Embedded_AI/gcc-linaro-7.5.0-2019.12-x86_64_arm-linux-gnueabihf/arm-linux-gnueabihf/libc/usr/share/info/libc.info-10

This is libc.info, produced by makeinfo version 5.2 from libc.texinfo.

This file documents the GNU C Library.

   This is ‘The GNU C Library Reference Manual’, for version 2.25.

   Copyright © 1993–2017 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 1.3 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 GNU
Manual”, and with the Back-Cover Texts as in (a) below.  A copy of the
license is included in the section entitled "GNU Free Documentation
License".

   (a) The FSF’s Back-Cover Text is: “You have the freedom to copy and
modify this GNU manual.  Buying copies from the FSF supports it in
developing GNU and promoting software freedom.”
INFO-DIR-SECTION Software 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
* a64l: (libc)Encode Binary Data.
* abort: (libc)Aborting a Program.
* abs: (libc)Absolute Value.
* accept: (libc)Accepting Connections.
* access: (libc)Testing File Access.
* acosf: (libc)Inverse Trig Functions.
* acoshf: (libc)Hyperbolic Functions.
* acosh: (libc)Hyperbolic Functions.
* acoshl: (libc)Hyperbolic Functions.
* acos: (libc)Inverse Trig 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.
* aligned_alloc: (libc)Aligned Memory Blocks.
* alloca: (libc)Variable Size Automatic.
* alphasort64: (libc)Scanning Directory Content.
* alphasort: (libc)Scanning Directory Content.
* ALTWERASE: (libc)Local Modes.
* ARG_MAX: (libc)General Limits.
* argp_error: (libc)Argp Helper Functions.
* ARGP_ERR_UNKNOWN: (libc)Argp Parser 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.
* asinf: (libc)Inverse Trig Functions.
* asinhf: (libc)Hyperbolic Functions.
* asinh: (libc)Hyperbolic Functions.
* asinhl: (libc)Hyperbolic Functions.
* asin: (libc)Inverse Trig Functions.
* asinl: (libc)Inverse Trig Functions.
* asprintf: (libc)Dynamic Output.
* assert: (libc)Consistency Checking.
* assert_perror: (libc)Consistency Checking.
* atan2f: (libc)Inverse Trig Functions.
* atan2: (libc)Inverse Trig Functions.
* atan2l: (libc)Inverse Trig Functions.
* atanf: (libc)Inverse Trig Functions.
* atanhf: (libc)Hyperbolic Functions.
* atanh: (libc)Hyperbolic Functions.
* atanhl: (libc)Hyperbolic Functions.
* atan: (libc)Inverse Trig 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_fd: (libc)Backtraces.
* backtrace_symbols: (libc)Backtraces.
* basename: (libc)Finding Tokens in a String.
* basename: (libc)Finding Tokens in a String.
* BC_BASE_MAX: (libc)Utility Limits.
* BC_DIM_MAX: (libc)Utility Limits.
* bcmp: (libc)String/Array Comparison.
* bcopy: (libc)Copying Strings and Arrays.
* BC_SCALE_MAX: (libc)Utility Limits.
* BC_STRING_MAX: (libc)Utility Limits.
* bind: (libc)Setting Address.
* bind_textdomain_codeset: (libc)Charset conversion in gettext.
* bindtextdomain: (libc)Locating gettext catalog.
* BRKINT: (libc)Input Modes.
* brk: (libc)Resizing the Data Segment.
* bsearch: (libc)Array Search Function.
* btowc: (libc)Converting a Character.
* BUFSIZ: (libc)Controlling Buffering.
* bzero: (libc)Copying Strings and Arrays.
* cabsf: (libc)Absolute Value.
* cabs: (libc)Absolute Value.
* cabsl: (libc)Absolute Value.
* cacosf: (libc)Inverse Trig Functions.
* cacoshf: (libc)Hyperbolic Functions.
* cacosh: (libc)Hyperbolic Functions.
* cacoshl: (libc)Hyperbolic Functions.
* cacos: (libc)Inverse Trig Functions.
* cacosl: (libc)Inverse Trig Functions.
* calloc: (libc)Allocating Cleared Space.
* canonicalize_file_name: (libc)Symbolic Links.
* canonicalizef: (libc)FP Bit Twiddling.
* canonicalize: (libc)FP Bit Twiddling.
* canonicalizel: (libc)FP Bit Twiddling.
* cargf: (libc)Operations on Complex.
* carg: (libc)Operations on Complex.
* cargl: (libc)Operations on Complex.
* casinf: (libc)Inverse Trig Functions.
* casinhf: (libc)Hyperbolic Functions.
* casinh: (libc)Hyperbolic Functions.
* casinhl: (libc)Hyperbolic Functions.
* casin: (libc)Inverse Trig Functions.
* casinl: (libc)Inverse Trig Functions.
* catanf: (libc)Inverse Trig Functions.
* catanhf: (libc)Hyperbolic Functions.
* catanh: (libc)Hyperbolic Functions.
* catanhl: (libc)Hyperbolic Functions.
* catan: (libc)Inverse Trig 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.
* cbrtf: (libc)Exponents and Logarithms.
* cbrt: (libc)Exponents and Logarithms.
* cbrtl: (libc)Exponents and Logarithms.
* ccosf: (libc)Trig Functions.
* ccoshf: (libc)Hyperbolic Functions.
* ccosh: (libc)Hyperbolic Functions.
* ccoshl: (libc)Hyperbolic Functions.
* ccos: (libc)Trig Functions.
* ccosl: (libc)Trig Functions.
* CCTS_OFLOW: (libc)Control Modes.
* ceilf: (libc)Rounding Functions.
* ceil: (libc)Rounding Functions.
* ceill: (libc)Rounding Functions.
* cexpf: (libc)Exponents and Logarithms.
* cexp: (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.
* CHILD_MAX: (libc)General Limits.
* chmod: (libc)Setting Permissions.
* chown: (libc)File Owner.
* CIGNORE: (libc)Control Modes.
* cimagf: (libc)Operations on Complex.
* cimag: (libc)Operations on Complex.
* cimagl: (libc)Operations on Complex.
* clearenv: (libc)Environment Access.
* clearerr: (libc)Error Recovery.
* clearerr_unlocked: (libc)Error Recovery.
* CLK_TCK: (libc)Processor Time.
* CLOCAL: (libc)Control Modes.
* clock: (libc)CPU Time.
* CLOCKS_PER_SEC: (libc)CPU Time.
* clog10f: (libc)Exponents and Logarithms.
* clog10: (libc)Exponents and Logarithms.
* clog10l: (libc)Exponents and Logarithms.
* clogf: (libc)Exponents and Logarithms.
* clog: (libc)Exponents and Logarithms.
* clogl: (libc)Exponents and Logarithms.
* closedir: (libc)Reading/Closing Directory.
* close: (libc)Opening and Closing Files.
* closelog: (libc)closelog.
* COLL_WEIGHTS_MAX: (libc)Utility Limits.
* _Complex_I: (libc)Complex Numbers.
* confstr: (libc)String Parameters.
* conjf: (libc)Operations on Complex.
* conj: (libc)Operations on Complex.
* conjl: (libc)Operations on Complex.
* connect: (libc)Connecting.
* copysignf: (libc)FP Bit Twiddling.
* copysign: (libc)FP Bit Twiddling.
* copysignl: (libc)FP Bit Twiddling.
* cosf: (libc)Trig Functions.
* coshf: (libc)Hyperbolic Functions.
* cosh: (libc)Hyperbolic Functions.
* coshl: (libc)Hyperbolic Functions.
* cos: (libc)Trig Functions.
* cosl: (libc)Trig Functions.
* cpowf: (libc)Exponents and Logarithms.
* cpow: (libc)Exponents and Logarithms.
* cpowl: (libc)Exponents and Logarithms.
* cprojf: (libc)Operations on Complex.
* cproj: (libc)Operations on Complex.
* cprojl: (libc)Operations on Complex.
* 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.
* crealf: (libc)Operations on Complex.
* creal: (libc)Operations on Complex.
* creall: (libc)Operations on Complex.
* creat64: (libc)Opening and Closing Files.
* creat: (libc)Opening and Closing Files.
* CRTS_IFLOW: (libc)Control Modes.
* crypt: (libc)crypt.
* crypt_r: (libc)crypt.
* CS5: (libc)Control Modes.
* CS6: (libc)Control Modes.
* CS7: (libc)Control Modes.
* CS8: (libc)Control Modes.
* csinf: (libc)Trig Functions.
* csinhf: (libc)Hyperbolic Functions.
* csinh: (libc)Hyperbolic Functions.
* csinhl: (libc)Hyperbolic Functions.
* csin: (libc)Trig Functions.
* csinl: (libc)Trig Functions.
* CSIZE: (libc)Control Modes.
* csqrtf: (libc)Exponents and Logarithms.
* csqrt: (libc)Exponents and Logarithms.
* csqrtl: (libc)Exponents and Logarithms.
* CSTOPB: (libc)Control Modes.
* ctanf: (libc)Trig Functions.
* ctanhf: (libc)Hyperbolic Functions.
* ctanh: (libc)Hyperbolic Functions.
* ctanhl: (libc)Hyperbolic Functions.
* ctan: (libc)Trig 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_FAILED: (libc)DES Encryption.
* 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.
* dremf: (libc)Remainder Functions.
* drem: (libc)Remainder Functions.
* dreml: (libc)Remainder Functions.
* DTTOIF: (libc)Directory Entries.
* dup2: (libc)Duplicating Descriptors.
* dup: (libc)Duplicating Descriptors.
* 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.
* EBADFD: (libc)Error Codes.
* EBADF: (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.
* ecb_crypt: (libc)DES Encryption.
* ECHILD: (libc)Error Codes.
* ECHOCTL: (libc)Local Modes.
* ECHOE: (libc)Local Modes.
* ECHOKE: (libc)Local Modes.
* ECHOK: (libc)Local Modes.
* ECHO: (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.
* ecvt: (libc)System V Number Conversion.
* ecvt_r: (libc)System V Number Conversion.
* EDEADLK: (libc)Error Codes.
* EDEADLOCK: (libc)Error Codes.
* EDESTADDRREQ: (libc)Error Codes.
* EDIED: (libc)Error Codes.
* ED: (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.
* EHWPOISON: (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.
* EKEYEXPIRED: (libc)Error Codes.
* EKEYREJECTED: (libc)Error Codes.
* EKEYREVOKED: (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.
* 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.
* 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.
