There are also two global variables
that argp can use defined here, `argp_program_version' and
`argp_program_bug_address'.  They are considered global variables
because they will almost always be constant for a given program, even
if they use different argument parsers for various tasks.

     /* Argp example #2 - a pretty minimal program using argp */

     /* This program doesn't use any options or arguments, but uses
        argp to be compliant with the GNU standard command line
        format.

        In addition to making sure no arguments are given, and
        implementing a -help option, this example will have a
        -version option, and will put the given documentation string
        and bug address in the -help output, as per GNU standards.

        The variable ARGP contains the argument parser specification;
        adding fields to this structure is the way most parameters are
        passed to argp_parse (the first three fields are usually used,
        but not in this small program).  There are also two global
        variables that argp knows about defined here,
        ARGP_PROGRAM_VERSION and ARGP_PROGRAM_BUG_ADDRESS (they are
        global variables because they will almost always be constant
        for a given program, even if it uses different argument
        parsers for various tasks). */

     #include <argp.h>

     const char *argp_program_version =
       "argp-ex2 1.0";
     const char *argp_program_bug_address =
       "<bug-gnu-utils@gnu.org>";

     /* Program documentation. */
     static char doc[] =
       "Argp example #2 -- a pretty minimal program using argp";

     /* Our argument parser.  The `options', `parser', and
        `args_doc' fields are zero because we have neither options or
        arguments; `doc' and `argp_program_bug_address' will be
        used in the output for `--help', and the `--version'
        option will print out `argp_program_version'. */
     static struct argp argp = { 0, 0, 0, doc };

     int main (int argc, char **argv)
     {
       argp_parse (&argp, argc, argv, 0, 0, 0);
       exit (0);
     }


File: libc.info,  Node: Argp Example 3,  Next: Argp Example 4,  Prev: Argp Example 2,  Up: Argp Examples

25.3.11.3 A Program Using Argp with User Options
................................................

This program uses the same features as example 2, adding user options
and arguments.

   We now use the first four fields in `argp' (*note Argp Parsers::)
and specify `parse_opt' as the parser function.  *Note Argp Parser
Functions::.

   Note that in this example, `main' uses a structure to communicate
with the `parse_opt' function, a pointer to which it passes in the
`input' argument to `argp_parse'.  *Note Argp::.  It is retrieved by
`parse_opt' through the `input' field in its `state' argument.  *Note
Argp Parsing State::.  Of course, it's also possible to use global
variables instead, but using a structure like this is somewhat more
flexible and clean.

     /* Argp example #3 - a program with options and arguments using argp */

     /* This program uses the same features as example 2, and uses options and
        arguments.

        We now use the first four fields in ARGP, so here's a description of them:
          OPTIONS  - A pointer to a vector of struct argp_option (see below)
          PARSER   - A function to parse a single option, called by argp
          ARGS_DOC - A string describing how the non-option arguments should look
          DOC      - A descriptive string about this program; if it contains a
                      vertical tab character (\v), the part after it will be
                      printed *following* the options

        The function PARSER takes the following arguments:
          KEY  - An integer specifying which option this is (taken
                  from the KEY field in each struct argp_option), or
                  a special key specifying something else; the only
                  special keys we use here are ARGP_KEY_ARG, meaning
                  a non-option argument, and ARGP_KEY_END, meaning
                  that all arguments have been parsed
          ARG  - For an option KEY, the string value of its
                  argument, or NULL if it has none
          STATE- A pointer to a struct argp_state, containing
                  various useful information about the parsing state; used here
                  are the INPUT field, which reflects the INPUT argument to
                  argp_parse, and the ARG_NUM field, which is the number of the
                  current non-option argument being parsed
        It should return either 0, meaning success, ARGP_ERR_UNKNOWN, meaning the
        given KEY wasn't recognized, or an errno value indicating some other
        error.

        Note that in this example, main uses a structure to communicate with the
        parse_opt function, a pointer to which it passes in the INPUT argument to
        argp_parse.  Of course, it's also possible to use global variables
        instead, but this is somewhat more flexible.

        The OPTIONS field contains a pointer to a vector of struct argp_option's;
        that structure has the following fields (if you assign your option
        structures using array initialization like this example, unspecified
        fields will be defaulted to 0, and need not be specified):
          NAME   - The name of this option's long option (may be zero)
          KEY    - The KEY to pass to the PARSER function when parsing this option,
                    *and* the name of this option's short option, if it is a
                    printable ascii character
          ARG    - The name of this option's argument, if any
          FLAGS  - Flags describing this option; some of them are:
                      OPTION_ARG_OPTIONAL - The argument to this option is optional
                      OPTION_ALIAS        - This option is an alias for the
                                             previous option
                      OPTION_HIDDEN       - Don't show this option in -help output
          DOC    - A documentation string for this option, shown in -help output

        An options vector should be terminated by an option with all fields zero. */

     #include <argp.h>

     const char *argp_program_version =
       "argp-ex3 1.0";
     const char *argp_program_bug_address =
       "<bug-gnu-utils@gnu.org>";

     /* Program documentation. */
     static char doc[] =
       "Argp example #3 -- a program with options and arguments using argp";

     /* A description of the arguments we accept. */
     static char args_doc[] = "ARG1 ARG2";

