This is libc.info, produced by makeinfo version 4.13 from libc.texinfo.

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

   This file documents the GNU C library.

   This is Edition 0.12, last updated 2007-10-27, of `The GNU C Library
Reference Manual', for version 2.8.

   Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
2003, 2007, 2008 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 1.2 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."


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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

     The default action is to terminate the process.

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

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

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

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


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

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

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

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

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

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

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

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

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

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


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

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

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

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

* Menu:

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

     #include <signal.h>

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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


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

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

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

     #include <signal.h>

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

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

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

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

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

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

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

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

     struct sigaction query_action;

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


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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

     ...
     struct sigaction temp;

     sigaction (SIGHUP, NULL, &temp);

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


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

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

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

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

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

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

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

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

* Menu:

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


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

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

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

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

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

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

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

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

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

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

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

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

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

       return EXIT_SUCCESS;
     }


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

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

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

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

     volatile sig_atomic_t fatal_error_in_progress = 0;

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

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

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


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

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

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

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

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

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

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

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

     jmp_buf return_to_top_level;

     volatile sig_atomic_t waiting_for_input;

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

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

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


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

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

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

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

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

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

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


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

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

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

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

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

     struct process *process_list;

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

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

   Here is the handler itself:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

     sig_atomic_t process_status_change;

     sig_atomic_t last_process_status_change;

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

* Menu:

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


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

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

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

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

     volatile struct two_words { int a, b; } memory;

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

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

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

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

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


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

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

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

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

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

   In practice, you can assume that `int' is atomic.  You can also
assume that pointer types are atomic; that is very convenient.  Both of
these assumptions are true on all of the machines that the GNU C
library supports and on all POSIX systems we know of.


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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

* Menu:

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


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

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

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

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

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

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

     #include <signal.h>

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

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

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

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

     /* Enable both handlers during program initialization. */

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

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


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

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

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

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

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

   * Two processes need to synchronize while working together.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       return 0;
     }

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


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

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

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

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

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

* Menu:

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


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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

    `EINVAL'
          The HOW argument is invalid.

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

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


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

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

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

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

     int
     main (void)
     {
       sigset_t block_alarm;

       ...

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

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

           ...
         }
     }


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

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

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

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

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

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

   Here is an example:

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

     void catch_stop ();

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

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

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

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

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


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

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

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

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

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

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

   Here is an example.

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

     sigset_t base_mask, waiting_mask;

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

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

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

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

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


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

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

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

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

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

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

     ...

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

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

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

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

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

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

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

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

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


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

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

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

* Menu:

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


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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

     sigset_t mask, oldmask;

     ...

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

     ...

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

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

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

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


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

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

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

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

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

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

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

    `size_t ss_size'
          This is the size (in bytes) of the signal stack which `ss_sp'
          points to.  You should set this to however much space you
          allocated for the stack.

          There are two macros defined in `signal.h' that you should
          use in calculating this size:

         `SIGSTKSZ'
               This is the canonical size for a signal stack.  It is
               judged to be sufficient for normal uses.

         `MINSIGSTKSZ'
               This is the amount of signal stack space the operating
               system needs just to implement signal delivery.  The
               size of a signal stack *must* be greater than this.

               For most cases, just using `SIGSTKSZ' for `ss_size' is
               sufficient.  But if you know how much stack space your
               program's signal handlers will need, you may want to use
               a different size.  In this case, you should allocate
               `MINSIGSTKSZ' additional bytes for the signal stack and
               increase `ss_size' accordingly.

    `int ss_flags'
          This field contains the bitwise OR of these flags:

         `SS_DISABLE'
               This tells the system that it should not use the signal
               stack.

         `SS_ONSTACK'
               This is set by the system, and indicates that the signal
               stack is currently in use.  If this bit is not set, then
               signals will be delivered on the normal user stack.

 -- Function: int sigaltstack (const stack_t *restrict STACK, stack_t
          *restrict OLDSTACK)
     The `sigaltstack' function specifies an alternate stack for use
     during signal handling.  When a signal is received by the process
     and its action indicates that the signal stack is used, the system
     arranges a switch to the currently installed signal stack while
     the handler for that signal is executed.

     If OLDSTACK is not a null pointer, information about the currently
     installed signal stack is returned in the location it points to.
     If STACK is not a null pointer, then this is installed as the new
     stack for use by signal handlers.

     The return value is `0' on success and `-1' on failure.  If
     `sigaltstack' fails, it sets `errno' to one of these values:

    `EINVAL'
          You tried to disable a stack that was in fact currently in
          use.

    `ENOMEM'
          The size of the alternate stack was too small.  It must be
          greater than `MINSIGSTKSZ'.

   Here is the older `sigstack' interface.  You should use
`sigaltstack' instead on systems that have it.