* ENOKEY: (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.
* ENOTRECOVERABLE: (libc)Error Codes.
* ENOTSOCK: (libc)Error Codes.
* ENOTSUP: (libc)Error Codes.
* ENOTTY: (libc)Error Codes.
* ENOTUNIQ: (libc)Error Codes.
* envz_add: (libc)Envz Functions.
* envz_entry: (libc)Envz Functions.
* envz_get: (libc)Envz Functions.
* envz_merge: (libc)Envz Functions.
* envz_remove: (libc)Envz Functions.
* envz_strip: (libc)Envz Functions.
* ENXIO: (libc)Error Codes.
* EOF: (libc)EOF and Errors.
* EOPNOTSUPP: (libc)Error Codes.
* EOVERFLOW: (libc)Error Codes.
* EOWNERDEAD: (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.
* erand48: (libc)SVID Random.
* erand48_r: (libc)SVID Random.
* ERANGE: (libc)Error Codes.
* EREMCHG: (libc)Error Codes.
* EREMOTEIO: (libc)Error Codes.
* EREMOTE: (libc)Error Codes.
* ERESTART: (libc)Error Codes.
* erfcf: (libc)Special Functions.
* erfc: (libc)Special Functions.
* erfcl: (libc)Special Functions.
* erff: (libc)Special Functions.
* ERFKILL: (libc)Error Codes.
* erf: (libc)Special Functions.
* erfl: (libc)Special Functions.
* EROFS: (libc)Error Codes.
* ERPCMISMATCH: (libc)Error Codes.
* err: (libc)Error Messages.
* errno: (libc)Checking for Errors.
* error_at_line: (libc)Error Messages.
* error: (libc)Error Messages.
* errx: (libc)Error Messages.
* 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.
* ETIMEDOUT: (libc)Error Codes.
* ETIME: (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.
* execle: (libc)Executing a File.
* execl: (libc)Executing a File.
* execlp: (libc)Executing a File.
* execve: (libc)Executing a File.
* execv: (libc)Executing a File.
* execvp: (libc)Executing a File.
* EXFULL: (libc)Error Codes.
* EXIT_FAILURE: (libc)Exit Status.
* exit: (libc)Normal Termination.
* _exit: (libc)Termination Internals.
* _Exit: (libc)Termination Internals.
* EXIT_SUCCESS: (libc)Exit Status.
* exp10f: (libc)Exponents and Logarithms.
* exp10: (libc)Exponents and Logarithms.
* exp10l: (libc)Exponents and Logarithms.
* exp2f: (libc)Exponents and Logarithms.
* exp2: (libc)Exponents and Logarithms.
* exp2l: (libc)Exponents and Logarithms.
* expf: (libc)Exponents and Logarithms.
* exp: (libc)Exponents and Logarithms.
* explicit_bzero: (libc)Erasing Sensitive Data.
* expl: (libc)Exponents and Logarithms.
* expm1f: (libc)Exponents and Logarithms.
* expm1: (libc)Exponents and Logarithms.
* expm1l: (libc)Exponents and Logarithms.
* EXPR_NEST_MAX: (libc)Utility Limits.
* fabsf: (libc)Absolute Value.
* fabs: (libc)Absolute Value.
* fabsl: (libc)Absolute Value.
* __fbufsize: (libc)Controlling Buffering.
* fchdir: (libc)Working Directory.
* fchmod: (libc)Setting Permissions.
* fchown: (libc)File Owner.
* fcloseall: (libc)Closing Streams.
* fclose: (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.
* FD_CLOEXEC: (libc)Descriptor Flags.
* FD_CLR: (libc)Waiting for I/O.
* fdimf: (libc)Misc FP Arithmetic.
* fdim: (libc)Misc FP Arithmetic.
* fdiml: (libc)Misc FP Arithmetic.
* FD_ISSET: (libc)Waiting for I/O.
* fdopendir: (libc)Opening a Directory.
* fdopen: (libc)Descriptors and Streams.
* FD_SET: (libc)Waiting for I/O.
* FD_SETSIZE: (libc)Waiting for I/O.
* F_DUPFD: (libc)Duplicating Descriptors.
* FD_ZERO: (libc)Waiting for I/O.
* feclearexcept: (libc)Status bit operations.
* fedisableexcept: (libc)Control Functions.
* feenableexcept: (libc)Control Functions.
* fegetenv: (libc)Control Functions.
* fegetexceptflag: (libc)Status bit operations.
* fegetexcept: (libc)Control Functions.
* fegetmode: (libc)Control Functions.
* 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.
* fesetexcept: (libc)Status bit operations.
* fesetmode: (libc)Control Functions.
* fesetround: (libc)Rounding.
* FE_SNANS_ALWAYS_SIGNAL: (libc)Infinity and NaN.
* fetestexceptflag: (libc)Status bit operations.
* 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.
* F_GETFD: (libc)Descriptor Flags.
* F_GETFL: (libc)Getting File Status Flags.
* fgetgrent: (libc)Scanning All Groups.
* fgetgrent_r: (libc)Scanning All Groups.
* F_GETLK: (libc)File Locks.
* F_GETOWN: (libc)Interrupt Input.
* 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.
* FILENAME_MAX: (libc)Limits for Files.
* fileno: (libc)Descriptors and Streams.
* fileno_unlocked: (libc)Descriptors and Streams.
* finitef: (libc)Floating Point Classes.
* finite: (libc)Floating Point Classes.
* finitel: (libc)Floating Point Classes.
* __flbf: (libc)Controlling Buffering.
* flockfile: (libc)Streams and Threads.
* floorf: (libc)Rounding Functions.
* floor: (libc)Rounding Functions.
* floorl: (libc)Rounding Functions.
* _flushlbf: (libc)Flushing Buffers.
* FLUSHO: (libc)Local Modes.
* fmaf: (libc)Misc FP Arithmetic.
* fma: (libc)Misc FP Arithmetic.
* fmal: (libc)Misc FP Arithmetic.
* fmaxf: (libc)Misc FP Arithmetic.
* fmax: (libc)Misc FP Arithmetic.
* fmaxl: (libc)Misc FP Arithmetic.
* fmaxmagf: (libc)Misc FP Arithmetic.
* fmaxmag: (libc)Misc FP Arithmetic.
* fmaxmagl: (libc)Misc FP Arithmetic.
* fmemopen: (libc)String Streams.
* fminf: (libc)Misc FP Arithmetic.
* fmin: (libc)Misc FP Arithmetic.
* fminl: (libc)Misc FP Arithmetic.
* fminmagf: (libc)Misc FP Arithmetic.
* fminmag: (libc)Misc FP Arithmetic.
* fminmagl: (libc)Misc FP Arithmetic.
* fmodf: (libc)Remainder Functions.
* fmod: (libc)Remainder Functions.
* fmodl: (libc)Remainder Functions.
* fmtmsg: (libc)Printing Formatted Messages.
* fnmatch: (libc)Wildcard Matching.
* F_OFD_GETLK: (libc)Open File Description Locks.
* F_OFD_SETLK: (libc)Open File Description Locks.
* F_OFD_SETLKW: (libc)Open File Description Locks.
* F_OK: (libc)Testing File Access.
* fopen64: (libc)Opening Streams.
* fopencookie: (libc)Streams and Cookies.
* fopen: (libc)Opening Streams.
* FOPEN_MAX: (libc)Opening Streams.
* fork: (libc)Creating a Process.
* forkpty: (libc)Pseudo-Terminal Pairs.
* fpathconf: (libc)Pathconf.
* fpclassify: (libc)Floating Point Classes.
* __fpending: (libc)Controlling Buffering.
* FP_ILOGB0: (libc)Exponents and Logarithms.
* FP_ILOGBNAN: (libc)Exponents and Logarithms.
* FP_LLOGB0: (libc)Exponents and Logarithms.
* FP_LLOGBNAN: (libc)Exponents and Logarithms.
* fprintf: (libc)Formatted Output Functions.
* __fpurge: (libc)Flushing Buffers.
* 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.
* __freadable: (libc)Opening Streams.
* __freading: (libc)Opening Streams.
* fread: (libc)Block Input/Output.
* fread_unlocked: (libc)Block Input/Output.
* free: (libc)Freeing after Malloc.
* freopen64: (libc)Opening Streams.
* freopen: (libc)Opening Streams.
* frexpf: (libc)Normalization Functions.
* frexp: (libc)Normalization Functions.
* frexpl: (libc)Normalization Functions.
* fromfpf: (libc)Rounding Functions.
* fromfp: (libc)Rounding Functions.
* fromfpl: (libc)Rounding Functions.
* fromfpxf: (libc)Rounding Functions.
* fromfpx: (libc)Rounding Functions.
* fromfpxl: (libc)Rounding Functions.
* fscanf: (libc)Formatted Input Functions.
* fseek: (libc)File Positioning.
* fseeko64: (libc)File Positioning.
* fseeko: (libc)File Positioning.
* F_SETFD: (libc)Descriptor Flags.
* F_SETFL: (libc)Getting File Status Flags.
* F_SETLK: (libc)File Locks.
* F_SETLKW: (libc)File Locks.
* __fsetlocking: (libc)Streams and Threads.
* F_SETOWN: (libc)Interrupt Input.
* 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.
* __fwritable: (libc)Opening Streams.
* fwrite: (libc)Block Input/Output.
* fwrite_unlocked: (libc)Block Input/Output.
* __fwriting: (libc)Opening Streams.
* fwscanf: (libc)Formatted Input Functions.
* gammaf: (libc)Special Functions.
* gamma: (libc)Special Functions.
* gammal: (libc)Special Functions.
* __gconv_end_fct: (libc)glibc iconv Implementation.
* __gconv_fct: (libc)glibc iconv Implementation.
* __gconv_init_fct: (libc)glibc iconv Implementation.
* gcvt: (libc)System V Number Conversion.
* getauxval: (libc)Auxiliary Vector.
* get_avphys_pages: (libc)Query Memory Parameters.
* getchar: (libc)Character Input.
* getchar_unlocked: (libc)Character Input.
* getc: (libc)Character Input.
* getcontext: (libc)System V contexts.
* getc_unlocked: (libc)Character Input.
* get_current_dir_name: (libc)Working Directory.
* 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.
* getentropy: (libc)Unpredictable Bytes.
* 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.
* get_nprocs_conf: (libc)Processor Resources.
* get_nprocs: (libc)Processor Resources.
* 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.
* getpayloadf: (libc)FP Bit Twiddling.
* getpayload: (libc)FP Bit Twiddling.
* getpayloadl: (libc)FP Bit Twiddling.
* getpeername: (libc)Who is Connected.
* getpgid: (libc)Process Group Functions.
* getpgrp: (libc)Process Group Functions.
* get_phys_pages: (libc)Query Memory Parameters.
* 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.
* getrandom: (libc)Unpredictable Bytes.
* getrlimit64: (libc)Limits on Resources.
* getrlimit: (libc)Limits on Resources.
* getrusage: (libc)Resource Usage.