     /* The options we understand. */
     static struct argp_option options[] = {
       {"verbose",  'v', 0,      0,  "Produce verbose output" },
       {"quiet",    'q', 0,      0,  "Don't produce any output" },
       {"silent",   's', 0,      OPTION_ALIAS },
       {"output",   'o', "FILE", 0,
        "Output to FILE instead of standard output" },
       { 0 }
     };

     /* Used by `main' to communicate with `parse_opt'. */
     struct arguments
     {
       char *args[2];                /* ARG1 & ARG2 */
       int silent, verbose;
       char *output_file;
     };

     /* Parse a single option. */
     static error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       /* Get the INPUT argument from `argp_parse', which we
          know is a pointer to our arguments structure. */
       struct arguments *arguments = state->input;

       switch (key)
         {
         case 'q': case 's':
           arguments->silent = 1;
           break;
         case 'v':
           arguments->verbose = 1;
           break;
         case 'o':
           arguments->output_file = arg;
           break;

         case ARGP_KEY_ARG:
           if (state->arg_num >= 2)
             /* Too many arguments. */
             argp_usage (state);

           arguments->args[state->arg_num] = arg;

           break;

         case ARGP_KEY_END:
           if (state->arg_num < 2)
             /* Not enough arguments. */
             argp_usage (state);
           break;

         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }

     /* Our argp parser. */
     static struct argp argp = { options, parse_opt, args_doc, doc };

     int main (int argc, char **argv)
     {
       struct arguments arguments;

       /* Default values. */
       arguments.silent = 0;
       arguments.verbose = 0;
       arguments.output_file = "-";

       /* Parse our arguments; every option seen by `parse_opt' will
          be reflected in `arguments'. */
       argp_parse (&argp, argc, argv, 0, 0, &arguments);

       printf ("ARG1 = %s\nARG2 = %s\nOUTPUT_FILE = %s\n"
               "VERBOSE = %s\nSILENT = %s\n",
               arguments.args[0], arguments.args[1],
               arguments.output_file,
               arguments.verbose ? "yes" : "no",
               arguments.silent ? "yes" : "no");

       exit (0);
     }


File: libc.info,  Node: Argp Example 4,  Prev: Argp Example 3,  Up: Argp Examples

25.3.11.4 A Program Using Multiple Combined Argp Parsers
........................................................

This program uses the same features as example 3, but has more options,
and presents more structure in the `--help' output.  It also
illustrates how you can `steal' the remainder of the input arguments
past a certain point for programs that accept a list of items.  It also
illustrates the KEY value `ARGP_KEY_NO_ARGS', which is only given if no
non-option arguments were supplied to the program.  *Note Argp Special
Keys::.

   For structuring help output, two features are used: _headers_ and a
two part option string.  The _headers_ are entries in the options
vector.  *Note Argp Option Vectors::.  The first four fields are zero.
The two part documentation string are in the variable `doc', which
allows documentation both before and after the options.  *Note Argp
Parsers::, the two parts of `doc' are separated by a vertical-tab
character (`'\v'', or `'\013'').  By convention, the documentation
before the options is a short string stating what the program does, and
after any options it is longer, describing the behavior in more detail.
All documentation strings are automatically filled for output,
although newlines may be included to force a line break at a particular
point.  In addition, documentation strings are passed to the `gettext'
function, for possible translation into the current locale.

     /* Argp example #4 - a program with somewhat more complicated options */

     /* This program uses the same features as example 3, but has more
        options, and somewhat more structure in the -help output.  It
        also shows how you can `steal' the remainder of the input
        arguments past a certain point, for programs that accept a
        list of items.  It also shows the special argp KEY value
        ARGP_KEY_NO_ARGS, which is only given if no non-option
        arguments were supplied to the program.

        For structuring the help output, two features are used,
        *headers* which are entries in the options vector with the
        first four fields being zero, and a two part documentation
        string (in the variable DOC), which allows documentation both
        before and after the options; the two parts of DOC are
        separated by a vertical-tab character ('\v', or '\013').  By
        convention, the documentation before the options is just a
        short string saying what the program does, and that afterwards
        is longer, describing the behavior in more detail.  All
        documentation strings are automatically filled for output,
        although newlines may be included to force a line break at a
        particular point.  All documentation strings are also passed to
        the `gettext' function, for possible translation into the
        current locale. */

     #include <stdlib.h>
     #include <error.h>
     #include <argp.h>

     const char *argp_program_version =
       "argp-ex4 1.0";
     const char *argp_program_bug_address =
       "<bug-gnu-utils@prep.ai.mit.edu>";

     /* Program documentation. */
     static char doc[] =
       "Argp example #4 -- a program with somewhat more complicated\
     options\
     \vThis part of the documentation comes *after* the options;\
      note that the text is automatically filled, but it's possible\
      to force a line-break, e.g.\n<-- here.";

     /* A description of the arguments we accept. */
     static char args_doc[] = "ARG1 [STRING...]";

     /* Keys for options without short-options. */
     #define OPT_ABORT  1            /* -abort */

     /* The options we understand. */
     static struct argp_option options[] = {
       {"verbose",  'v', 0,       0, "Produce verbose output" },
       {"quiet",    'q', 0,       0, "Don't produce any output" },
       {"silent",   's', 0,       OPTION_ALIAS },
       {"output",   'o', "FILE",  0,
        "Output to FILE instead of standard output" },

