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: Operating Modes,  Next: Getting File Status Flags,  Prev: Open-time Flags,  Up: File Status Flags

13.14.3 I/O Operating Modes
---------------------------

The operating modes affect how input and output operations using a file
descriptor work.  These flags are set by ‘open’ and can be fetched and
changed with ‘fcntl’.

 -- Macro: int O_APPEND
     The bit that enables append mode for the file.  If set, then all
     ‘write’ operations write the data at the end of the file, extending
     it, regardless of the current file position.  This is the only
     reliable way to append to a file.  In append mode, you are
     guaranteed that the data you write will always go to the current
     end of the file, regardless of other processes writing to the file.
     Conversely, if you simply set the file position to the end of file
     and write, then another process can extend the file after you set
     the file position but before you write, resulting in your data
     appearing someplace before the real end of file.

 -- Macro: int O_NONBLOCK
     The bit that enables nonblocking mode for the file.  If this bit is
     set, ‘read’ requests on the file can return immediately with a
     failure status if there is no input immediately available, instead
     of blocking.  Likewise, ‘write’ requests can also return
     immediately with a failure status if the output can’t be written
     immediately.

     Note that the ‘O_NONBLOCK’ flag is overloaded as both an I/O
     operating mode and a file name translation flag; *note Open-time
     Flags::.

 -- Macro: int O_NDELAY
     This is an obsolete name for ‘O_NONBLOCK’, provided for
     compatibility with BSD. It is not defined by the POSIX.1 standard.

   The remaining operating modes are BSD and GNU extensions.  They exist
only on some systems.  On other systems, these macros are not defined.

 -- Macro: int O_ASYNC
     The bit that enables asynchronous input mode.  If set, then ‘SIGIO’
     signals will be generated when input is available.  *Note Interrupt
     Input::.

     Asynchronous input mode is a BSD feature.

 -- Macro: int O_FSYNC
     The bit that enables synchronous writing for the file.  If set,
     each ‘write’ call will make sure the data is reliably stored on
     disk before returning.

     Synchronous writing is a BSD feature.

 -- Macro: int O_SYNC
     This is another name for ‘O_FSYNC’.  They have the same value.

 -- Macro: int O_NOATIME
     If this bit is set, ‘read’ will not update the access time of the
     file.  *Note File Times::.  This is used by programs that do
     backups, so that backing a file up does not count as reading it.
     Only the owner of the file or the superuser may use this bit.

     This is a GNU extension.


File: libc.info,  Node: Getting File Status Flags,  Prev: Operating Modes,  Up: File Status Flags

13.14.4 Getting and Setting File Status Flags
---------------------------------------------

The ‘fcntl’ function can fetch or change file status flags.

 -- Macro: int F_GETFL
     This macro is used as the COMMAND argument to ‘fcntl’, to read the
     file status flags for the open file with descriptor FILEDES.

     The normal return value from ‘fcntl’ with this command is a
     nonnegative number which can be interpreted as the bitwise OR of
     the individual flags.  Since the file access modes are not
     single-bit values, you can mask off other bits in the returned
     flags with ‘O_ACCMODE’ to compare them.

     In case of an error, ‘fcntl’ returns -1.  The following ‘errno’
     error conditions are defined for this command:

     ‘EBADF’
          The FILEDES argument is invalid.

 -- Macro: int F_SETFL
     This macro is used as the COMMAND argument to ‘fcntl’, to set the
     file status flags for the open file corresponding to the FILEDES
     argument.  This command requires a third ‘int’ argument to specify
     the new flags, so the call looks like this:

          fcntl (FILEDES, F_SETFL, NEW-FLAGS)

     You can’t change the access mode for the file in this way; that is,
     whether the file descriptor was opened for reading or writing.

     The normal return value from ‘fcntl’ with this command is an
     unspecified value other than -1, which indicates an error.  The
     error conditions are the same as for the ‘F_GETFL’ command.

   If you want to modify the file status flags, you should get the
current flags with ‘F_GETFL’ and modify the value.  Don’t assume that
the flags listed here are the only ones that are implemented; your
program may be run years from now and more flags may exist then.  For
example, here is a function to set or clear the flag ‘O_NONBLOCK’
without altering any other flags:

     /* Set the ‘O_NONBLOCK’ flag of DESC if VALUE is nonzero,
        or clear the flag if VALUE is 0.
        Return 0 on success, or -1 on error with ‘errno’ set. */

     int
     set_nonblock_flag (int desc, int value)
     {
       int oldflags = fcntl (desc, F_GETFL, 0);
       /* If reading the flags failed, return error indication now. */
       if (oldflags == -1)
         return -1;
       /* Set just the flag we want to set. */
       if (value != 0)
         oldflags |= O_NONBLOCK;
       else
         oldflags &= ~O_NONBLOCK;
       /* Store modified flag word in the descriptor. */
       return fcntl (desc, F_SETFL, oldflags);
     }


File: libc.info,  Node: File Locks,  Next: Open File Description Locks,  Prev: File Status Flags,  Up: Low-Level I/O

13.15 File Locks
================

This section describes record locks that are associated with the
process.  There is also a different type of record lock that is
associated with the open file description instead of the process.  *Note
Open File Description Locks::.

   The remaining ‘fcntl’ commands are used to support "record locking",
which permits multiple cooperating programs to prevent each other from
simultaneously accessing parts of a file in error-prone ways.

   An "exclusive" or "write" lock gives a process exclusive access for
writing to the specified part of the file.  While a write lock is in
place, no other process can lock that part of the file.

   A "shared" or "read" lock prohibits any other process from requesting
a write lock on the specified part of the file.  However, other
processes can request read locks.

   The ‘read’ and ‘write’ functions do not actually check to see whether
there are any locks in place.  If you want to implement a locking
protocol for a file shared by multiple processes, your application must
do explicit ‘fcntl’ calls to request and clear locks at the appropriate
points.

   Locks are associated with processes.  A process can only have one
kind of lock set for each byte of a given file.  When any file
descriptor for that file is closed by the process, all of the locks that
process holds on that file are released, even if the locks were made
using other descriptors that remain open.  Likewise, locks are released
when a process exits, and are not inherited by child processes created
using ‘fork’ (*note Creating a Process::).

   When making a lock, use a ‘struct flock’ to specify what kind of lock
and where.  This data type and the associated macros for the ‘fcntl’
function are declared in the header file ‘fcntl.h’.

 -- Data Type: struct flock
     This structure is used with the ‘fcntl’ function to describe a file
     lock.  It has these members:

     ‘short int l_type’
          Specifies the type of the lock; one of ‘F_RDLCK’, ‘F_WRLCK’,
          or ‘F_UNLCK’.

     ‘short int l_whence’
          This corresponds to the WHENCE argument to ‘fseek’ or ‘lseek’,
          and specifies what the offset is relative to.  Its value can
          be one of ‘SEEK_SET’, ‘SEEK_CUR’, or ‘SEEK_END’.

     ‘off_t l_start’
          This specifies the offset of the start of the region to which
          the lock applies, and is given in bytes relative to the point
          specified by the ‘l_whence’ member.

     ‘off_t l_len’
          This specifies the length of the region to be locked.  A value
          of ‘0’ is treated specially; it means the region extends to
          the end of the file.

     ‘pid_t l_pid’
          This field is the process ID (*note Process Creation
          Concepts::) of the process holding the lock.  It is filled in
          by calling ‘fcntl’ with the ‘F_GETLK’ command, but is ignored
          when making a lock.  If the conflicting lock is an open file
          description lock (*note Open File Description Locks::), then
          this field will be set to -1.

 -- Macro: int F_GETLK
     This macro is used as the COMMAND argument to ‘fcntl’, to specify
     that it should get information about a lock.  This command requires
     a third argument of type ‘struct flock *’ to be passed to ‘fcntl’,
     so that the form of the call is:

          fcntl (FILEDES, F_GETLK, LOCKP)

     If there is a lock already in place that would block the lock
     described by the LOCKP argument, information about that lock
     overwrites ‘*LOCKP’.  Existing locks are not reported if they are
     compatible with making a new lock as specified.  Thus, you should
     specify a lock type of ‘F_WRLCK’ if you want to find out about both
     read and write locks, or ‘F_RDLCK’ if you want to find out about
     write locks only.

     There might be more than one lock affecting the region specified by
     the LOCKP argument, but ‘fcntl’ only returns information about one
     of them.  The ‘l_whence’ member of the LOCKP structure is set to
     ‘SEEK_SET’ and the ‘l_start’ and ‘l_len’ fields set to identify the
     locked region.

     If no lock applies, the only change to the LOCKP structure is to
     update the ‘l_type’ to a value of ‘F_UNLCK’.

     The normal return value from ‘fcntl’ with this command is an
     unspecified value other than -1, which is reserved to indicate an
     error.  The following ‘errno’ error conditions are defined for this
     command:

     ‘EBADF’
          The FILEDES argument is invalid.

     ‘EINVAL’
          Either the LOCKP argument doesn’t specify valid lock
          information, or the file associated with FILEDES doesn’t
          support locks.

 -- Macro: int F_SETLK
     This macro is used as the COMMAND argument to ‘fcntl’, to specify
     that it should set or clear a lock.  This command requires a third
     argument of type ‘struct flock *’ to be passed to ‘fcntl’, so that
     the form of the call is:

          fcntl (FILEDES, F_SETLK, LOCKP)

     If the process already has a lock on any part of the region, the
     old lock on that part is replaced with the new lock.  You can
     remove a lock by specifying a lock type of ‘F_UNLCK’.

     If the lock cannot be set, ‘fcntl’ returns immediately with a value
     of -1.  This function does not block while waiting for other
     processes to release locks.  If ‘fcntl’ succeeds, it returns a
     value other than -1.

     The following ‘errno’ error conditions are defined for this
     function:

     ‘EAGAIN’
     ‘EACCES’
          The lock cannot be set because it is blocked by an existing
          lock on the file.  Some systems use ‘EAGAIN’ in this case, and
          other systems use ‘EACCES’; your program should treat them
          alike, after ‘F_SETLK’.  (GNU/Linux and GNU/Hurd systems
          always use ‘EAGAIN’.)

     ‘EBADF’
          Either: the FILEDES argument is invalid; you requested a read
          lock but the FILEDES is not open for read access; or, you
          requested a write lock but the FILEDES is not open for write
          access.

     ‘EINVAL’
          Either the LOCKP argument doesn’t specify valid lock
          information, or the file associated with FILEDES doesn’t
          support locks.

     ‘ENOLCK’
          The system has run out of file lock resources; there are
          already too many file locks in place.

          Well-designed file systems never report this error, because
          they have no limitation on the number of locks.  However, you
          must still take account of the possibility of this error, as
          it could result from network access to a file system on
          another machine.

 -- Macro: int F_SETLKW
     This macro is used as the COMMAND argument to ‘fcntl’, to specify
     that it should set or clear a lock.  It is just like the ‘F_SETLK’
     command, but causes the process to block (or wait) until the
     request can be specified.

     This command requires a third argument of type ‘struct flock *’, as
     for the ‘F_SETLK’ command.

     The ‘fcntl’ return values and errors are the same as for the
     ‘F_SETLK’ command, but these additional ‘errno’ error conditions
     are defined for this command:

     ‘EINTR’
          The function was interrupted by a signal while it was waiting.
          *Note Interrupted Primitives::.

     ‘EDEADLK’
          The specified region is being locked by another process.  But
          that process is waiting to lock a region which the current
          process has locked, so waiting for the lock would result in
          deadlock.  The system does not guarantee that it will detect
          all such conditions, but it lets you know if it notices one.

   The following macros are defined for use as values for the ‘l_type’
member of the ‘flock’ structure.  The values are integer constants.

‘F_RDLCK’
     This macro is used to specify a read (or shared) lock.

‘F_WRLCK’
     This macro is used to specify a write (or exclusive) lock.

‘F_UNLCK’
     This macro is used to specify that the region is unlocked.

   As an example of a situation where file locking is useful, consider a
program that can be run simultaneously by several different users, that
logs status information to a common file.  One example of such a program
might be a game that uses a file to keep track of high scores.  Another
example might be a program that records usage or accounting information
for billing purposes.

   Having multiple copies of the program simultaneously writing to the
file could cause the contents of the file to become mixed up.  But you
can prevent this kind of problem by setting a write lock on the file
before actually writing to the file.

   If the program also needs to read the file and wants to make sure
that the contents of the file are in a consistent state, then it can
also use a read lock.  While the read lock is set, no other process can
lock that part of the file for writing.

   Remember that file locks are only an _advisory_ protocol for
controlling access to a file.  There is still potential for access to
the file by programs that don’t use the lock protocol.


File: libc.info,  Node: Open File Description Locks,  Next: Open File Description Locks Example,  Prev: File Locks,  Up: Low-Level I/O

13.16 Open File Description Locks
=================================

In contrast to process-associated record locks (*note File Locks::),
open file description record locks are associated with an open file
description rather than a process.

   Using ‘fcntl’ to apply an open file description lock on a region that
already has an existing open file description lock that was created via
the same file descriptor will never cause a lock conflict.

   Open file description locks are also inherited by child processes
across ‘fork’, or ‘clone’ with ‘CLONE_FILES’ set (*note Creating a
Process::), along with the file descriptor.

   It is important to distinguish between the open file _description_
(an instance of an open file, usually created by a call to ‘open’) and
an open file _descriptor_, which is a numeric value that refers to the
open file description.  The locks described here are associated with the
open file _description_ and not the open file _descriptor_.

   Using ‘dup’ (*note Duplicating Descriptors::) to copy a file
descriptor does not give you a new open file description, but rather
copies a reference to an existing open file description and assigns it
to a new file descriptor.  Thus, open file description locks set on a
file descriptor cloned by ‘dup’ will never conflict with open file
description locks set on the original descriptor since they refer to the
same open file description.  Depending on the range and type of lock
involved, the original lock may be modified by a ‘F_OFD_SETLK’ or
‘F_OFD_SETLKW’ command in this situation however.

   Open file description locks always conflict with process-associated
locks, even if acquired by the same process or on the same open file
descriptor.

   Open file description locks use the same ‘struct flock’ as
process-associated locks as an argument (*note File Locks::) and the
macros for the ‘command’ values are also declared in the header file
‘fcntl.h’.  To use them, the macro ‘_GNU_SOURCE’ must be defined prior
to including any header file.

   In contrast to process-associated locks, any ‘struct flock’ used as
an argument to open file description lock commands must have the ‘l_pid’
value set to 0.  Also, when returning information about an open file
description lock in a ‘F_GETLK’ or ‘F_OFD_GETLK’ request, the ‘l_pid’
field in ‘struct flock’ will be set to -1 to indicate that the lock is
not associated with a process.

   When the same ‘struct flock’ is reused as an argument to a
‘F_OFD_SETLK’ or ‘F_OFD_SETLKW’ request after being used for an
‘F_OFD_GETLK’ request, it is necessary to inspect and reset the ‘l_pid’
field to 0.

 -- Macro: int F_OFD_GETLK
     This macro is used as the COMMAND argument to ‘fcntl’, to specify
     that it should get information about a lock.  This command requires
     a third argument of type ‘struct flock *’ to be passed to ‘fcntl’,
     so that the form of the call is:

          fcntl (FILEDES, F_OFD_GETLK, LOCKP)

     If there is a lock already in place that would block the lock
     described by the LOCKP argument, information about that lock is
     written to ‘*LOCKP’.  Existing locks are not reported if they are
     compatible with making a new lock as specified.  Thus, you should
     specify a lock type of ‘F_WRLCK’ if you want to find out about both
     read and write locks, or ‘F_RDLCK’ if you want to find out about
     write locks only.

     There might be more than one lock affecting the region specified by
     the LOCKP argument, but ‘fcntl’ only returns information about one
     of them.  Which lock is returned in this situation is undefined.

     The ‘l_whence’ member of the LOCKP structure are set to ‘SEEK_SET’
     and the ‘l_start’ and ‘l_len’ fields are set to identify the locked
     region.

     If no conflicting lock exists, the only change to the LOCKP
     structure is to update the ‘l_type’ field to the value ‘F_UNLCK’.

     The normal return value from ‘fcntl’ with this command is either 0
     on success or -1, which indicates an error.  The following ‘errno’
     error conditions are defined for this command:

     ‘EBADF’
          The FILEDES argument is invalid.

     ‘EINVAL’
          Either the LOCKP argument doesn’t specify valid lock
          information, the operating system kernel doesn’t support open
          file description locks, or the file associated with FILEDES
          doesn’t support locks.

 -- Macro: int F_OFD_SETLK
     This macro is used as the COMMAND argument to ‘fcntl’, to specify
     that it should set or clear a lock.  This command requires a third
     argument of type ‘struct flock *’ to be passed to ‘fcntl’, so that
     the form of the call is:

          fcntl (FILEDES, F_OFD_SETLK, LOCKP)

     If the open file already has a lock on any part of the region, the
     old lock on that part is replaced with the new lock.  You can
     remove a lock by specifying a lock type of ‘F_UNLCK’.

     If the lock cannot be set, ‘fcntl’ returns immediately with a value
     of -1.  This command does not wait for other tasks to release
     locks.  If ‘fcntl’ succeeds, it returns 0.

     The following ‘errno’ error conditions are defined for this
     command:

     ‘EAGAIN’
          The lock cannot be set because it is blocked by an existing
          lock on the file.

     ‘EBADF’
          Either: the FILEDES argument is invalid; you requested a read
          lock but the FILEDES is not open for read access; or, you
          requested a write lock but the FILEDES is not open for write
          access.

     ‘EINVAL’
          Either the LOCKP argument doesn’t specify valid lock
          information, the operating system kernel doesn’t support open
          file description locks, or the file associated with FILEDES
          doesn’t support locks.

     ‘ENOLCK’
          The system has run out of file lock resources; there are
          already too many file locks in place.

          Well-designed file systems never report this error, because
          they have no limitation on the number of locks.  However, you
          must still take account of the possibility of this error, as
          it could result from network access to a file system on
          another machine.

 -- Macro: int F_OFD_SETLKW
     This macro is used as the COMMAND argument to ‘fcntl’, to specify
     that it should set or clear a lock.  It is just like the
     ‘F_OFD_SETLK’ command, but causes the process to wait until the
     request can be completed.

     This command requires a third argument of type ‘struct flock *’, as
     for the ‘F_OFD_SETLK’ command.

     The ‘fcntl’ return values and errors are the same as for the
     ‘F_OFD_SETLK’ command, but these additional ‘errno’ error
     conditions are defined for this command:

     ‘EINTR’
          The function was interrupted by a signal while it was waiting.
          *Note Interrupted Primitives::.

   Open file description locks are useful in the same sorts of
situations as process-associated locks.  They can also be used to
synchronize file access between threads within the same process by
having each thread perform its own ‘open’ of the file, to obtain its own
open file description.

   Because open file description locks are automatically freed only upon
closing the last file descriptor that refers to the open file
description, this locking mechanism avoids the possibility that locks
are inadvertently released due to a library routine opening and closing
a file without the application being aware.

   As with process-associated locks, open file description locks are
advisory.


File: libc.info,  Node: Open File Description Locks Example,  Next: Interrupt Input,  Prev: Open File Description Locks,  Up: Low-Level I/O

13.17 Open File Description Locks Example
=========================================

Here is an example of using open file description locks in a threaded
program.  If this program used process-associated locks, then it would
be subject to data corruption because process-associated locks are
shared by the threads inside a process, and thus cannot be used by one
thread to lock out another thread in the same process.

   Proper error handling has been omitted in the following program for
brevity.


     #define _GNU_SOURCE
     #include <stdio.h>
     #include <sys/types.h>
     #include <sys/stat.h>
     #include <unistd.h>
     #include <fcntl.h>
     #include <pthread.h>

     #define FILENAME        "/tmp/foo"
     #define NUM_THREADS     3
     #define ITERATIONS      5

     void *
     thread_start (void *arg)
     {
       int i, fd, len;
       long tid = (long) arg;
       char buf[256];
       struct flock lck = {
         .l_whence = SEEK_SET,
         .l_start = 0,
         .l_len = 1,
       };

       fd = open ("/tmp/foo", O_RDWR | O_CREAT, 0666);

       for (i = 0; i < ITERATIONS; i++)
         {
           lck.l_type = F_WRLCK;
           fcntl (fd, F_OFD_SETLKW, &lck);

           len = sprintf (buf, "%d: tid=%ld fd=%d\n", i, tid, fd);

           lseek (fd, 0, SEEK_END);
           write (fd, buf, len);
           fsync (fd);

           lck.l_type = F_UNLCK;
           fcntl (fd, F_OFD_SETLK, &lck);

           /* sleep to ensure lock is yielded to another thread */
           usleep (1);
         }
       pthread_exit (NULL);
     }

     int
     main (int argc, char **argv)
     {
       long i;
       pthread_t threads[NUM_THREADS];

       truncate (FILENAME, 0);

       for (i = 0; i < NUM_THREADS; i++)
         pthread_create (&threads[i], NULL, thread_start, (void *) i);

       pthread_exit (NULL);
       return 0;
     }

   This example creates three threads each of which loops five times,
appending to the file.  Access to the file is serialized via open file
description locks.  If we compile and run the above program, we’ll end
up with /tmp/foo that has 15 lines in it.

   If we, however, were to replace the ‘F_OFD_SETLK’ and ‘F_OFD_SETLKW’
commands with their process-associated lock equivalents, the locking
essentially becomes a noop since it is all done within the context of
the same process.  That leads to data corruption (typically manifested
as missing lines) as some threads race in and overwrite the data written
by others.


File: libc.info,  Node: Interrupt Input,  Next: IOCTLs,  Prev: Open File Description Locks Example,  Up: Low-Level I/O

13.18 Interrupt-Driven Input
============================

If you set the ‘O_ASYNC’ status flag on a file descriptor (*note File
Status Flags::), a ‘SIGIO’ signal is sent whenever input or output
becomes possible on that file descriptor.  The process or process group
to receive the signal can be selected by using the ‘F_SETOWN’ command to
the ‘fcntl’ function.  If the file descriptor is a socket, this also
selects the recipient of ‘SIGURG’ signals that are delivered when
out-of-band data arrives on that socket; see *note Out-of-Band Data::.
(‘SIGURG’ is sent in any situation where ‘select’ would report the
socket as having an “exceptional condition”.  *Note Waiting for I/O::.)

   If the file descriptor corresponds to a terminal device, then ‘SIGIO’
signals are sent to the foreground process group of the terminal.  *Note
Job Control::.

   The symbols in this section are defined in the header file ‘fcntl.h’.

 -- Macro: int F_GETOWN
     This macro is used as the COMMAND argument to ‘fcntl’, to specify
     that it should get information about the process or process group
     to which ‘SIGIO’ signals are sent.  (For a terminal, this is
     actually the foreground process group ID, which you can get using
     ‘tcgetpgrp’; see *note Terminal Access Functions::.)

     The return value is interpreted as a process ID; if negative, its
     absolute value is the process group ID.

     The following ‘errno’ error condition is defined for this command:

     ‘EBADF’
          The FILEDES argument is invalid.

 -- Macro: int F_SETOWN
     This macro is used as the COMMAND argument to ‘fcntl’, to specify
     that it should set the process or process group to which ‘SIGIO’
     signals are sent.  This command requires a third argument of type
     ‘pid_t’ to be passed to ‘fcntl’, so that the form of the call is:

          fcntl (FILEDES, F_SETOWN, PID)

     The PID argument should be a process ID. You can also pass a
     negative number whose absolute value is a process group ID.

     The return value from ‘fcntl’ with this command is -1 in case of
     error and some other value if successful.  The following ‘errno’
     error conditions are defined for this command:

     ‘EBADF’
          The FILEDES argument is invalid.

     ‘ESRCH’
          There is no process or process group corresponding to PID.


File: libc.info,  Node: IOCTLs,  Prev: Interrupt Input,  Up: Low-Level I/O

13.19 Generic I/O Control operations
====================================

GNU systems can handle most input/output operations on many different
devices and objects in terms of a few file primitives - ‘read’, ‘write’
and ‘lseek’.  However, most devices also have a few peculiar operations
which do not fit into this model.  Such as:

   • Changing the character font used on a terminal.