* getservbyname: (libc)Services Database.
* getservbyport: (libc)Services Database.
* getservent: (libc)Services Database.
* getsid: (libc)Process Group Functions.
* gets: (libc)Line Input.
* 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.
* getwchar: (libc)Character Input.
* getwchar_unlocked: (libc)Character Input.
* getwc: (libc)Character Input.
* getwc_unlocked: (libc)Character Input.
* getwd: (libc)Working Directory.
* getw: (libc)Character Input.
* glob64: (libc)Calling Glob.
* globfree64: (libc)More Flags for Globbing.
* globfree: (libc)More Flags for Globbing.
* glob: (libc)Calling Glob.
* 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.
* HUGE_VALF: (libc)Math Error Reporting.
* HUGE_VAL: (libc)Math Error Reporting.
* HUGE_VALL: (libc)Math Error Reporting.
* HUPCL: (libc)Control Modes.
* hypotf: (libc)Exponents and Logarithms.
* hypot: (libc)Exponents and Logarithms.
* hypotl: (libc)Exponents and Logarithms.
* ICANON: (libc)Local Modes.
* iconv_close: (libc)Generic Conversion Interface.
* iconv: (libc)Generic Conversion Interface.
* iconv_open: (libc)Generic Conversion Interface.
* ICRNL: (libc)Input Modes.
* IEXTEN: (libc)Local Modes.
* if_freenameindex: (libc)Interface Naming.
* if_indextoname: (libc)Interface Naming.
* if_nameindex: (libc)Interface Naming.
* if_nametoindex: (libc)Interface Naming.
* IFNAMSIZ: (libc)Interface Naming.
* IFTODT: (libc)Directory Entries.
* IGNBRK: (libc)Input Modes.
* IGNCR: (libc)Input Modes.
* IGNPAR: (libc)Input Modes.
* I: (libc)Complex Numbers.
* ilogbf: (libc)Exponents and Logarithms.
* ilogb: (libc)Exponents and Logarithms.
* ilogbl: (libc)Exponents and Logarithms.
* _Imaginary_I: (libc)Complex Numbers.
* imaxabs: (libc)Absolute Value.
* IMAXBEL: (libc)Input Modes.
* imaxdiv: (libc)Integer Division.
* in6addr_any: (libc)Host Address Data Type.
* in6addr_loopback: (libc)Host Address Data Type.
* 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.
* 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.
* INFINITY: (libc)Infinity and NaN.
* initgroups: (libc)Setting Groups.
* initstate: (libc)BSD Random.
* initstate_r: (libc)BSD Random.
* INLCR: (libc)Input Modes.
* innetgr: (libc)Netgroup Membership.
* INPCK: (libc)Input Modes.
* ioctl: (libc)IOCTLs.
* _IOFBF: (libc)Controlling Buffering.
* _IOLBF: (libc)Controlling Buffering.
* _IONBF: (libc)Controlling Buffering.
* IPPORT_RESERVED: (libc)Ports.
* IPPORT_USERRESERVED: (libc)Ports.
* 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.
* iscanonical: (libc)Floating Point Classes.
* iscntrl: (libc)Classification of Characters.
* isdigit: (libc)Classification of Characters.
* iseqsig: (libc)FP Comparison Functions.
* isfinite: (libc)Floating Point Classes.
* isgraph: (libc)Classification of Characters.
* isgreaterequal: (libc)FP Comparison Functions.
* isgreater: (libc)FP Comparison Functions.
* ISIG: (libc)Local Modes.
* isinff: (libc)Floating Point Classes.
* isinf: (libc)Floating Point Classes.
* isinfl: (libc)Floating Point Classes.
* islessequal: (libc)FP Comparison Functions.
* islessgreater: (libc)FP Comparison Functions.
* isless: (libc)FP Comparison Functions.
* islower: (libc)Classification of Characters.
* isnanf: (libc)Floating Point Classes.
* isnan: (libc)Floating Point Classes.
* isnan: (libc)Floating Point Classes.
* isnanl: (libc)Floating Point Classes.
* isnormal: (libc)Floating Point Classes.
* isprint: (libc)Classification of Characters.
* ispunct: (libc)Classification of Characters.
* issignaling: (libc)Floating Point Classes.
* isspace: (libc)Classification of Characters.
* issubnormal: (libc)Floating Point Classes.
* ISTRIP: (libc)Input Modes.
* 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.
* iszero: (libc)Floating Point Classes.
* IXANY: (libc)Input Modes.
* IXOFF: (libc)Input Modes.
* IXON: (libc)Input Modes.
* j0f: (libc)Special Functions.
* j0: (libc)Special Functions.
* j0l: (libc)Special Functions.
* j1f: (libc)Special Functions.
* j1: (libc)Special Functions.
* j1l: (libc)Special Functions.
* jnf: (libc)Special Functions.
* jn: (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.
* L_ctermid: (libc)Identifying the Terminal.
* L_cuserid: (libc)Who Logged In.
* ldexpf: (libc)Normalization Functions.
* ldexp: (libc)Normalization Functions.
* ldexpl: (libc)Normalization Functions.
* ldiv: (libc)Integer Division.
* lfind: (libc)Array Search Function.
* lgammaf: (libc)Special Functions.
* lgammaf_r: (libc)Special Functions.
* lgamma: (libc)Special Functions.
* lgammal: (libc)Special Functions.
* lgammal_r: (libc)Special Functions.
* lgamma_r: (libc)Special Functions.
* LINE_MAX: (libc)Utility Limits.
* link: (libc)Hard Links.
* LINK_MAX: (libc)Limits for Files.
* lio_listio64: (libc)Asynchronous Reads/Writes.
* lio_listio: (libc)Asynchronous Reads/Writes.
* listen: (libc)Listening.
* llabs: (libc)Absolute Value.
* lldiv: (libc)Integer Division.
* llogbf: (libc)Exponents and Logarithms.
* llogb: (libc)Exponents and Logarithms.
* llogbl: (libc)Exponents and Logarithms.
* llrintf: (libc)Rounding Functions.
* llrint: (libc)Rounding Functions.
* llrintl: (libc)Rounding Functions.
* llroundf: (libc)Rounding Functions.
* llround: (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.
* log10f: (libc)Exponents and Logarithms.
* log10: (libc)Exponents and Logarithms.
* log10l: (libc)Exponents and Logarithms.
* log1pf: (libc)Exponents and Logarithms.
* log1p: (libc)Exponents and Logarithms.
* log1pl: (libc)Exponents and Logarithms.
* log2f: (libc)Exponents and Logarithms.
* log2: (libc)Exponents and Logarithms.
* log2l: (libc)Exponents and Logarithms.
* logbf: (libc)Exponents and Logarithms.
* logb: (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.
* log: (libc)Exponents and Logarithms.
* 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.
* lrintf: (libc)Rounding Functions.
* lrint: (libc)Rounding Functions.
* lrintl: (libc)Rounding Functions.
* lroundf: (libc)Rounding Functions.
* lround: (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.
* L_tmpnam: (libc)Temporary Files.
* 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.
* MAX_CANON: (libc)Limits for Files.
* MAX_INPUT: (libc)Limits for Files.
* MAXNAMLEN: (libc)Limits for Files.
* MAXSYMLINKS: (libc)Symbolic Links.
* MB_CUR_MAX: (libc)Selecting the Conversion.
* mblen: (libc)Non-reentrant Character Conversion.
* MB_LEN_MAX: (libc)Selecting the 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.
* MDMBUF: (libc)Control Modes.
* memalign: (libc)Aligned Memory Blocks.
* memccpy: (libc)Copying Strings and Arrays.
* memchr: (libc)Search Functions.
* memcmp: (libc)String/Array Comparison.
* memcpy: (libc)Copying Strings and Arrays.
* memfrob: (libc)Trivial Encryption.
* memmem: (libc)Search Functions.
* memmove: (libc)Copying Strings and Arrays.
* mempcpy: (libc)Copying Strings and Arrays.
* memrchr: (libc)Search Functions.
* memset: (libc)Copying Strings and Arrays.
* 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.
* mlockall: (libc)Page Lock Functions.
* mlock: (libc)Page Lock Functions.
* mmap64: (libc)Memory-mapped I/O.
* mmap: (libc)Memory-mapped I/O.
* modff: (libc)Rounding Functions.
* modf: (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.
* MSG_DONTROUTE: (libc)Socket Data Options.
* MSG_OOB: (libc)Socket Data Options.
* MSG_PEEK: (libc)Socket Data Options.
* msync: (libc)Memory-mapped I/O.
* mtrace: (libc)Tracing malloc.
* munlockall: (libc)Page Lock Functions.
* munlock: (libc)Page Lock Functions.
* munmap: (libc)Memory-mapped I/O.
* muntrace: (libc)Tracing malloc.
* NAME_MAX: (libc)Limits for Files.
* nanf: (libc)FP Bit Twiddling.
* nan: (libc)FP Bit Twiddling.
* NAN: (libc)Infinity and NaN.
* nanl: (libc)FP Bit Twiddling.
* nanosleep: (libc)Sleeping.
* NCCS: (libc)Mode Data Types.
* nearbyintf: (libc)Rounding Functions.
* nearbyint: (libc)Rounding Functions.
* nearbyintl: (libc)Rounding Functions.
* nextafterf: (libc)FP Bit Twiddling.
* nextafter: (libc)FP Bit Twiddling.
* nextafterl: (libc)FP Bit Twiddling.
* nextdownf: (libc)FP Bit Twiddling.
* nextdown: (libc)FP Bit Twiddling.
* nextdownl: (libc)FP Bit Twiddling.
* nexttowardf: (libc)FP Bit Twiddling.
* nexttoward: (libc)FP Bit Twiddling.
* nexttowardl: (libc)FP Bit Twiddling.
* nextupf: (libc)FP Bit Twiddling.
* nextup: (libc)FP Bit Twiddling.
* nextupl: (libc)FP Bit Twiddling.
* nftw64: (libc)Working with Directory Trees.
* nftw: (libc)Working with Directory Trees.
* ngettext: (libc)Advanced gettext functions.
* NGROUPS_MAX: (libc)General Limits.
* nice: (libc)Traditional Scheduling Functions.
* nl_langinfo: (libc)The Elegant and Fast Way.
* NOFLSH: (libc)Local Modes.
* NOKERNINFO: (libc)Local Modes.
* nrand48: (libc)SVID Random.
* nrand48_r: (libc)SVID Random.
* NSIG: (libc)Standard Signals.
* ntohl: (libc)Byte Order.
* ntohs: (libc)Byte Order.
* ntp_adjtime: (libc)High Accuracy Clock.