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

    `void *ss_sp'
          This is the stack pointer.  If the stack grows downwards on
          your machine, this should point to the top of the area you
          allocated.  If the stack grows upwards, it should point to
          the bottom.

    `int ss_onstack'
          This field is true if the process is currently using this
          stack.

 -- Function: int sigstack (const struct sigstack *STACK, struct
          sigstack *OLDSTACK)
     The `sigstack' function specifies an alternate stack for use during
     signal handling.  When a signal is received by the process and its
     action indicates that the signal stack is used, the system
     arranges a switch to the currently installed signal stack while
     the handler for that signal is executed.

     If OLDSTACK is not a null pointer, information about the currently
     installed signal stack is returned in the location it points to.
     If STACK is not a null pointer, then this is installed as the new
     stack for use by signal handlers.

     The return value is `0' on success and `-1' on failure.


File: libc.info,  Node: BSD Signal Handling,  Prev: Signal Stack,  Up: Signal Handling

24.10 BSD Signal Handling
=========================

This section describes alternative signal handling functions derived
from BSD Unix.  These facilities were an advance, in their time; today,
they are mostly obsolete, and supported mainly for compatibility with
BSD Unix.

   There are many similarities between the BSD and POSIX signal handling
facilities, because the POSIX facilities were inspired by the BSD
facilities.  Besides having different names for all the functions to
avoid conflicts, the main differences between the two are:

   * BSD Unix represents signal masks as an `int' bit mask, rather than
     as a `sigset_t' object.

   * The BSD facilities use a different default for whether an
     interrupted primitive should fail or resume.  The POSIX facilities
     make system calls fail unless you specify that they should resume.
     With the BSD facility, the default is to make system calls resume
     unless you say they should fail.  *Note Interrupted Primitives::.

   The BSD facilities are declared in `signal.h'.  

* Menu:

* BSD Handler::                 BSD Function to Establish a Handler.
* Blocking in BSD::             BSD Functions for Blocking Signals.


File: libc.info,  Node: BSD Handler,  Next: Blocking in BSD,  Up: BSD Signal Handling

24.10.1 BSD Function to Establish a Handler
-------------------------------------------

 -- Data Type: struct sigvec
     This data type is the BSD equivalent of `struct sigaction' (*note
     Advanced Signal Handling::); it is used to specify signal actions
     to the `sigvec' function.  It contains the following members:

    `sighandler_t sv_handler'
          This is the handler function.

    `int sv_mask'
          This is the mask of additional signals to be blocked while
          the handler function is being called.

    `int sv_flags'
          This is a bit mask used to specify various flags which affect
          the behavior of the signal.  You can also refer to this field
          as `sv_onstack'.

These symbolic constants can be used to provide values for the
`sv_flags' field of a `sigvec' structure.  This field is a bit mask
value, so you bitwise-OR the flags of interest to you together.

 -- Macro: int SV_ONSTACK
     If this bit is set in the `sv_flags' field of a `sigvec'
     structure, it means to use the signal stack when delivering the
     signal.

 -- Macro: int SV_INTERRUPT
     If this bit is set in the `sv_flags' field of a `sigvec'
     structure, it means that system calls interrupted by this kind of
     signal should not be restarted if the handler returns; instead,
     the system calls should return with a `EINTR' error status.  *Note
     Interrupted Primitives::.

 -- Macro: int SV_RESETHAND
     If this bit is set in the `sv_flags' field of a `sigvec'
     structure, it means to reset the action for the signal back to
     `SIG_DFL' when the signal is received.

 -- Function: int sigvec (int SIGNUM, const struct sigvec
          *ACTION,struct sigvec *OLD-ACTION)
     This function is the equivalent of `sigaction' (*note Advanced
     Signal Handling::); it installs the action ACTION for the signal
     SIGNUM, returning information about the previous action in effect
     for that signal in OLD-ACTION.

 -- Function: int siginterrupt (int SIGNUM, int FAILFLAG)
     This function specifies which approach to use when certain
     primitives are interrupted by handling signal SIGNUM.  If FAILFLAG
     is false, signal SIGNUM restarts primitives.  If FAILFLAG is true,
     handling SIGNUM causes these primitives to fail with error code
     `EINTR'.  *Note Interrupted Primitives::.


File: libc.info,  Node: Blocking in BSD,  Prev: BSD Handler,  Up: BSD Signal Handling

24.10.2 BSD Functions for Blocking Signals
------------------------------------------

 -- Macro: int sigmask (int SIGNUM)
     This macro returns a signal mask that has the bit for signal SIGNUM
     set.  You can bitwise-OR the results of several calls to `sigmask'
     together to specify more than one signal.  For example,

          (sigmask (SIGTSTP) | sigmask (SIGSTOP)
           | sigmask (SIGTTIN) | sigmask (SIGTTOU))

     specifies a mask that includes all the job-control stop signals.