   • Telling a magnetic tape system to rewind or fast forward.  (Since
     they cannot move in byte increments, ‘lseek’ is inapplicable).

   • Ejecting a disk from a drive.

   • Playing an audio track from a CD-ROM drive.

   • Maintaining routing tables for a network.

   Although some such objects such as sockets and terminals (1) have
special functions of their own, it would not be practical to create
functions for all these cases.

   Instead these minor operations, known as "IOCTL"s, are assigned code
numbers and multiplexed through the ‘ioctl’ function, defined in
‘sys/ioctl.h’.  The code numbers themselves are defined in many
different headers.

 -- Function: int ioctl (int FILEDES, int COMMAND, …)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘ioctl’ function performs the generic I/O operation COMMAND on
     FILEDES.

     A third argument is usually present, either a single number or a
     pointer to a structure.  The meaning of this argument, the returned
     value, and any error codes depends upon the command used.  Often -1
     is returned for a failure.

   On some systems, IOCTLs used by different devices share the same
numbers.  Thus, although use of an inappropriate IOCTL _usually_ only
produces an error, you should not attempt to use device-specific IOCTLs
on an unknown device.

   Most IOCTLs are OS-specific and/or only used in special system
utilities, and are thus beyond the scope of this document.  For an
example of the use of an IOCTL, see *note Out-of-Band Data::.

   ---------- Footnotes ----------

   (1) Actually, the terminal-specific functions are implemented with
IOCTLs on many platforms.


File: libc.info,  Node: File System Interface,  Next: Pipes and FIFOs,  Prev: Low-Level I/O,  Up: Top

14 File System Interface
************************

This chapter describes the GNU C Library’s functions for manipulating
files.  Unlike the input and output functions (*note I/O on Streams::;
*note Low-Level I/O::), these functions are concerned with operating on
the files themselves rather than on their contents.

   Among the facilities described in this chapter are functions for
examining or modifying directories, functions for renaming and deleting
files, and functions for examining and setting file attributes such as
access permissions and modification times.

* Menu:

* Working Directory::           This is used to resolve relative
				 file names.
* Accessing Directories::       Finding out what files a directory
				 contains.
* Working with Directory Trees:: Apply actions to all files or a selectable
                                 subset of a directory hierarchy.
* Hard Links::                  Adding alternate names to a file.
* Symbolic Links::              A file that “points to” a file name.
* Deleting Files::              How to delete a file, and what that means.
* Renaming Files::              Changing a file’s name.
* Creating Directories::        A system call just for creating a directory.
* File Attributes::             Attributes of individual files.
* Making Special Files::        How to create special files.
* Temporary Files::             Naming and creating temporary files.


File: libc.info,  Node: Working Directory,  Next: Accessing Directories,  Up: File System Interface

14.1 Working Directory
======================

Each process has associated with it a directory, called its "current
working directory" or simply "working directory", that is used in the
resolution of relative file names (*note File Name Resolution::).

   When you log in and begin a new session, your working directory is
initially set to the home directory associated with your login account
in the system user database.  You can find any user’s home directory
using the ‘getpwuid’ or ‘getpwnam’ functions; see *note User Database::.

   Users can change the working directory using shell commands like
‘cd’.  The functions described in this section are the primitives used
by those commands and by other programs for examining and changing the
working directory.

   Prototypes for these functions are declared in the header file
‘unistd.h’.

 -- Function: char * getcwd (char *BUFFER, size_t SIZE)
     Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | *Note
     POSIX Safety Concepts::.

     The ‘getcwd’ function returns an absolute file name representing
     the current working directory, storing it in the character array
     BUFFER that you provide.  The SIZE argument is how you tell the
     system the allocation size of BUFFER.

     The GNU C Library version of this function also permits you to
     specify a null pointer for the BUFFER argument.  Then ‘getcwd’
     allocates a buffer automatically, as with ‘malloc’ (*note
     Unconstrained Allocation::).  If the SIZE is greater than zero,
     then the buffer is that large; otherwise, the buffer is as large as
     necessary to hold the result.

     The return value is BUFFER on success and a null pointer on
     failure.  The following ‘errno’ error conditions are defined for
     this function:

     ‘EINVAL’
          The SIZE argument is zero and BUFFER is not a null pointer.

     ‘ERANGE’
          The SIZE argument is less than the length of the working
          directory name.  You need to allocate a bigger array and try
          again.

     ‘EACCES’
          Permission to read or search a component of the file name was
          denied.

   You could implement the behavior of GNU’s ‘getcwd (NULL, 0)’ using
only the standard behavior of ‘getcwd’:

     char *
     gnu_getcwd ()
     {
       size_t size = 100;

       while (1)
         {
           char *buffer = (char *) xmalloc (size);
           if (getcwd (buffer, size) == buffer)
             return buffer;
           free (buffer);
           if (errno != ERANGE)
             return 0;
           size *= 2;
         }
     }

*Note Malloc Examples::, for information about ‘xmalloc’, which is not a
library function but is a customary name used in most GNU software.

 -- Deprecated Function: char * getwd (char *BUFFER)
     Preliminary: | MT-Safe | AS-Unsafe heap i18n | AC-Unsafe mem fd |
     *Note POSIX Safety Concepts::.

     This is similar to ‘getcwd’, but has no way to specify the size of
     the buffer.  The GNU C Library provides ‘getwd’ only for backwards
     compatibility with BSD.

     The BUFFER argument should be a pointer to an array at least
     ‘PATH_MAX’ bytes long (*note Limits for Files::).  On GNU/Hurd
     systems there is no limit to the size of a file name, so this is
     not necessarily enough space to contain the directory name.  That
     is why this function is deprecated.

 -- Function: char * get_current_dir_name (void)
     Preliminary: | MT-Safe env | AS-Unsafe heap | AC-Unsafe mem fd |
     *Note POSIX Safety Concepts::.

     This ‘get_current_dir_name’ function is basically equivalent to
     ‘getcwd (NULL, 0)’.  The only difference is that the value of the
     ‘PWD’ variable is returned if this value is correct.  This is a
     subtle difference which is visible if the path described by the
     ‘PWD’ value is using one or more symbol links in which case the
     value returned by ‘getcwd’ can resolve the symbol links and
     therefore yield a different result.

     This function is a GNU extension.

 -- Function: int chdir (const char *FILENAME)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is used to set the process’s working directory to
     FILENAME.

     The normal, successful return value from ‘chdir’ is ‘0’.  A value
     of ‘-1’ is returned to indicate an error.  The ‘errno’ error
     conditions defined for this function are the usual file name syntax
     errors (*note File Name Errors::), plus ‘ENOTDIR’ if the file
     FILENAME is not a directory.

 -- Function: int fchdir (int FILEDES)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is used to set the process’s working directory to
     directory associated with the file descriptor FILEDES.

     The normal, successful return value from ‘fchdir’ is ‘0’.  A value
     of ‘-1’ is returned to indicate an error.  The following ‘errno’
     error conditions are defined for this function:

     ‘EACCES’
          Read permission is denied for the directory named by
          ‘dirname’.

     ‘EBADF’
          The FILEDES argument is not a valid file descriptor.

     ‘ENOTDIR’
          The file descriptor FILEDES is not associated with a
          directory.

     ‘EINTR’
          The function call was interrupt by a signal.

     ‘EIO’
          An I/O error occurred.


File: libc.info,  Node: Accessing Directories,  Next: Working with Directory Trees,  Prev: Working Directory,  Up: File System Interface

14.2 Accessing Directories
==========================

The facilities described in this section let you read the contents of a
directory file.  This is useful if you want your program to list all the
files in a directory, perhaps as part of a menu.

   The ‘opendir’ function opens a "directory stream" whose elements are
directory entries.  Alternatively ‘fdopendir’ can be used which can have
advantages if the program needs to have more control over the way the
directory is opened for reading.  This allows, for instance, to pass the
‘O_NOATIME’ flag to ‘open’.

   You use the ‘readdir’ function on the directory stream to retrieve
these entries, represented as ‘struct dirent’ objects.  The name of the
file for each entry is stored in the ‘d_name’ member of this structure.
There are obvious parallels here to the stream facilities for ordinary
files, described in *note I/O on Streams::.

* Menu:

* Directory Entries::           Format of one directory entry.
* Opening a Directory::         How to open a directory stream.
* Reading/Closing Directory::   How to read directory entries from the stream.
* Simple Directory Lister::     A very simple directory listing program.
* Random Access Directory::     Rereading part of the directory
                                 already read with the same stream.
* Scanning Directory Content::  Get entries for user selected subset of
                                 contents in given directory.
* Simple Directory Lister Mark II::  Revised version of the program.


File: libc.info,  Node: Directory Entries,  Next: Opening a Directory,  Up: Accessing Directories

14.2.1 Format of a Directory Entry
----------------------------------

This section describes what you find in a single directory entry, as you
might obtain it from a directory stream.  All the symbols are declared
in the header file ‘dirent.h’.

 -- Data Type: struct dirent
     This is a structure type used to return information about directory
     entries.  It contains the following fields:

     ‘char d_name[]’
          This is the null-terminated file name component.  This is the
          only field you can count on in all POSIX systems.

     ‘ino_t d_fileno’
          This is the file serial number.  For BSD compatibility, you
          can also refer to this member as ‘d_ino’.  On GNU/Linux and
          GNU/Hurd systems and most POSIX systems, for most files this
          the same as the ‘st_ino’ member that ‘stat’ will return for
          the file.  *Note File Attributes::.

     ‘unsigned char d_namlen’
          This is the length of the file name, not including the
          terminating null character.  Its type is ‘unsigned char’
          because that is the integer type of the appropriate size.
          This member is a BSD extension.  The symbol
          ‘_DIRENT_HAVE_D_NAMLEN’ is defined if this member is
          available.

     ‘unsigned char d_type’
          This is the type of the file, possibly unknown.  The following
          constants are defined for its value:

          ‘DT_UNKNOWN’
               The type is unknown.  Only some filesystems have full
               support to return the type of the file, others might
               always return this value.

          ‘DT_REG’
               A regular file.

          ‘DT_DIR’
               A directory.

          ‘DT_FIFO’
               A named pipe, or FIFO. *Note FIFO Special Files::.

          ‘DT_SOCK’
               A local-domain socket.

          ‘DT_CHR’
               A character device.

          ‘DT_BLK’
               A block device.

          ‘DT_LNK’
               A symbolic link.

          This member is a BSD extension.  The symbol
          ‘_DIRENT_HAVE_D_TYPE’ is defined if this member is available.
          On systems where it is used, it corresponds to the file type
          bits in the ‘st_mode’ member of ‘struct stat’.  If the value
          cannot be determined the member value is DT_UNKNOWN. These two
          macros convert between ‘d_type’ values and ‘st_mode’ values:

           -- Function: int IFTODT (mode_t MODE)
               Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX
               Safety Concepts::.

               This returns the ‘d_type’ value corresponding to MODE.

           -- Function: mode_t DTTOIF (int DTYPE)
               Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX
               Safety Concepts::.

               This returns the ‘st_mode’ value corresponding to DTYPE.

     This structure may contain additional members in the future.  Their
     availability is always announced in the compilation environment by
     a macro named ‘_DIRENT_HAVE_D_XXX’ where XXX is replaced by the
     name of the new member.  For instance, the member ‘d_reclen’
     available on some systems is announced through the macro
     ‘_DIRENT_HAVE_D_RECLEN’.

     When a file has multiple names, each name has its own directory
     entry.  The only way you can tell that the directory entries belong
     to a single file is that they have the same value for the
     ‘d_fileno’ field.

     File attributes such as size, modification times etc., are part of
     the file itself, not of any particular directory entry.  *Note File
     Attributes::.


File: libc.info,  Node: Opening a Directory,  Next: Reading/Closing Directory,  Prev: Directory Entries,  Up: Accessing Directories

14.2.2 Opening a Directory Stream
---------------------------------

This section describes how to open a directory stream.  All the symbols
are declared in the header file ‘dirent.h’.

 -- Data Type: DIR
     The ‘DIR’ data type represents a directory stream.

   You shouldn’t ever allocate objects of the ‘struct dirent’ or ‘DIR’
data types, since the directory access functions do that for you.
Instead, you refer to these objects using the pointers returned by the
following functions.

 -- Function: DIR * opendir (const char *DIRNAME)
     Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | *Note
     POSIX Safety Concepts::.

     The ‘opendir’ function opens and returns a directory stream for
     reading the directory whose file name is DIRNAME.  The stream has
     type ‘DIR *’.

     If unsuccessful, ‘opendir’ returns a null pointer.  In addition to
     the usual file name errors (*note File Name Errors::), the
     following ‘errno’ error conditions are defined for this function:

     ‘EACCES’
          Read permission is denied for the directory named by
          ‘dirname’.

     ‘EMFILE’
          The process has too many files open.

     ‘ENFILE’
          The entire system, or perhaps the file system which contains
          the directory, cannot support any additional open files at the
          moment.  (This problem cannot happen on GNU/Hurd systems.)

     ‘ENOMEM’
          Not enough memory available.

     The ‘DIR’ type is typically implemented using a file descriptor,
     and the ‘opendir’ function in terms of the ‘open’ function.  *Note
     Low-Level I/O::.  Directory streams and the underlying file
     descriptors are closed on ‘exec’ (*note Executing a File::).

   The directory which is opened for reading by ‘opendir’ is identified
by the name.  In some situations this is not sufficient.  Or the way
‘opendir’ implicitly creates a file descriptor for the directory is not
the way a program might want it.  In these cases an alternative
interface can be used.

 -- Function: DIR * fdopendir (int FD)
     Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | *Note
     POSIX Safety Concepts::.

     The ‘fdopendir’ function works just like ‘opendir’ but instead of
     taking a file name and opening a file descriptor for the directory
     the caller is required to provide a file descriptor.  This file
     descriptor is then used in subsequent uses of the returned
     directory stream object.

     The caller must make sure the file descriptor is associated with a
     directory and it allows reading.

     If the ‘fdopendir’ call returns successfully the file descriptor is
     now under the control of the system.  It can be used in the same
     way the descriptor implicitly created by ‘opendir’ can be used but
     the program must not close the descriptor.

     In case the function is unsuccessful it returns a null pointer and
     the file descriptor remains to be usable by the program.  The
     following ‘errno’ error conditions are defined for this function:

     ‘EBADF’
          The file descriptor is not valid.

     ‘ENOTDIR’
          The file descriptor is not associated with a directory.

     ‘EINVAL’
          The descriptor does not allow reading the directory content.

     ‘ENOMEM’
          Not enough memory available.

   In some situations it can be desirable to get hold of the file
descriptor which is created by the ‘opendir’ call.  For instance, to
switch the current working directory to the directory just read the
‘fchdir’ function could be used.  Historically the ‘DIR’ type was
exposed and programs could access the fields.  This does not happen in
the GNU C Library.  Instead a separate function is provided to allow
access.

 -- Function: int dirfd (DIR *DIRSTREAM)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The function ‘dirfd’ returns the file descriptor associated with
     the directory stream DIRSTREAM.  This descriptor can be used until
     the directory is closed with ‘closedir’.  If the directory stream
     implementation is not using file descriptors the return value is
     ‘-1’.


File: libc.info,  Node: Reading/Closing Directory,  Next: Simple Directory Lister,  Prev: Opening a Directory,  Up: Accessing Directories

14.2.3 Reading and Closing a Directory Stream
---------------------------------------------

This section describes how to read directory entries from a directory
stream, and how to close the stream when you are done with it.  All the
symbols are declared in the header file ‘dirent.h’.

 -- Function: struct dirent * readdir (DIR *DIRSTREAM)
     Preliminary: | MT-Unsafe race:dirstream | AS-Unsafe lock |
     AC-Unsafe lock | *Note POSIX Safety Concepts::.

     This function reads the next entry from the directory.  It normally
     returns a pointer to a structure containing information about the
     file.  This structure is associated with the DIRSTREAM handle and
     can be rewritten by a subsequent call.

     *Portability Note:* On some systems ‘readdir’ may not return
     entries for ‘.’ and ‘..’, even though these are always valid file
     names in any directory.  *Note File Name Resolution::.

     If there are no more entries in the directory or an error is
     detected, ‘readdir’ returns a null pointer.  The following ‘errno’
     error conditions are defined for this function:

     ‘EBADF’
          The DIRSTREAM argument is not valid.

     To distinguish between an end-of-directory condition or an error,
     you must set ‘errno’ to zero before calling ‘readdir’.  To avoid
     entering an infinite loop, you should stop reading from the
     directory after the first error.

     In POSIX.1-2008, ‘readdir’ is not thread-safe.  In the GNU C
     Library implementation, it is safe to call ‘readdir’ concurrently
     on different DIRSTREAMs, but multiple threads accessing the same
     DIRSTREAM result in undefined behavior.  ‘readdir_r’ is a fully
     thread-safe alternative, but suffers from poor portability (see
     below).  It is recommended that you use ‘readdir’, with external
     locking if multiple threads access the same DIRSTREAM.

 -- Function: int readdir_r (DIR *DIRSTREAM, struct dirent *ENTRY,
          struct dirent **RESULT)
     Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock | *Note
     POSIX Safety Concepts::.

     This function is a version of ‘readdir’ which performs internal
     locking.  Like ‘readdir’ it returns the next entry from the
     directory.  To prevent conflicts between simultaneously running
     threads the result is stored inside the ENTRY object.

     *Portability Note:* ‘readdir_r’ is deprecated.  It is recommended
     to use ‘readdir’ instead of ‘readdir_r’ for the following reasons:

        • On systems which do not define ‘NAME_MAX’, it may not be
          possible to use ‘readdir_r’ safely because the caller does not
          specify the length of the buffer for the directory entry.

        • On some systems, ‘readdir_r’ cannot read directory entries
          with very long names.  If such a name is encountered, the GNU
          C Library implementation of ‘readdir_r’ returns with an error
          code of ‘ENAMETOOLONG’ after the final directory entry has
          been read.  On other systems, ‘readdir_r’ may return
          successfully, but the ‘d_name’ member may not be
          NUL-terminated or may be truncated.

        • POSIX-1.2008 does not guarantee that ‘readdir’ is thread-safe,
          even when access to the same DIRSTREAM is serialized.  But in
          current implementations (including the GNU C Library), it is
          safe to call ‘readdir’ concurrently on different DIRSTREAMs,
          so there is no need to use ‘readdir_r’ in most multi-threaded
          programs.  In the rare case that multiple threads need to read
          from the same DIRSTREAM, it is still better to use ‘readdir’
          and external synchronization.

        • It is expected that future versions of POSIX will obsolete
          ‘readdir_r’ and mandate the level of thread safety for
          ‘readdir’ which is provided by the GNU C Library and other
          implementations today.

     Normally ‘readdir_r’ returns zero and sets ‘*RESULT’ to ENTRY.  If
     there are no more entries in the directory or an error is detected,
     ‘readdir_r’ sets ‘*RESULT’ to a null pointer and returns a nonzero
     error code, also stored in ‘errno’, as described for ‘readdir’.

     It is also important to look at the definition of the ‘struct
     dirent’ type.  Simply passing a pointer to an object of this type
     for the second parameter of ‘readdir_r’ might not be enough.  Some
     systems don’t define the ‘d_name’ element sufficiently long.  In
     this case the user has to provide additional space.  There must be
     room for at least ‘NAME_MAX + 1’ characters in the ‘d_name’ array.
     Code to call ‘readdir_r’ could look like this:

            union
            {
              struct dirent d;
              char b[offsetof (struct dirent, d_name) + NAME_MAX + 1];
            } u;

            if (readdir_r (dir, &u.d, &res) == 0)
              …

   To support large filesystems on 32-bit machines there are LFS
variants of the last two functions.

 -- Function: struct dirent64 * readdir64 (DIR *DIRSTREAM)
     Preliminary: | MT-Unsafe race:dirstream | AS-Unsafe lock |
     AC-Unsafe lock | *Note POSIX Safety Concepts::.

     The ‘readdir64’ function is just like the ‘readdir’ function except
     that it returns a pointer to a record of type ‘struct dirent64’.
     Some of the members of this data type (notably ‘d_ino’) might have
     a different size to allow large filesystems.

     In all other aspects this function is equivalent to ‘readdir’.

 -- Function: int readdir64_r (DIR *DIRSTREAM, struct dirent64 *ENTRY,
          struct dirent64 **RESULT)
     Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock | *Note
     POSIX Safety Concepts::.

     The deprecated ‘readdir64_r’ function is equivalent to the
     ‘readdir_r’ function except that it takes parameters of base type
     ‘struct dirent64’ instead of ‘struct dirent’ in the second and
     third position.  The same precautions mentioned in the
     documentation of ‘readdir_r’ also apply here.

 -- Function: int closedir (DIR *DIRSTREAM)
     Preliminary: | MT-Safe | AS-Unsafe heap lock/hurd | AC-Unsafe mem
     fd lock/hurd | *Note POSIX Safety Concepts::.

     This function closes the directory stream DIRSTREAM.  It returns
     ‘0’ on success and ‘-1’ on failure.

     The following ‘errno’ error conditions are defined for this
     function:

     ‘EBADF’
          The DIRSTREAM argument is not valid.


File: libc.info,  Node: Simple Directory Lister,  Next: Random Access Directory,  Prev: Reading/Closing Directory,  Up: Accessing Directories

14.2.4 Simple Program to List a Directory
-----------------------------------------

Here’s a simple program that prints the names of the files in the
current working directory:


     #include <stdio.h>
     #include <sys/types.h>
     #include <dirent.h>

     int
     main (void)
     {
       DIR *dp;
       struct dirent *ep;

       dp = opendir ("./");
       if (dp != NULL)
         {
           while (ep = readdir (dp))
             puts (ep->d_name);
           (void) closedir (dp);
         }
       else
         perror ("Couldn't open the directory");

       return 0;
     }

   The order in which files appear in a directory tends to be fairly
random.  A more useful program would sort the entries (perhaps by
alphabetizing them) before printing them; see *note Scanning Directory
Content::, and *note Array Sort Function::.


File: libc.info,  Node: Random Access Directory,  Next: Scanning Directory Content,  Prev: Simple Directory Lister,  Up: Accessing Directories

14.2.5 Random Access in a Directory Stream
------------------------------------------

This section describes how to reread parts of a directory that you have
already read from an open directory stream.  All the symbols are
declared in the header file ‘dirent.h’.

 -- Function: void rewinddir (DIR *DIRSTREAM)
     Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock | *Note
     POSIX Safety Concepts::.

     The ‘rewinddir’ function is used to reinitialize the directory
     stream DIRSTREAM, so that if you call ‘readdir’ it returns
     information about the first entry in the directory again.  This
     function also notices if files have been added or removed to the
     directory since it was opened with ‘opendir’.  (Entries for these
     files might or might not be returned by ‘readdir’ if they were
     added or removed since you last called ‘opendir’ or ‘rewinddir’.)

 -- Function: long int telldir (DIR *DIRSTREAM)
     Preliminary: | MT-Safe | AS-Unsafe heap/bsd lock/bsd | AC-Unsafe
     mem/bsd lock/bsd | *Note POSIX Safety Concepts::.

     The ‘telldir’ function returns the file position of the directory
     stream DIRSTREAM.  You can use this value with ‘seekdir’ to restore
     the directory stream to that position.

 -- Function: void seekdir (DIR *DIRSTREAM, long int POS)
     Preliminary: | MT-Safe | AS-Unsafe heap/bsd lock/bsd | AC-Unsafe
     mem/bsd lock/bsd | *Note POSIX Safety Concepts::.

     The ‘seekdir’ function sets the file position of the directory
     stream DIRSTREAM to POS.  The value POS must be the result of a
     previous call to ‘telldir’ on this particular stream; closing and
     reopening the directory can invalidate values returned by
     ‘telldir’.


File: libc.info,  Node: Scanning Directory Content,  Next: Simple Directory Lister Mark II,  Prev: Random Access Directory,  Up: Accessing Directories

14.2.6 Scanning the Content of a Directory
------------------------------------------

A higher-level interface to the directory handling functions is the
‘scandir’ function.  With its help one can select a subset of the
entries in a directory, possibly sort them and get a list of names as
the result.