* ntp_gettime: (libc)High Accuracy Clock.
* NULL: (libc)Null Pointer Constant.
* O_ACCMODE: (libc)Access Modes.
* O_APPEND: (libc)Operating Modes.
* O_ASYNC: (libc)Operating Modes.
* obstack_1grow_fast: (libc)Extra Fast Growing.
* obstack_1grow: (libc)Growing Objects.
* obstack_alignment_mask: (libc)Obstacks Data Alignment.
* obstack_alloc: (libc)Allocation in an Obstack.
* obstack_base: (libc)Status of an Obstack.
* obstack_blank_fast: (libc)Extra Fast Growing.
* obstack_blank: (libc)Growing Objects.
* 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_fast: (libc)Extra Fast Growing.
* obstack_int_grow: (libc)Growing Objects.
* 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_fast: (libc)Extra Fast Growing.
* obstack_ptr_grow: (libc)Growing Objects.
* obstack_room: (libc)Extra Fast Growing.
* obstack_vprintf: (libc)Variable Arguments Output.
* O_CREAT: (libc)Open-time Flags.
* O_EXCL: (libc)Open-time Flags.
* O_EXEC: (libc)Access Modes.
* O_EXLOCK: (libc)Open-time Flags.
* offsetof: (libc)Structure Measurement.
* O_FSYNC: (libc)Operating Modes.
* O_IGNORE_CTTY: (libc)Open-time Flags.
* O_NDELAY: (libc)Operating Modes.
* on_exit: (libc)Cleanups on Exit.
* ONLCR: (libc)Output Modes.
* O_NOATIME: (libc)Operating Modes.
* O_NOCTTY: (libc)Open-time Flags.
* ONOEOT: (libc)Output Modes.
* O_NOLINK: (libc)Open-time Flags.
* O_NONBLOCK: (libc)Open-time Flags.
* O_NONBLOCK: (libc)Operating Modes.
* O_NOTRANS: (libc)Open-time Flags.
* open64: (libc)Opening and Closing Files.
* opendir: (libc)Opening a Directory.
* open: (libc)Opening and Closing Files.
* openlog: (libc)openlog.
* OPEN_MAX: (libc)General Limits.
* open_memstream: (libc)String Streams.
* openpty: (libc)Pseudo-Terminal Pairs.
* OPOST: (libc)Output Modes.
* 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.
* OXTABS: (libc)Output Modes.
* PA_FLAG_MASK: (libc)Parsing a Template String.
* PARENB: (libc)Control Modes.
* PARMRK: (libc)Input Modes.
* PARODD: (libc)Control Modes.
* parse_printf_format: (libc)Parsing a Template String.
* pathconf: (libc)Pathconf.
* PATH_MAX: (libc)Limits for Files.
* _PATH_UTMP: (libc)Manipulating the Database.
* _PATH_WTMP: (libc)Manipulating the Database.
* pause: (libc)Using Pause.
* pclose: (libc)Pipe to a Subprocess.
* PENDIN: (libc)Local Modes.
* perror: (libc)Error Messages.
* 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.
* pipe: (libc)Creating a Pipe.
* popen: (libc)Pipe to a Subprocess.
* _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_fallocate64: (libc)Storage Allocation.
* posix_fallocate: (libc)Storage Allocation.
* _POSIX_JOB_CONTROL: (libc)System Options.
* posix_memalign: (libc)Aligned Memory Blocks.
* _POSIX_NO_TRUNC: (libc)Options for Files.
* _POSIX_SAVED_IDS: (libc)System Options.
* _POSIX_VDISABLE: (libc)Options for Files.
* _POSIX_VERSION: (libc)Version Supported.
* pow10f: (libc)Exponents and Logarithms.
* pow10: (libc)Exponents and Logarithms.
* pow10l: (libc)Exponents and Logarithms.
* powf: (libc)Exponents and Logarithms.
* pow: (libc)Exponents and Logarithms.
* powl: (libc)Exponents and Logarithms.
* __ppc_get_timebase_freq: (libc)PowerPC.
* __ppc_get_timebase: (libc)PowerPC.
* __ppc_mdoio: (libc)PowerPC.
* __ppc_mdoom: (libc)PowerPC.
* __ppc_set_ppr_low: (libc)PowerPC.
* __ppc_set_ppr_med_high: (libc)PowerPC.
* __ppc_set_ppr_med: (libc)PowerPC.
* __ppc_set_ppr_med_low: (libc)PowerPC.
* __ppc_set_ppr_very_low: (libc)PowerPC.
* __ppc_yield: (libc)PowerPC.
* pread64: (libc)I/O Primitives.
* pread: (libc)I/O Primitives.
* printf: (libc)Formatted Output Functions.
* printf_size_info: (libc)Predefined Printf Handlers.
* printf_size: (libc)Predefined Printf Handlers.
* psignal: (libc)Signal Messages.
* pthread_getattr_default_np: (libc)Default Thread Attributes.
* pthread_getspecific: (libc)Thread-specific Data.
* pthread_key_create: (libc)Thread-specific Data.
* pthread_key_delete: (libc)Thread-specific Data.
* pthread_setattr_default_np: (libc)Default Thread Attributes.
* pthread_setspecific: (libc)Thread-specific Data.
* P_tmpdir: (libc)Temporary Files.
* ptsname: (libc)Allocation.
* ptsname_r: (libc)Allocation.
* putchar: (libc)Simple Output.
* putchar_unlocked: (libc)Simple Output.
* putc: (libc)Simple Output.
* putc_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.
* putwchar: (libc)Simple Output.
* putwchar_unlocked: (libc)Simple Output.
* putwc: (libc)Simple Output.
* putwc_unlocked: (libc)Simple Output.
* putw: (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_MAX: (libc)ISO Random.
* random: (libc)BSD Random.
* random_r: (libc)BSD Random.
* rand_r: (libc)ISO Random.
* rawmemchr: (libc)Search Functions.
* readdir64: (libc)Reading/Closing Directory.
* readdir64_r: (libc)Reading/Closing Directory.
* readdir: (libc)Reading/Closing Directory.
* readdir_r: (libc)Reading/Closing Directory.
* read: (libc)I/O Primitives.
* readlink: (libc)Symbolic Links.
* readv: (libc)Scatter-Gather.
* realloc: (libc)Changing Block Size.
* realpath: (libc)Symbolic Links.
* recvfrom: (libc)Receiving Datagrams.
* recv: (libc)Receiving Data.
* recvmsg: (libc)Receiving Datagrams.
* RE_DUP_MAX: (libc)General Limits.
* 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.
* remainderf: (libc)Remainder Functions.
* remainder: (libc)Remainder Functions.
* remainderl: (libc)Remainder Functions.
* remove: (libc)Deleting Files.
* rename: (libc)Renaming Files.
* rewinddir: (libc)Random Access Directory.
* rewind: (libc)File Positioning.
* rindex: (libc)Search Functions.
* rintf: (libc)Rounding Functions.
* rint: (libc)Rounding Functions.
* rintl: (libc)Rounding Functions.
* RLIM_INFINITY: (libc)Limits on Resources.
* rmdir: (libc)Deleting Files.
* R_OK: (libc)Testing File Access.
* roundevenf: (libc)Rounding Functions.
* roundeven: (libc)Rounding Functions.
* roundevenl: (libc)Rounding Functions.
* roundf: (libc)Rounding Functions.
* round: (libc)Rounding Functions.
* roundl: (libc)Rounding Functions.
* rpmatch: (libc)Yes-or-No Questions.
* SA_NOCLDSTOP: (libc)Flags for Sigaction.
* SA_ONSTACK: (libc)Flags for Sigaction.
* SA_RESTART: (libc)Flags for Sigaction.
* sbrk: (libc)Resizing the Data Segment.
* scalbf: (libc)Normalization Functions.
* scalb: (libc)Normalization Functions.
* scalbl: (libc)Normalization Functions.
* scalblnf: (libc)Normalization Functions.
* scalbln: (libc)Normalization Functions.
* scalblnl: (libc)Normalization Functions.
* scalbnf: (libc)Normalization Functions.
* scalbn: (libc)Normalization Functions.
* scalbnl: (libc)Normalization Functions.
* scandir64: (libc)Scanning Directory Content.
* scandir: (libc)Scanning Directory Content.
* scanf: (libc)Formatted Input Functions.
* sched_getaffinity: (libc)CPU Affinity.
* sched_getparam: (libc)Basic Scheduling Functions.
* sched_get_priority_max: (libc)Basic Scheduling Functions.
* sched_get_priority_min: (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.
* secure_getenv: (libc)Environment Access.
* seed48: (libc)SVID Random.
* seed48_r: (libc)SVID Random.
* SEEK_CUR: (libc)File Positioning.
* seekdir: (libc)Random Access Directory.
* SEEK_END: (libc)File Positioning.
* SEEK_SET: (libc)File Positioning.
* select: (libc)Waiting for I/O.
* sem_close: (libc)Semaphores.
* semctl: (libc)Semaphores.
* sem_destroy: (libc)Semaphores.
* semget: (libc)Semaphores.
* sem_getvalue: (libc)Semaphores.
* sem_init: (libc)Semaphores.
* sem_open: (libc)Semaphores.
* semop: (libc)Semaphores.
* sem_post: (libc)Semaphores.
* semtimedop: (libc)Semaphores.
* sem_timedwait: (libc)Semaphores.
* sem_trywait: (libc)Semaphores.
* sem_unlink: (libc)Semaphores.
* sem_wait: (libc)Semaphores.
* send: (libc)Sending Data.
* sendmsg: (libc)Receiving Datagrams.
* sendto: (libc)Sending Datagrams.
* setbuffer: (libc)Controlling Buffering.
* setbuf: (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.
* setpayloadf: (libc)FP Bit Twiddling.
* setpayload: (libc)FP Bit Twiddling.
* setpayloadl: (libc)FP Bit Twiddling.
* setpayloadsigf: (libc)FP Bit Twiddling.
* setpayloadsig: (libc)FP Bit Twiddling.
* setpayloadsigl: (libc)FP Bit Twiddling.
* 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.
* shm_open: (libc)Memory-mapped I/O.
* shm_unlink: (libc)Memory-mapped I/O.
* shutdown: (libc)Closing a Socket.
* S_IFMT: (libc)Testing File Type.
* SIGABRT: (libc)Program Error Signals.
* sigaction: (libc)Advanced Signal Handling.
* sigaddset: (libc)Signal Sets.
* SIGALRM: (libc)Alarm Signals.
* sigaltstack: (libc)Signal Stack.
* sigblock: (libc)BSD Signal Handling.
* SIGBUS: (libc)Program Error Signals.
* SIGCHLD: (libc)Job Control Signals.
* SIGCLD: (libc)Job Control Signals.
* SIGCONT: (libc)Job Control Signals.
* sigdelset: (libc)Signal Sets.
* sigemptyset: (libc)Signal Sets.
* SIGEMT: (libc)Program Error Signals.
* SIG_ERR: (libc)Basic Signal Handling.
* sigfillset: (libc)Signal Sets.
* SIGFPE: (libc)Program Error Signals.