       {0,0,0,0, "The following options should be grouped together:" },
       {"repeat",   'r', "COUNT", OPTION_ARG_OPTIONAL,
        "Repeat the output COUNT (default 10) times"},
       {"abort",    OPT_ABORT, 0, 0, "Abort before showing any output"},

       { 0 }
     };

     /* Used by `main' to communicate with `parse_opt'. */
     struct arguments
     {
       char *arg1;                   /* ARG1 */
       char **strings;               /* [STRING...] */
       int silent, verbose, abort;   /* `-s', `-v', `--abort' */
       char *output_file;            /* FILE arg to `--output' */
       int repeat_count;             /* COUNT arg to `--repeat' */
     };

     /* Parse a single option. */
     static error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       /* Get the `input' argument from `argp_parse', which we
          know is a pointer to our arguments structure. */
       struct arguments *arguments = state->input;

       switch (key)
         {
         case 'q': case 's':
           arguments->silent = 1;
           break;
         case 'v':
           arguments->verbose = 1;
           break;
         case 'o':
           arguments->output_file = arg;
           break;
         case 'r':
           arguments->repeat_count = arg ? atoi (arg) : 10;
           break;
         case OPT_ABORT:
           arguments->abort = 1;
           break;

         case ARGP_KEY_NO_ARGS:
           argp_usage (state);

         case ARGP_KEY_ARG:
           /* Here we know that `state->arg_num == 0', since we
              force argument parsing to end before any more arguments can
              get here. */
           arguments->arg1 = arg;

           /* Now we consume all the rest of the arguments.
              `state->next' is the index in `state->argv' of the
              next argument to be parsed, which is the first STRING
              we're interested in, so we can just use
              `&state->argv[state->next]' as the value for
              arguments->strings.

              _In addition_, by setting `state->next' to the end
              of the arguments, we can force argp to stop parsing here and
              return. */
           arguments->strings = &state->argv[state->next];
           state->next = state->argc;

           break;

         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }

     /* Our argp parser. */
     static struct argp argp = { options, parse_opt, args_doc, doc };

     int main (int argc, char **argv)
     {
       int i, j;
       struct arguments arguments;

       /* Default values. */
       arguments.silent = 0;
       arguments.verbose = 0;
       arguments.output_file = "-";
       arguments.repeat_count = 1;
       arguments.abort = 0;

       /* Parse our arguments; every option seen by `parse_opt' will be
          reflected in `arguments'. */
       argp_parse (&argp, argc, argv, 0, 0, &arguments);

       if (arguments.abort)
         error (10, 0, "ABORTED");

       for (i = 0; i < arguments.repeat_count; i++)
         {
           printf ("ARG1 = %s\n", arguments.arg1);
           printf ("STRINGS = ");
           for (j = 0; arguments.strings[j]; j++)
             printf (j == 0 ? "%s" : ", %s", arguments.strings[j]);
           printf ("\n");
           printf ("OUTPUT_FILE = %s\nVERBOSE = %s\nSILENT = %s\n",
                   arguments.output_file,
                   arguments.verbose ? "yes" : "no",
                   arguments.silent ? "yes" : "no");
         }

       exit (0);
     }


File: libc.info,  Node: Argp User Customization,  Prev: Argp Examples,  Up: Argp

25.3.12 Argp User Customization
-------------------------------

The formatting of argp `--help' output may be controlled to some extent
by a program's users, by setting the `ARGP_HELP_FMT' environment
variable to a comma-separated list of tokens.  Whitespace is ignored:

`dup-args'
`no-dup-args'
     These turn "duplicate-argument-mode" on or off.  In duplicate
     argument mode, if an option that accepts an argument has multiple
     names, the argument is shown for each name.  Otherwise, it is only
     shown for the first long option.  A note is subsequently printed
     so the user knows that it applies to other names as well.  The
     default is `no-dup-args', which is less consistent, but prettier.

`dup-args-note'

`no-dup-args-note'
     These will enable or disable the note informing the user of
     suppressed option argument duplication.  The default is
     `dup-args-note'.

`short-opt-col=N'
     This prints the first short option in column N.  The default is 2.

`long-opt-col=N'
     This prints the first long option in column N.  The default is 6.

`doc-opt-col=N'
     This prints `documentation options' (*note Argp Option Flags::) in
     column N.  The default is 2.

`opt-doc-col=N'
     This prints the documentation for options starting in column N.
     The default is 29.

`header-col=N'
     This will indent the group headers that document groups of options
     to column N.  The default is 1.

`usage-indent=N'
     This will indent continuation lines in `Usage:' messages to column
     N.  The default is 12.

`rmargin=N'
     This will word wrap help output at or before column N.  The default
     is 79.


File: libc.info,  Node: Suboptions,  Next: Suboptions Example,  Prev: Argp,  Up: Parsing Program Arguments

25.3.12.1 Parsing of Suboptions
...............................

Having a single level of options is sometimes not enough.  There might
be too many options which have to be available or a set of options is
closely related.