 -- Function: int scandir (const char *DIR, struct dirent ***NAMELIST,
          int (*SELECTOR) (const struct dirent *), int (*CMP) (const
          struct dirent **, const struct dirent **))
     Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | *Note
     POSIX Safety Concepts::.

     The ‘scandir’ function scans the contents of the directory selected
     by DIR.  The result in *NAMELIST is an array of pointers to
     structures of type ‘struct dirent’ which describe all selected
     directory entries and which is allocated using ‘malloc’.  Instead
     of always getting all directory entries returned, the user supplied
     function SELECTOR can be used to decide which entries are in the
     result.  Only the entries for which SELECTOR returns a non-zero
     value are selected.

     Finally the entries in *NAMELIST are sorted using the user-supplied
     function CMP.  The arguments passed to the CMP function are of type
     ‘struct dirent **’, therefore one cannot directly use the ‘strcmp’
     or ‘strcoll’ functions; instead see the functions ‘alphasort’ and
     ‘versionsort’ below.

     The return value of the function is the number of entries placed in
     *NAMELIST.  If it is ‘-1’ an error occurred (either the directory
     could not be opened for reading or the malloc call failed) and the
     global variable ‘errno’ contains more information on the error.

   As described above, the fourth argument to the ‘scandir’ function
must be a pointer to a sorting function.  For the convenience of the
programmer the GNU C Library contains implementations of functions which
are very helpful for this purpose.

 -- Function: int alphasort (const struct dirent **A, const struct
          dirent **B)
     Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem |
     *Note POSIX Safety Concepts::.

     The ‘alphasort’ function behaves like the ‘strcoll’ function (*note
     String/Array Comparison::).  The difference is that the arguments
     are not string pointers but instead they are of type ‘struct dirent
     **’.

     The return value of ‘alphasort’ is less than, equal to, or greater
     than zero depending on the order of the two entries A and B.

 -- Function: int versionsort (const struct dirent **A, const struct
          dirent **B)
     Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | *Note POSIX
     Safety Concepts::.

     The ‘versionsort’ function is like ‘alphasort’ except that it uses
     the ‘strverscmp’ function internally.

   If the filesystem supports large files we cannot use the ‘scandir’
anymore since the ‘dirent’ structure might not able to contain all the
information.  The LFS provides the new type ‘struct dirent64’.  To use
this we need a new function.

 -- Function: int scandir64 (const char *DIR, struct dirent64
          ***NAMELIST, int (*SELECTOR) (const struct dirent64 *), int
          (*CMP) (const struct dirent64 **, const struct dirent64 **))
     Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | *Note
     POSIX Safety Concepts::.

     The ‘scandir64’ function works like the ‘scandir’ function except
     that the directory entries it returns are described by elements of
     type ‘struct dirent64’.  The function pointed to by SELECTOR is
     again used to select the desired entries, except that SELECTOR now
     must point to a function which takes a ‘struct dirent64 *’
     parameter.

     Similarly the CMP function should expect its two arguments to be of
     type ‘struct dirent64 **’.

   As CMP is now a function of a different type, the functions
‘alphasort’ and ‘versionsort’ cannot be supplied for that argument.
Instead we provide the two replacement functions below.

 -- Function: int alphasort64 (const struct dirent64 **A, const struct
          dirent **B)
     Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem |
     *Note POSIX Safety Concepts::.

     The ‘alphasort64’ function behaves like the ‘strcoll’ function
     (*note String/Array Comparison::).  The difference is that the
     arguments are not string pointers but instead they are of type
     ‘struct dirent64 **’.

     Return value of ‘alphasort64’ is less than, equal to, or greater
     than zero depending on the order of the two entries A and B.

 -- Function: int versionsort64 (const struct dirent64 **A, const struct
          dirent64 **B)
     Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | *Note POSIX
     Safety Concepts::.

     The ‘versionsort64’ function is like ‘alphasort64’, excepted that
     it uses the ‘strverscmp’ function internally.

   It is important not to mix the use of ‘scandir’ and the 64-bit
comparison functions or vice versa.  There are systems on which this
works but on others it will fail miserably.


File: libc.info,  Node: Simple Directory Lister Mark II,  Prev: Scanning Directory Content,  Up: Accessing Directories

14.2.7 Simple Program to List a Directory, Mark II
--------------------------------------------------

Here is a revised version of the directory lister found above (*note
Simple Directory Lister::).  Using the ‘scandir’ function we can avoid
the functions which work directly with the directory contents.  After
the call the returned entries are available for direct use.


     #include <stdio.h>
     #include <dirent.h>

     static int
     one (const struct dirent *unused)
     {
       return 1;
     }

     int
     main (void)
     {
       struct dirent **eps;
       int n;

       n = scandir ("./", &eps, one, alphasort);
       if (n >= 0)
         {
           int cnt;
           for (cnt = 0; cnt < n; ++cnt)
             puts (eps[cnt]->d_name);
         }
       else
         perror ("Couldn't open the directory");

       return 0;
     }

   Note the simple selector function in this example.  Since we want to
see all directory entries we always return ‘1’.


File: libc.info,  Node: Working with Directory Trees,  Next: Hard Links,  Prev: Accessing Directories,  Up: File System Interface

14.3 Working with Directory Trees
=================================

The functions described so far for handling the files in a directory
have allowed you to either retrieve the information bit by bit, or to
process all the files as a group (see ‘scandir’).  Sometimes it is
useful to process whole hierarchies of directories and their contained
files.  The X/Open specification defines two functions to do this.  The
simpler form is derived from an early definition in System V systems and
therefore this function is available on SVID-derived systems.  The
prototypes and required definitions can be found in the ‘ftw.h’ header.

   There are four functions in this family: ‘ftw’, ‘nftw’ and their
64-bit counterparts ‘ftw64’ and ‘nftw64’.  These functions take as one
of their arguments a pointer to a callback function of the appropriate
type.

 -- Data Type: __ftw_func_t

          int (*) (const char *, const struct stat *, int)

     The type of callback functions given to the ‘ftw’ function.  The
     first parameter points to the file name, the second parameter to an
     object of type ‘struct stat’ which is filled in for the file named
     in the first parameter.

     The last parameter is a flag giving more information about the
     current file.  It can have the following values:

     ‘FTW_F’
          The item is either a normal file or a file which does not fit
          into one of the following categories.  This could be special
          files, sockets etc.
     ‘FTW_D’
          The item is a directory.
     ‘FTW_NS’
          The ‘stat’ call failed and so the information pointed to by
          the second parameter is invalid.
     ‘FTW_DNR’
          The item is a directory which cannot be read.
     ‘FTW_SL’
          The item is a symbolic link.  Since symbolic links are
          normally followed seeing this value in a ‘ftw’ callback
          function means the referenced file does not exist.  The
          situation for ‘nftw’ is different.

          This value is only available if the program is compiled with
          ‘_XOPEN_EXTENDED’ defined before including the first header.
          The original SVID systems do not have symbolic links.

     If the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ this
     type is in fact ‘__ftw64_func_t’ since this mode changes ‘struct
     stat’ to be ‘struct stat64’.

   For the LFS interface and for use in the function ‘ftw64’, the header
‘ftw.h’ defines another function type.

 -- Data Type: __ftw64_func_t

          int (*) (const char *, const struct stat64 *, int)

     This type is used just like ‘__ftw_func_t’ for the callback
     function, but this time is called from ‘ftw64’.  The second
     parameter to the function is a pointer to a variable of type
     ‘struct stat64’ which is able to represent the larger values.

 -- Data Type: __nftw_func_t

          int (*) (const char *, const struct stat *, int, struct FTW *)

     The first three arguments are the same as for the ‘__ftw_func_t’
     type.  However for the third argument some additional values are
     defined to allow finer differentiation:
     ‘FTW_DP’
          The current item is a directory and all subdirectories have
          already been visited and reported.  This flag is returned
          instead of ‘FTW_D’ if the ‘FTW_DEPTH’ flag is passed to ‘nftw’
          (see below).
     ‘FTW_SLN’
          The current item is a stale symbolic link.  The file it points
          to does not exist.

     The last parameter of the callback function is a pointer to a
     structure with some extra information as described below.

     If the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ this
     type is in fact ‘__nftw64_func_t’ since this mode changes ‘struct
     stat’ to be ‘struct stat64’.

   For the LFS interface there is also a variant of this data type
available which has to be used with the ‘nftw64’ function.

 -- Data Type: __nftw64_func_t

          int (*) (const char *, const struct stat64 *, int, struct FTW *)

     This type is used just like ‘__nftw_func_t’ for the callback
     function, but this time is called from ‘nftw64’.  The second
     parameter to the function is this time a pointer to a variable of
     type ‘struct stat64’ which is able to represent the larger values.

 -- Data Type: struct FTW
     The information contained in this structure helps in interpreting
     the name parameter and gives some information about the current
     state of the traversal of the directory hierarchy.

     ‘int base’
          The value is the offset into the string passed in the first
          parameter to the callback function of the beginning of the
          file name.  The rest of the string is the path of the file.
          This information is especially important if the ‘FTW_CHDIR’
          flag was set in calling ‘nftw’ since then the current
          directory is the one the current item is found in.
     ‘int level’
          Whilst processing, the code tracks how many directories down
          it has gone to find the current file.  This nesting level
          starts at 0 for files in the initial directory (or is zero for
          the initial file if a file was passed).

 -- Function: int ftw (const char *FILENAME, __ftw_func_t FUNC, int
          DESCRIPTORS)
     Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | *Note
     POSIX Safety Concepts::.

     The ‘ftw’ function calls the callback function given in the
     parameter FUNC for every item which is found in the directory
     specified by FILENAME and all directories below.  The function
     follows symbolic links if necessary but does not process an item
     twice.  If FILENAME is not a directory then it itself is the only
     object returned to the callback function.

     The file name passed to the callback function is constructed by
     taking the FILENAME parameter and appending the names of all passed
     directories and then the local file name.  So the callback function
     can use this parameter to access the file.  ‘ftw’ also calls ‘stat’
     for the file and passes that information on to the callback
     function.  If this ‘stat’ call is not successful the failure is
     indicated by setting the third argument of the callback function to
     ‘FTW_NS’.  Otherwise it is set according to the description given
     in the account of ‘__ftw_func_t’ above.

     The callback function is expected to return 0 to indicate that no
     error occurred and that processing should continue.  If an error
     occurred in the callback function or it wants ‘ftw’ to return
     immediately, the callback function can return a value other than 0.
     This is the only correct way to stop the function.  The program
     must not use ‘setjmp’ or similar techniques to continue from
     another place.  This would leave resources allocated by the ‘ftw’
     function unfreed.

     The DESCRIPTORS parameter to ‘ftw’ specifies how many file
     descriptors it is allowed to consume.  The function runs faster the
     more descriptors it can use.  For each level in the directory
     hierarchy at most one descriptor is used, but for very deep ones
     any limit on open file descriptors for the process or the system
     may be exceeded.  Moreover, file descriptor limits in a
     multi-threaded program apply to all the threads as a group, and
     therefore it is a good idea to supply a reasonable limit to the
     number of open descriptors.

     The return value of the ‘ftw’ function is 0 if all callback
     function calls returned 0 and all actions performed by the ‘ftw’
     succeeded.  If a function call failed (other than calling ‘stat’ on
     an item) the function returns -1.  If a callback function returns a
     value other than 0 this value is returned as the return value of
     ‘ftw’.

     When the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ on a
     32-bit system this function is in fact ‘ftw64’, i.e., the LFS
     interface transparently replaces the old interface.

 -- Function: int ftw64 (const char *FILENAME, __ftw64_func_t FUNC, int
          DESCRIPTORS)
     Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | *Note
     POSIX Safety Concepts::.

     This function is similar to ‘ftw’ but it can work on filesystems
     with large files.  File information is reported using a variable of
     type ‘struct stat64’ which is passed by reference to the callback
     function.

     When the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ on a
     32-bit system this function is available under the name ‘ftw’ and
     transparently replaces the old implementation.

 -- Function: int nftw (const char *FILENAME, __nftw_func_t FUNC, int
          DESCRIPTORS, int FLAG)
     Preliminary: | MT-Safe cwd | AS-Unsafe heap | AC-Unsafe mem fd cwd
     | *Note POSIX Safety Concepts::.

     The ‘nftw’ function works like the ‘ftw’ functions.  They call the
     callback function FUNC for all items found in the directory
     FILENAME and below.  At most DESCRIPTORS file descriptors are
     consumed during the ‘nftw’ call.

     One difference is that the callback function is of a different
     type.  It is of type ‘struct FTW *’ and provides the callback
     function with the extra information described above.

     A second difference is that ‘nftw’ takes a fourth argument, which
     is 0 or a bitwise-OR combination of any of the following values.

     ‘FTW_PHYS’
          While traversing the directory symbolic links are not
          followed.  Instead symbolic links are reported using the
          ‘FTW_SL’ value for the type parameter to the callback
          function.  If the file referenced by a symbolic link does not
          exist ‘FTW_SLN’ is returned instead.
     ‘FTW_MOUNT’
          The callback function is only called for items which are on
          the same mounted filesystem as the directory given by the
          FILENAME parameter to ‘nftw’.
     ‘FTW_CHDIR’
          If this flag is given the current working directory is changed
          to the directory of the reported object before the callback
          function is called.  When ‘ntfw’ finally returns the current
          directory is restored to its original value.
     ‘FTW_DEPTH’
          If this option is specified then all subdirectories and files
          within them are processed before processing the top directory
          itself (depth-first processing).  This also means the type
          flag given to the callback function is ‘FTW_DP’ and not
          ‘FTW_D’.
     ‘FTW_ACTIONRETVAL’
          If this option is specified then return values from callbacks
          are handled differently.  If the callback returns
          ‘FTW_CONTINUE’, walking continues normally.  ‘FTW_STOP’ means
          walking stops and ‘FTW_STOP’ is returned to the caller.  If
          ‘FTW_SKIP_SUBTREE’ is returned by the callback with ‘FTW_D’
          argument, the subtree is skipped and walking continues with
          next sibling of the directory.  If ‘FTW_SKIP_SIBLINGS’ is
          returned by the callback, all siblings of the current entry
          are skipped and walking continues in its parent.  No other
          return values should be returned from the callbacks if this
          option is set.  This option is a GNU extension.

     The return value is computed in the same way as for ‘ftw’.  ‘nftw’
     returns 0 if no failures occurred and all callback functions
     returned 0.  In case of internal errors, such as memory problems,
     the return value is -1 and ERRNO is set accordingly.  If the return
     value of a callback invocation was non-zero then that value is
     returned.

     When the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ on a
     32-bit system this function is in fact ‘nftw64’, i.e., the LFS
     interface transparently replaces the old interface.

 -- Function: int nftw64 (const char *FILENAME, __nftw64_func_t FUNC,
          int DESCRIPTORS, int FLAG)
     Preliminary: | MT-Safe cwd | AS-Unsafe heap | AC-Unsafe mem fd cwd
     | *Note POSIX Safety Concepts::.

     This function is similar to ‘nftw’ but it can work on filesystems
     with large files.  File information is reported using a variable of
     type ‘struct stat64’ which is passed by reference to the callback
     function.

     When the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ on a
     32-bit system this function is available under the name ‘nftw’ and
     transparently replaces the old implementation.


File: libc.info,  Node: Hard Links,  Next: Symbolic Links,  Prev: Working with Directory Trees,  Up: File System Interface

14.4 Hard Links
===============

In POSIX systems, one file can have many names at the same time.  All of
the names are equally real, and no one of them is preferred to the
others.

   To add a name to a file, use the ‘link’ function.  (The new name is
also called a "hard link" to the file.)  Creating a new link to a file
does not copy the contents of the file; it simply makes a new name by
which the file can be known, in addition to the file’s existing name or
names.

   One file can have names in several directories, so the organization
of the file system is not a strict hierarchy or tree.

   In most implementations, it is not possible to have hard links to the
same file in multiple file systems.  ‘link’ reports an error if you try
to make a hard link to the file from another file system when this
cannot be done.

   The prototype for the ‘link’ function is declared in the header file
‘unistd.h’.

 -- Function: int link (const char *OLDNAME, const char *NEWNAME)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘link’ function makes a new link to the existing file named by
     OLDNAME, under the new name NEWNAME.

     This function returns a value of ‘0’ if it is successful and ‘-1’
     on failure.  In addition to the usual file name errors (*note File
     Name Errors::) for both OLDNAME and NEWNAME, the following ‘errno’
     error conditions are defined for this function:

     ‘EACCES’
          You are not allowed to write to the directory in which the new
          link is to be written.

     ‘EEXIST’
          There is already a file named NEWNAME.  If you want to replace
          this link with a new link, you must remove the old link
          explicitly first.

     ‘EMLINK’
          There are already too many links to the file named by OLDNAME.
          (The maximum number of links to a file is ‘LINK_MAX’; see
          *note Limits for Files::.)

     ‘ENOENT’
          The file named by OLDNAME doesn’t exist.  You can’t make a
          link to a file that doesn’t exist.

     ‘ENOSPC’
          The directory or file system that would contain the new link
          is full and cannot be extended.

     ‘EPERM’
          On GNU/Linux and GNU/Hurd systems and some others, you cannot
          make links to directories.  Many systems allow only privileged
          users to do so.  This error is used to report the problem.

     ‘EROFS’
          The directory containing the new link can’t be modified
          because it’s on a read-only file system.

     ‘EXDEV’
          The directory specified in NEWNAME is on a different file
          system than the existing file.

     ‘EIO’
          A hardware error occurred while trying to read or write the to
          filesystem.


File: libc.info,  Node: Symbolic Links,  Next: Deleting Files,  Prev: Hard Links,  Up: File System Interface

14.5 Symbolic Links
===================

GNU systems support "soft links" or "symbolic links".  This is a kind of
“file” that is essentially a pointer to another file name.  Unlike hard
links, symbolic links can be made to directories or across file systems
with no restrictions.  You can also make a symbolic link to a name which
is not the name of any file.  (Opening this link will fail until a file
by that name is created.)  Likewise, if the symbolic link points to an
existing file which is later deleted, the symbolic link continues to
point to the same file name even though the name no longer names any
file.

   The reason symbolic links work the way they do is that special things
happen when you try to open the link.  The ‘open’ function realizes you
have specified the name of a link, reads the file name contained in the
link, and opens that file name instead.  The ‘stat’ function likewise
operates on the file that the symbolic link points to, instead of on the
link itself.

   By contrast, other operations such as deleting or renaming the file
operate on the link itself.  The functions ‘readlink’ and ‘lstat’ also
refrain from following symbolic links, because their purpose is to
obtain information about the link.  ‘link’, the function that makes a
hard link, does too.  It makes a hard link to the symbolic link, which
one rarely wants.

   Some systems have, for some functions operating on files, a limit on
how many symbolic links are followed when resolving a path name.  The
limit if it exists is published in the ‘sys/param.h’ header file.

 -- Macro: int MAXSYMLINKS

     The macro ‘MAXSYMLINKS’ specifies how many symlinks some function
     will follow before returning ‘ELOOP’.  Not all functions behave the
     same and this value is not the same as that returned for
     ‘_SC_SYMLOOP’ by ‘sysconf’.  In fact, the ‘sysconf’ result can
     indicate that there is no fixed limit although ‘MAXSYMLINKS’ exists
     and has a finite value.

   Prototypes for most of the functions listed in this section are in
‘unistd.h’.

 -- Function: int symlink (const char *OLDNAME, const char *NEWNAME)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘symlink’ function makes a symbolic link to OLDNAME named
     NEWNAME.

     The normal return value from ‘symlink’ is ‘0’.  A return value of
     ‘-1’ indicates an error.  In addition to the usual file name syntax
     errors (*note File Name Errors::), the following ‘errno’ error
     conditions are defined for this function:

     ‘EEXIST’
          There is already an existing file named NEWNAME.

     ‘EROFS’
          The file NEWNAME would exist on a read-only file system.

     ‘ENOSPC’
          The directory or file system cannot be extended to make the
          new link.

     ‘EIO’
          A hardware error occurred while reading or writing data on the
          disk.

 -- Function: ssize_t readlink (const char *FILENAME, char *BUFFER,
          size_t SIZE)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘readlink’ function gets the value of the symbolic link
     FILENAME.  The file name that the link points to is copied into
     BUFFER.  This file name string is _not_ null-terminated; ‘readlink’
     normally returns the number of characters copied.  The SIZE
     argument specifies the maximum number of characters to copy,
     usually the allocation size of BUFFER.

     If the return value equals SIZE, you cannot tell whether or not
     there was room to return the entire name.  So make a bigger buffer
     and call ‘readlink’ again.  Here is an example:

          char *
          readlink_malloc (const char *filename)
          {
            int size = 100;
            char *buffer = NULL;

            while (1)
              {
                buffer = (char *) xrealloc (buffer, size);
                int nchars = readlink (filename, buffer, size);
                if (nchars < 0)
                  {
                    free (buffer);
                    return NULL;
                  }
                if (nchars < size)
                  return buffer;
                size *= 2;
              }
          }

     A value of ‘-1’ is returned in case of error.  In addition to the
     usual file name errors (*note File Name Errors::), the following
     ‘errno’ error conditions are defined for this function:

     ‘EINVAL’
          The named file is not a symbolic link.

     ‘EIO’
          A hardware error occurred while reading or writing data on the
          disk.

   In some situations it is desirable to resolve all the symbolic links
to get the real name of a file where no prefix names a symbolic link
which is followed and no filename in the path is ‘.’ or ‘..’.  This is
for instance desirable if files have to be compared in which case
different names can refer to the same inode.

 -- Function: char * canonicalize_file_name (const char *NAME)
     Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | *Note
     POSIX Safety Concepts::.

     The ‘canonicalize_file_name’ function returns the absolute name of
     the file named by NAME which contains no ‘.’, ‘..’ components nor
     any repeated path separators (‘/’) or symlinks.  The result is
     passed back as the return value of the function in a block of
     memory allocated with ‘malloc’.  If the result is not used anymore
     the memory should be freed with a call to ‘free’.

     If any of the path components are missing the function returns a
     NULL pointer.  This is also what is returned if the length of the
     path reaches or exceeds ‘PATH_MAX’ characters.  In any case ‘errno’
     is set accordingly.

     ‘ENAMETOOLONG’
          The resulting path is too long.  This error only occurs on
          systems which have a limit on the file name length.

     ‘EACCES’
          At least one of the path components is not readable.

     ‘ENOENT’
          The input file name is empty.

     ‘ENOENT’
          At least one of the path components does not exist.

     ‘ELOOP’
          More than ‘MAXSYMLINKS’ many symlinks have been followed.

     This function is a GNU extension and is declared in ‘stdlib.h’.

   The Unix standard includes a similar function which differs from
‘canonicalize_file_name’ in that the user has to provide the buffer
where the result is placed in.

 -- Function: char * realpath (const char *restrict NAME, char *restrict
          RESOLVED)
     Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | *Note
     POSIX Safety Concepts::.

     A call to ‘realpath’ where the RESOLVED parameter is ‘NULL’ behaves
     exactly like ‘canonicalize_file_name’.  The function allocates a
     buffer for the file name and returns a pointer to it.  If RESOLVED
     is not ‘NULL’ it points to a buffer into which the result is
     copied.  It is the callers responsibility to allocate a buffer
     which is large enough.  On systems which define ‘PATH_MAX’ this
     means the buffer must be large enough for a pathname of this size.
     For systems without limitations on the pathname length the
     requirement cannot be met and programs should not call ‘realpath’
     with anything but ‘NULL’ for the second parameter.

     One other difference is that the buffer RESOLVED (if nonzero) will
     contain the part of the path component which does not exist or is
     not readable if the function returns ‘NULL’ and ‘errno’ is set to
     ‘EACCES’ or ‘ENOENT’.

     This function is declared in ‘stdlib.h’.

   The advantage of using this function is that it is more widely
available.  The drawback is that it reports failures for long paths on
systems which have no limits on the file name length.


File: libc.info,  Node: Deleting Files,  Next: Renaming Files,  Prev: Symbolic Links,  Up: File System Interface

14.6 Deleting Files
===================

You can delete a file with ‘unlink’ or ‘remove’.