 -- Function: int sigblock (int MASK)
     This function is equivalent to `sigprocmask' (*note Process Signal
     Mask::) with a HOW argument of `SIG_BLOCK': it adds the signals
     specified by MASK to the calling process's set of blocked signals.
     The return value is the previous set of blocked signals.

 -- Function: int sigsetmask (int MASK)
     This function equivalent to `sigprocmask' (*note Process Signal
     Mask::) with a HOW argument of `SIG_SETMASK': it sets the calling
     process's signal mask to MASK.  The return value is the previous
     set of blocked signals.

 -- Function: int sigpause (int MASK)
     This function is the equivalent of `sigsuspend' (*note Waiting for
     a Signal::):  it sets the calling process's signal mask to MASK,
     and waits for a signal to arrive.  On return the previous set of
     blocked signals is restored.


File: libc.info,  Node: Program Basics,  Next: Processes,  Prev: Signal Handling,  Up: Top

25 The Basic Program/System Interface
*************************************

"Processes" are the primitive units for allocation of system resources.
Each process has its own address space and (usually) one thread of
control.  A process executes a program; you can have multiple processes
executing the same program, but each process has its own copy of the
program within its own address space and executes it independently of
the other copies.  Though it may have multiple threads of control
within the same program and a program may be composed of multiple
logically separate modules, a process always executes exactly one
program.

   Note that we are using a specific definition of "program" for the
purposes of this manual, which corresponds to a common definition in the
context of Unix system.  In popular usage, "program" enjoys a much
broader definition; it can refer for example to a system's kernel, an
editor macro, a complex package of software, or a discrete section of
code executing within a process.

   Writing the program is what this manual is all about.  This chapter
explains the most basic interface between your program and the system
that runs, or calls, it.  This includes passing of parameters (arguments
and environment) from the system, requesting basic services from the
system, and telling the system the program is done.

   A program starts another program with the `exec' family of system
calls.  This chapter looks at program startup from the execee's point
of view.  To see the event from the execor's point of view, see *note
Executing a File::.

* Menu:

* Program Arguments::           Parsing your program's command-line arguments.
* Environment Variables::       Less direct parameters affecting your program
* System Calls::                Requesting service from the system
* Program Termination::         Telling the system you're done; return status


File: libc.info,  Node: Program Arguments,  Next: Environment Variables,  Up: Program Basics

25.1 Program Arguments
======================

The system starts a C program by calling the function `main'.  It is up
to you to write a function named `main'--otherwise, you won't even be
able to link your program without errors.

   In ISO C you can define `main' either to take no arguments, or to
take two arguments that represent the command line arguments to the
program, like this:

     int main (int ARGC, char *ARGV[])

   The command line arguments are the whitespace-separated tokens given
in the shell command used to invoke the program; thus, in `cat foo
bar', the arguments are `foo' and `bar'.  The only way a program can
look at its command line arguments is via the arguments of `main'.  If
`main' doesn't take arguments, then you cannot get at the command line.

   The value of the ARGC argument is the number of command line
arguments.  The ARGV argument is a vector of C strings; its elements
are the individual command line argument strings.  The file name of the
program being run is also included in the vector as the first element;
the value of ARGC counts this element.  A null pointer always follows
the last element: `ARGV[ARGC]' is this null pointer.

   For the command `cat foo bar', ARGC is 3 and ARGV has three
elements, `"cat"', `"foo"' and `"bar"'.

   In Unix systems you can define `main' a third way, using three
arguments:

     int main (int ARGC, char *ARGV[], char *ENVP[])

   The first two arguments are just the same.  The third argument ENVP
gives the program's environment; it is the same as the value of
`environ'.  *Note Environment Variables::.  POSIX.1 does not allow this
three-argument form, so to be portable it is best to write `main' to
take two arguments, and use the value of `environ'.

* Menu:

* Argument Syntax::             By convention, options start with a hyphen.
* Parsing Program Arguments::   Ways to parse program options and arguments.


File: libc.info,  Node: Argument Syntax,  Next: Parsing Program Arguments,  Up: Program Arguments

25.1.1 Program Argument Syntax Conventions
------------------------------------------

POSIX recommends these conventions for command line arguments.
`getopt' (*note Getopt::) and `argp_parse' (*note Argp::) make it easy
to implement them.

   * Arguments are options if they begin with a hyphen delimiter (`-').

   * Multiple options may follow a hyphen delimiter in a single token if
     the options do not take arguments.  Thus, `-abc' is equivalent to
     `-a -b -c'.

   * Option names are single alphanumeric characters (as for `isalnum';
     *note Classification of Characters::).

   * Certain options require an argument.  For example, the `-o' command
     of the `ld' command requires an argument--an output file name.