   For this case some programs use suboptions.  One of the most
prominent programs is certainly `mount'(8).  The `-o' option take one
argument which itself is a comma separated list of options.  To ease the
programming of code like this the function `getsubopt' is available.

 -- Function: int getsubopt (char **OPTIONP, const char* const *TOKENS,
          char **VALUEP)
     The OPTIONP parameter must be a pointer to a variable containing
     the address of the string to process.  When the function returns
     the reference is updated to point to the next suboption or to the
     terminating `\0' character if there is no more suboption available.

     The TOKENS parameter references an array of strings containing the
     known suboptions.  All strings must be `\0' terminated and to mark
     the end a null pointer must be stored.  When `getsubopt' finds a
     possible legal suboption it compares it with all strings available
     in the TOKENS array and returns the index in the string as the
     indicator.

     In case the suboption has an associated value introduced by a `='
     character, a pointer to the value is returned in VALUEP.  The
     string is `\0' terminated.  If no argument is available VALUEP is
     set to the null pointer.  By doing this the caller can check
     whether a necessary value is given or whether no unexpected value
     is present.

     In case the next suboption in the string is not mentioned in the
     TOKENS array the starting address of the suboption including a
     possible value is returned in VALUEP and the return value of the
     function is `-1'.


File: libc.info,  Node: Suboptions Example,  Prev: Suboptions,  Up: Parsing Program Arguments

25.3.13 Parsing of Suboptions Example
-------------------------------------

The code which might appear in the `mount'(8) program is a perfect
example of the use of `getsubopt':

     #include <stdio.h>
     #include <stdlib.h>
     #include <unistd.h>

     int do_all;
     const char *type;
     int read_size;
     int write_size;
     int read_only;

     enum
     {
       RO_OPTION = 0,
       RW_OPTION,
       READ_SIZE_OPTION,
       WRITE_SIZE_OPTION,
       THE_END
     };

     const char *mount_opts[] =
     {
       [RO_OPTION] = "ro",
       [RW_OPTION] = "rw",
       [READ_SIZE_OPTION] = "rsize",
       [WRITE_SIZE_OPTION] = "wsize",
       [THE_END] = NULL
     };

     int
     main (int argc, char *argv[])
     {
       char *subopts, *value;
       int opt;

       while ((opt = getopt (argc, argv, "at:o:")) != -1)
         switch (opt)
           {
           case 'a':
             do_all = 1;
             break;
           case 't':
             type = optarg;
             break;
           case 'o':
             subopts = optarg;
             while (*subopts != '\0')
               switch (getsubopt (&subopts, mount_opts, &value))
                 {
                 case RO_OPTION:
                   read_only = 1;
                   break;
                 case RW_OPTION:
                   read_only = 0;
                   break;
                 case READ_SIZE_OPTION:
                   if (value == NULL)
                     abort ();
                   read_size = atoi (value);
                   break;
                 case WRITE_SIZE_OPTION:
                   if (value == NULL)
                     abort ();
                   write_size = atoi (value);
                   break;
                 default:
                   /* Unknown suboption. */
                   printf ("Unknown suboption `%s'\n", value);
                   break;
                 }
             break;
           default:
             abort ();
           }

       /* Do the real work. */

       return 0;
     }


File: libc.info,  Node: Environment Variables,  Next: System Calls,  Prev: Program Arguments,  Up: Program Basics

25.4 Environment Variables
==========================

When a program is executed, it receives information about the context in
which it was invoked in two ways.  The first mechanism uses the ARGV
and ARGC arguments to its `main' function, and is discussed in *Note
Program Arguments::.  The second mechanism uses "environment variables"
and is discussed in this section.

   The ARGV mechanism is typically used to pass command-line arguments
specific to the particular program being invoked.  The environment, on
the other hand, keeps track of information that is shared by many
programs, changes infrequently, and that is less frequently used.

   The environment variables discussed in this section are the same
environment variables that you set using assignments and the `export'
command in the shell.  Programs executed from the shell inherit all of
the environment variables from the shell.

   Standard environment variables are used for information about the
user's home directory, terminal type, current locale, and so on; you
can define additional variables for other purposes.  The set of all
environment variables that have values is collectively known as the
"environment".

   Names of environment variables are case-sensitive and must not
contain the character `='.  System-defined environment variables are
invariably uppercase.

   The values of environment variables can be anything that can be
represented as a string.  A value must not contain an embedded null
character, since this is assumed to terminate the string.

* Menu:

* Environment Access::          How to get and set the values of
				 environment variables.
* Standard Environment::        These environment variables have
                		 standard interpretations.


File: libc.info,  Node: Environment Access,  Next: Standard Environment,  Up: Environment Variables

25.4.1 Environment Access
-------------------------

The value of an environment variable can be accessed with the `getenv'
function.  This is declared in the header file `stdlib.h'.  All of the
following functions can be safely used in multi-threaded programs.  It
is made sure that concurrent modifications to the environment do not
lead to errors.  

 -- Function: char * getenv (const char *NAME)
     This function returns a string that is the value of the environment
     variable NAME.  You must not modify this string.  In some non-Unix
     systems not using the GNU library, it might be overwritten by
     subsequent calls to `getenv' (but not by any other library
     function).  If the environment variable NAME is not defined, the
     value is a null pointer.