   Deletion actually deletes a file name.  If this is the file’s only
name, then the file is deleted as well.  If the file has other remaining
names (*note Hard Links::), it remains accessible under those names.

 -- Function: int unlink (const char *FILENAME)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘unlink’ function deletes the file name FILENAME.  If this is a
     file’s sole name, the file itself is also deleted.  (Actually, if
     any process has the file open when this happens, deletion is
     postponed until all processes have closed the file.)

     The function ‘unlink’ is declared in the header file ‘unistd.h’.

     This function returns ‘0’ on successful completion, and ‘-1’ on
     error.  In addition to the usual file name errors (*note File Name
     Errors::), the following ‘errno’ error conditions are defined for
     this function:

     ‘EACCES’
          Write permission is denied for the directory from which the
          file is to be removed, or the directory has the sticky bit set
          and you do not own the file.

     ‘EBUSY’
          This error indicates that the file is being used by the system
          in such a way that it can’t be unlinked.  For example, you
          might see this error if the file name specifies the root
          directory or a mount point for a file system.

     ‘ENOENT’
          The file name to be deleted doesn’t exist.

     ‘EPERM’
          On some systems ‘unlink’ cannot be used to delete the name of
          a directory, or at least can only be used this way by a
          privileged user.  To avoid such problems, use ‘rmdir’ to
          delete directories.  (On GNU/Linux and GNU/Hurd systems
          ‘unlink’ can never delete the name of a directory.)

     ‘EROFS’
          The directory containing the file name to be deleted is on a
          read-only file system and can’t be modified.

 -- Function: int rmdir (const char *FILENAME)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘rmdir’ function deletes a directory.  The directory must be
     empty before it can be removed; in other words, it can only contain
     entries for ‘.’ and ‘..’.

     In most other respects, ‘rmdir’ behaves like ‘unlink’.  There are
     two additional ‘errno’ error conditions defined for ‘rmdir’:

     ‘ENOTEMPTY’
     ‘EEXIST’
          The directory to be deleted is not empty.

     These two error codes are synonymous; some systems use one, and
     some use the other.  GNU/Linux and GNU/Hurd systems always use
     ‘ENOTEMPTY’.

     The prototype for this function is declared in the header file
     ‘unistd.h’.

 -- Function: int remove (const char *FILENAME)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This is the ISO C function to remove a file.  It works like
     ‘unlink’ for files and like ‘rmdir’ for directories.  ‘remove’ is
     declared in ‘stdio.h’.


File: libc.info,  Node: Renaming Files,  Next: Creating Directories,  Prev: Deleting Files,  Up: File System Interface

14.7 Renaming Files
===================

The ‘rename’ function is used to change a file’s name.

 -- Function: int rename (const char *OLDNAME, const char *NEWNAME)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘rename’ function renames the file OLDNAME to NEWNAME.  The
     file formerly accessible under the name OLDNAME is afterwards
     accessible as NEWNAME instead.  (If the file had any other names
     aside from OLDNAME, it continues to have those names.)

     The directory containing the name NEWNAME must be on the same file
     system as the directory containing the name OLDNAME.

     One special case for ‘rename’ is when OLDNAME and NEWNAME are two
     names for the same file.  The consistent way to handle this case is
     to delete OLDNAME.  However, in this case POSIX requires that
     ‘rename’ do nothing and report success—which is inconsistent.  We
     don’t know what your operating system will do.

     If OLDNAME is not a directory, then any existing file named NEWNAME
     is removed during the renaming operation.  However, if NEWNAME is
     the name of a directory, ‘rename’ fails in this case.

     If OLDNAME is a directory, then either NEWNAME must not exist or it
     must name a directory that is empty.  In the latter case, the
     existing directory named NEWNAME is deleted first.  The name
     NEWNAME must not specify a subdirectory of the directory ‘oldname’
     which is being renamed.

     One useful feature of ‘rename’ is that the meaning of NEWNAME
     changes “atomically” from any previously existing file by that name
     to its new meaning (i.e., the file that was called OLDNAME).  There
     is no instant at which NEWNAME is non-existent “in between” the old
     meaning and the new meaning.  If there is a system crash during the
     operation, it is possible for both names to still exist; but
     NEWNAME will always be intact if it exists at all.

     If ‘rename’ fails, it returns ‘-1’.  In addition to the usual file
     name errors (*note File Name Errors::), the following ‘errno’ error
     conditions are defined for this function:

     ‘EACCES’
          One of the directories containing NEWNAME or OLDNAME refuses
          write permission; or NEWNAME and OLDNAME are directories and
          write permission is refused for one of them.

     ‘EBUSY’
          A directory named by OLDNAME or NEWNAME is being used by the
          system in a way that prevents the renaming from working.  This
          includes directories that are mount points for filesystems,
          and directories that are the current working directories of
          processes.

     ‘ENOTEMPTY’
     ‘EEXIST’
          The directory NEWNAME isn’t empty.  GNU/Linux and GNU/Hurd
          systems always return ‘ENOTEMPTY’ for this, but some other
          systems return ‘EEXIST’.

     ‘EINVAL’
          OLDNAME is a directory that contains NEWNAME.

     ‘EISDIR’
          NEWNAME is a directory but the OLDNAME isn’t.

     ‘EMLINK’
          The parent directory of NEWNAME would have too many links
          (entries).

     ‘ENOENT’
          The file OLDNAME doesn’t exist.

     ‘ENOSPC’
          The directory that would contain NEWNAME has no room for
          another entry, and there is no space left in the file system
          to expand it.

     ‘EROFS’
          The operation would involve writing to a directory on a
          read-only file system.

     ‘EXDEV’
          The two file names NEWNAME and OLDNAME are on different file
          systems.


File: libc.info,  Node: Creating Directories,  Next: File Attributes,  Prev: Renaming Files,  Up: File System Interface

14.8 Creating Directories
=========================

Directories are created with the ‘mkdir’ function.  (There is also a
shell command ‘mkdir’ which does the same thing.)

 -- Function: int mkdir (const char *FILENAME, mode_t MODE)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘mkdir’ function creates a new, empty directory with name
     FILENAME.

     The argument MODE specifies the file permissions for the new
     directory file.  *Note Permission Bits::, for more information
     about this.

     A return value of ‘0’ indicates successful completion, and ‘-1’
     indicates failure.  In addition to the usual file name syntax
     errors (*note File Name Errors::), the following ‘errno’ error
     conditions are defined for this function:

     ‘EACCES’
          Write permission is denied for the parent directory in which
          the new directory is to be added.

     ‘EEXIST’
          A file named FILENAME already exists.

     ‘EMLINK’
          The parent directory has too many links (entries).

          Well-designed file systems never report this error, because
          they permit more links than your disk could possibly hold.
          However, you must still take account of the possibility of
          this error, as it could result from network access to a file
          system on another machine.

     ‘ENOSPC’
          The file system doesn’t have enough room to create the new
          directory.

     ‘EROFS’
          The parent directory of the directory being created is on a
          read-only file system and cannot be modified.

     To use this function, your program should include the header file
     ‘sys/stat.h’.


File: libc.info,  Node: File Attributes,  Next: Making Special Files,  Prev: Creating Directories,  Up: File System Interface

14.9 File Attributes
====================

When you issue an ‘ls -l’ shell command on a file, it gives you
information about the size of the file, who owns it, when it was last
modified, etc.  These are called the "file attributes", and are
associated with the file itself and not a particular one of its names.

   This section contains information about how you can inquire about and
modify the attributes of a file.

* Menu:

* Attribute Meanings::          The names of the file attributes,
                                 and what their values mean.
* Reading Attributes::          How to read the attributes of a file.
* Testing File Type::           Distinguishing ordinary files,
                                 directories, links…
* File Owner::                  How ownership for new files is determined,
			         and how to change it.
* Permission Bits::             How information about a file’s access
                                 mode is stored.
* Access Permission::           How the system decides who can access a file.
* Setting Permissions::         How permissions for new files are assigned,
			         and how to change them.
* Testing File Access::         How to find out if your process can
                                 access a file.
* File Times::                  About the time attributes of a file.
* File Size::			Manually changing the size of a file.
* Storage Allocation::          Allocate backing storage for files.


File: libc.info,  Node: Attribute Meanings,  Next: Reading Attributes,  Up: File Attributes

14.9.1 The meaning of the File Attributes
-----------------------------------------

When you read the attributes of a file, they come back in a structure
called ‘struct stat’.  This section describes the names of the
attributes, their data types, and what they mean.  For the functions to
read the attributes of a file, see *note Reading Attributes::.

   The header file ‘sys/stat.h’ declares all the symbols defined in this
section.

 -- Data Type: struct stat
     The ‘stat’ structure type is used to return information about the
     attributes of a file.  It contains at least the following members:

     ‘mode_t st_mode’
          Specifies the mode of the file.  This includes file type
          information (*note Testing File Type::) and the file
          permission bits (*note Permission Bits::).

     ‘ino_t st_ino’
          The file serial number, which distinguishes this file from all
          other files on the same device.

     ‘dev_t st_dev’
          Identifies the device containing the file.  The ‘st_ino’ and
          ‘st_dev’, taken together, uniquely identify the file.  The
          ‘st_dev’ value is not necessarily consistent across reboots or
          system crashes, however.

     ‘nlink_t st_nlink’
          The number of hard links to the file.  This count keeps track
          of how many directories have entries for this file.  If the
          count is ever decremented to zero, then the file itself is
          discarded as soon as no process still holds it open.  Symbolic
          links are not counted in the total.

     ‘uid_t st_uid’
          The user ID of the file’s owner.  *Note File Owner::.

     ‘gid_t st_gid’
          The group ID of the file.  *Note File Owner::.

     ‘off_t st_size’
          This specifies the size of a regular file in bytes.  For files
          that are really devices this field isn’t usually meaningful.
          For symbolic links this specifies the length of the file name
          the link refers to.

     ‘time_t st_atime’
          This is the last access time for the file.  *Note File
          Times::.

     ‘unsigned long int st_atime_usec’
          This is the fractional part of the last access time for the
          file.  *Note File Times::.

     ‘time_t st_mtime’
          This is the time of the last modification to the contents of
          the file.  *Note File Times::.

     ‘unsigned long int st_mtime_usec’
          This is the fractional part of the time of the last
          modification to the contents of the file.  *Note File Times::.

     ‘time_t st_ctime’
          This is the time of the last modification to the attributes of
          the file.  *Note File Times::.

     ‘unsigned long int st_ctime_usec’
          This is the fractional part of the time of the last
          modification to the attributes of the file.  *Note File
          Times::.

     ‘blkcnt_t st_blocks’
          This is the amount of disk space that the file occupies,
          measured in units of 512-byte blocks.

          The number of disk blocks is not strictly proportional to the
          size of the file, for two reasons: the file system may use
          some blocks for internal record keeping; and the file may be
          sparse—it may have “holes” which contain zeros but do not
          actually take up space on the disk.

          You can tell (approximately) whether a file is sparse by
          comparing this value with ‘st_size’, like this:

               (st.st_blocks * 512 < st.st_size)

          This test is not perfect because a file that is just slightly
          sparse might not be detected as sparse at all.  For practical
          applications, this is not a problem.

     ‘unsigned int st_blksize’
          The optimal block size for reading or writing this file, in
          bytes.  You might use this size for allocating the buffer
          space for reading or writing the file.  (This is unrelated to
          ‘st_blocks’.)

   The extensions for the Large File Support (LFS) require, even on
32-bit machines, types which can handle file sizes up to 2^63.
Therefore a new definition of ‘struct stat’ is necessary.

 -- Data Type: struct stat64
     The members of this type are the same and have the same names as
     those in ‘struct stat’.  The only difference is that the members
     ‘st_ino’, ‘st_size’, and ‘st_blocks’ have a different type to
     support larger values.

     ‘mode_t st_mode’
          Specifies the mode of the file.  This includes file type
          information (*note Testing File Type::) and the file
          permission bits (*note Permission Bits::).

     ‘ino64_t st_ino’
          The file serial number, which distinguishes this file from all
          other files on the same device.

     ‘dev_t st_dev’
          Identifies the device containing the file.  The ‘st_ino’ and
          ‘st_dev’, taken together, uniquely identify the file.  The
          ‘st_dev’ value is not necessarily consistent across reboots or
          system crashes, however.

     ‘nlink_t st_nlink’
          The number of hard links to the file.  This count keeps track
          of how many directories have entries for this file.  If the
          count is ever decremented to zero, then the file itself is
          discarded as soon as no process still holds it open.  Symbolic
          links are not counted in the total.

     ‘uid_t st_uid’
          The user ID of the file’s owner.  *Note File Owner::.

     ‘gid_t st_gid’
          The group ID of the file.  *Note File Owner::.

     ‘off64_t st_size’
          This specifies the size of a regular file in bytes.  For files
          that are really devices this field isn’t usually meaningful.
          For symbolic links this specifies the length of the file name
          the link refers to.

     ‘time_t st_atime’
          This is the last access time for the file.  *Note File
          Times::.

     ‘unsigned long int st_atime_usec’
          This is the fractional part of the last access time for the
          file.  *Note File Times::.

     ‘time_t st_mtime’
          This is the time of the last modification to the contents of
          the file.  *Note File Times::.

     ‘unsigned long int st_mtime_usec’
          This is the fractional part of the time of the last
          modification to the contents of the file.  *Note File Times::.

     ‘time_t st_ctime’
          This is the time of the last modification to the attributes of
          the file.  *Note File Times::.

     ‘unsigned long int st_ctime_usec’
          This is the fractional part of the time of the last
          modification to the attributes of the file.  *Note File
          Times::.

     ‘blkcnt64_t st_blocks’
          This is the amount of disk space that the file occupies,
          measured in units of 512-byte blocks.

     ‘unsigned int st_blksize’
          The optimal block size for reading of writing this file, in
          bytes.  You might use this size for allocating the buffer
          space for reading of writing the file.  (This is unrelated to
          ‘st_blocks’.)

   Some of the file attributes have special data type names which exist
specifically for those attributes.  (They are all aliases for well-known
integer types that you know and love.)  These typedef names are defined
in the header file ‘sys/types.h’ as well as in ‘sys/stat.h’.  Here is a
list of them.

 -- Data Type: mode_t
     This is an integer data type used to represent file modes.  In the
     GNU C Library, this is an unsigned type no narrower than ‘unsigned
     int’.

 -- Data Type: ino_t
     This is an unsigned integer type used to represent file serial
     numbers.  (In Unix jargon, these are sometimes called "inode
     numbers".)  In the GNU C Library, this type is no narrower than
     ‘unsigned int’.

     If the source is compiled with ‘_FILE_OFFSET_BITS == 64’ this type
     is transparently replaced by ‘ino64_t’.

 -- Data Type: ino64_t
     This is an unsigned integer type used to represent file serial
     numbers for the use in LFS. In the GNU C Library, this type is no
     narrower than ‘unsigned int’.

     When compiling with ‘_FILE_OFFSET_BITS == 64’ this type is
     available under the name ‘ino_t’.

 -- Data Type: dev_t
     This is an arithmetic data type used to represent file device
     numbers.  In the GNU C Library, this is an integer type no narrower
     than ‘int’.

 -- Data Type: nlink_t
     This is an integer type used to represent file link counts.

 -- Data Type: blkcnt_t
     This is a signed integer type used to represent block counts.  In
     the GNU C Library, this type is no narrower than ‘int’.

     If the source is compiled with ‘_FILE_OFFSET_BITS == 64’ this type
     is transparently replaced by ‘blkcnt64_t’.

 -- Data Type: blkcnt64_t
     This is a signed integer type used to represent block counts for
     the use in LFS. In the GNU C Library, this type is no narrower than
     ‘int’.

     When compiling with ‘_FILE_OFFSET_BITS == 64’ this type is
     available under the name ‘blkcnt_t’.


File: libc.info,  Node: Reading Attributes,  Next: Testing File Type,  Prev: Attribute Meanings,  Up: File Attributes

14.9.2 Reading the Attributes of a File
---------------------------------------

To examine the attributes of files, use the functions ‘stat’, ‘fstat’
and ‘lstat’.  They return the attribute information in a ‘struct stat’
object.  All three functions are declared in the header file
‘sys/stat.h’.

 -- Function: int stat (const char *FILENAME, struct stat *BUF)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘stat’ function returns information about the attributes of the
     file named by FILENAME in the structure pointed to by BUF.

     If FILENAME is the name of a symbolic link, the attributes you get
     describe the file that the link points to.  If the link points to a
     nonexistent file name, then ‘stat’ fails reporting a nonexistent
     file.

     The return value is ‘0’ if the operation is successful, or ‘-1’ on
     failure.  In addition to the usual file name errors (*note File
     Name Errors::, the following ‘errno’ error conditions are defined
     for this function:

     ‘ENOENT’
          The file named by FILENAME doesn’t exist.

     When the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ this
     function is in fact ‘stat64’ since the LFS interface transparently
     replaces the normal implementation.

 -- Function: int stat64 (const char *FILENAME, struct stat64 *BUF)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is similar to ‘stat’ but it is also able to work on
     files larger than 2^31 bytes on 32-bit systems.  To be able to do
     this the result is stored in a variable of type ‘struct stat64’ to
     which BUF must point.

     When the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ this
     function is available under the name ‘stat’ and so transparently
     replaces the interface for small files on 32-bit machines.

 -- Function: int fstat (int FILEDES, struct stat *BUF)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘fstat’ function is like ‘stat’, except that it takes an open
     file descriptor as an argument instead of a file name.  *Note
     Low-Level I/O::.

     Like ‘stat’, ‘fstat’ returns ‘0’ on success and ‘-1’ on failure.
     The following ‘errno’ error conditions are defined for ‘fstat’:

     ‘EBADF’
          The FILEDES argument is not a valid file descriptor.

     When the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ this
     function is in fact ‘fstat64’ since the LFS interface transparently
     replaces the normal implementation.

 -- Function: int fstat64 (int FILEDES, struct stat64 *BUF)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is similar to ‘fstat’ but is able to work on large
     files on 32-bit platforms.  For large files the file descriptor
     FILEDES should be obtained by ‘open64’ or ‘creat64’.  The BUF
     pointer points to a variable of type ‘struct stat64’ which is able
     to represent the larger values.

     When the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ this
     function is available under the name ‘fstat’ and so transparently
     replaces the interface for small files on 32-bit machines.

 -- Function: int lstat (const char *FILENAME, struct stat *BUF)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘lstat’ function is like ‘stat’, except that it does not follow
     symbolic links.  If FILENAME is the name of a symbolic link,
     ‘lstat’ returns information about the link itself; otherwise
     ‘lstat’ works like ‘stat’.  *Note Symbolic Links::.

     When the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ this
     function is in fact ‘lstat64’ since the LFS interface transparently
     replaces the normal implementation.

 -- Function: int lstat64 (const char *FILENAME, struct stat64 *BUF)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is similar to ‘lstat’ but it is also able to work on
     files larger than 2^31 bytes on 32-bit systems.  To be able to do
     this the result is stored in a variable of type ‘struct stat64’ to
     which BUF must point.

     When the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ this
     function is available under the name ‘lstat’ and so transparently
     replaces the interface for small files on 32-bit machines.


File: libc.info,  Node: Testing File Type,  Next: File Owner,  Prev: Reading Attributes,  Up: File Attributes

14.9.3 Testing the Type of a File
---------------------------------

The "file mode", stored in the ‘st_mode’ field of the file attributes,
contains two kinds of information: the file type code, and the access
permission bits.  This section discusses only the type code, which you
can use to tell whether the file is a directory, socket, symbolic link,
and so on.  For details about access permissions see *note Permission
Bits::.

   There are two ways you can access the file type information in a file
mode.  Firstly, for each file type there is a "predicate macro" which
examines a given file mode and returns whether it is of that type or
not.  Secondly, you can mask out the rest of the file mode to leave just
the file type code, and compare this against constants for each of the
supported file types.

   All of the symbols listed in this section are defined in the header
file ‘sys/stat.h’.

   The following predicate macros test the type of a file, given the
value M which is the ‘st_mode’ field returned by ‘stat’ on that file:

 -- Macro: int S_ISDIR (mode_t M)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This macro returns non-zero if the file is a directory.

 -- Macro: int S_ISCHR (mode_t M)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This macro returns non-zero if the file is a character special file
     (a device like a terminal).

 -- Macro: int S_ISBLK (mode_t M)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This macro returns non-zero if the file is a block special file (a
     device like a disk).

 -- Macro: int S_ISREG (mode_t M)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This macro returns non-zero if the file is a regular file.

 -- Macro: int S_ISFIFO (mode_t M)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This macro returns non-zero if the file is a FIFO special file, or
     a pipe.  *Note Pipes and FIFOs::.

 -- Macro: int S_ISLNK (mode_t M)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This macro returns non-zero if the file is a symbolic link.  *Note
     Symbolic Links::.

 -- Macro: int S_ISSOCK (mode_t M)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This macro returns non-zero if the file is a socket.  *Note
     Sockets::.

   An alternate non-POSIX method of testing the file type is supported
for compatibility with BSD. The mode can be bitwise AND-ed with ‘S_IFMT’
to extract the file type code, and compared to the appropriate constant.
For example,

     S_ISCHR (MODE)

is equivalent to:

     ((MODE & S_IFMT) == S_IFCHR)

 -- Macro: int S_IFMT
     This is a bit mask used to extract the file type code from a mode
     value.

   These are the symbolic names for the different file type codes:

‘S_IFDIR’
     This is the file type constant of a directory file.

‘S_IFCHR’
     This is the file type constant of a character-oriented device file.

‘S_IFBLK’
     This is the file type constant of a block-oriented device file.

‘S_IFREG’
     This is the file type constant of a regular file.

‘S_IFLNK’
     This is the file type constant of a symbolic link.

‘S_IFSOCK’
     This is the file type constant of a socket.

‘S_IFIFO’
     This is the file type constant of a FIFO or pipe.

   The POSIX.1b standard introduced a few more objects which possibly
can be implemented as objects in the filesystem.  These are message
queues, semaphores, and shared memory objects.  To allow differentiating
these objects from other files the POSIX standard introduced three new
test macros.  But unlike the other macros they do not take the value of
the ‘st_mode’ field as the parameter.  Instead they expect a pointer to
the whole ‘struct stat’ structure.

 -- Macro: int S_TYPEISMQ (struct stat *S)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     If the system implements POSIX message queues as distinct objects
     and the file is a message queue object, this macro returns a
     non-zero value.  In all other cases the result is zero.

 -- Macro: int S_TYPEISSEM (struct stat *S)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     If the system implements POSIX semaphores as distinct objects and
     the file is a semaphore object, this macro returns a non-zero
     value.  In all other cases the result is zero.

 -- Macro: int S_TYPEISSHM (struct stat *S)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     If the system implements POSIX shared memory objects as distinct
     objects and the file is a shared memory object, this macro returns
     a non-zero value.  In all other cases the result is zero.


File: libc.info,  Node: File Owner,  Next: Permission Bits,  Prev: Testing File Type,  Up: File Attributes

14.9.4 File Owner
-----------------

Every file has an "owner" which is one of the registered user names
defined on the system.  Each file also has a "group" which is one of the
defined groups.  The file owner can often be useful for showing you who
edited the file (especially when you edit with GNU Emacs), but its main
purpose is for access control.

   The file owner and group play a role in determining access because
the file has one set of access permission bits for the owner, another
set that applies to users who belong to the file’s group, and a third
set of bits that applies to everyone else.  *Note Access Permission::,
for the details of how access is decided based on this data.

   When a file is created, its owner is set to the effective user ID of
the process that creates it (*note Process Persona::).  The file’s group
ID may be set to either the effective group ID of the process, or the
group ID of the directory that contains the file, depending on the
system where the file is stored.  When you access a remote file system,
it behaves according to its own rules, not according to the system your
program is running on.  Thus, your program must be prepared to encounter
either kind of behavior no matter what kind of system you run it on.

   You can change the owner and/or group owner of an existing file using
the ‘chown’ function.  This is the primitive for the ‘chown’ and ‘chgrp’
shell commands.

   The prototype for this function is declared in ‘unistd.h’.