* SIGHUP: (libc)Termination Signals.
* SIGILL: (libc)Program Error Signals.
* SIGINFO: (libc)Miscellaneous Signals.
* siginterrupt: (libc)BSD Signal Handling.
* SIGINT: (libc)Termination Signals.
* SIGIO: (libc)Asynchronous I/O Signals.
* SIGIOT: (libc)Program Error Signals.
* sigismember: (libc)Signal Sets.
* SIGKILL: (libc)Termination Signals.
* siglongjmp: (libc)Non-Local Exits and Signals.
* SIGLOST: (libc)Operation Error Signals.
* sigmask: (libc)BSD Signal Handling.
* signal: (libc)Basic Signal Handling.
* signbit: (libc)FP Bit Twiddling.
* significandf: (libc)Normalization Functions.
* significand: (libc)Normalization Functions.
* significandl: (libc)Normalization Functions.
* sigpause: (libc)BSD Signal Handling.
* sigpending: (libc)Checking for Pending Signals.
* SIGPIPE: (libc)Operation Error Signals.
* SIGPOLL: (libc)Asynchronous I/O Signals.
* sigprocmask: (libc)Process Signal Mask.
* SIGPROF: (libc)Alarm Signals.
* SIGQUIT: (libc)Termination Signals.
* SIGSEGV: (libc)Program Error Signals.
* sigsetjmp: (libc)Non-Local Exits and Signals.
* sigsetmask: (libc)BSD Signal Handling.
* sigstack: (libc)Signal Stack.
* SIGSTOP: (libc)Job Control Signals.
* sigsuspend: (libc)Sigsuspend.
* 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.
* sincosf: (libc)Trig Functions.
* sincos: (libc)Trig Functions.
* sincosl: (libc)Trig Functions.
* sinf: (libc)Trig Functions.
* sinhf: (libc)Hyperbolic Functions.
* sinh: (libc)Hyperbolic Functions.
* sinhl: (libc)Hyperbolic Functions.
* sin: (libc)Trig Functions.
* sinl: (libc)Trig Functions.
* 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.
* sleep: (libc)Sleeping.
* SNANF: (libc)Infinity and NaN.
* SNAN: (libc)Infinity and NaN.
* SNANL: (libc)Infinity and NaN.
* snprintf: (libc)Formatted Output Functions.
* SOCK_DGRAM: (libc)Communication Styles.
* socket: (libc)Creating a Socket.
* socketpair: (libc)Socket Pairs.
* 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.
* sprintf: (libc)Formatted Output Functions.
* sqrtf: (libc)Exponents and Logarithms.
* sqrt: (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.
* SSIZE_MAX: (libc)General Limits.
* stat64: (libc)Reading Attributes.
* stat: (libc)Reading Attributes.
* stime: (libc)Simple Calendar Time.
* stpcpy: (libc)Copying Strings and Arrays.
* stpncpy: (libc)Truncating Strings.
* strcasecmp: (libc)String/Array Comparison.
* strcasestr: (libc)Search Functions.
* strcat: (libc)Concatenating Strings.
* strchr: (libc)Search Functions.
* strchrnul: (libc)Search Functions.
* strcmp: (libc)String/Array Comparison.
* strcoll: (libc)Collation Functions.
* strcpy: (libc)Copying Strings and Arrays.
* strcspn: (libc)Search Functions.
* strdupa: (libc)Copying Strings and Arrays.
* strdup: (libc)Copying Strings and Arrays.
* STREAM_MAX: (libc)General Limits.
* strerror: (libc)Error Messages.
* strerror_r: (libc)Error Messages.
* strfmon: (libc)Formatting Numbers.
* strfromd: (libc)Printing of Floats.
* strfromf: (libc)Printing of Floats.
* strfroml: (libc)Printing of Floats.
* strfry: (libc)strfry.
* strftime: (libc)Formatting Calendar Time.
* strlen: (libc)String Length.
* strncasecmp: (libc)String/Array Comparison.
* strncat: (libc)Truncating Strings.
* strncmp: (libc)String/Array Comparison.
* strncpy: (libc)Truncating Strings.
* strndupa: (libc)Truncating Strings.
* strndup: (libc)Truncating Strings.
* 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.
* strtold: (libc)Parsing of Floats.
* strtol: (libc)Parsing of Integers.
* 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.
* S_TYPEISMQ: (libc)Testing File Type.
* S_TYPEISSEM: (libc)Testing File Type.
* S_TYPEISSHM: (libc)Testing File Type.
* SUN_LEN: (libc)Local Namespace Details.
* 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.
* tanf: (libc)Trig Functions.
* tanhf: (libc)Hyperbolic Functions.
* tanh: (libc)Hyperbolic Functions.
* tanhl: (libc)Hyperbolic Functions.
* tan: (libc)Trig 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.
* telldir: (libc)Random Access Directory.
* tempnam: (libc)Temporary Files.
* textdomain: (libc)Locating gettext catalog.
* tfind: (libc)Tree Search Function.
* tgammaf: (libc)Special Functions.
* tgamma: (libc)Special Functions.
* tgammal: (libc)Special Functions.
* timegm: (libc)Broken-down Time.
* time: (libc)Simple Calendar Time.
* timelocal: (libc)Broken-down Time.
* times: (libc)Processor Time.
* tmpfile64: (libc)Temporary Files.
* tmpfile: (libc)Temporary Files.
* TMP_MAX: (libc)Temporary Files.
* tmpnam: (libc)Temporary Files.
* tmpnam_r: (libc)Temporary Files.
* toascii: (libc)Case Conversion.
* _tolower: (libc)Case Conversion.
* tolower: (libc)Case Conversion.
* TOSTOP: (libc)Local Modes.
* totalorderf: (libc)FP Comparison Functions.
* totalorder: (libc)FP Comparison Functions.
* totalorderl: (libc)FP Comparison Functions.
* totalordermagf: (libc)FP Comparison Functions.
* totalordermag: (libc)FP Comparison Functions.
* totalordermagl: (libc)FP Comparison Functions.
* _toupper: (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.
* truncate64: (libc)File Size.
* truncate: (libc)File Size.
* truncf: (libc)Rounding Functions.
* trunc: (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.
* TZNAME_MAX: (libc)General Limits.
* tzset: (libc)Time Zone Functions.
* ufromfpf: (libc)Rounding Functions.
* ufromfp: (libc)Rounding Functions.
* ufromfpl: (libc)Rounding Functions.
* ufromfpxf: (libc)Rounding Functions.
* ufromfpx: (libc)Rounding Functions.
* ufromfpxl: (libc)Rounding 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_copy: (libc)Argument Macros.
* va_copy: (libc)Argument Macros.
* va_end: (libc)Argument Macros.
* valloc: (libc)Aligned Memory Blocks.
* vasprintf: (libc)Variable Arguments Output.
* va_start: (libc)Argument Macros.
* VDISCARD: (libc)Other Special.
* VDSUSP: (libc)Signal Characters.
* VEOF: (libc)Editing Characters.
* VEOL2: (libc)Editing Characters.
* VEOL: (libc)Editing Characters.
* VERASE: (libc)Editing Characters.
* 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.
* VINTR: (libc)Signal Characters.
* VKILL: (libc)Editing Characters.
* vlimit: (libc)Limits on Resources.
* VLNEXT: (libc)Other Special.
* VMIN: (libc)Noncanonical Input.
* vprintf: (libc)Variable Arguments Output.
* VQUIT: (libc)Signal Characters.
* VREPRINT: (libc)Editing Characters.
* vscanf: (libc)Variable Arguments Input.
* vsnprintf: (libc)Variable Arguments Output.
* vsprintf: (libc)Variable Arguments Output.
* vsscanf: (libc)Variable Arguments Input.
* VSTART: (libc)Start/Stop Characters.
* VSTATUS: (libc)Other Special.
* VSTOP: (libc)Start/Stop Characters.
* VSUSP: (libc)Signal Characters.
* vswprintf: (libc)Variable Arguments Output.
* vswscanf: (libc)Variable Arguments Input.
* vsyslog: (libc)syslog; vsyslog.
* VTIME: (libc)Noncanonical Input.
* vtimes: (libc)Resource Usage.
* vwarn: (libc)Error Messages.
* vwarnx: (libc)Error Messages.
* VWERASE: (libc)Editing Characters.
* 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.
* WCHAR_MAX: (libc)Extended Char Intro.
* WCHAR_MIN: (libc)Extended Char Intro.
* WCOREDUMP: (libc)Process Completion Status.
* wcpcpy: (libc)Copying Strings and Arrays.
* wcpncpy: (libc)Truncating Strings.
* wcrtomb: (libc)Converting a Character.
* wcscasecmp: (libc)String/Array Comparison.
* wcscat: (libc)Concatenating Strings.
* wcschr: (libc)Search Functions.
* wcschrnul: (libc)Search Functions.
* wcscmp: (libc)String/Array Comparison.
* wcscoll: (libc)Collation Functions.
* wcscpy: (libc)Copying Strings and Arrays.
* wcscspn: (libc)Search Functions.
* wcsdup: (libc)Copying Strings and Arrays.
* wcsftime: (libc)Formatting Calendar Time.
* wcslen: (libc)String Length.
* wcsncasecmp: (libc)String/Array Comparison.
* wcsncat: (libc)Truncating Strings.
* wcsncmp: (libc)String/Array Comparison.
* wcsncpy: (libc)Truncating Strings.
* 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.
* wcstold: (libc)Parsing of Floats.
* wcstol: (libc)Parsing of Integers.
* 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.
* 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.
* wmemchr: (libc)Search Functions.
* wmemcmp: (libc)String/Array Comparison.
* wmemcpy: (libc)Copying Strings and Arrays.
* wmemmove: (libc)Copying Strings and Arrays.
* wmempcpy: (libc)Copying Strings and Arrays.
* wmemset: (libc)Copying Strings and Arrays.
* W_OK: (libc)Testing File Access.
* 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.
* WSTOPSIG: (libc)Process Completion Status.
* WTERMSIG: (libc)Process Completion Status.
* X_OK: (libc)Testing File Access.
* y0f: (libc)Special Functions.
* y0: (libc)Special Functions.
* y0l: (libc)Special Functions.
* y1f: (libc)Special Functions.
* y1: (libc)Special Functions.
* y1l: (libc)Special Functions.
* ynf: (libc)Special Functions.
* yn: (libc)Special Functions.
* ynl: (libc)Special Functions.
END-INFO-DIR-ENTRY