 -- Function: int putenv (char *STRING)
     The `putenv' function adds or removes definitions from the
     environment.  If the STRING is of the form `NAME=VALUE', the
     definition is added to the environment.  Otherwise, the STRING is
     interpreted as the name of an environment variable, and any
     definition for this variable in the environment is removed.

     The difference to the `setenv' function is that the exact string
     given as the parameter STRING is put into the environment.  If the
     user should change the string after the `putenv' call this will
     reflect in automatically in the environment.  This also requires
     that STRING is no automatic variable which scope is left before the
     variable is removed from the environment.  The same applies of
     course to dynamically allocated variables which are freed later.

     This function is part of the extended Unix interface.  Since it
     was also available in old SVID libraries you should define either
     _XOPEN_SOURCE or _SVID_SOURCE before including any header.

 -- Function: int setenv (const char *NAME, const char *VALUE, int
          REPLACE)
     The `setenv' function can be used to add a new definition to the
     environment.  The entry with the name NAME is replaced by the
     value `NAME=VALUE'.  Please note that this is also true if VALUE
     is the empty string.  To do this a new string is created and the
     strings NAME and VALUE are copied.  A null pointer for the VALUE
     parameter is illegal.  If the environment already contains an
     entry with key NAME the REPLACE parameter controls the action.  If
     replace is zero, nothing happens.  Otherwise the old entry is
     replaced by the new one.

     Please note that you cannot remove an entry completely using this
     function.

     This function was originally part of the BSD library but is now
     part of the Unix standard.

 -- Function: int unsetenv (const char *NAME)
     Using this function one can remove an entry completely from the
     environment.  If the environment contains an entry with the key
     NAME this whole entry is removed.  A call to this function is
     equivalent to a call to `putenv' when the VALUE part of the string
     is empty.

     The function return `-1' if NAME is a null pointer, points to an
     empty string, or points to a string containing a `=' character.
     It returns `0' if the call succeeded.

     This function was originally part of the BSD library but is now
     part of the Unix standard.  The BSD version had no return value,
     though.

   There is one more function to modify the whole environment.  This
function is said to be used in the POSIX.9 (POSIX bindings for Fortran
77) and so one should expect it did made it into POSIX.1.  But this
never happened.  But we still provide this function as a GNU extension
to enable writing standard compliant Fortran environments.

 -- Function: int clearenv (void)
     The `clearenv' function removes all entries from the environment.
     Using `putenv' and `setenv' new entries can be added again later.

     If the function is successful it returns `0'.  Otherwise the return
     value is nonzero.

   You can deal directly with the underlying representation of
environment objects to add more variables to the environment (for
example, to communicate with another program you are about to execute;
*note Executing a File::).

 -- Variable: char ** environ
     The environment is represented as an array of strings.  Each
     string is of the format `NAME=VALUE'.  The order in which strings
     appear in the environment is not significant, but the same NAME
     must not appear more than once.  The last element of the array is
     a null pointer.

     This variable is declared in the header file `unistd.h'.

     If you just want to get the value of an environment variable, use
     `getenv'.

   Unix systems, and the GNU system, pass the initial value of
`environ' as the third argument to `main'.  *Note Program Arguments::.


File: libc.info,  Node: Standard Environment,  Prev: Environment Access,  Up: Environment Variables

25.4.2 Standard Environment Variables
-------------------------------------

These environment variables have standard meanings.  This doesn't mean
that they are always present in the environment; but if these variables
_are_ present, they have these meanings.  You shouldn't try to use
these environment variable names for some other purpose.

`HOME'
     This is a string representing the user's "home directory", or
     initial default working directory.

     The user can set `HOME' to any value.  If you need to make sure to
     obtain the proper home directory for a particular user, you should
     not use `HOME'; instead, look up the user's name in the user
     database (*note User Database::).

     For most purposes, it is better to use `HOME', precisely because
     this lets the user specify the value.

`LOGNAME'
     This is the name that the user used to log in.  Since the value in
     the environment can be tweaked arbitrarily, this is not a reliable
     way to identify the user who is running a program; a function like
     `getlogin' (*note Who Logged In::) is better for that purpose.

     For most purposes, it is better to use `LOGNAME', precisely because
     this lets the user specify the value.

`PATH'
     A "path" is a sequence of directory names which is used for
     searching for a file.  The variable `PATH' holds a path used for
     searching for programs to be run.

     The `execlp' and `execvp' functions (*note Executing a File::) use
     this environment variable, as do many shells and other utilities
     which are implemented in terms of those functions.

     The syntax of a path is a sequence of directory names separated by
     colons.  An empty string instead of a directory name stands for the
     current directory (*note Working Directory::).

     A typical value for this environment variable might be a string
     like:

          :/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local/bin

     This means that if the user tries to execute a program named `foo',
     the system will look for files named `foo', `/bin/foo',
     `/etc/foo', and so on.  The first of these files that exists is
     the one that is executed.

`TERM'
     This specifies the kind of terminal that is receiving program
     output.  Some programs can make use of this information to take
     advantage of special escape sequences or terminal modes supported
     by particular kinds of terminals.  Many programs which use the
     termcap library (*note Find: (termcap)Finding a Terminal
     Description.) use the `TERM' environment variable, for example.