 -- Function: int chown (const char *FILENAME, uid_t OWNER, gid_t GROUP)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘chown’ function changes the owner of the file FILENAME to
     OWNER, and its group owner to GROUP.

     Changing the owner of the file on certain systems clears the
     set-user-ID and set-group-ID permission bits.  (This is because
     those bits may not be appropriate for the new owner.)  Other file
     permission bits are not changed.

     The return value is ‘0’ on success and ‘-1’ on failure.  In
     addition to the usual file name errors (*note File Name Errors::),
     the following ‘errno’ error conditions are defined for this
     function:

     ‘EPERM’
          This process lacks permission to make the requested change.

          Only privileged users or the file’s owner can change the
          file’s group.  On most file systems, only privileged users can
          change the file owner; some file systems allow you to change
          the owner if you are currently the owner.  When you access a
          remote file system, the behavior you encounter is determined
          by the system that actually holds the file, not by the system
          your program is running on.

          *Note Options for Files::, for information about the
          ‘_POSIX_CHOWN_RESTRICTED’ macro.

     ‘EROFS’
          The file is on a read-only file system.

 -- Function: int fchown (int FILEDES, uid_t OWNER, gid_t GROUP)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This is like ‘chown’, except that it changes the owner of the open
     file with descriptor FILEDES.

     The return value from ‘fchown’ is ‘0’ on success and ‘-1’ on
     failure.  The following ‘errno’ error codes are defined for this
     function:

     ‘EBADF’
          The FILEDES argument is not a valid file descriptor.

     ‘EINVAL’
          The FILEDES argument corresponds to a pipe or socket, not an
          ordinary file.

     ‘EPERM’
          This process lacks permission to make the requested change.
          For details see ‘chmod’ above.

     ‘EROFS’
          The file resides on a read-only file system.


File: libc.info,  Node: Permission Bits,  Next: Access Permission,  Prev: File Owner,  Up: File Attributes

14.9.5 The Mode Bits for Access Permission
------------------------------------------

The "file mode", stored in the ‘st_mode’ field of the file attributes,
contains two kinds of information: the file type code, and the access
permission bits.  This section discusses only the access permission
bits, which control who can read or write the file.  *Note Testing File
Type::, for information about the file type code.

   All of the symbols listed in this section are defined in the header
file ‘sys/stat.h’.

   These symbolic constants are defined for the file mode bits that
control access permission for the file:

‘S_IRUSR’
‘S_IREAD’
     Read permission bit for the owner of the file.  On many systems
     this bit is 0400.  ‘S_IREAD’ is an obsolete synonym provided for
     BSD compatibility.

‘S_IWUSR’
‘S_IWRITE’
     Write permission bit for the owner of the file.  Usually 0200.  ‘S_IWRITE’
     is an obsolete synonym provided for BSD compatibility.

‘S_IXUSR’
‘S_IEXEC’
     Execute (for ordinary files) or search (for directories) permission
     bit for the owner of the file.  Usually 0100.  ‘S_IEXEC’ is an
     obsolete synonym provided for BSD compatibility.

‘S_IRWXU’
     This is equivalent to ‘(S_IRUSR | S_IWUSR | S_IXUSR)’.

‘S_IRGRP’
     Read permission bit for the group owner of the file.  Usually 040.

‘S_IWGRP’
     Write permission bit for the group owner of the file.  Usually 020.

‘S_IXGRP’
     Execute or search permission bit for the group owner of the file.
     Usually 010.

‘S_IRWXG’
     This is equivalent to ‘(S_IRGRP | S_IWGRP | S_IXGRP)’.

‘S_IROTH’
     Read permission bit for other users.  Usually 04.

‘S_IWOTH’
     Write permission bit for other users.  Usually 02.

‘S_IXOTH’
     Execute or search permission bit for other users.  Usually 01.

‘S_IRWXO’
     This is equivalent to ‘(S_IROTH | S_IWOTH | S_IXOTH)’.

‘S_ISUID’
     This is the set-user-ID on execute bit, usually 04000.  *Note How
     Change Persona::.

‘S_ISGID’
     This is the set-group-ID on execute bit, usually 02000.  *Note How
     Change Persona::.

‘S_ISVTX’
     This is the "sticky" bit, usually 01000.

     For a directory it gives permission to delete a file in that
     directory only if you own that file.  Ordinarily, a user can either
     delete all the files in a directory or cannot delete any of them
     (based on whether the user has write permission for the directory).
     The same restriction applies—you must have both write permission
     for the directory and own the file you want to delete.  The one
     exception is that the owner of the directory can delete any file in
     the directory, no matter who owns it (provided the owner has given
     himself write permission for the directory).  This is commonly used
     for the ‘/tmp’ directory, where anyone may create files but not
     delete files created by other users.

     Originally the sticky bit on an executable file modified the
     swapping policies of the system.  Normally, when a program
     terminated, its pages in core were immediately freed and reused.
     If the sticky bit was set on the executable file, the system kept
     the pages in core for a while as if the program were still running.
     This was advantageous for a program likely to be run many times in
     succession.  This usage is obsolete in modern systems.  When a
     program terminates, its pages always remain in core as long as
     there is no shortage of memory in the system.  When the program is
     next run, its pages will still be in core if no shortage arose
     since the last run.

     On some modern systems where the sticky bit has no useful meaning
     for an executable file, you cannot set the bit at all for a
     non-directory.  If you try, ‘chmod’ fails with ‘EFTYPE’; *note
     Setting Permissions::.

     Some systems (particularly SunOS) have yet another use for the
     sticky bit.  If the sticky bit is set on a file that is _not_
     executable, it means the opposite: never cache the pages of this
     file at all.  The main use of this is for the files on an NFS
     server machine which are used as the swap area of diskless client
     machines.  The idea is that the pages of the file will be cached in
     the client’s memory, so it is a waste of the server’s memory to
     cache them a second time.  With this usage the sticky bit also
     implies that the filesystem may fail to record the file’s
     modification time onto disk reliably (the idea being that no-one
     cares for a swap file).

     This bit is only available on BSD systems (and those derived from
     them).  Therefore one has to use the ‘_GNU_SOURCE’ feature select
     macro, or not define any feature test macros, to get the definition
     (*note Feature Test Macros::).

   The actual bit values of the symbols are listed in the table above so
you can decode file mode values when debugging your programs.  These bit
values are correct for most systems, but they are not guaranteed.

   *Warning:* Writing explicit numbers for file permissions is bad
practice.  Not only is it not portable, it also requires everyone who
reads your program to remember what the bits mean.  To make your program
clean use the symbolic names.


File: libc.info,  Node: Access Permission,  Next: Setting Permissions,  Prev: Permission Bits,  Up: File Attributes

14.9.6 How Your Access to a File is Decided
-------------------------------------------

Recall that the operating system normally decides access permission for
a file based on the effective user and group IDs of the process and its
supplementary group IDs, together with the file’s owner, group and
permission bits.  These concepts are discussed in detail in *note
Process Persona::.

   If the effective user ID of the process matches the owner user ID of
the file, then permissions for read, write, and execute/search are
controlled by the corresponding “user” (or “owner”) bits.  Likewise, if
any of the effective group ID or supplementary group IDs of the process
matches the group owner ID of the file, then permissions are controlled
by the “group” bits.  Otherwise, permissions are controlled by the
“other” bits.

   Privileged users, like ‘root’, can access any file regardless of its
permission bits.  As a special case, for a file to be executable even by
a privileged user, at least one of its execute bits must be set.


File: libc.info,  Node: Setting Permissions,  Next: Testing File Access,  Prev: Access Permission,  Up: File Attributes

14.9.7 Assigning File Permissions
---------------------------------

The primitive functions for creating files (for example, ‘open’ or
‘mkdir’) take a MODE argument, which specifies the file permissions to
give the newly created file.  This mode is modified by the process’s
"file creation mask", or "umask", before it is used.

   The bits that are set in the file creation mask identify permissions
that are always to be disabled for newly created files.  For example, if
you set all the “other” access bits in the mask, then newly created
files are not accessible at all to processes in the “other” category,
even if the MODE argument passed to the create function would permit
such access.  In other words, the file creation mask is the complement
of the ordinary access permissions you want to grant.

   Programs that create files typically specify a MODE argument that
includes all the permissions that make sense for the particular file.
For an ordinary file, this is typically read and write permission for
all classes of users.  These permissions are then restricted as
specified by the individual user’s own file creation mask.

   To change the permission of an existing file given its name, call
‘chmod’.  This function uses the specified permission bits and ignores
the file creation mask.

   In normal use, the file creation mask is initialized by the user’s
login shell (using the ‘umask’ shell command), and inherited by all
subprocesses.  Application programs normally don’t need to worry about
the file creation mask.  It will automatically do what it is supposed to
do.

   When your program needs to create a file and bypass the umask for its
access permissions, the easiest way to do this is to use ‘fchmod’ after
opening the file, rather than changing the umask.  In fact, changing the
umask is usually done only by shells.  They use the ‘umask’ function.

   The functions in this section are declared in ‘sys/stat.h’.

 -- Function: mode_t umask (mode_t MASK)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘umask’ function sets the file creation mask of the current
     process to MASK, and returns the previous value of the file
     creation mask.

     Here is an example showing how to read the mask with ‘umask’
     without changing it permanently:

          mode_t
          read_umask (void)
          {
            mode_t mask = umask (0);
            umask (mask);
            return mask;
          }

     However, on GNU/Hurd systems it is better to use ‘getumask’ if you
     just want to read the mask value, because it is reentrant.

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

     Return the current value of the file creation mask for the current
     process.  This function is a GNU extension and is only available on
     GNU/Hurd systems.

 -- Function: int chmod (const char *FILENAME, mode_t MODE)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘chmod’ function sets the access permission bits for the file
     named by FILENAME to MODE.

     If FILENAME is a symbolic link, ‘chmod’ changes the permissions of
     the file pointed to by the link, not those of the link itself.

     This function returns ‘0’ if successful and ‘-1’ if not.  In
     addition to the usual file name errors (*note File Name Errors::),
     the following ‘errno’ error conditions are defined for this
     function:

     ‘ENOENT’
          The named file doesn’t exist.

     ‘EPERM’
          This process does not have permission to change the access
          permissions of this file.  Only the file’s owner (as judged by
          the effective user ID of the process) or a privileged user can
          change them.

     ‘EROFS’
          The file resides on a read-only file system.

     ‘EFTYPE’
          MODE has the ‘S_ISVTX’ bit (the “sticky bit”) set, and the
          named file is not a directory.  Some systems do not allow
          setting the sticky bit on non-directory files, and some do
          (and only some of those assign a useful meaning to the bit for
          non-directory files).

          You only get ‘EFTYPE’ on systems where the sticky bit has no
          useful meaning for non-directory files, so it is always safe
          to just clear the bit in MODE and call ‘chmod’ again.  *Note
          Permission Bits::, for full details on the sticky bit.

 -- Function: int fchmod (int FILEDES, mode_t MODE)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This is like ‘chmod’, except that it changes the permissions of the
     currently open file given by FILEDES.

     The return value from ‘fchmod’ is ‘0’ on success and ‘-1’ on
     failure.  The following ‘errno’ error codes are defined for this
     function:

     ‘EBADF’
          The FILEDES argument is not a valid file descriptor.

     ‘EINVAL’
          The FILEDES argument corresponds to a pipe or socket, or
          something else that doesn’t really have access permissions.

     ‘EPERM’
          This process does not have permission to change the access
          permissions of this file.  Only the file’s owner (as judged by
          the effective user ID of the process) or a privileged user can
          change them.

     ‘EROFS’
          The file resides on a read-only file system.


File: libc.info,  Node: Testing File Access,  Next: File Times,  Prev: Setting Permissions,  Up: File Attributes

14.9.8 Testing Permission to Access a File
------------------------------------------

In some situations it is desirable to allow programs to access files or
devices even if this is not possible with the permissions granted to the
user.  One possible solution is to set the setuid-bit of the program
file.  If such a program is started the _effective_ user ID of the
process is changed to that of the owner of the program file.  So to
allow write access to files like ‘/etc/passwd’, which normally can be
written only by the super-user, the modifying program will have to be
owned by ‘root’ and the setuid-bit must be set.

   But besides the files the program is intended to change the user
should not be allowed to access any file to which s/he would not have
access anyway.  The program therefore must explicitly check whether _the
user_ would have the necessary access to a file, before it reads or
writes the file.

   To do this, use the function ‘access’, which checks for access
permission based on the process’s _real_ user ID rather than the
effective user ID. (The setuid feature does not alter the real user ID,
so it reflects the user who actually ran the program.)

   There is another way you could check this access, which is easy to
describe, but very hard to use.  This is to examine the file mode bits
and mimic the system’s own access computation.  This method is
undesirable because many systems have additional access control
features; your program cannot portably mimic them, and you would not
want to try to keep track of the diverse features that different systems
have.  Using ‘access’ is simple and automatically does whatever is
appropriate for the system you are using.

   ‘access’ is _only_ appropriate to use in setuid programs.  A
non-setuid program will always use the effective ID rather than the real
ID.

   The symbols in this section are declared in ‘unistd.h’.

 -- Function: int access (const char *FILENAME, int HOW)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘access’ function checks to see whether the file named by
     FILENAME can be accessed in the way specified by the HOW argument.
     The HOW argument either can be the bitwise OR of the flags ‘R_OK’,
     ‘W_OK’, ‘X_OK’, or the existence test ‘F_OK’.

     This function uses the _real_ user and group IDs of the calling
     process, rather than the _effective_ IDs, to check for access
     permission.  As a result, if you use the function from a ‘setuid’
     or ‘setgid’ program (*note How Change Persona::), it gives
     information relative to the user who actually ran the program.

     The return value is ‘0’ if the access is permitted, and ‘-1’
     otherwise.  (In other words, treated as a predicate function,
     ‘access’ returns true if the requested access is _denied_.)

     In addition to the usual file name errors (*note File Name
     Errors::), the following ‘errno’ error conditions are defined for
     this function:

     ‘EACCES’
          The access specified by HOW is denied.

     ‘ENOENT’
          The file doesn’t exist.

     ‘EROFS’
          Write permission was requested for a file on a read-only file
          system.

   These macros are defined in the header file ‘unistd.h’ for use as the
HOW argument to the ‘access’ function.  The values are integer
constants.

 -- Macro: int R_OK
     Flag meaning test for read permission.

 -- Macro: int W_OK
     Flag meaning test for write permission.

 -- Macro: int X_OK
     Flag meaning test for execute/search permission.

 -- Macro: int F_OK
     Flag meaning test for existence of the file.


File: libc.info,  Node: File Times,  Next: File Size,  Prev: Testing File Access,  Up: File Attributes

14.9.9 File Times
-----------------

Each file has three time stamps associated with it: its access time, its
modification time, and its attribute modification time.  These
correspond to the ‘st_atime’, ‘st_mtime’, and ‘st_ctime’ members of the
‘stat’ structure; see *note File Attributes::.

   All of these times are represented in calendar time format, as
‘time_t’ objects.  This data type is defined in ‘time.h’.  For more
information about representation and manipulation of time values, see
*note Calendar Time::.

   Reading from a file updates its access time attribute, and writing
updates its modification time.  When a file is created, all three time
stamps for that file are set to the current time.  In addition, the
attribute change time and modification time fields of the directory that
contains the new entry are updated.

   Adding a new name for a file with the ‘link’ function updates the
attribute change time field of the file being linked, and both the
attribute change time and modification time fields of the directory
containing the new name.  These same fields are affected if a file name
is deleted with ‘unlink’, ‘remove’ or ‘rmdir’.  Renaming a file with
‘rename’ affects only the attribute change time and modification time
fields of the two parent directories involved, and not the times for the
file being renamed.

   Changing the attributes of a file (for example, with ‘chmod’) updates
its attribute change time field.

   You can also change some of the time stamps of a file explicitly
using the ‘utime’ function—all except the attribute change time.  You
need to include the header file ‘utime.h’ to use this facility.

 -- Data Type: struct utimbuf
     The ‘utimbuf’ structure is used with the ‘utime’ function to
     specify new access and modification times for a file.  It contains
     the following members:

     ‘time_t actime’
          This is the access time for the file.

     ‘time_t modtime’
          This is the modification time for the file.

 -- Function: int utime (const char *FILENAME, const struct utimbuf
          *TIMES)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is used to modify the file times associated with the
     file named FILENAME.

     If TIMES is a null pointer, then the access and modification times
     of the file are set to the current time.  Otherwise, they are set
     to the values from the ‘actime’ and ‘modtime’ members
     (respectively) of the ‘utimbuf’ structure pointed to by TIMES.

     The attribute modification time for the file is set to the current
     time in either case (since changing the time stamps is itself a
     modification of the file attributes).

     The ‘utime’ function returns ‘0’ if successful and ‘-1’ on failure.
     In addition to the usual file name errors (*note File Name
     Errors::), the following ‘errno’ error conditions are defined for
     this function:

     ‘EACCES’
          There is a permission problem in the case where a null pointer
          was passed as the TIMES argument.  In order to update the time
          stamp on the file, you must either be the owner of the file,
          have write permission for the file, or be a privileged user.

     ‘ENOENT’
          The file doesn’t exist.

     ‘EPERM’
          If the TIMES argument is not a null pointer, you must either
          be the owner of the file or be a privileged user.

     ‘EROFS’
          The file lives on a read-only file system.

   Each of the three time stamps has a corresponding microsecond part,
which extends its resolution.  These fields are called ‘st_atime_usec’,
‘st_mtime_usec’, and ‘st_ctime_usec’; each has a value between 0 and
999,999, which indicates the time in microseconds.  They correspond to
the ‘tv_usec’ field of a ‘timeval’ structure; see *note High-Resolution
Calendar::.

   The ‘utimes’ function is like ‘utime’, but also lets you specify the
fractional part of the file times.  The prototype for this function is
in the header file ‘sys/time.h’.

 -- Function: int utimes (const char *FILENAME, const struct timeval
          TVP[2])
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function sets the file access and modification times of the
     file FILENAME.  The new file access time is specified by ‘TVP[0]’,
     and the new modification time by ‘TVP[1]’.  Similar to ‘utime’, if
     TVP is a null pointer then the access and modification times of the
     file are set to the current time.  This function comes from BSD.

     The return values and error conditions are the same as for the
     ‘utime’ function.

 -- Function: int lutimes (const char *FILENAME, const struct timeval
          TVP[2])
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is like ‘utimes’, except that it does not follow
     symbolic links.  If FILENAME is the name of a symbolic link,
     ‘lutimes’ sets the file access and modification times of the
     symbolic link special file itself (as seen by ‘lstat’; *note
     Symbolic Links::) while ‘utimes’ sets the file access and
     modification times of the file the symbolic link refers to.  This
     function comes from FreeBSD, and is not available on all platforms
     (if not available, it will fail with ‘ENOSYS’).

     The return values and error conditions are the same as for the
     ‘utime’ function.

 -- Function: int futimes (int FD, const struct timeval TVP[2])
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is like ‘utimes’, except that it takes an open file
     descriptor as an argument instead of a file name.  *Note Low-Level
     I/O::.  This function comes from FreeBSD, and is not available on
     all platforms (if not available, it will fail with ‘ENOSYS’).

     Like ‘utimes’, ‘futimes’ returns ‘0’ on success and ‘-1’ on
     failure.  The following ‘errno’ error conditions are defined for
     ‘futimes’:

     ‘EACCES’
          There is a permission problem in the case where a null pointer
          was passed as the TIMES argument.  In order to update the time
          stamp on the file, you must either be the owner of the file,
          have write permission for the file, or be a privileged user.

     ‘EBADF’
          The FILEDES argument is not a valid file descriptor.

     ‘EPERM’
          If the TIMES argument is not a null pointer, you must either
          be the owner of the file or be a privileged user.

     ‘EROFS’
          The file lives on a read-only file system.


File: libc.info,  Node: File Size,  Next: Storage Allocation,  Prev: File Times,  Up: File Attributes

14.9.10 File Size
-----------------

Normally file sizes are maintained automatically.  A file begins with a
size of 0 and is automatically extended when data is written past its
end.  It is also possible to empty a file completely by an ‘open’ or
‘fopen’ call.

   However, sometimes it is necessary to _reduce_ the size of a file.
This can be done with the ‘truncate’ and ‘ftruncate’ functions.  They
were introduced in BSD Unix.  ‘ftruncate’ was later added to POSIX.1.

   Some systems allow you to extend a file (creating holes) with these
functions.  This is useful when using memory-mapped I/O (*note
Memory-mapped I/O::), where files are not automatically extended.
However, it is not portable but must be implemented if ‘mmap’ allows
mapping of files (i.e., ‘_POSIX_MAPPED_FILES’ is defined).

   Using these functions on anything other than a regular file gives
_undefined_ results.  On many systems, such a call will appear to
succeed, without actually accomplishing anything.

 -- Function: int truncate (const char *FILENAME, off_t LENGTH)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘truncate’ function changes the size of FILENAME to LENGTH.  If
     LENGTH is shorter than the previous length, data at the end will be
     lost.  The file must be writable by the user to perform this
     operation.

     If LENGTH is longer, holes will be added to the end.  However, some
     systems do not support this feature and will leave the file
     unchanged.

     When the source file is compiled with ‘_FILE_OFFSET_BITS == 64’ the
     ‘truncate’ function is in fact ‘truncate64’ and the type ‘off_t’
     has 64 bits which makes it possible to handle files up to 2^63
     bytes in length.

     The return value is 0 for success, or -1 for an error.  In addition
     to the usual file name errors, the following errors may occur:

     ‘EACCES’
          The file is a directory or not writable.

     ‘EINVAL’
          LENGTH is negative.

     ‘EFBIG’
          The operation would extend the file beyond the limits of the
          operating system.

     ‘EIO’
          A hardware I/O error occurred.

     ‘EPERM’
          The file is "append-only" or "immutable".

     ‘EINTR’
          The operation was interrupted by a signal.

 -- Function: int truncate64 (const char *NAME, off64_t LENGTH)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is similar to the ‘truncate’ function.  The
     difference is that the LENGTH argument is 64 bits wide even on 32
     bits machines, which allows the handling of files with sizes up to
     2^63 bytes.

     When the source file is compiled with ‘_FILE_OFFSET_BITS == 64’ on
     a 32 bits machine this function is actually available under the
     name ‘truncate’ and so transparently replaces the 32 bits
     interface.

 -- Function: int ftruncate (int FD, off_t LENGTH)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This is like ‘truncate’, but it works on a file descriptor FD for
     an opened file instead of a file name to identify the object.  The
     file must be opened for writing to successfully carry out the
     operation.

     The POSIX standard leaves it implementation defined what happens if
     the specified new LENGTH of the file is bigger than the original
     size.  The ‘ftruncate’ function might simply leave the file alone
     and do nothing or it can increase the size to the desired size.  In
     this later case the extended area should be zero-filled.  So using
     ‘ftruncate’ is no reliable way to increase the file size but if it
     is possible it is probably the fastest way.  The function also
     operates on POSIX shared memory segments if these are implemented
     by the system.

     ‘ftruncate’ is especially useful in combination with ‘mmap’.  Since
     the mapped region must have a fixed size one cannot enlarge the
     file by writing something beyond the last mapped page.  Instead one
     has to enlarge the file itself and then remap the file with the new
     size.  The example below shows how this works.

     When the source file is compiled with ‘_FILE_OFFSET_BITS == 64’ the
     ‘ftruncate’ function is in fact ‘ftruncate64’ and the type ‘off_t’
     has 64 bits which makes it possible to handle files up to 2^63
     bytes in length.

     The return value is 0 for success, or -1 for an error.  The
     following errors may occur:

     ‘EBADF’
          FD does not correspond to an open file.

     ‘EACCES’
          FD is a directory or not open for writing.

     ‘EINVAL’
          LENGTH is negative.

     ‘EFBIG’
          The operation would extend the file beyond the limits of the
          operating system.

     ‘EIO’
          A hardware I/O error occurred.

     ‘EPERM’
          The file is "append-only" or "immutable".

     ‘EINTR’
          The operation was interrupted by a signal.