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 is to re-initialize the crucial data structures in the
signal handler, or to 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>

     volatile 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’ is 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 ensure 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 C 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 C 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 Signal Handling::.

   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 C Library is to make primitives fail with ‘EINTR’.

   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)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     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)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     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)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     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.

     ‘ESRCH’
          The PID argument does not refer to an existing process or
          group.

 -- Function: int killpg (int PGID, int SIGNUM)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     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)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function initializes the signal set SET to exclude all of the
     defined signals.  It always returns ‘0’.

 -- Function: int sigfillset (sigset_t *SET)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     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)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     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)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     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)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     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 “unspecified”.  Instead, use
‘pthread_sigmask’.

 -- Function: int sigprocmask (int HOW, const sigset_t *restrict SET,
          sigset_t *restrict OLDSET)
     Preliminary: | MT-Unsafe race:sigprocmask/bsd(SIG_UNBLOCK) |
     AS-Unsafe lock/hurd | AC-Unsafe lock/hurd | *Note POSIX Safety
     Concepts::.

     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)
     Preliminary: | MT-Safe | AS-Unsafe lock/hurd | AC-Unsafe lock/hurd
     | *Note POSIX Safety Concepts::.

     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 (void)
     Preliminary: | MT-Unsafe race:sigprocmask/!bsd!linux | AS-Unsafe
     lock/hurd | AC-Unsafe lock/hurd | *Note POSIX Safety Concepts::.

     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)
     Preliminary: | MT-Unsafe race:sigprocmask/!bsd!linux | AS-Unsafe
     lock/hurd | AC-Unsafe lock/hurd | *Note POSIX Safety Concepts::.

     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)
     Preliminary: | MT-Safe | AS-Unsafe lock/hurd | AC-Unsafe lock/hurd
     | *Note POSIX Safety Concepts::.

     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 (struct sigstack *STACK, struct sigstack
          *OLDSTACK)
     Preliminary: | MT-Safe | AS-Unsafe lock/hurd | AC-Unsafe lock/hurd
     | *Note POSIX Safety Concepts::.

     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 difference between the two is that BSD Unix
represents signal masks as an ‘int’ bit mask, rather than as a
‘sigset_t’ object.

   The BSD facilities are declared in ‘signal.h’.

 -- Function: int siginterrupt (int SIGNUM, int FAILFLAG)
     Preliminary: | MT-Unsafe const:sigintr | AS-Unsafe | AC-Unsafe
     corrupt | *Note POSIX Safety Concepts::.

     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::.

 -- Macro: int sigmask (int SIGNUM)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     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)
     Preliminary: | MT-Safe | AS-Unsafe lock/hurd | AC-Unsafe lock/hurd
     | *Note POSIX Safety Concepts::.

     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)
     Preliminary: | MT-Safe | AS-Unsafe lock/hurd | AC-Unsafe lock/hurd
     | *Note POSIX Safety Concepts::.

     This function is 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)
     Preliminary: | MT-Unsafe race:sigprocmask/!bsd!linux | AS-Unsafe
     lock/hurd | AC-Unsafe lock/hurd | *Note POSIX Safety Concepts::.

     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 systems.  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, see *note
Executing a File::.

* Menu:

* Program Arguments::           Parsing your program’s command-line arguments
* Environment Variables::       Less direct parameters affecting your program
* Auxiliary Vector::            Least 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, GNU systems 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 *const *ARGV, const char
          *OPTIONS)
     Preliminary: | MT-Unsafe race:getopt env | AS-Unsafe heap i18n lock
     corrupt | AC-Unsafe mem lock corrupt | *Note POSIX Safety
     Concepts::.

     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)
     Preliminary: | MT-Unsafe race:getopt env | AS-Unsafe heap i18n lock
     corrupt | AC-Unsafe mem lock corrupt | *Note POSIX Safety
     Concepts::.

     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 option’s 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 ‘getopt_long’ 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)
     Preliminary: | MT-Unsafe race:getopt env | AS-Unsafe heap i18n lock
     corrupt | AC-Unsafe mem lock corrupt | *Note POSIX Safety
     Concepts::.

     The ‘getopt_long_only’ function is equivalent to the ‘getopt_long’
     function but it allows 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 (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)
     Preliminary: | MT-Unsafe race:argpbuf locale env | AS-Unsafe heap
     i18n lock corrupt | AC-Unsafe mem lock corrupt | *Note POSIX Safety
     Concepts::.

     The ‘argp_parse’ function parses the arguments in ARGV, of length
     ARGC, using the argp parser ARGP.  *Note Argp Parsers::.  Passing a
     null pointer for ARGP is the same as using 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_child’ 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 its 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 Parsing State,  Next: Argp Helper Functions,  Prev: Argp Special Keys,  Up: Argp Parser Functions

25.3.5.2 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 Helper Functions,  Prev: Argp Parsing State,  Up: Argp Parser Functions

25.3.5.3 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)
     Preliminary: | MT-Unsafe race:argpbuf env locale | AS-Unsafe heap
     i18n corrupt | AC-Unsafe mem corrupt lock | *Note POSIX Safety
     Concepts::.

     Outputs the standard usage message for the argp parser referred to
     by STATE to ‘STATE->err_stream’ and terminates the program with
     ‘exit (argp_err_exit_status)’.  *Note Argp Global Variables::.

 -- Function: void argp_error (const struct argp_state *STATE, const
          char *FMT, …)
     Preliminary: | MT-Unsafe race:argpbuf env locale | AS-Unsafe heap
     i18n corrupt | AC-Unsafe mem corrupt lock | *Note POSIX Safety
     Concepts::.

     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, …)
     Preliminary: | MT-Safe | AS-Unsafe corrupt heap | AC-Unsafe lock
     corrupt mem | *Note POSIX Safety Concepts::.

     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)
     Preliminary: | MT-Unsafe race:argpbuf env locale | AS-Unsafe heap
     i18n corrupt | AC-Unsafe mem corrupt lock | *Note POSIX Safety
     Concepts::.

     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 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)
     Preliminary: | MT-Unsafe race:argpbuf env locale | AS-Unsafe heap
     i18n corrupt | AC-Unsafe mem corrupt lock | *Note POSIX Safety
     Concepts::.

     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 message and exit when there are any
arguments, and prints a rather pointless message for ‘--help’.


     /* 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 <stdlib.h>
     #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.


     /* 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 <stdlib.h>
     #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.


     /* 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 <stdlib.h>
     #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.


     /* 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, char *const *TOKENS, char
          **VALUEP)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     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 are no more suboptions
     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: Auxiliary Vector,  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’.

   Libraries should use ‘secure_getenv’ instead of ‘getenv’, so that
they do not accidentally use untrusted environment variables.
Modifications of environment variables are not allowed in multi-threaded
programs.  The ‘getenv’ and ‘secure_getenv’ functions can be safely used
in multi-threaded programs.

 -- Function: char * getenv (const char *NAME)
     Preliminary: | MT-Safe env | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     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 C 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: char * secure_getenv (const char *NAME)
     Preliminary: | MT-Safe env | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is similar to ‘getenv’, but it returns a null pointer
     if the environment is untrusted.  This happens when the program
     file has SUID or SGID bits set.  General-purpose libraries should
     always prefer this function over ‘getenv’ to avoid vulnerabilities
     if the library is referenced from a SUID/SGID program.

     This function is a GNU extension.

 -- Function: int putenv (char *STRING)
     Preliminary: | MT-Unsafe const:env | AS-Unsafe heap lock |
     AC-Unsafe corrupt lock mem | *Note POSIX Safety Concepts::.

     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.

     If the function is successful it returns ‘0’.  Otherwise the return
     value is nonzero and ‘errno’ is set to indicate the error.

     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 automatically in the environment.  This also requires that
     STRING not be an automatic variable whose 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.  You should
     define _XOPEN_SOURCE before including any header.

 -- Function: int setenv (const char *NAME, const char *VALUE, int
          REPLACE)
     Preliminary: | MT-Unsafe const:env | AS-Unsafe heap lock |
     AC-Unsafe corrupt lock mem | *Note POSIX Safety Concepts::.

     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.

     If the function is successful it returns ‘0’.  Otherwise the
     environment is unchanged and the return value is ‘-1’ and ‘errno’
     is set.

     This function was originally part of the BSD library but is now
     part of the Unix standard.

 -- Function: int unsetenv (const char *NAME)
     Preliminary: | MT-Unsafe const:env | AS-Unsafe lock | AC-Unsafe
     lock | *Note POSIX Safety Concepts::.

     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 returns ‘-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)
     Preliminary: | MT-Unsafe const:env | AS-Unsafe heap lock |
     AC-Unsafe lock mem | *Note POSIX Safety Concepts::.

     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 GNU systems, 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: Auxiliary Vector,  Next: System Calls,  Prev: Environment Variables,  Up: Program Basics

25.5 Auxiliary Vector
=====================

When a program is executed, it receives information from the operating
system about the environment in which it is operating.  The form of this
information is a table of key-value pairs, where the keys are from the
set of ‘AT_’ values in ‘elf.h’.  Some of the data is provided by the
kernel for libc consumption, and may be obtained by ordinary interfaces,
such as ‘sysconf’.  However, on a platform-by-platform basis there may
be information that is not available any other way.

25.5.1 Definition of ‘getauxval’
--------------------------------

 -- Function: unsigned long int getauxval (unsigned long int TYPE)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is used to inquire about the entries in the auxiliary
     vector.  The TYPE argument should be one of the ‘AT_’ symbols
     defined in ‘elf.h’.  If a matching entry is found, the value is
     returned; if the entry is not found, zero is returned and ‘errno’
     is set to ‘ENOENT’.

   For some platforms, the key ‘AT_HWCAP’ is the easiest way to inquire
about any instruction set extensions available at runtime.  In this
case, there will (of necessity) be a platform-specific set of ‘HWCAP_’
values masked together that describe the capabilities of the cpu on
which the program is being executed.


File: libc.info,  Node: System Calls,  Next: Program Termination,  Prev: Auxiliary Vector,  Up: Program Basics

25.6 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, …)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     ‘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

25.7 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

25.7.1 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)
     Preliminary: | MT-Unsafe race:exit | AS-Unsafe corrupt | AC-Unsafe
     corrupt lock | *Note POSIX Safety Concepts::.

     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

25.7.2 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
its 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

25.7.3 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))
     Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe lock mem |
     *Note POSIX Safety Concepts::.

     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)
     Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe lock mem |
     *Note POSIX Safety Concepts::.

     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

25.7.4 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)
     Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Unsafe lock corrupt
     | *Note POSIX Safety Concepts::.