`TZ'
     This specifies the time zone.  *Note TZ Variable::, for
     information about the format of this string and how it is used.

`LANG'
     This specifies the default locale to use for attribute categories
     where neither `LC_ALL' nor the specific environment variable for
     that category is set.  *Note Locales::, for more information about
     locales.

`LC_ALL'
     If this environment variable is set it overrides the selection for
     all the locales done using the other `LC_*' environment variables.
     The value of the other `LC_*' environment variables is simply
     ignored in this case.

`LC_COLLATE'
     This specifies what locale to use for string sorting.

`LC_CTYPE'
     This specifies what locale to use for character sets and character
     classification.

`LC_MESSAGES'
     This specifies what locale to use for printing messages and to
     parse responses.

`LC_MONETARY'
     This specifies what locale to use for formatting monetary values.

`LC_NUMERIC'
     This specifies what locale to use for formatting numbers.

`LC_TIME'
     This specifies what locale to use for formatting date/time values.

`NLSPATH'
     This specifies the directories in which the `catopen' function
     looks for message translation catalogs.

`_POSIX_OPTION_ORDER'
     If this environment variable is defined, it suppresses the usual
     reordering of command line arguments by `getopt' and `argp_parse'.
     *Note Argument Syntax::.



File: libc.info,  Node: System Calls,  Next: Program Termination,  Prev: Environment Variables,  Up: Program Basics

25.5 System Calls
=================

A system call is a request for service that a program makes of the
kernel.  The service is generally something that only the kernel has
the privilege to do, such as doing I/O.  Programmers don't normally
need to be concerned with system calls because there are functions in
the GNU C library to do virtually everything that system calls do.
These functions work by making system calls themselves.  For example,
there is a system call that changes the permissions of a file, but you
don't need to know about it because you can just use the GNU C
library's `chmod' function.

   System calls are sometimes called kernel calls.

   However, there are times when you want to make a system call
explicitly, and for that, the GNU C library provides the `syscall'
function.  `syscall' is harder to use and less portable than functions
like `chmod', but easier and more portable than coding the system call
in assembler instructions.

   `syscall' is most useful when you are working with a system call
which is special to your system or is newer than the GNU C library you
are using.  `syscall' is implemented in an entirely generic way; the
function does not know anything about what a particular system call
does or even if it is valid.

   The description of `syscall' in this section assumes a certain
protocol for system calls on the various platforms on which the GNU C
library runs.  That protocol is not defined by any strong authority, but
we won't describe it here either because anyone who is coding `syscall'
probably won't accept anything less than kernel and C library source
code as a specification of the interface between them anyway.

   `syscall' is declared in `unistd.h'.

 -- Function: long int syscall (long int SYSNO, ...)
     `syscall' performs a generic system call.

     SYSNO is the system call number.  Each kind of system call is
     identified by a number.  Macros for all the possible system call
     numbers are defined in `sys/syscall.h'

     The remaining arguments are the arguments for the system call, in
     order, and their meanings depend on the kind of system call.  Each
     kind of system call has a definite number of arguments, from zero
     to five.  If you code more arguments than the system call takes,
     the extra ones to the right are ignored.

     The return value is the return value from the system call, unless
     the system call failed.  In that case, `syscall' returns `-1' and
     sets `errno' to an error code that the system call returned.  Note
     that system calls do not return `-1' when they succeed.  

     If you specify an invalid SYSNO, `syscall' returns `-1' with
     `errno' = `ENOSYS'.

     Example:


          #include <unistd.h>
          #include <sys/syscall.h>
          #include <errno.h>

          ...

          int rc;

          rc = syscall(SYS_chmod, "/etc/passwd", 0444);

          if (rc == -1)
             fprintf(stderr, "chmod failed, errno = %d\n", errno);

     This, if all the compatibility stars are aligned, is equivalent to
     the following preferable code:


          #include <sys/types.h>
          #include <sys/stat.h>
          #include <errno.h>

          ...

          int rc;

          rc = chmod("/etc/passwd", 0444);
          if (rc == -1)
             fprintf(stderr, "chmod failed, errno = %d\n", errno);



File: libc.info,  Node: Program Termination,  Prev: System Calls,  Up: Program Basics

25.6 Program Termination
========================

The usual way for a program to terminate is simply for its `main'
function to return.  The "exit status value" returned from the `main'
function is used to report information back to the process's parent
process or shell.

   A program can also terminate normally by calling the `exit' function.

   In addition, programs can be terminated by signals; this is
discussed in more detail in *Note Signal Handling::.  The `abort'
function causes a signal that kills the program.

* Menu:

* Normal Termination::          If a program calls `exit', a
                                 process terminates normally.
* Exit Status::                 The `exit status' provides information
                                 about why the process terminated.
* Cleanups on Exit::            A process can run its own cleanup
                                 functions upon normal termination.
* Aborting a Program::          The `abort' function causes
                                 abnormal program termination.
* Termination Internals::       What happens when a process terminates.


File: libc.info,  Node: Normal Termination,  Next: Exit Status,  Up: Program Termination

25.6.1 Normal Termination
-------------------------

A process terminates normally when its program signals it is done by
calling `exit'.  Returning from `main' is equivalent to calling `exit',
and the value that `main' returns is used as the argument to `exit'.