 -- Function: int ftruncate64 (int ID, off64_t LENGTH)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is similar to the ‘ftruncate’ function.  The
     difference is that the LENGTH argument is 64 bits wide even on 32
     bits machines which allows the handling of files with sizes up to
     2^63 bytes.

     When the source file is compiled with ‘_FILE_OFFSET_BITS == 64’ on
     a 32 bits machine this function is actually available under the
     name ‘ftruncate’ and so transparently replaces the 32 bits
     interface.

   As announced here is a little example of how to use ‘ftruncate’ in
combination with ‘mmap’:

     int fd;
     void *start;
     size_t len;

     int
     add (off_t at, void *block, size_t size)
     {
       if (at + size > len)
         {
           /* Resize the file and remap.  */
           size_t ps = sysconf (_SC_PAGESIZE);
           size_t ns = (at + size + ps - 1) & ~(ps - 1);
           void *np;
           if (ftruncate (fd, ns) < 0)
             return -1;
           np = mmap (NULL, ns, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
           if (np == MAP_FAILED)
             return -1;
           start = np;
           len = ns;
         }
       memcpy ((char *) start + at, block, size);
       return 0;
     }

   The function ‘add’ writes a block of memory at an arbitrary position
in the file.  If the current size of the file is too small it is
extended.  Note that it is extended by a whole number of pages.  This is
a requirement of ‘mmap’.  The program has to keep track of the real
size, and when it has finished a final ‘ftruncate’ call should set the
real size of the file.


File: libc.info,  Node: Storage Allocation,  Prev: File Size,  Up: File Attributes

14.9.11 Storage Allocation
--------------------------

Most file systems support allocating large files in a non-contiguous
fashion: the file is split into _fragments_ which are allocated
sequentially, but the fragments themselves can be scattered across the
disk.  File systems generally try to avoid such fragmentation because it
decreases performance, but if a file gradually increases in size, there
might be no other option than to fragment it.  In addition, many file
systems support _sparse files_ with _holes_: regions of null bytes for
which no backing storage has been allocated by the file system.  When
the holes are finally overwritten with data, fragmentation can occur as
well.

   Explicit allocation of storage for yet-unwritten parts of the file
can help the system to avoid fragmentation.  Additionally, if storage
pre-allocation fails, it is possible to report the out-of-disk error
early, often without filling up the entire disk.  However, due to
deduplication, copy-on-write semantics, and file compression, such
pre-allocation may not reliably prevent the out-of-disk-space error from
occurring later.  Checking for write errors is still required, and
writes to memory-mapped regions created with ‘mmap’ can still result in
‘SIGBUS’.

 -- Function: int posix_fallocate (int FD, off_t OFFSET, off_t LENGTH)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     Allocate backing store for the region of LENGTH bytes starting at
     byte OFFSET in the file for the descriptor FD.  The file length is
     increased to ‘LENGTH + OFFSET’ if necessary.

     FD must be a regular file opened for writing, or ‘EBADF’ is
     returned.  If there is insufficient disk space to fulfill the
     allocation request, ‘ENOSPC’ is returned.

     *Note:* If ‘fallocate’ is not available (because the file system
     does not support it), ‘posix_fallocate’ is emulated, which has the
     following drawbacks:

        • It is very inefficient because all file system blocks in the
          requested range need to be examined (even if they have been
          allocated before) and potentially rewritten.  In contrast,
          with proper ‘fallocate’ support (see below), the file system
          can examine the internal file allocation data structures and
          eliminate holes directly, maybe even using unwritten extents
          (which are pre-allocated but uninitialized on disk).

        • There is a race condition if another thread or process
          modifies the underlying file in the to-be-allocated area.
          Non-null bytes could be overwritten with null bytes.

        • If FD has been opened with the ‘O_WRONLY’ flag, the function
          will fail with an ‘errno’ value of ‘EBADF’.

        • If FD has been opened with the ‘O_APPEND’ flag, the function
          will fail with an ‘errno’ value of ‘EBADF’.

        • If LENGTH is zero, ‘ftruncate’ is used to increase the file
          size as requested, without allocating file system blocks.
          There is a race condition which means that ‘ftruncate’ can
          accidentally truncate the file if it has been extended
          concurrently.

     On Linux, if an application does not benefit from emulation or if
     the emulation is harmful due to its inherent race conditions, the
     application can use the Linux-specific ‘fallocate’ function, with a
     zero flag argument.  For the ‘fallocate’ function, the GNU C
     Library does not perform allocation emulation if the file system
     does not support allocation.  Instead, an ‘EOPNOTSUPP’ is returned
     to the caller.

 -- Function: int posix_fallocate64 (int FD, off64_t OFFSET, off64_t
          LENGTH)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is a variant of ‘posix_fallocate64’ which accepts
     64-bit file offsets on all platforms.


File: libc.info,  Node: Making Special Files,  Next: Temporary Files,  Prev: File Attributes,  Up: File System Interface

14.10 Making Special Files
==========================

The ‘mknod’ function is the primitive for making special files, such as
files that correspond to devices.  The GNU C Library includes this
function for compatibility with BSD.

   The prototype for ‘mknod’ is declared in ‘sys/stat.h’.

 -- Function: int mknod (const char *FILENAME, mode_t MODE, dev_t DEV)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘mknod’ function makes a special file with name FILENAME.  The
     MODE specifies the mode of the file, and may include the various
     special file bits, such as ‘S_IFCHR’ (for a character special file)
     or ‘S_IFBLK’ (for a block special file).  *Note Testing File
     Type::.

     The DEV argument specifies which device the special file refers to.
     Its exact interpretation depends on the kind of special file being
     created.

     The return value is ‘0’ on success and ‘-1’ on error.  In addition
     to the usual file name errors (*note File Name Errors::), the
     following ‘errno’ error conditions are defined for this function:

     ‘EPERM’
          The calling process is not privileged.  Only the superuser can
          create special files.

     ‘ENOSPC’
          The directory or file system that would contain the new file
          is full and cannot be extended.

     ‘EROFS’
          The directory containing the new file can’t be modified
          because it’s on a read-only file system.

     ‘EEXIST’
          There is already a file named FILENAME.  If you want to
          replace this file, you must remove the old file explicitly
          first.


File: libc.info,  Node: Temporary Files,  Prev: Making Special Files,  Up: File System Interface

14.11 Temporary Files
=====================

If you need to use a temporary file in your program, you can use the
‘tmpfile’ function to open it.  Or you can use the ‘tmpnam’ (better:
‘tmpnam_r’) function to provide a name for a temporary file and then you
can open it in the usual way with ‘fopen’.

   The ‘tempnam’ function is like ‘tmpnam’ but lets you choose what
directory temporary files will go in, and something about what their
file names will look like.  Important for multi-threaded programs is
that ‘tempnam’ is reentrant, while ‘tmpnam’ is not since it returns a
pointer to a static buffer.

   These facilities are declared in the header file ‘stdio.h’.

 -- Function: FILE * tmpfile (void)
     Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe mem fd
     lock | *Note POSIX Safety Concepts::.

     This function creates a temporary binary file for update mode, as
     if by calling ‘fopen’ with mode ‘"wb+"’.  The file is deleted
     automatically when it is closed or when the program terminates.
     (On some other ISO C systems the file may fail to be deleted if the
     program terminates abnormally).

     This function is reentrant.

     When the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ on a
     32-bit system this function is in fact ‘tmpfile64’, i.e., the LFS
     interface transparently replaces the old interface.

 -- Function: FILE * tmpfile64 (void)
     Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe mem fd
     lock | *Note POSIX Safety Concepts::.

     This function is similar to ‘tmpfile’, but the stream it returns a
     pointer to was opened using ‘tmpfile64’.  Therefore this stream can
     be used for files larger than 2^31 bytes on 32-bit machines.

     Please note that the return type is still ‘FILE *’.  There is no
     special ‘FILE’ type for the LFS interface.

     If the sources are compiled with ‘_FILE_OFFSET_BITS == 64’ on a 32
     bits machine this function is available under the name ‘tmpfile’
     and so transparently replaces the old interface.

 -- Function: char * tmpnam (char *RESULT)
     Preliminary: | MT-Unsafe race:tmpnam/!result | AS-Unsafe | AC-Safe
     | *Note POSIX Safety Concepts::.

     This function constructs and returns a valid file name that does
     not refer to any existing file.  If the RESULT argument is a null
     pointer, the return value is a pointer to an internal static
     string, which might be modified by subsequent calls and therefore
     makes this function non-reentrant.  Otherwise, the RESULT argument
     should be a pointer to an array of at least ‘L_tmpnam’ characters,
     and the result is written into that array.

     It is possible for ‘tmpnam’ to fail if you call it too many times
     without removing previously-created files.  This is because the
     limited length of the temporary file names gives room for only a
     finite number of different names.  If ‘tmpnam’ fails it returns a
     null pointer.

     *Warning:* Between the time the pathname is constructed and the
     file is created another process might have created a file with the
     same name using ‘tmpnam’, leading to a possible security hole.  The
     implementation generates names which can hardly be predicted, but
     when opening the file you should use the ‘O_EXCL’ flag.  Using
     ‘tmpfile’ or ‘mkstemp’ is a safe way to avoid this problem.

 -- Function: char * tmpnam_r (char *RESULT)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function is nearly identical to the ‘tmpnam’ function, except
     that if RESULT is a null pointer it returns a null pointer.

     This guarantees reentrancy because the non-reentrant situation of
     ‘tmpnam’ cannot happen here.

     *Warning*: This function has the same security problems as
     ‘tmpnam’.

 -- Macro: int L_tmpnam
     The value of this macro is an integer constant expression that
     represents the minimum size of a string large enough to hold a file
     name generated by the ‘tmpnam’ function.

 -- Macro: int TMP_MAX
     The macro ‘TMP_MAX’ is a lower bound for how many temporary names
     you can create with ‘tmpnam’.  You can rely on being able to call
     ‘tmpnam’ at least this many times before it might fail saying you
     have made too many temporary file names.

     With the GNU C Library, you can create a very large number of
     temporary file names.  If you actually created the files, you would
     probably run out of disk space before you ran out of names.  Some
     other systems have a fixed, small limit on the number of temporary
     files.  The limit is never less than ‘25’.

 -- Function: char * tempnam (const char *DIR, const char *PREFIX)
     Preliminary: | MT-Safe env | AS-Unsafe heap | AC-Unsafe mem | *Note
     POSIX Safety Concepts::.

     This function generates a unique temporary file name.  If PREFIX is
     not a null pointer, up to five characters of this string are used
     as a prefix for the file name.  The return value is a string newly
     allocated with ‘malloc’, so you should release its storage with
     ‘free’ when it is no longer needed.

     Because the string is dynamically allocated this function is
     reentrant.

     The directory prefix for the temporary file name is determined by
     testing each of the following in sequence.  The directory must
     exist and be writable.

        • The environment variable ‘TMPDIR’, if it is defined.  For
          security reasons this only happens if the program is not SUID
          or SGID enabled.

        • The DIR argument, if it is not a null pointer.

        • The value of the ‘P_tmpdir’ macro.

        • The directory ‘/tmp’.

     This function is defined for SVID compatibility.

     *Warning:* Between the time the pathname is constructed and the
     file is created another process might have created a file with the
     same name using ‘tempnam’, leading to a possible security hole.
     The implementation generates names which can hardly be predicted,
     but when opening the file you should use the ‘O_EXCL’ flag.  Using
     ‘tmpfile’ or ‘mkstemp’ is a safe way to avoid this problem.

 -- SVID Macro: char * P_tmpdir
     This macro is the name of the default directory for temporary
     files.

   Older Unix systems did not have the functions just described.
Instead they used ‘mktemp’ and ‘mkstemp’.  Both of these functions work
by modifying a file name template string you pass.  The last six
characters of this string must be ‘XXXXXX’.  These six ‘X’s are replaced
with six characters which make the whole string a unique file name.
Usually the template string is something like ‘/tmp/PREFIXXXXXXX’, and
each program uses a unique PREFIX.

   *NB:* Because ‘mktemp’ and ‘mkstemp’ modify the template string, you
_must not_ pass string constants to them.  String constants are normally
in read-only storage, so your program would crash when ‘mktemp’ or
‘mkstemp’ tried to modify the string.  These functions are declared in
the header file ‘stdlib.h’.

 -- Function: char * mktemp (char *TEMPLATE)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘mktemp’ function generates a unique file name by modifying
     TEMPLATE as described above.  If successful, it returns TEMPLATE as
     modified.  If ‘mktemp’ cannot find a unique file name, it makes
     TEMPLATE an empty string and returns that.  If TEMPLATE does not
     end with ‘XXXXXX’, ‘mktemp’ returns a null pointer.

     *Warning:* Between the time the pathname is constructed and the
     file is created another process might have created a file with the
     same name using ‘mktemp’, leading to a possible security hole.  The
     implementation generates names which can hardly be predicted, but
     when opening the file you should use the ‘O_EXCL’ flag.  Using
     ‘mkstemp’ is a safe way to avoid this problem.

 -- Function: int mkstemp (char *TEMPLATE)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe fd | *Note POSIX Safety
     Concepts::.

     The ‘mkstemp’ function generates a unique file name just as
     ‘mktemp’ does, but it also opens the file for you with ‘open’
     (*note Opening and Closing Files::).  If successful, it modifies
     TEMPLATE in place and returns a file descriptor for that file open
     for reading and writing.  If ‘mkstemp’ cannot create a
     uniquely-named file, it returns ‘-1’.  If TEMPLATE does not end
     with ‘XXXXXX’, ‘mkstemp’ returns ‘-1’ and does not modify TEMPLATE.

     The file is opened using mode ‘0600’.  If the file is meant to be
     used by other users this mode must be changed explicitly.

   Unlike ‘mktemp’, ‘mkstemp’ is actually guaranteed to create a unique
file that cannot possibly clash with any other program trying to create
a temporary file.  This is because it works by calling ‘open’ with the
‘O_EXCL’ flag, which says you want to create a new file and get an error
if the file already exists.

 -- Function: char * mkdtemp (char *TEMPLATE)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘mkdtemp’ function creates a directory with a unique name.  If
     it succeeds, it overwrites TEMPLATE with the name of the directory,
     and returns TEMPLATE.  As with ‘mktemp’ and ‘mkstemp’, TEMPLATE
     should be a string ending with ‘XXXXXX’.

     If ‘mkdtemp’ cannot create an uniquely named directory, it returns
     ‘NULL’ and sets ERRNO appropriately.  If TEMPLATE does not end with
     ‘XXXXXX’, ‘mkdtemp’ returns ‘NULL’ and does not modify TEMPLATE.
     ERRNO will be set to ‘EINVAL’ in this case.

     The directory is created using mode ‘0700’.

   The directory created by ‘mkdtemp’ cannot clash with temporary files
or directories created by other users.  This is because directory
creation always works like ‘open’ with ‘O_EXCL’.  *Note Creating
Directories::.

   The ‘mkdtemp’ function comes from OpenBSD.


File: libc.info,  Node: Pipes and FIFOs,  Next: Sockets,  Prev: File System Interface,  Up: Top

15 Pipes and FIFOs
******************

A "pipe" is a mechanism for interprocess communication; data written to
the pipe by one process can be read by another process.  The data is
handled in a first-in, first-out (FIFO) order.  The pipe has no name; it
is created for one use and both ends must be inherited from the single
process which created the pipe.

   A "FIFO special file" is similar to a pipe, but instead of being an
anonymous, temporary connection, a FIFO has a name or names like any
other file.  Processes open the FIFO by name in order to communicate
through it.

   A pipe or FIFO has to be open at both ends simultaneously.  If you
read from a pipe or FIFO file that doesn’t have any processes writing to
it (perhaps because they have all closed the file, or exited), the read
returns end-of-file.  Writing to a pipe or FIFO that doesn’t have a
reading process is treated as an error condition; it generates a
‘SIGPIPE’ signal, and fails with error code ‘EPIPE’ if the signal is
handled or blocked.

   Neither pipes nor FIFO special files allow file positioning.  Both
reading and writing operations happen sequentially; reading from the
beginning of the file and writing at the end.

* Menu:

* Creating a Pipe::             Making a pipe with the ‘pipe’ function.
* Pipe to a Subprocess::        Using a pipe to communicate with a
				 child process.
* FIFO Special Files::          Making a FIFO special file.
* Pipe Atomicity::		When pipe (or FIFO) I/O is atomic.


File: libc.info,  Node: Creating a Pipe,  Next: Pipe to a Subprocess,  Up: Pipes and FIFOs

15.1 Creating a Pipe
====================

The primitive for creating a pipe is the ‘pipe’ function.  This creates
both the reading and writing ends of the pipe.  It is not very useful
for a single process to use a pipe to talk to itself.  In typical use, a
process creates a pipe just before it forks one or more child processes
(*note Creating a Process::).  The pipe is then used for communication
either between the parent or child processes, or between two sibling
processes.

   The ‘pipe’ function is declared in the header file ‘unistd.h’.

 -- Function: int pipe (int FILEDES[2])
     Preliminary: | MT-Safe | AS-Safe | AC-Safe fd | *Note POSIX Safety
     Concepts::.

     The ‘pipe’ function creates a pipe and puts the file descriptors
     for the reading and writing ends of the pipe (respectively) into
     ‘FILEDES[0]’ and ‘FILEDES[1]’.

     An easy way to remember that the input end comes first is that file
     descriptor ‘0’ is standard input, and file descriptor ‘1’ is
     standard output.

     If successful, ‘pipe’ returns a value of ‘0’.  On failure, ‘-1’ is
     returned.  The following ‘errno’ error conditions are defined for
     this function:

     ‘EMFILE’
          The process has too many files open.

     ‘ENFILE’
          There are too many open files in the entire system.  *Note
          Error Codes::, for more information about ‘ENFILE’.  This
          error never occurs on GNU/Hurd systems.

   Here is an example of a simple program that creates a pipe.  This
program uses the ‘fork’ function (*note Creating a Process::) to create
a child process.  The parent process writes data to the pipe, which is
read by the child process.


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

     /* Read characters from the pipe and echo them to ‘stdout’. */

     void
     read_from_pipe (int file)
     {
       FILE *stream;
       int c;
       stream = fdopen (file, "r");
       while ((c = fgetc (stream)) != EOF)
         putchar (c);
       fclose (stream);
     }

     /* Write some random text to the pipe. */

     void
     write_to_pipe (int file)
     {
       FILE *stream;
       stream = fdopen (file, "w");
       fprintf (stream, "hello, world!\n");
       fprintf (stream, "goodbye, world!\n");
       fclose (stream);
     }

     int
     main (void)
     {
       pid_t pid;
       int mypipe[2];

       /* Create the pipe. */
       if (pipe (mypipe))
         {
           fprintf (stderr, "Pipe failed.\n");
           return EXIT_FAILURE;
         }

       /* Create the child process. */
       pid = fork ();
       if (pid == (pid_t) 0)
         {
           /* This is the child process.
              Close other end first. */
           close (mypipe[1]);
           read_from_pipe (mypipe[0]);
           return EXIT_SUCCESS;
         }
       else if (pid < (pid_t) 0)
         {
           /* The fork failed. */
           fprintf (stderr, "Fork failed.\n");
           return EXIT_FAILURE;
         }
       else
         {
           /* This is the parent process.
              Close other end first. */
           close (mypipe[0]);
           write_to_pipe (mypipe[1]);
           return EXIT_SUCCESS;
         }
     }


File: libc.info,  Node: Pipe to a Subprocess,  Next: FIFO Special Files,  Prev: Creating a Pipe,  Up: Pipes and FIFOs

15.2 Pipe to a Subprocess
=========================

A common use of pipes is to send data to or receive data from a program
being run as a subprocess.  One way of doing this is by using a
combination of ‘pipe’ (to create the pipe), ‘fork’ (to create the
subprocess), ‘dup2’ (to force the subprocess to use the pipe as its
standard input or output channel), and ‘exec’ (to execute the new
program).  Or, you can use ‘popen’ and ‘pclose’.

   The advantage of using ‘popen’ and ‘pclose’ is that the interface is
much simpler and easier to use.  But it doesn’t offer as much
flexibility as using the low-level functions directly.

 -- Function: FILE * popen (const char *COMMAND, const char *MODE)
     Preliminary: | MT-Safe | AS-Unsafe heap corrupt | AC-Unsafe corrupt
     lock fd mem | *Note POSIX Safety Concepts::.

     The ‘popen’ function is closely related to the ‘system’ function;
     see *note Running a Command::.  It executes the shell command
     COMMAND as a subprocess.  However, instead of waiting for the
     command to complete, it creates a pipe to the subprocess and
     returns a stream that corresponds to that pipe.

     If you specify a MODE argument of ‘"r"’, you can read from the
     stream to retrieve data from the standard output channel of the
     subprocess.  The subprocess inherits its standard input channel
     from the parent process.

     Similarly, if you specify a MODE argument of ‘"w"’, you can write
     to the stream to send data to the standard input channel of the
     subprocess.  The subprocess inherits its standard output channel
     from the parent process.

     In the event of an error ‘popen’ returns a null pointer.  This
     might happen if the pipe or stream cannot be created, if the
     subprocess cannot be forked, or if the program cannot be executed.

 -- Function: int pclose (FILE *STREAM)
     Preliminary: | MT-Safe | AS-Unsafe heap plugin corrupt lock |
     AC-Unsafe corrupt lock fd mem | *Note POSIX Safety Concepts::.

     The ‘pclose’ function is used to close a stream created by ‘popen’.
     It waits for the child process to terminate and returns its status
     value, as for the ‘system’ function.

   Here is an example showing how to use ‘popen’ and ‘pclose’ to filter
output through another program, in this case the paging program ‘more’.


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

     void
     write_data (FILE * stream)
     {
       int i;
       for (i = 0; i < 100; i++)
         fprintf (stream, "%d\n", i);
       if (ferror (stream))
         {
           fprintf (stderr, "Output to stream failed.\n");
           exit (EXIT_FAILURE);
         }
     }

     int
     main (void)
     {
       FILE *output;

       output = popen ("more", "w");
       if (!output)
         {
           fprintf (stderr,
                    "incorrect parameters or too many files.\n");
           return EXIT_FAILURE;
         }
       write_data (output);
       if (pclose (output) != 0)
         {
           fprintf (stderr,
                    "Could not run more or other error.\n");
         }
       return EXIT_SUCCESS;
     }


File: libc.info,  Node: FIFO Special Files,  Next: Pipe Atomicity,  Prev: Pipe to a Subprocess,  Up: Pipes and FIFOs

15.3 FIFO Special Files
=======================

A FIFO special file is similar to a pipe, except that it is created in a
different way.  Instead of being an anonymous communications channel, a
FIFO special file is entered into the file system by calling ‘mkfifo’.

   Once you have created a FIFO special file in this way, any process
can open it for reading or writing, in the same way as an ordinary file.
However, it has to be open at both ends simultaneously before you can
proceed to do any input or output operations on it.  Opening a FIFO for
reading normally blocks until some other process opens the same FIFO for
writing, and vice versa.

   The ‘mkfifo’ function is declared in the header file ‘sys/stat.h’.

 -- Function: int mkfifo (const char *FILENAME, mode_t MODE)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘mkfifo’ function makes a FIFO special file with name FILENAME.
     The MODE argument is used to set the file’s permissions; see *note
     Setting Permissions::.

     The normal, successful return value from ‘mkfifo’ is ‘0’.  In the
     case of an error, ‘-1’ is returned.  In addition to the usual file
     name errors (*note File Name Errors::), the following ‘errno’ error
     conditions are defined for this function:

     ‘EEXIST’
          The named file already exists.

     ‘ENOSPC’
          The directory or file system cannot be extended.

     ‘EROFS’
          The directory that would contain the file resides on a
          read-only file system.


File: libc.info,  Node: Pipe Atomicity,  Prev: FIFO Special Files,  Up: Pipes and FIFOs

15.4 Atomicity of Pipe I/O
==========================

Reading or writing pipe data is "atomic" if the size of data written is
not greater than ‘PIPE_BUF’.  This means that the data transfer seems to
be an instantaneous unit, in that nothing else in the system can observe
a state in which it is partially complete.  Atomic I/O may not begin
right away (it may need to wait for buffer space or for data), but once
it does begin it finishes immediately.