     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

25.7.5 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)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     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)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     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: Inter-Process Communication,  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)
     Preliminary: | MT-Safe | AS-Unsafe plugin heap lock | AC-Unsafe
     lock mem | *Note POSIX Safety Concepts::.

     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.


File: libc.info,  Node: Process Creation Concepts,  Next: Process Identification,  Prev: Running a Command,  Up: Processes

26.2 Process Creation Concepts
==============================

This section gives an overview of processes and of the steps involved in
creating a process and making it run another program.

   Each process is named by a "process ID" number.  A unique process ID
is allocated to each process when it is created.  The "lifetime" of a
process ends when its termination is reported to its parent process; at
that time, all of the process resources, including its process ID, are
freed.

   Processes are created with the ‘fork’ system call (so the operation
of creating a new process is sometimes called "forking" a process).  The
"child process" created by ‘fork’ is a copy of the original "parent
process", except that it has its own process ID.

   After forking a child process, both the parent and child processes
continue to execute normally.  If you want your program to wait for a
child process to finish executing before continuing, you must do this
explicitly after the fork operation, by calling ‘wait’ or ‘waitpid’
(*note Process Completion::).  These functions give you limited
information about why the child terminated—for example, its exit status
code.

   A newly forked child process continues to execute the same program as
its parent process, at the point where the ‘fork’ call returns.  You can
use the return value from ‘fork’ to tell whether the program is running
in the parent process or the child.

   Having several processes run the same program is only occasionally
useful.  But the child can execute another program using one of the
‘exec’ functions; see *note Executing a File::.  The program that the
process is executing is called its "process image".  Starting execution
of a new program causes the process to forget all about its previous
process image; when the new program exits, the process exits too,
instead of returning to the previous process image.


File: libc.info,  Node: Process Identification,  Next: Creating a Process,  Prev: Process Creation Concepts,  Up: Processes

26.3 Process Identification
===========================

The ‘pid_t’ data type represents process IDs.  You can get the process
ID of a process by calling ‘getpid’.  The function ‘getppid’ returns the
process ID of the parent of the current process (this is also known as
the "parent process ID"). Your program should include the header files
‘unistd.h’ and ‘sys/types.h’ to use these functions.

 -- Data Type: pid_t
     The ‘pid_t’ data type is a signed integer type which is capable of
     representing a process ID. In the GNU C Library, this is an ‘int’.

 -- Function: pid_t getpid (void)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘getpid’ function returns the process ID of the current
     process.

 -- Function: pid_t getppid (void)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘getppid’ function returns the process ID of the parent of the
     current process.


File: libc.info,  Node: Creating a Process,  Next: Executing a File,  Prev: Process Identification,  Up: Processes

26.4 Creating a Process
=======================

The ‘fork’ function is the primitive for creating a process.  It is
declared in the header file ‘unistd.h’.

 -- Function: pid_t fork (void)
     Preliminary: | MT-Safe | AS-Unsafe plugin | AC-Unsafe lock | *Note
     POSIX Safety Concepts::.

     The ‘fork’ function creates a new process.

     If the operation is successful, there are then both parent and
     child processes and both see ‘fork’ return, but with different
     values: it returns a value of ‘0’ in the child process and returns
     the child’s process ID in the parent process.

     If process creation failed, ‘fork’ returns a value of ‘-1’ in the
     parent process.  The following ‘errno’ error conditions are defined
     for ‘fork’:

     ‘EAGAIN’
          There aren’t enough system resources to create another
          process, or the user already has too many processes running.
          This means exceeding the ‘RLIMIT_NPROC’ resource limit, which
          can usually be increased; *note Limits on Resources::.

     ‘ENOMEM’
          The process requires more space than the system can supply.

   The specific attributes of the child process that differ from the
parent process are:

   • The child process has its own unique process ID.

   • The parent process ID of the child process is the process ID of its
     parent process.

   • The child process gets its own copies of the parent process’s open
     file descriptors.  Subsequently changing attributes of the file
     descriptors in the parent process won’t affect the file descriptors
     in the child, and vice versa.  *Note Control Operations::.
     However, the file position associated with each descriptor is
     shared by both processes; *note File Position::.

   • The elapsed processor times for the child process are set to zero;
     see *note Processor Time::.

   • The child doesn’t inherit file locks set by the parent process.
     *Note Control Operations::.

   • The child doesn’t inherit alarms set by the parent process.  *Note
     Setting an Alarm::.

   • The set of pending signals (*note Delivery of Signal::) for the
     child process is cleared.  (The child process inherits its mask of
     blocked signals and signal actions from the parent process.)

 -- Function: pid_t vfork (void)
     Preliminary: | MT-Safe | AS-Unsafe plugin | AC-Unsafe lock | *Note
     POSIX Safety Concepts::.

     The ‘vfork’ function is similar to ‘fork’ but on some systems it is
     more efficient; however, there are restrictions you must follow to
     use it safely.

     While ‘fork’ makes a complete copy of the calling process’s address
     space and allows both the parent and child to execute
     independently, ‘vfork’ does not make this copy.  Instead, the child
     process created with ‘vfork’ shares its parent’s address space
     until it calls ‘_exit’ or one of the ‘exec’ functions.  In the
     meantime, the parent process suspends execution.

     You must be very careful not to allow the child process created
     with ‘vfork’ to modify any global data or even local variables
     shared with the parent.  Furthermore, the child process cannot
     return from (or do a long jump out of) the function that called
     ‘vfork’!  This would leave the parent process’s control information
     very confused.  If in doubt, use ‘fork’ instead.

     Some operating systems don’t really implement ‘vfork’.  The GNU C
     Library permits you to use ‘vfork’ on all systems, but actually
     executes ‘fork’ if ‘vfork’ isn’t available.  If you follow the
     proper precautions for using ‘vfork’, your program will still work
     even if the system uses ‘fork’ instead.


File: libc.info,  Node: Executing a File,  Next: Process Completion,  Prev: Creating a Process,  Up: Processes

26.5 Executing a File
=====================

This section describes the ‘exec’ family of functions, for executing a
file as a process image.  You can use these functions to make a child
process execute a new program after it has been forked.

   To see the effects of ‘exec’ from the point of view of the called
program, see *note Program Basics::.

   The functions in this family differ in how you specify the arguments,
but otherwise they all do the same thing.  They are declared in the
header file ‘unistd.h’.

 -- Function: int execv (const char *FILENAME, char *const ARGV[])
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘execv’ function executes the file named by FILENAME as a new
     process image.

     The ARGV argument is an array of null-terminated strings that is
     used to provide a value for the ‘argv’ argument to the ‘main’
     function of the program to be executed.  The last element of this
     array must be a null pointer.  By convention, the first element of
     this array is the file name of the program sans directory names.
     *Note Program Arguments::, for full details on how programs can
     access these arguments.

     The environment for the new process image is taken from the
     ‘environ’ variable of the current process image; see *note
     Environment Variables::, for information about environments.

 -- Function: int execl (const char *FILENAME, const char *ARG0, …)
     Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | *Note
     POSIX Safety Concepts::.

     This is similar to ‘execv’, but the ARGV strings are specified
     individually instead of as an array.  A null pointer must be passed
     as the last such argument.

 -- Function: int execve (const char *FILENAME, char *const ARGV[], char
          *const ENV[])
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This is similar to ‘execv’, but permits you to specify the
     environment for the new program explicitly as the ENV argument.
     This should be an array of strings in the same format as for the
     ‘environ’ variable; see *note Environment Access::.

 -- Function: int execle (const char *FILENAME, const char *ARG0, …,
          char *const ENV[])
     Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | *Note
     POSIX Safety Concepts::.

     This is similar to ‘execl’, but permits you to specify the
     environment for the new program explicitly.  The environment
     argument is passed following the null pointer that marks the last
     ARGV argument, and should be an array of strings in the same format
     as for the ‘environ’ variable.

 -- Function: int execvp (const char *FILENAME, char *const ARGV[])
     Preliminary: | MT-Safe env | AS-Unsafe heap | AC-Unsafe mem | *Note
     POSIX Safety Concepts::.

     The ‘execvp’ function is similar to ‘execv’, except that it
     searches the directories listed in the ‘PATH’ environment variable
     (*note Standard Environment::) to find the full file name of a file
     from FILENAME if FILENAME does not contain a slash.

     This function is useful for executing system utility programs,
     because it looks for them in the places that the user has chosen.
     Shells use it to run the commands that users type.

 -- Function: int execlp (const char *FILENAME, const char *ARG0, …)
     Preliminary: | MT-Safe env | AS-Unsafe heap | AC-Unsafe mem | *Note
     POSIX Safety Concepts::.

     This function is like ‘execl’, except that it performs the same
     file name searching as the ‘execvp’ function.

   The size of the argument list and environment list taken together
must not be greater than ‘ARG_MAX’ bytes.  *Note General Limits::.  On
GNU/Hurd systems, the size (which compares against ‘ARG_MAX’) includes,
for each string, the number of characters in the string, plus the size
of a ‘char *’, plus one, rounded up to a multiple of the size of a ‘char
*’.  Other systems may have somewhat different rules for counting.

   These functions normally don’t return, since execution of a new
program causes the currently executing program to go away completely.  A
value of ‘-1’ is returned in the event of a failure.  In addition to the
usual file name errors (*note File Name Errors::), the following ‘errno’
error conditions are defined for these functions:

‘E2BIG’
     The combined size of the new program’s argument list and
     environment list is larger than ‘ARG_MAX’ bytes.  GNU/Hurd systems
     have no specific limit on the argument list size, so this error
     code cannot result, but you may get ‘ENOMEM’ instead if the
     arguments are too big for available memory.

‘ENOEXEC’
     The specified file can’t be executed because it isn’t in the right
     format.

‘ENOMEM’
     Executing the specified file requires more storage than is
     available.

   If execution of the new file succeeds, it updates the access time
field of the file as if the file had been read.  *Note File Times::, for
more details about access times of files.

   The point at which the file is closed again is not specified, but is
at some point before the process exits or before another process image
is executed.

   Executing a new process image completely changes the contents of
memory, copying only the argument and environment strings to new
locations.  But many other attributes of the process are unchanged:

   • The process ID and the parent process ID. *Note Process Creation
     Concepts::.

   • Session and process group membership.  *Note Concepts of Job
     Control::.

   • Real user ID and group ID, and supplementary group IDs.  *Note
     Process Persona::.

   • Pending alarms.  *Note Setting an Alarm::.

   • Current working directory and root directory.  *Note Working
     Directory::.  On GNU/Hurd systems, the root directory is not copied
     when executing a setuid program; instead the system default root
     directory is used for the new program.

   • File mode creation mask.  *Note Setting Permissions::.