 -- Function: void exit (int STATUS)
     The `exit' function tells the system that the program is done,
     which causes it to terminate the process.

     STATUS is the program's exit status, which becomes part of the
     process' termination status.  This function does not return.

   Normal termination causes the following actions:

  1. Functions that were registered with the `atexit' or `on_exit'
     functions are called in the reverse order of their registration.
     This mechanism allows your application to specify its own
     "cleanup" actions to be performed at program termination.
     Typically, this is used to do things like saving program state
     information in a file, or unlocking locks in shared data bases.

  2. All open streams are closed, writing out any buffered output data.
     See *Note Closing Streams::.  In addition, temporary files opened
     with the `tmpfile' function are removed; see *Note Temporary
     Files::.

  3. `_exit' is called, terminating the program.  *Note Termination
     Internals::.


File: libc.info,  Node: Exit Status,  Next: Cleanups on Exit,  Prev: Normal Termination,  Up: Program Termination

25.6.2 Exit Status
------------------

When a program exits, it can return to the parent process a small
amount of information about the cause of termination, using the "exit
status".  This is a value between 0 and 255 that the exiting process
passes as an argument to `exit'.

   Normally you should use the exit status to report very broad
information about success or failure.  You can't provide a lot of
detail about the reasons for the failure, and most parent processes
would not want much detail anyway.

   There are conventions for what sorts of status values certain
programs should return.  The most common convention is simply 0 for
success and 1 for failure.  Programs that perform comparison use a
different convention: they use status 1 to indicate a mismatch, and
status 2 to indicate an inability to compare.  Your program should
follow an existing convention if an existing convention makes sense for
it.

   A general convention reserves status values 128 and up for special
purposes.  In particular, the value 128 is used to indicate failure to
execute another program in a subprocess.  This convention is not
universally obeyed, but it is a good idea to follow it in your programs.

   *Warning:* Don't try to use the number of errors as the exit status.
This is actually not very useful; a parent process would generally not
care how many errors occurred.  Worse than that, it does not work,
because the status value is truncated to eight bits.  Thus, if the
program tried to report 256 errors, the parent would receive a report
of 0 errors--that is, success.

   For the same reason, it does not work to use the value of `errno' as
the exit status--these can exceed 255.

   *Portability note:* Some non-POSIX systems use different conventions
for exit status values.  For greater portability, you can use the
macros `EXIT_SUCCESS' and `EXIT_FAILURE' for the conventional status
value for success and failure, respectively.  They are declared in the
file `stdlib.h'.  

 -- Macro: int EXIT_SUCCESS
     This macro can be used with the `exit' function to indicate
     successful program completion.

     On POSIX systems, the value of this macro is `0'.  On other
     systems, the value might be some other (possibly non-constant)
     integer expression.

 -- Macro: int EXIT_FAILURE
     This macro can be used with the `exit' function to indicate
     unsuccessful program completion in a general sense.

     On POSIX systems, the value of this macro is `1'.  On other
     systems, the value might be some other (possibly non-constant)
     integer expression.  Other nonzero status values also indicate
     failures.  Certain programs use different nonzero status values to
     indicate particular kinds of "non-success".  For example, `diff'
     uses status value `1' to mean that the files are different, and
     `2' or more to mean that there was difficulty in opening the files.

   Don't confuse a program's exit status with a process' termination
status.  There are lots of ways a process can terminate besides having
it's program finish.  In the event that the process termination _is_
caused by program termination (i.e. `exit'), though, the program's exit
status becomes part of the process' termination status.


File: libc.info,  Node: Cleanups on Exit,  Next: Aborting a Program,  Prev: Exit Status,  Up: Program Termination

25.6.3 Cleanups on Exit
-----------------------

Your program can arrange to run its own cleanup functions if normal
termination happens.  If you are writing a library for use in various
application programs, then it is unreliable to insist that all
applications call the library's cleanup functions explicitly before
exiting.  It is much more robust to make the cleanup invisible to the
application, by setting up a cleanup function in the library itself
using `atexit' or `on_exit'.

 -- Function: int atexit (void (*FUNCTION) (void))
     The `atexit' function registers the function FUNCTION to be called
     at normal program termination.  The FUNCTION is called with no
     arguments.

     The return value from `atexit' is zero on success and nonzero if
     the function cannot be registered.

 -- Function: int on_exit (void (*FUNCTION)(int STATUS, void *ARG),
          void *ARG)
     This function is a somewhat more powerful variant of `atexit'.  It
     accepts two arguments, a function FUNCTION and an arbitrary
     pointer ARG.  At normal program termination, the FUNCTION is
     called with two arguments:  the STATUS value passed to `exit', and
     the ARG.

     This function is included in the GNU C library only for
     compatibility for SunOS, and may not be supported by other
     implementations.

   Here's a trivial program that illustrates the use of `exit' and
`atexit':

     #include <stdio.h>
     #include <stdlib.h>

     void
     bye (void)
     {
       puts ("Goodbye, cruel world....");
     }

     int
     main (void)
     {
       atexit (bye);
       exit (EXIT_SUCCESS);
     }

When this program is executed, it just prints the message and exits.