   Reading or writing a larger amount of data may not be atomic; for
example, output data from other processes sharing the descriptor may be
interspersed.  Also, once ‘PIPE_BUF’ characters have been written,
further writes will block until some characters are read.

   *Note Limits for Files::, for information about the ‘PIPE_BUF’
parameter.


File: libc.info,  Node: Sockets,  Next: Low-Level Terminal Interface,  Prev: Pipes and FIFOs,  Up: Top

16 Sockets
**********

This chapter describes the GNU facilities for interprocess communication
using sockets.

   A "socket" is a generalized interprocess communication channel.  Like
a pipe, a socket is represented as a file descriptor.  Unlike pipes
sockets support communication between unrelated processes, and even
between processes running on different machines that communicate over a
network.  Sockets are the primary means of communicating with other
machines; ‘telnet’, ‘rlogin’, ‘ftp’, ‘talk’ and the other familiar
network programs use sockets.

   Not all operating systems support sockets.  In the GNU C Library, the
header file ‘sys/socket.h’ exists regardless of the operating system,
and the socket functions always exist, but if the system does not really
support sockets these functions always fail.

   *Incomplete:* We do not currently document the facilities for
broadcast messages or for configuring Internet interfaces.  The
reentrant functions and some newer functions that are related to IPv6
aren’t documented either so far.

* Menu:

* Socket Concepts::	Basic concepts you need to know about.
* Communication Styles::Stream communication, datagrams and other styles.
* Socket Addresses::	How socket names (“addresses”) work.
* Interface Naming::	Identifying specific network interfaces.
* Local Namespace::	Details about the local namespace.
* Internet Namespace::	Details about the Internet namespace.
* Misc Namespaces::	Other namespaces not documented fully here.
* Open/Close Sockets::  Creating sockets and destroying them.
* Connections::		Operations on sockets with connection state.
* Datagrams::		Operations on datagram sockets.
* Inetd::		Inetd is a daemon that starts servers on request.
			   The most convenient way to write a server
			   is to make it work with Inetd.
* Socket Options::	Miscellaneous low-level socket options.
* Networks Database::   Accessing the database of network names.


File: libc.info,  Node: Socket Concepts,  Next: Communication Styles,  Up: Sockets

16.1 Socket Concepts
====================

When you create a socket, you must specify the style of communication
you want to use and the type of protocol that should implement it.  The
"communication style" of a socket defines the user-level semantics of
sending and receiving data on the socket.  Choosing a communication
style specifies the answers to questions such as these:

   • *What are the units of data transmission?*  Some communication
     styles regard the data as a sequence of bytes with no larger
     structure; others group the bytes into records (which are known in
     this context as "packets").

   • *Can data be lost during normal operation?*  Some communication
     styles guarantee that all the data sent arrives in the order it was
     sent (barring system or network crashes); other styles occasionally
     lose data as a normal part of operation, and may sometimes deliver
     packets more than once or in the wrong order.

     Designing a program to use unreliable communication styles usually
     involves taking precautions to detect lost or misordered packets
     and to retransmit data as needed.

   • *Is communication entirely with one partner?*  Some communication
     styles are like a telephone call—you make a "connection" with one
     remote socket and then exchange data freely.  Other styles are like
     mailing letters—you specify a destination address for each message
     you send.

   You must also choose a "namespace" for naming the socket.  A socket
name (“address”) is meaningful only in the context of a particular
namespace.  In fact, even the data type to use for a socket name may
depend on the namespace.  Namespaces are also called “domains”, but we
avoid that word as it can be confused with other usage of the same term.
Each namespace has a symbolic name that starts with ‘PF_’.  A
corresponding symbolic name starting with ‘AF_’ designates the address
format for that namespace.

   Finally you must choose the "protocol" to carry out the
communication.  The protocol determines what low-level mechanism is used
to transmit and receive data.  Each protocol is valid for a particular
namespace and communication style; a namespace is sometimes called a
"protocol family" because of this, which is why the namespace names
start with ‘PF_’.

   The rules of a protocol apply to the data passing between two
programs, perhaps on different computers; most of these rules are
handled by the operating system and you need not know about them.  What
you do need to know about protocols is this:

   • In order to have communication between two sockets, they must
     specify the _same_ protocol.

   • Each protocol is meaningful with particular style/namespace
     combinations and cannot be used with inappropriate combinations.
     For example, the TCP protocol fits only the byte stream style of
     communication and the Internet namespace.

   • For each combination of style and namespace there is a "default
     protocol", which you can request by specifying 0 as the protocol
     number.  And that’s what you should normally do—use the default.

   Throughout the following description at various places
variables/parameters to denote sizes are required.  And here the trouble
starts.  In the first implementations the type of these variables was
simply ‘int’.  On most machines at that time an ‘int’ was 32 bits wide,
which created a _de facto_ standard requiring 32-bit variables.  This is
important since references to variables of this type are passed to the
kernel.

   Then the POSIX people came and unified the interface with the words
"all size values are of type ‘size_t’".  On 64-bit machines ‘size_t’ is
64 bits wide, so pointers to variables were no longer possible.

   The Unix98 specification provides a solution by introducing a type
‘socklen_t’.  This type is used in all of the cases that POSIX changed
to use ‘size_t’.  The only requirement of this type is that it be an
unsigned type of at least 32 bits.  Therefore, implementations which
require that references to 32-bit variables be passed can be as happy as
implementations which use 64-bit values.


File: libc.info,  Node: Communication Styles,  Next: Socket Addresses,  Prev: Socket Concepts,  Up: Sockets

16.2 Communication Styles
=========================

The GNU C Library includes support for several different kinds of
sockets, each with different characteristics.  This section describes
the supported socket types.  The symbolic constants listed here are
defined in ‘sys/socket.h’.

 -- Macro: int SOCK_STREAM
     The ‘SOCK_STREAM’ style is like a pipe (*note Pipes and FIFOs::).
     It operates over a connection with a particular remote socket and
     transmits data reliably as a stream of bytes.

     Use of this style is covered in detail in *note Connections::.

 -- Macro: int SOCK_DGRAM
     The ‘SOCK_DGRAM’ style is used for sending individually-addressed
     packets unreliably.  It is the diametrical opposite of
     ‘SOCK_STREAM’.

     Each time you write data to a socket of this kind, that data
     becomes one packet.  Since ‘SOCK_DGRAM’ sockets do not have
     connections, you must specify the recipient address with each
     packet.

     The only guarantee that the system makes about your requests to
     transmit data is that it will try its best to deliver each packet
     you send.  It may succeed with the sixth packet after failing with
     the fourth and fifth packets; the seventh packet may arrive before
     the sixth, and may arrive a second time after the sixth.

     The typical use for ‘SOCK_DGRAM’ is in situations where it is
     acceptable to simply re-send a packet if no response is seen in a
     reasonable amount of time.

     *Note Datagrams::, for detailed information about how to use
     datagram sockets.

 -- Macro: int SOCK_RAW
     This style provides access to low-level network protocols and
     interfaces.  Ordinary user programs usually have no need to use
     this style.


File: libc.info,  Node: Socket Addresses,  Next: Interface Naming,  Prev: Communication Styles,  Up: Sockets

16.3 Socket Addresses
=====================

The name of a socket is normally called an "address".  The functions and
symbols for dealing with socket addresses were named inconsistently,
sometimes using the term “name” and sometimes using “address”.  You can
regard these terms as synonymous where sockets are concerned.

   A socket newly created with the ‘socket’ function has no address.
Other processes can find it for communication only if you give it an
address.  We call this "binding" the address to the socket, and the way
to do it is with the ‘bind’ function.

   You need only be concerned with the address of a socket if other
processes are to find it and start communicating with it.  You can
specify an address for other sockets, but this is usually pointless; the
first time you send data from a socket, or use it to initiate a
connection, the system assigns an address automatically if you have not
specified one.

   Occasionally a client needs to specify an address because the server
discriminates based on address; for example, the rsh and rlogin
protocols look at the client’s socket address and only bypass password
checking if it is less than ‘IPPORT_RESERVED’ (*note Ports::).

   The details of socket addresses vary depending on what namespace you
are using.  *Note Local Namespace::, or *note Internet Namespace::, for
specific information.

   Regardless of the namespace, you use the same functions ‘bind’ and
‘getsockname’ to set and examine a socket’s address.  These functions
use a phony data type, ‘struct sockaddr *’, to accept the address.  In
practice, the address lives in a structure of some other data type
appropriate to the address format you are using, but you cast its
address to ‘struct sockaddr *’ when you pass it to ‘bind’.

* Menu:

* Address Formats::		About ‘struct sockaddr’.
* Setting Address::		Binding an address to a socket.
* Reading Address::		Reading the address of a socket.


File: libc.info,  Node: Address Formats,  Next: Setting Address,  Up: Socket Addresses

16.3.1 Address Formats
----------------------

The functions ‘bind’ and ‘getsockname’ use the generic data type ‘struct
sockaddr *’ to represent a pointer to a socket address.  You can’t use
this data type effectively to interpret an address or construct one; for
that, you must use the proper data type for the socket’s namespace.

   Thus, the usual practice is to construct an address of the proper
namespace-specific type, then cast a pointer to ‘struct sockaddr *’ when
you call ‘bind’ or ‘getsockname’.

   The one piece of information that you can get from the ‘struct
sockaddr’ data type is the "address format designator".  This tells you
which data type to use to understand the address fully.

   The symbols in this section are defined in the header file
‘sys/socket.h’.

 -- Data Type: struct sockaddr
     The ‘struct sockaddr’ type itself has the following members:

     ‘short int sa_family’
          This is the code for the address format of this address.  It
          identifies the format of the data which follows.

     ‘char sa_data[14]’
          This is the actual socket address data, which is
          format-dependent.  Its length also depends on the format, and
          may well be more than 14.  The length 14 of ‘sa_data’ is
          essentially arbitrary.

   Each address format has a symbolic name which starts with ‘AF_’.
Each of them corresponds to a ‘PF_’ symbol which designates the
corresponding namespace.  Here is a list of address format names:

‘AF_LOCAL’
     This designates the address format that goes with the local
     namespace.  (‘PF_LOCAL’ is the name of that namespace.)  *Note
     Local Namespace Details::, for information about this address
     format.

‘AF_UNIX’
     This is a synonym for ‘AF_LOCAL’.  Although ‘AF_LOCAL’ is mandated
     by POSIX.1g, ‘AF_UNIX’ is portable to more systems.  ‘AF_UNIX’ was
     the traditional name stemming from BSD, so even most POSIX systems
     support it.  It is also the name of choice in the Unix98
     specification.  (The same is true for ‘PF_UNIX’ vs.  ‘PF_LOCAL’).

‘AF_FILE’
     This is another synonym for ‘AF_LOCAL’, for compatibility.
     (‘PF_FILE’ is likewise a synonym for ‘PF_LOCAL’.)

‘AF_INET’
     This designates the address format that goes with the Internet
     namespace.  (‘PF_INET’ is the name of that namespace.)  *Note
     Internet Address Formats::.

‘AF_INET6’
     This is similar to ‘AF_INET’, but refers to the IPv6 protocol.
     (‘PF_INET6’ is the name of the corresponding namespace.)

‘AF_UNSPEC’
     This designates no particular address format.  It is used only in
     rare cases, such as to clear out the default destination address of
     a “connected” datagram socket.  *Note Sending Datagrams::.

     The corresponding namespace designator symbol ‘PF_UNSPEC’ exists
     for completeness, but there is no reason to use it in a program.

   ‘sys/socket.h’ defines symbols starting with ‘AF_’ for many different
kinds of networks, most or all of which are not actually implemented.
We will document those that really work as we receive information about
how to use them.


File: libc.info,  Node: Setting Address,  Next: Reading Address,  Prev: Address Formats,  Up: Socket Addresses

16.3.2 Setting the Address of a Socket
--------------------------------------

Use the ‘bind’ function to assign an address to a socket.  The prototype
for ‘bind’ is in the header file ‘sys/socket.h’.  For examples of use,
see *note Local Socket Example::, or see *note Inet Example::.

 -- Function: int bind (int SOCKET, struct sockaddr *ADDR, socklen_t
          LENGTH)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The ‘bind’ function assigns an address to the socket SOCKET.  The
     ADDR and LENGTH arguments specify the address; the detailed format
     of the address depends on the namespace.  The first part of the
     address is always the format designator, which specifies a
     namespace, and says that the address is in the format of that
     namespace.

     The return value is ‘0’ on success and ‘-1’ on failure.  The
     following ‘errno’ error conditions are defined for this function:

     ‘EBADF’
          The SOCKET argument is not a valid file descriptor.

     ‘ENOTSOCK’
          The descriptor SOCKET is not a socket.

     ‘EADDRNOTAVAIL’
          The specified address is not available on this machine.

     ‘EADDRINUSE’
          Some other socket is already using the specified address.

     ‘EINVAL’
          The socket SOCKET already has an address.

     ‘EACCES’
          You do not have permission to access the requested address.
          (In the Internet domain, only the super-user is allowed to
          specify a port number in the range 0 through ‘IPPORT_RESERVED’
          minus one; see *note Ports::.)

     Additional conditions may be possible depending on the particular
     namespace of the socket.


File: libc.info,  Node: Reading Address,  Prev: Setting Address,  Up: Socket Addresses

16.3.3 Reading the Address of a Socket
--------------------------------------

Use the function ‘getsockname’ to examine the address of an Internet
socket.  The prototype for this function is in the header file
‘sys/socket.h’.

 -- Function: int getsockname (int SOCKET, struct sockaddr *ADDR,
          socklen_t *LENGTH-PTR)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe mem/hurd | *Note POSIX
     Safety Concepts::.

     The ‘getsockname’ function returns information about the address of
     the socket SOCKET in the locations specified by the ADDR and
     LENGTH-PTR arguments.  Note that the LENGTH-PTR is a pointer; you
     should initialize it to be the allocation size of ADDR, and on
     return it contains the actual size of the address data.

     The format of the address data depends on the socket namespace.
     The length of the information is usually fixed for a given
     namespace, so normally you can know exactly how much space is
     needed and can provide that much.  The usual practice is to
     allocate a place for the value using the proper data type for the
     socket’s namespace, then cast its address to ‘struct sockaddr *’ to
     pass it to ‘getsockname’.

     The return value is ‘0’ on success and ‘-1’ on error.  The
     following ‘errno’ error conditions are defined for this function:

     ‘EBADF’
          The SOCKET argument is not a valid file descriptor.

     ‘ENOTSOCK’
          The descriptor SOCKET is not a socket.

     ‘ENOBUFS’
          There are not enough internal buffers available for the
          operation.

   You can’t read the address of a socket in the file namespace.  This
is consistent with the rest of the system; in general, there’s no way to
find a file’s name from a descriptor for that file.


File: libc.info,  Node: Interface Naming,  Next: Local Namespace,  Prev: Socket Addresses,  Up: Sockets

16.4 Interface Naming
=====================

Each network interface has a name.  This usually consists of a few
letters that relate to the type of interface, which may be followed by a
number if there is more than one interface of that type.  Examples might
be ‘lo’ (the loopback interface) and ‘eth0’ (the first Ethernet
interface).

   Although such names are convenient for humans, it would be clumsy to
have to use them whenever a program needs to refer to an interface.  In
such situations an interface is referred to by its "index", which is an
arbitrarily-assigned small positive integer.

   The following functions, constants and data types are declared in the
header file ‘net/if.h’.

 -- Constant: size_t IFNAMSIZ
     This constant defines the maximum buffer size needed to hold an
     interface name, including its terminating zero byte.

 -- Function: unsigned int if_nametoindex (const char *IFNAME)
     Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd | *Note
     POSIX Safety Concepts::.

     This function yields the interface index corresponding to a
     particular name.  If no interface exists with the name given, it
     returns 0.

 -- Function: char * if_indextoname (unsigned int IFINDEX, char *IFNAME)
     Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd | *Note
     POSIX Safety Concepts::.

     This function maps an interface index to its corresponding name.
     The returned name is placed in the buffer pointed to by ‘ifname’,
     which must be at least ‘IFNAMSIZ’ bytes in length.  If the index
     was invalid, the function’s return value is a null pointer,
     otherwise it is ‘ifname’.

 -- Data Type: struct if_nameindex
     This data type is used to hold the information about a single
     interface.  It has the following members:

     ‘unsigned int if_index;’
          This is the interface index.

     ‘char *if_name’
          This is the null-terminated index name.

 -- Function: struct if_nameindex * if_nameindex (void)
     Preliminary: | MT-Safe | AS-Unsafe heap lock/hurd | AC-Unsafe
     lock/hurd fd mem | *Note POSIX Safety Concepts::.

     This function returns an array of ‘if_nameindex’ structures, one
     for every interface that is present.  The end of the list is
     indicated by a structure with an interface of 0 and a null name
     pointer.  If an error occurs, this function returns a null pointer.

     The returned structure must be freed with ‘if_freenameindex’ after
     use.

 -- Function: void if_freenameindex (struct if_nameindex *PTR)
     Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | *Note
     POSIX Safety Concepts::.

     This function frees the structure returned by an earlier call to
     ‘if_nameindex’.


File: libc.info,  Node: Local Namespace,  Next: Internet Namespace,  Prev: Interface Naming,  Up: Sockets

16.5 The Local Namespace
========================

This section describes the details of the local namespace, whose
symbolic name (required when you create a socket) is ‘PF_LOCAL’.  The
local namespace is also known as “Unix domain sockets”.  Another name is
file namespace since socket addresses are normally implemented as file
names.

* Menu:

* Concepts: Local Namespace Concepts. What you need to understand.
* Details: Local Namespace Details.   Address format, symbolic names, etc.
* Example: Local Socket Example.      Example of creating a socket.


File: libc.info,  Node: Local Namespace Concepts,  Next: Local Namespace Details,  Up: Local Namespace

16.5.1 Local Namespace Concepts
-------------------------------

In the local namespace socket addresses are file names.  You can specify
any file name you want as the address of the socket, but you must have
write permission on the directory containing it.  It’s common to put
these files in the ‘/tmp’ directory.

   One peculiarity of the local namespace is that the name is only used
when opening the connection; once open the address is not meaningful and
may not exist.

   Another peculiarity is that you cannot connect to such a socket from
another machine–not even if the other machine shares the file system
which contains the name of the socket.  You can see the socket in a
directory listing, but connecting to it never succeeds.  Some programs
take advantage of this, such as by asking the client to send its own
process ID, and using the process IDs to distinguish between clients.
However, we recommend you not use this method in protocols you design,
as we might someday permit connections from other machines that mount
the same file systems.  Instead, send each new client an identifying
number if you want it to have one.

   After you close a socket in the local namespace, you should delete
the file name from the file system.  Use ‘unlink’ or ‘remove’ to do
this; see *note Deleting Files::.

   The local namespace supports just one protocol for any communication
style; it is protocol number ‘0’.


File: libc.info,  Node: Local Namespace Details,  Next: Local Socket Example,  Prev: Local Namespace Concepts,  Up: Local Namespace

16.5.2 Details of Local Namespace
---------------------------------

To create a socket in the local namespace, use the constant ‘PF_LOCAL’
as the NAMESPACE argument to ‘socket’ or ‘socketpair’.  This constant is
defined in ‘sys/socket.h’.

 -- Macro: int PF_LOCAL
     This designates the local namespace, in which socket addresses are
     local names, and its associated family of protocols.  ‘PF_LOCAL’ is
     the macro used by POSIX.1g.

 -- Macro: int PF_UNIX
     This is a synonym for ‘PF_LOCAL’, for compatibility’s sake.

 -- Macro: int PF_FILE
     This is a synonym for ‘PF_LOCAL’, for compatibility’s sake.

   The structure for specifying socket names in the local namespace is
defined in the header file ‘sys/un.h’:

 -- Data Type: struct sockaddr_un
     This structure is used to specify local namespace socket addresses.
     It has the following members:

     ‘short int sun_family’
          This identifies the address family or format of the socket
          address.  You should store the value ‘AF_LOCAL’ to designate
          the local namespace.  *Note Socket Addresses::.

     ‘char sun_path[108]’
          This is the file name to use.

          *Incomplete:* Why is 108 a magic number?  RMS suggests making
          this a zero-length array and tweaking the following example to
          use ‘alloca’ to allocate an appropriate amount of storage
          based on the length of the filename.

   You should compute the LENGTH parameter for a socket address in the
local namespace as the sum of the size of the ‘sun_family’ component and
the string length (_not_ the allocation size!)  of the file name string.
This can be done using the macro ‘SUN_LEN’:

 -- Macro: int SUN_LEN (_struct sockaddr_un *_ PTR)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This macro computes the length of the socket address in the local
     namespace.


File: libc.info,  Node: Local Socket Example,  Prev: Local Namespace Details,  Up: Local Namespace

16.5.3 Example of Local-Namespace Sockets
-----------------------------------------

Here is an example showing how to create and name a socket in the local
namespace.


     #include <stddef.h>
     #include <stdio.h>
     #include <errno.h>
     #include <stdlib.h>
     #include <string.h>
     #include <sys/socket.h>
     #include <sys/un.h>

     int
     make_named_socket (const char *filename)
     {
       struct sockaddr_un name;
       int sock;
       size_t size;

       /* Create the socket. */
       sock = socket (PF_LOCAL, SOCK_DGRAM, 0);
       if (sock < 0)
         {
           perror ("socket");
           exit (EXIT_FAILURE);
         }

       /* Bind a name to the socket. */
       name.sun_family = AF_LOCAL;
       strncpy (name.sun_path, filename, sizeof (name.sun_path));
       name.sun_path[sizeof (name.sun_path) - 1] = '\0';

       /* The size of the address is
          the offset of the start of the filename,
          plus its length (not including the terminating null byte).
          Alternatively you can just do:
          size = SUN_LEN (&name);
      */
       size = (offsetof (struct sockaddr_un, sun_path)
               + strlen (name.sun_path));

       if (bind (sock, (struct sockaddr *) &name, size) < 0)
         {
           perror ("bind");
           exit (EXIT_FAILURE);
         }

       return sock;
     }


File: libc.info,  Node: Internet Namespace,  Next: Misc Namespaces,  Prev: Local Namespace,  Up: Sockets

16.6 The Internet Namespace
===========================

This section describes the details of the protocols and socket naming
conventions used in the Internet namespace.

   Originally the Internet namespace used only IP version 4 (IPv4).
With the growing number of hosts on the Internet, a new protocol with a
larger address space was necessary: IP version 6 (IPv6).  IPv6
introduces 128-bit addresses (IPv4 has 32-bit addresses) and other
features, and will eventually replace IPv4.

   To create a socket in the IPv4 Internet namespace, use the symbolic
name ‘PF_INET’ of this namespace as the NAMESPACE argument to ‘socket’
or ‘socketpair’.  For IPv6 addresses you need the macro ‘PF_INET6’.
These macros are defined in ‘sys/socket.h’.

 -- Macro: int PF_INET
     This designates the IPv4 Internet namespace and associated family
     of protocols.

 -- Macro: int PF_INET6
     This designates the IPv6 Internet namespace and associated family
     of protocols.

   A socket address for the Internet namespace includes the following
components:

   • The address of the machine you want to connect to.  Internet
     addresses can be specified in several ways; these are discussed in
     *note Internet Address Formats::, *note Host Addresses:: and *note
     Host Names::.

   • A port number for that machine.  *Note Ports::.

   You must ensure that the address and port number are represented in a
canonical format called "network byte order".  *Note Byte Order::, for
information about this.

* Menu:

* Internet Address Formats::    How socket addresses are specified in the
                                 Internet namespace.
* Host Addresses::	        All about host addresses of Internet host.
* Ports::			Internet port numbers.
* Services Database::           Ports may have symbolic names.
* Byte Order::		        Different hosts may use different byte
                                 ordering conventions; you need to
                                 canonicalize host address and port number.
* Protocols Database::		Referring to protocols by name.
* Inet Example::	        Putting it all together.