   • Process signal mask; see *note Process Signal Mask::.

   • Pending signals; see *note Blocking Signals::.

   • Elapsed processor time associated with the process; see *note
     Processor Time::.

   If the set-user-ID and set-group-ID mode bits of the process image
file are set, this affects the effective user ID and effective group ID
(respectively) of the process.  These concepts are discussed in detail
in *note Process Persona::.

   Signals that are set to be ignored in the existing process image are
also set to be ignored in the new process image.  All other signals are
set to the default action in the new process image.  For more
information about signals, see *note Signal Handling::.

   File descriptors open in the existing process image remain open in
the new process image, unless they have the ‘FD_CLOEXEC’ (close-on-exec)
flag set.  The files that remain open inherit all attributes of the open
file descriptors from the existing process image, including file locks.
File descriptors are discussed in *note Low-Level I/O::.

   Streams, by contrast, cannot survive through ‘exec’ functions,
because they are located in the memory of the process itself.  The new
process image has no streams except those it creates afresh.  Each of
the streams in the pre-‘exec’ process image has a descriptor inside it,
and these descriptors do survive through ‘exec’ (provided that they do
not have ‘FD_CLOEXEC’ set).  The new process image can reconnect these
to new streams using ‘fdopen’ (*note Descriptors and Streams::).


File: libc.info,  Node: Process Completion,  Next: Process Completion Status,  Prev: Executing a File,  Up: Processes

26.6 Process Completion
=======================

The functions described in this section are used to wait for a child
process to terminate or stop, and determine its status.  These functions
are declared in the header file ‘sys/wait.h’.

 -- Function: pid_t waitpid (pid_t PID, int *STATUS-PTR, int OPTIONS)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘waitpid’ function is used to request status information from a
     child process whose process ID is PID.  Normally, the calling
     process is suspended until the child process makes status
     information available by terminating.

     Other values for the PID argument have special interpretations.  A
     value of ‘-1’ or ‘WAIT_ANY’ requests status information for any
     child process; a value of ‘0’ or ‘WAIT_MYPGRP’ requests information
     for any child process in the same process group as the calling
     process; and any other negative value − PGID requests information
     for any child process whose process group ID is PGID.

     If status information for a child process is available immediately,
     this function returns immediately without waiting.  If more than
     one eligible child process has status information available, one of
     them is chosen randomly, and its status is returned immediately.
     To get the status from the other eligible child processes, you need
     to call ‘waitpid’ again.

     The OPTIONS argument is a bit mask.  Its value should be the
     bitwise OR (that is, the ‘|’ operator) of zero or more of the
     ‘WNOHANG’ and ‘WUNTRACED’ flags.  You can use the ‘WNOHANG’ flag to
     indicate that the parent process shouldn’t wait; and the
     ‘WUNTRACED’ flag to request status information from stopped
     processes as well as processes that have terminated.

     The status information from the child process is stored in the
     object that STATUS-PTR points to, unless STATUS-PTR is a null
     pointer.

     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
     ‘waitpid’ is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this calls to
     ‘waitpid’ should be protected using cancellation handlers.

     The return value is normally the process ID of the child process
     whose status is reported.  If there are child processes but none of
     them is waiting to be noticed, ‘waitpid’ will block until one is.
     However, if the ‘WNOHANG’ option was specified, ‘waitpid’ will
     return zero instead of blocking.

     If a specific PID to wait for was given to ‘waitpid’, it will
     ignore all other children (if any).  Therefore if there are
     children waiting to be noticed but the child whose PID was
     specified is not one of them, ‘waitpid’ will block or return zero
     as described above.

     A value of ‘-1’ is returned in case of error.  The following
     ‘errno’ error conditions are defined for this function:

     ‘EINTR’
          The function was interrupted by delivery of a signal to the
          calling process.  *Note Interrupted Primitives::.

     ‘ECHILD’
          There are no child processes to wait for, or the specified PID
          is not a child of the calling process.

     ‘EINVAL’
          An invalid value was provided for the OPTIONS argument.

   These symbolic constants are defined as values for the PID argument
to the ‘waitpid’ function.

‘WAIT_ANY’

     This constant macro (whose value is ‘-1’) specifies that ‘waitpid’
     should return status information about any child process.

‘WAIT_MYPGRP’
     This constant (with value ‘0’) specifies that ‘waitpid’ should
     return status information about any child process in the same
     process group as the calling process.

   These symbolic constants are defined as flags for the OPTIONS
argument to the ‘waitpid’ function.  You can bitwise-OR the flags
together to obtain a value to use as the argument.

‘WNOHANG’

     This flag specifies that ‘waitpid’ should return immediately
     instead of waiting, if there is no child process ready to be
     noticed.

‘WUNTRACED’

     This flag specifies that ‘waitpid’ should report the status of any
     child processes that have been stopped as well as those that have
     terminated.

 -- Function: pid_t wait (int *STATUS-PTR)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This is a simplified version of ‘waitpid’, and is used to wait
     until any one child process terminates.  The call:

          wait (&status)

     is exactly equivalent to:

          waitpid (-1, &status, 0)

     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
     ‘wait’ is called.  If the thread gets canceled these resources stay
     allocated until the program ends.  To avoid this calls to ‘wait’
     should be protected using cancellation handlers.

 -- Function: pid_t wait4 (pid_t PID, int *STATUS-PTR, int OPTIONS,
          struct rusage *USAGE)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     If USAGE is a null pointer, ‘wait4’ is equivalent to ‘waitpid (PID,
     STATUS-PTR, OPTIONS)’.

     If USAGE is not null, ‘wait4’ stores usage figures for the child
     process in ‘*RUSAGE’ (but only if the child has terminated, not if
     it has stopped).  *Note Resource Usage::.

     This function is a BSD extension.

   Here’s an example of how to use ‘waitpid’ to get the status from all
child processes that have terminated, without ever waiting.  This
function is designed to be a handler for ‘SIGCHLD’, the signal that
indicates that at least one child process has terminated.

     void
     sigchld_handler (int signum)
     {
       int pid, status, serrno;
       serrno = errno;
       while (1)
         {
           pid = waitpid (WAIT_ANY, &status, WNOHANG);
           if (pid < 0)
             {
               perror ("waitpid");
               break;
             }
           if (pid == 0)
             break;
           notice_termination (pid, status);
         }
       errno = serrno;
     }


File: libc.info,  Node: Process Completion Status,  Next: BSD Wait Functions,  Prev: Process Completion,  Up: Processes

26.7 Process Completion Status
==============================

If the exit status value (*note Program Termination::) of the child
process is zero, then the status value reported by ‘waitpid’ or ‘wait’
is also zero.  You can test for other kinds of information encoded in
the returned status value using the following macros.  These macros are
defined in the header file ‘sys/wait.h’.

 -- Macro: int WIFEXITED (int STATUS)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This macro returns a nonzero value if the child process terminated
     normally with ‘exit’ or ‘_exit’.

 -- Macro: int WEXITSTATUS (int STATUS)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     If ‘WIFEXITED’ is true of STATUS, this macro returns the low-order
     8 bits of the exit status value from the child process.  *Note Exit
     Status::.

 -- Macro: int WIFSIGNALED (int STATUS)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This macro returns a nonzero value if the child process terminated
     because it received a signal that was not handled.  *Note Signal
     Handling::.

 -- Macro: int WTERMSIG (int STATUS)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     If ‘WIFSIGNALED’ is true of STATUS, this macro returns the signal
     number of the signal that terminated the child process.

 -- Macro: int WCOREDUMP (int STATUS)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This macro returns a nonzero value if the child process terminated
     and produced a core dump.

 -- Macro: int WIFSTOPPED (int STATUS)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This macro returns a nonzero value if the child process is stopped.

 -- Macro: int WSTOPSIG (int STATUS)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     If ‘WIFSTOPPED’ is true of STATUS, this macro returns the signal
     number of the signal that caused the child process to stop.


File: libc.info,  Node: BSD Wait Functions,  Next: Process Creation Example,  Prev: Process Completion Status,  Up: Processes

26.8 BSD Process Wait Function
==============================

The GNU C Library also provides the ‘wait3’ function for compatibility
with BSD. This function is declared in ‘sys/wait.h’.  It is the
predecessor to ‘wait4’, which is more flexible.  ‘wait3’ is now
obsolete.

 -- Function: pid_t wait3 (int *STATUS-PTR, int OPTIONS, struct rusage
          *USAGE)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     If USAGE is a null pointer, ‘wait3’ is equivalent to ‘waitpid (-1,
     STATUS-PTR, OPTIONS)’.

     If USAGE is not null, ‘wait3’ stores usage figures for the child
     process in ‘*RUSAGE’ (but only if the child has terminated, not if
     it has stopped).  *Note Resource Usage::.


File: libc.info,  Node: Process Creation Example,  Prev: BSD Wait Functions,  Up: Processes

26.9 Process Creation Example
=============================

Here is an example program showing how you might write a function
similar to the built-in ‘system’.  It executes its COMMAND argument
using the equivalent of ‘sh -c COMMAND’.

     #include <stddef.h>
     #include <stdlib.h>
     #include <unistd.h>
     #include <sys/types.h>
     #include <sys/wait.h>

     /* Execute the command using this shell program.  */
     #define SHELL "/bin/sh"

     int
     my_system (const char *command)
     {
       int status;
       pid_t pid;

       pid = fork ();
       if (pid == 0)
         {
           /* This is the child process.  Execute the shell command. */
           execl (SHELL, SHELL, "-c", command, NULL);
           _exit (EXIT_FAILURE);
         }
       else if (pid < 0)
         /* The fork failed.  Report failure.  */
         status = -1;
       else
         /* This is the parent process.  Wait for the child to complete.  */
         if (waitpid (pid, &status, 0) != pid)
           status = -1;
       return status;
     }

   There are a couple of things you should pay attention to in this
example.

   Remember that the first ‘argv’ argument supplied to the program
represents the name of the program being executed.  That is why, in the
call to ‘execl’, ‘SHELL’ is supplied once to name the program to execute
and a second time to supply a value for ‘argv[0]’.

   The ‘execl’ call in the child process doesn’t return if it is
successful.  If it fails, you must do something to make the child
process terminate.  Just returning a bad status code with ‘return’ would
leave two processes running the original program.  Instead, the right
behavior is for the child process to report failure to its parent
process.

   Call ‘_exit’ to accomplish this.  The reason for using ‘_exit’
instead of ‘exit’ is to avoid flushing fully buffered streams such as
‘stdout’.  The buffers of these streams probably contain data that was
copied from the parent process by the ‘fork’, data that will be output
eventually by the parent process.  Calling ‘exit’ in the child would
output the data twice.  *Note Termination Internals::.