File: libc.info,  Node: Aborting a Program,  Next: Termination Internals,  Prev: Cleanups on Exit,  Up: Program Termination

25.6.4 Aborting a Program
-------------------------

You can abort your program using the `abort' function.  The prototype
for this function is in `stdlib.h'.  

 -- Function: void abort (void)
     The `abort' function causes abnormal program termination.  This
     does not execute cleanup functions registered with `atexit' or
     `on_exit'.

     This function actually terminates the process by raising a
     `SIGABRT' signal, and your program can include a handler to
     intercept this signal; see *Note Signal Handling::.

   *Future Change Warning:* Proposed Federal censorship regulations may
prohibit us from giving you information about the possibility of
calling this function.  We would be required to say that this is not an
acceptable way of terminating a program.


File: libc.info,  Node: Termination Internals,  Prev: Aborting a Program,  Up: Program Termination

25.6.5 Termination Internals
----------------------------

The `_exit' function is the primitive used for process termination by
`exit'.  It is declared in the header file `unistd.h'.  

 -- Function: void _exit (int STATUS)
     The `_exit' function is the primitive for causing a process to
     terminate with status STATUS.  Calling this function does not
     execute cleanup functions registered with `atexit' or `on_exit'.

 -- Function: void _Exit (int STATUS)
     The `_Exit' function is the ISO C equivalent to `_exit'.  The
     ISO C committee members were not sure whether the definitions of
     `_exit' and `_Exit' were compatible so they have not used the
     POSIX name.

     This function was introduced in ISO C99 and is declared in
     `stdlib.h'.

   When a process terminates for any reason--either because the program
terminates, or as a result of a signal--the following things happen:

   * All open file descriptors in the process are closed.  *Note
     Low-Level I/O::.  Note that streams are not flushed automatically
     when the process terminates; see *Note I/O on Streams::.

   * A process exit status is saved to be reported back to the parent
     process via `wait' or `waitpid'; see *Note Process Completion::.
     If the program exited, this status includes as its low-order 8
     bits the program exit status.

   * Any child processes of the process being terminated are assigned a
     new parent process.  (On most systems, including GNU, this is the
     `init' process, with process ID 1.)

   * A `SIGCHLD' signal is sent to the parent process.

   * If the process is a session leader that has a controlling
     terminal, then a `SIGHUP' signal is sent to each process in the
     foreground job, and the controlling terminal is disassociated from
     that session.  *Note Job Control::.

   * If termination of a process causes a process group to become
     orphaned, and any member of that process group is stopped, then a
     `SIGHUP' signal and a `SIGCONT' signal are sent to each process in
     the group.  *Note Job Control::.


File: libc.info,  Node: Processes,  Next: Job Control,  Prev: Program Basics,  Up: Top

26 Processes
************

"Processes" are the primitive units for allocation of system resources.
Each process has its own address space and (usually) one thread of
control.  A process executes a program; you can have multiple processes
executing the same program, but each process has its own copy of the
program within its own address space and executes it independently of
the other copies.

   Processes are organized hierarchically.  Each process has a "parent
process" which explicitly arranged to create it.  The processes created
by a given parent are called its "child processes".  A child inherits
many of its attributes from the parent process.

   This chapter describes how a program can create, terminate, and
control child processes.  Actually, there are three distinct operations
involved: creating a new child process, causing the new process to
execute a program, and coordinating the completion of the child process
with the original program.

   The `system' function provides a simple, portable mechanism for
running another program; it does all three steps automatically.  If you
need more control over the details of how this is done, you can use the
primitive functions to do each step individually instead.

* Menu:

* Running a Command::           The easy way to run another program.
* Process Creation Concepts::   An overview of the hard way to do it.
* Process Identification::      How to get the process ID of a process.
* Creating a Process::          How to fork a child process.
* Executing a File::            How to make a process execute another program.
* Process Completion::          How to tell when a child process has completed.
* Process Completion Status::   How to interpret the status value
                                 returned from a child process.
* BSD Wait Functions::  	More functions, for backward compatibility.
* Process Creation Example::    A complete example program.


File: libc.info,  Node: Running a Command,  Next: Process Creation Concepts,  Up: Processes

26.1 Running a Command
======================

The easy way to run another program is to use the `system' function.
This function does all the work of running a subprogram, but it doesn't
give you much control over the details: you have to wait until the
subprogram terminates before you can do anything else.

 -- Function: int system (const char *COMMAND)
     This function executes COMMAND as a shell command.  In the GNU C
     library, it always uses the default shell `sh' to run the command.
     In particular, it searches the directories in `PATH' to find
     programs to execute.  The return value is `-1' if it wasn't
     possible to create the shell process, and otherwise is the status
     of the shell process.  *Note Process Completion::, for details on
     how this status code can be interpreted.

     If the COMMAND argument is a null pointer, a return value of zero
     indicates that no command processor is available.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `system' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this calls to
     `system' should be protected using cancellation handlers.

     The `system' function is declared in the header file `stdlib.h'.

   *Portability Note:* Some C implementations may not have any notion
of a command processor that can execute other programs.  You can
determine whether a command processor exists by executing
`system (NULL)'; if the return value is nonzero, a command processor is
available.

   The `popen' and `pclose' functions (*note Pipe to a Subprocess::)
are closely related to the `system' function.  They allow the parent
process to communicate with the standard input and output channels of
the command being executed.