File: libc.info,  Node: Internet Address Formats,  Next: Host Addresses,  Up: Internet Namespace

16.6.1 Internet Socket Address Formats
--------------------------------------

In the Internet namespace, for both IPv4 (‘AF_INET’) and IPv6
(‘AF_INET6’), a socket address consists of a host address and a port on
that host.  In addition, the protocol you choose serves effectively as a
part of the address because local port numbers are meaningful only
within a particular protocol.

   The data types for representing socket addresses in the Internet
namespace are defined in the header file ‘netinet/in.h’.

 -- Data Type: struct sockaddr_in
     This is the data type used to represent socket addresses in the
     Internet namespace.  It has the following members:

     ‘sa_family_t sin_family’
          This identifies the address family or format of the socket
          address.  You should store the value ‘AF_INET’ in this member.
          *Note Socket Addresses::.

     ‘struct in_addr sin_addr’
          This is the Internet address of the host machine.  *Note Host
          Addresses::, and *note Host Names::, for how to get a value to
          store here.

     ‘unsigned short int sin_port’
          This is the port number.  *Note Ports::.

   When you call ‘bind’ or ‘getsockname’, you should specify ‘sizeof
(struct sockaddr_in)’ as the LENGTH parameter if you are using an IPv4
Internet namespace socket address.

 -- Data Type: struct sockaddr_in6
     This is the data type used to represent socket addresses in the
     IPv6 namespace.  It has the following members:

     ‘sa_family_t sin6_family’
          This identifies the address family or format of the socket
          address.  You should store the value of ‘AF_INET6’ in this
          member.  *Note Socket Addresses::.

     ‘struct in6_addr sin6_addr’
          This is the IPv6 address of the host machine.  *Note Host
          Addresses::, and *note Host Names::, for how to get a value to
          store here.

     ‘uint32_t sin6_flowinfo’
          This is a currently unimplemented field.

     ‘uint16_t sin6_port’
          This is the port number.  *Note Ports::.


File: libc.info,  Node: Host Addresses,  Next: Ports,  Prev: Internet Address Formats,  Up: Internet Namespace

16.6.2 Host Addresses
---------------------

Each computer on the Internet has one or more "Internet addresses",
numbers which identify that computer among all those on the Internet.
Users typically write IPv4 numeric host addresses as sequences of four
numbers, separated by periods, as in ‘128.52.46.32’, and IPv6 numeric
host addresses as sequences of up to eight numbers separated by colons,
as in ‘5f03:1200:836f:c100::1’.

   Each computer also has one or more "host names", which are strings of
words separated by periods, as in ‘www.gnu.org’.

   Programs that let the user specify a host typically accept both
numeric addresses and host names.  To open a connection a program needs
a numeric address, and so must convert a host name to the numeric
address it stands for.

* Menu:

* Abstract Host Addresses::	What a host number consists of.
* Data type: Host Address Data Type.	Data type for a host number.
* Functions: Host Address Functions.	Functions to operate on them.
* Names: Host Names.		Translating host names to host numbers.


File: libc.info,  Node: Abstract Host Addresses,  Next: Host Address Data Type,  Up: Host Addresses

16.6.2.1 Internet Host Addresses
................................

Each computer on the Internet has one or more Internet addresses,
numbers which identify that computer among all those on the Internet.

   An IPv4 Internet host address is a number containing four bytes of
data.  Historically these are divided into two parts, a "network number"
and a "local network address number" within that network.  In the
mid-1990s classless addresses were introduced which changed this
behavior.  Since some functions implicitly expect the old definitions,
we first describe the class-based network and will then describe
classless addresses.  IPv6 uses only classless addresses and therefore
the following paragraphs don’t apply.

   The class-based IPv4 network number consists of the first one, two or
three bytes; the rest of the bytes are the local address.

   IPv4 network numbers are registered with the Network Information
Center (NIC), and are divided into three classes—A, B and C. The local
network address numbers of individual machines are registered with the
administrator of the particular network.

   Class A networks have single-byte numbers in the range 0 to 127.
There are only a small number of Class A networks, but they can each
support a very large number of hosts.  Medium-sized Class B networks
have two-byte network numbers, with the first byte in the range 128 to
191.  Class C networks are the smallest; they have three-byte network
numbers, with the first byte in the range 192-255.  Thus, the first 1,
2, or 3 bytes of an Internet address specify a network.  The remaining
bytes of the Internet address specify the address within that network.

   The Class A network 0 is reserved for broadcast to all networks.  In
addition, the host number 0 within each network is reserved for
broadcast to all hosts in that network.  These uses are obsolete now but
for compatibility reasons you shouldn’t use network 0 and host number 0.

   The Class A network 127 is reserved for loopback; you can always use
the Internet address ‘127.0.0.1’ to refer to the host machine.

   Since a single machine can be a member of multiple networks, it can
have multiple Internet host addresses.  However, there is never supposed
to be more than one machine with the same host address.

   There are four forms of the "standard numbers-and-dots notation" for
Internet addresses:

‘A.B.C.D’
     This specifies all four bytes of the address individually and is
     the commonly used representation.

‘A.B.C’
     The last part of the address, C, is interpreted as a 2-byte
     quantity.  This is useful for specifying host addresses in a Class
     B network with network address number ‘A.B’.

‘A.B’
     The last part of the address, B, is interpreted as a 3-byte
     quantity.  This is useful for specifying host addresses in a Class
     A network with network address number A.

‘A’
     If only one part is given, this corresponds directly to the host
     address number.

   Within each part of the address, the usual C conventions for
specifying the radix apply.  In other words, a leading ‘0x’ or ‘0X’
implies hexadecimal radix; a leading ‘0’ implies octal; and otherwise
decimal radix is assumed.

Classless Addresses
...................

IPv4 addresses (and IPv6 addresses also) are now considered classless;
the distinction between classes A, B and C can be ignored.  Instead an
IPv4 host address consists of a 32-bit address and a 32-bit mask.  The
mask contains set bits for the network part and cleared bits for the
host part.  The network part is contiguous from the left, with the
remaining bits representing the host.  As a consequence, the netmask can
simply be specified as the number of set bits.  Classes A, B and C are
just special cases of this general rule.  For example, class A addresses
have a netmask of ‘255.0.0.0’ or a prefix length of 8.

   Classless IPv4 network addresses are written in numbers-and-dots
notation with the prefix length appended and a slash as separator.  For
example the class A network 10 is written as ‘10.0.0.0/8’.

IPv6 Addresses
..............

IPv6 addresses contain 128 bits (IPv4 has 32 bits) of data.  A host
address is usually written as eight 16-bit hexadecimal numbers that are
separated by colons.  Two colons are used to abbreviate strings of
consecutive zeros.  For example, the IPv6 loopback address
‘0:0:0:0:0:0:0:1’ can just be written as ‘::1’.


File: libc.info,  Node: Host Address Data Type,  Next: Host Address Functions,  Prev: Abstract Host Addresses,  Up: Host Addresses

16.6.2.2 Host Address Data Type
...............................

IPv4 Internet host addresses are represented in some contexts as
integers (type ‘uint32_t’).  In other contexts, the integer is packaged
inside a structure of type ‘struct in_addr’.  It would be better if the
usage were made consistent, but it is not hard to extract the integer
from the structure or put the integer into a structure.

   You will find older code that uses ‘unsigned long int’ for IPv4
Internet host addresses instead of ‘uint32_t’ or ‘struct in_addr’.
Historically ‘unsigned long int’ was a 32-bit number but with 64-bit
machines this has changed.  Using ‘unsigned long int’ might break the
code if it is used on machines where this type doesn’t have 32 bits.
‘uint32_t’ is specified by Unix98 and guaranteed to have 32 bits.

   IPv6 Internet host addresses have 128 bits and are packaged inside a
structure of type ‘struct in6_addr’.

   The following basic definitions for Internet addresses are declared
in the header file ‘netinet/in.h’:

 -- Data Type: struct in_addr
     This data type is used in certain contexts to contain an IPv4
     Internet host address.  It has just one field, named ‘s_addr’,
     which records the host address number as an ‘uint32_t’.

 -- Macro: uint32_t INADDR_LOOPBACK
     You can use this constant to stand for “the address of this
     machine,” instead of finding its actual address.  It is the IPv4
     Internet address ‘127.0.0.1’, which is usually called ‘localhost’.
     This special constant saves you the trouble of looking up the
     address of your own machine.  Also, the system usually implements
     ‘INADDR_LOOPBACK’ specially, avoiding any network traffic for the
     case of one machine talking to itself.

 -- Macro: uint32_t INADDR_ANY
     You can use this constant to stand for “any incoming address” when
     binding to an address.  *Note Setting Address::.  This is the usual
     address to give in the ‘sin_addr’ member of ‘struct sockaddr_in’
     when you want to accept Internet connections.

 -- Macro: uint32_t INADDR_BROADCAST
     This constant is the address you use to send a broadcast message.

 -- Macro: uint32_t INADDR_NONE
     This constant is returned by some functions to indicate an error.

 -- Data Type: struct in6_addr
     This data type is used to store an IPv6 address.  It stores 128
     bits of data, which can be accessed (via a union) in a variety of
     ways.

 -- Constant: struct in6_addr in6addr_loopback
     This constant is the IPv6 address ‘::1’, the loopback address.  See
     above for a description of what this means.  The macro
     ‘IN6ADDR_LOOPBACK_INIT’ is provided to allow you to initialize your
     own variables to this value.

 -- Constant: struct in6_addr in6addr_any
     This constant is the IPv6 address ‘::’, the unspecified address.
     See above for a description of what this means.  The macro
     ‘IN6ADDR_ANY_INIT’ is provided to allow you to initialize your own
     variables to this value.


File: libc.info,  Node: Host Address Functions,  Next: Host Names,  Prev: Host Address Data Type,  Up: Host Addresses

16.6.2.3 Host Address Functions
...............................

These additional functions for manipulating Internet addresses are
declared in the header file ‘arpa/inet.h’.  They represent Internet
addresses in network byte order, and network numbers and
local-address-within-network numbers in host byte order.  *Note Byte
Order::, for an explanation of network and host byte order.

 -- Function: int inet_aton (const char *NAME, struct in_addr *ADDR)
     Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | *Note POSIX
     Safety Concepts::.

     This function converts the IPv4 Internet host address NAME from the
     standard numbers-and-dots notation into binary data and stores it
     in the ‘struct in_addr’ that ADDR points to.  ‘inet_aton’ returns
     nonzero if the address is valid, zero if not.

 -- Function: uint32_t inet_addr (const char *NAME)
     Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | *Note POSIX
     Safety Concepts::.

     This function converts the IPv4 Internet host address NAME from the
     standard numbers-and-dots notation into binary data.  If the input
     is not valid, ‘inet_addr’ returns ‘INADDR_NONE’.  This is an
     obsolete interface to ‘inet_aton’, described immediately above.  It
     is obsolete because ‘INADDR_NONE’ is a valid address
     (255.255.255.255), and ‘inet_aton’ provides a cleaner way to
     indicate error return.

 -- Function: uint32_t inet_network (const char *NAME)
     Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | *Note POSIX
     Safety Concepts::.

     This function extracts the network number from the address NAME,
     given in the standard numbers-and-dots notation.  The returned
     address is in host order.  If the input is not valid,
     ‘inet_network’ returns ‘-1’.

     The function works only with traditional IPv4 class A, B and C
     network types.  It doesn’t work with classless addresses and
     shouldn’t be used anymore.

 -- Function: char * inet_ntoa (struct in_addr ADDR)
     Preliminary: | MT-Safe locale | AS-Unsafe race | AC-Safe | *Note
     POSIX Safety Concepts::.

     This function converts the IPv4 Internet host address ADDR to a
     string in the standard numbers-and-dots notation.  The return value
     is a pointer into a statically-allocated buffer.  Subsequent calls
     will overwrite the same buffer, so you should copy the string if
     you need to save it.

     In multi-threaded programs each thread has its own
     statically-allocated buffer.  But still subsequent calls of
     ‘inet_ntoa’ in the same thread will overwrite the result of the
     last call.

     Instead of ‘inet_ntoa’ the newer function ‘inet_ntop’ which is
     described below should be used since it handles both IPv4 and IPv6
     addresses.

 -- Function: struct in_addr inet_makeaddr (uint32_t NET, uint32_t
          LOCAL)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function makes an IPv4 Internet host address by combining the
     network number NET with the local-address-within-network number
     LOCAL.

 -- Function: uint32_t inet_lnaof (struct in_addr ADDR)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function returns the local-address-within-network part of the
     Internet host address ADDR.

     The function works only with traditional IPv4 class A, B and C
     network types.  It doesn’t work with classless addresses and
     shouldn’t be used anymore.

 -- Function: uint32_t inet_netof (struct in_addr ADDR)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This function returns the network number part of the Internet host
     address ADDR.

     The function works only with traditional IPv4 class A, B and C
     network types.  It doesn’t work with classless addresses and
     shouldn’t be used anymore.

 -- Function: int inet_pton (int AF, const char *CP, void *BUF)
     Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | *Note POSIX
     Safety Concepts::.

     This function converts an Internet address (either IPv4 or IPv6)
     from presentation (textual) to network (binary) format.  AF should
     be either ‘AF_INET’ or ‘AF_INET6’, as appropriate for the type of
     address being converted.  CP is a pointer to the input string, and
     BUF is a pointer to a buffer for the result.  It is the caller’s
     responsibility to make sure the buffer is large enough.

 -- Function: const char * inet_ntop (int AF, const void *CP, char *BUF,
          socklen_t LEN)
     Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | *Note POSIX
     Safety Concepts::.

     This function converts an Internet address (either IPv4 or IPv6)
     from network (binary) to presentation (textual) form.  AF should be
     either ‘AF_INET’ or ‘AF_INET6’, as appropriate.  CP is a pointer to
     the address to be converted.  BUF should be a pointer to a buffer
     to hold the result, and LEN is the length of this buffer.  The
     return value from the function will be this buffer address.


File: libc.info,  Node: Host Names,  Prev: Host Address Functions,  Up: Host Addresses

16.6.2.4 Host Names
...................

Besides the standard numbers-and-dots notation for Internet addresses,
you can also refer to a host by a symbolic name.  The advantage of a
symbolic name is that it is usually easier to remember.  For example,
the machine with Internet address ‘158.121.106.19’ is also known as
‘alpha.gnu.org’; and other machines in the ‘gnu.org’ domain can refer to
it simply as ‘alpha’.

   Internally, the system uses a database to keep track of the mapping
between host names and host numbers.  This database is usually either
the file ‘/etc/hosts’ or an equivalent provided by a name server.  The
functions and other symbols for accessing this database are declared in
‘netdb.h’.  They are BSD features, defined unconditionally if you
include ‘netdb.h’.

 -- Data Type: struct hostent
     This data type is used to represent an entry in the hosts database.
     It has the following members:

     ‘char *h_name’
          This is the “official” name of the host.

     ‘char **h_aliases’
          These are alternative names for the host, represented as a
          null-terminated vector of strings.

     ‘int h_addrtype’
          This is the host address type; in practice, its value is
          always either ‘AF_INET’ or ‘AF_INET6’, with the latter being
          used for IPv6 hosts.  In principle other kinds of addresses
          could be represented in the database as well as Internet
          addresses; if this were done, you might find a value in this
          field other than ‘AF_INET’ or ‘AF_INET6’.  *Note Socket
          Addresses::.

     ‘int h_length’
          This is the length, in bytes, of each address.

     ‘char **h_addr_list’
          This is the vector of addresses for the host.  (Recall that
          the host might be connected to multiple networks and have
          different addresses on each one.)  The vector is terminated by
          a null pointer.

     ‘char *h_addr’
          This is a synonym for ‘h_addr_list[0]’; in other words, it is
          the first host address.

   As far as the host database is concerned, each address is just a
block of memory ‘h_length’ bytes long.  But in other contexts there is
an implicit assumption that you can convert IPv4 addresses to a ‘struct
in_addr’ or an ‘uint32_t’.  Host addresses in a ‘struct hostent’
structure are always given in network byte order; see *note Byte
Order::.

   You can use ‘gethostbyname’, ‘gethostbyname2’ or ‘gethostbyaddr’ to
search the hosts database for information about a particular host.  The
information is returned in a statically-allocated structure; you must
copy the information if you need to save it across calls.  You can also
use ‘getaddrinfo’ and ‘getnameinfo’ to obtain this information.

 -- Function: struct hostent * gethostbyname (const char *NAME)
     Preliminary: | MT-Unsafe race:hostbyname env locale | AS-Unsafe
     dlopen plugin corrupt heap lock | AC-Unsafe lock corrupt mem fd |
     *Note POSIX Safety Concepts::.

     The ‘gethostbyname’ function returns information about the host
     named NAME.  If the lookup fails, it returns a null pointer.

 -- Function: struct hostent * gethostbyname2 (const char *NAME, int AF)
     Preliminary: | MT-Unsafe race:hostbyname2 env locale | AS-Unsafe
     dlopen plugin corrupt heap lock | AC-Unsafe lock corrupt mem fd |
     *Note POSIX Safety Concepts::.

     The ‘gethostbyname2’ function is like ‘gethostbyname’, but allows
     the caller to specify the desired address family (e.g. ‘AF_INET’ or
     ‘AF_INET6’) of the result.

 -- Function: struct hostent * gethostbyaddr (const void *ADDR,
          socklen_t LENGTH, int FORMAT)
     Preliminary: | MT-Unsafe race:hostbyaddr env locale | AS-Unsafe
     dlopen plugin corrupt heap lock | AC-Unsafe lock corrupt mem fd |
     *Note POSIX Safety Concepts::.

     The ‘gethostbyaddr’ function returns information about the host
     with Internet address ADDR.  The parameter ADDR is not really a
     pointer to char - it can be a pointer to an IPv4 or an IPv6
     address.  The LENGTH argument is the size (in bytes) of the address
     at ADDR.  FORMAT specifies the address format; for an IPv4 Internet
     address, specify a value of ‘AF_INET’; for an IPv6 Internet
     address, use ‘AF_INET6’.

     If the lookup fails, ‘gethostbyaddr’ returns a null pointer.

   If the name lookup by ‘gethostbyname’ or ‘gethostbyaddr’ fails, you
can find out the reason by looking at the value of the variable
‘h_errno’.  (It would be cleaner design for these functions to set
‘errno’, but use of ‘h_errno’ is compatible with other systems.)

   Here are the error codes that you may find in ‘h_errno’:

‘HOST_NOT_FOUND’
     No such host is known in the database.

‘TRY_AGAIN’
     This condition happens when the name server could not be contacted.
     If you try again later, you may succeed then.

‘NO_RECOVERY’
     A non-recoverable error occurred.

‘NO_ADDRESS’
     The host database contains an entry for the name, but it doesn’t
     have an associated Internet address.

   The lookup functions above all have one thing in common: they are not
reentrant and therefore unusable in multi-threaded applications.
Therefore provides the GNU C Library a new set of functions which can be
used in this context.

 -- Function: int gethostbyname_r (const char *restrict NAME, struct
          hostent *restrict RESULT_BUF, char *restrict BUF, size_t
          BUFLEN, struct hostent **restrict RESULT, int *restrict
          H_ERRNOP)
     Preliminary: | MT-Safe env locale | AS-Unsafe dlopen plugin corrupt
     heap lock | AC-Unsafe lock corrupt mem fd | *Note POSIX Safety
     Concepts::.

     The ‘gethostbyname_r’ function returns information about the host
     named NAME.  The caller must pass a pointer to an object of type
     ‘struct hostent’ in the RESULT_BUF parameter.  In addition the
     function may need extra buffer space and the caller must pass a
     pointer and the size of the buffer in the BUF and BUFLEN
     parameters.

     A pointer to the buffer, in which the result is stored, is
     available in ‘*RESULT’ after the function call successfully
     returned.  The buffer passed as the BUF parameter can be freed only
     once the caller has finished with the result hostent struct, or has
     copied it including all the other memory that it points to.  If an
     error occurs or if no entry is found, the pointer ‘*RESULT’ is a
     null pointer.  Success is signalled by a zero return value.  If the
     function failed the return value is an error number.  In addition
     to the errors defined for ‘gethostbyname’ it can also be ‘ERANGE’.
     In this case the call should be repeated with a larger buffer.
     Additional error information is not stored in the global variable
     ‘h_errno’ but instead in the object pointed to by H_ERRNOP.

     Here’s a small example:
          struct hostent *
          gethostname (char *host)
          {
            struct hostent *hostbuf, *hp;
            size_t hstbuflen;
            char *tmphstbuf;
            int res;
            int herr;

            hostbuf = malloc (sizeof (struct hostent));
            hstbuflen = 1024;
            tmphstbuf = malloc (hstbuflen);

            while ((res = gethostbyname_r (host, hostbuf, tmphstbuf, hstbuflen,
                                           &hp, &herr)) == ERANGE)
              {
                /* Enlarge the buffer.  */
                hstbuflen *= 2;
                tmphstbuf = realloc (tmphstbuf, hstbuflen);
              }

            free (tmphstbuf);
            /*  Check for errors.  */
            if (res || hp == NULL)
              return NULL;
            return hp;
          }

 -- Function: int gethostbyname2_r (const char *NAME, int AF, struct
          hostent *restrict RESULT_BUF, char *restrict BUF, size_t
          BUFLEN, struct hostent **restrict RESULT, int *restrict
          H_ERRNOP)
     Preliminary: | MT-Safe env locale | AS-Unsafe dlopen plugin corrupt
     heap lock | AC-Unsafe lock corrupt mem fd | *Note POSIX Safety
     Concepts::.

     The ‘gethostbyname2_r’ function is like ‘gethostbyname_r’, but
     allows the caller to specify the desired address family (e.g.
     ‘AF_INET’ or ‘AF_INET6’) for the result.

 -- Function: int gethostbyaddr_r (const void *ADDR, socklen_t LENGTH,
          int FORMAT, struct hostent *restrict RESULT_BUF, char
          *restrict BUF, size_t BUFLEN, struct hostent **restrict
          RESULT, int *restrict H_ERRNOP)
     Preliminary: | MT-Safe env locale | AS-Unsafe dlopen plugin corrupt
     heap lock | AC-Unsafe lock corrupt mem fd | *Note POSIX Safety
     Concepts::.

     The ‘gethostbyaddr_r’ function returns information about the host
     with Internet address ADDR.  The parameter ADDR is not really a
     pointer to char - it can be a pointer to an IPv4 or an IPv6
     address.  The LENGTH argument is the size (in bytes) of the address
     at ADDR.  FORMAT specifies the address format; for an IPv4 Internet
     address, specify a value of ‘AF_INET’; for an IPv6 Internet
     address, use ‘AF_INET6’.

     Similar to the ‘gethostbyname_r’ function, the caller must provide
     buffers for the result and memory used internally.  In case of
     success the function returns zero.  Otherwise the value is an error
     number where ‘ERANGE’ has the special meaning that the
     caller-provided buffer is too small.

   You can also scan the entire hosts database one entry at a time using
‘sethostent’, ‘gethostent’ and ‘endhostent’.  Be careful when using
these functions because they are not reentrant.

 -- Function: void sethostent (int STAYOPEN)
     Preliminary: | MT-Unsafe race:hostent env locale | AS-Unsafe dlopen
     plugin heap lock | AC-Unsafe corrupt lock fd mem | *Note POSIX
     Safety Concepts::.

     This function opens the hosts database to begin scanning it.  You
     can then call ‘gethostent’ to read the entries.

     If the STAYOPEN argument is nonzero, this sets a flag so that
     subsequent calls to ‘gethostbyname’ or ‘gethostbyaddr’ will not
     close the database (as they usually would).  This makes for more
     efficiency if you call those functions several times, by avoiding
     reopening the database for each call.

 -- Function: struct hostent * gethostent (void)
     Preliminary: | MT-Unsafe race:hostent race:hostentbuf env locale |
     AS-Unsafe dlopen plugin heap lock | AC-Unsafe corrupt lock fd mem |
     *Note POSIX Safety Concepts::.

     This function returns the next entry in the hosts database.  It
     returns a null pointer if there are no more entries.

 -- Function: void endhostent (void)
     Preliminary: | MT-Unsafe race:hostent env locale | AS-Unsafe dlopen
     plugin heap lock | AC-Unsafe corrupt lock fd mem | *Note POSIX
     Safety Concepts::.

     This function closes the hosts database.