   * An option and its argument may or may not appear as separate
     tokens.  (In other words, the whitespace separating them is
     optional.)  Thus, `-o foo' and `-ofoo' are equivalent.

   * Options typically precede other non-option arguments.

     The implementations of `getopt' and `argp_parse' in the GNU C
     library normally make it appear as if all the option arguments were
     specified before all the non-option arguments for the purposes of
     parsing, even if the user of your program intermixed option and
     non-option arguments.  They do this by reordering the elements of
     the ARGV array.  This behavior is nonstandard; if you want to
     suppress it, define the `_POSIX_OPTION_ORDER' environment variable.
     *Note Standard Environment::.

   * The argument `--' terminates all options; any following arguments
     are treated as non-option arguments, even if they begin with a
     hyphen.

   * A token consisting of a single hyphen character is interpreted as
     an ordinary non-option argument.  By convention, it is used to
     specify input from or output to the standard input and output
     streams.

   * Options may be supplied in any order, or appear multiple times.
     The interpretation is left up to the particular application
     program.

   GNU adds "long options" to these conventions.  Long options consist
of `--' followed by a name made of alphanumeric characters and dashes.
Option names are typically one to three words long, with hyphens to
separate words.  Users can abbreviate the option names as long as the
abbreviations are unique.

   To specify an argument for a long option, write `--NAME=VALUE'.
This syntax enables a long option to accept an argument that is itself
optional.

   Eventually, the GNU system will provide completion for long option
names in the shell.


File: libc.info,  Node: Parsing Program Arguments,  Prev: Argument Syntax,  Up: Program Arguments

25.1.2 Parsing Program Arguments
--------------------------------

If the syntax for the command line arguments to your program is simple
enough, you can simply pick the arguments off from ARGV by hand.  But
unless your program takes a fixed number of arguments, or all of the
arguments are interpreted in the same way (as file names, for example),
you are usually better off using `getopt' (*note Getopt::) or
`argp_parse' (*note Argp::) to do the parsing.

   `getopt' is more standard (the short-option only version of it is a
part of the POSIX standard), but using `argp_parse' is often easier,
both for very simple and very complex option structures, because it
does more of the dirty work for you.

* Menu:

* Getopt::                      Parsing program options using `getopt'.
* Argp::                        Parsing program options using `argp_parse'.
* Suboptions::                  Some programs need more detailed options.
* Suboptions Example::          This shows how it could be done for `mount'.


File: libc.info,  Node: Getopt,  Next: Argp,  Up: Parsing Program Arguments

25.2 Parsing program options using `getopt'
===========================================

The `getopt' and `getopt_long' functions automate some of the chore
involved in parsing typical unix command line options.

* Menu:

* Using Getopt::                Using the `getopt' function.
* Example of Getopt::           An example of parsing options with `getopt'.
* Getopt Long Options::         GNU suggests utilities accept long-named
                                 options; here is one way to do.
* Getopt Long Option Example::  An example of using `getopt_long'.


File: libc.info,  Node: Using Getopt,  Next: Example of Getopt,  Up: Getopt

25.2.1 Using the `getopt' function
----------------------------------

Here are the details about how to call the `getopt' function.  To use
this facility, your program must include the header file `unistd.h'.  

 -- Variable: int opterr
     If the value of this variable is nonzero, then `getopt' prints an
     error message to the standard error stream if it encounters an
     unknown option character or an option with a missing required
     argument.  This is the default behavior.  If you set this variable
     to zero, `getopt' does not print any messages, but it still
     returns the character `?' to indicate an error.

 -- Variable: int optopt
     When `getopt' encounters an unknown option character or an option
     with a missing required argument, it stores that option character
     in this variable.  You can use this for providing your own
     diagnostic messages.

 -- Variable: int optind
     This variable is set by `getopt' to the index of the next element
     of the ARGV array to be processed.  Once `getopt' has found all of
     the option arguments, you can use this variable to determine where
     the remaining non-option arguments begin.  The initial value of
     this variable is `1'.

 -- Variable: char * optarg
     This variable is set by `getopt' to point at the value of the
     option argument, for those options that accept arguments.

 -- Function: int getopt (int ARGC, char **ARGV, const char *OPTIONS)
     The `getopt' function gets the next option argument from the
     argument list specified by the ARGV and ARGC arguments.  Normally
     these values come directly from the arguments received by `main'.

     The OPTIONS argument is a string that specifies the option
     characters that are valid for this program.  An option character
     in this string can be followed by a colon (`:') to indicate that
     it takes a required argument.  If an option character is followed
     by two colons (`::'), its argument is optional; this is a GNU
     extension.

     `getopt' has three ways to deal with options that follow
     non-options ARGV elements.  The special argument `--' forces in
     all cases the end of option scanning.

        * The default is to permute the contents of ARGV while scanning
          it so that eventually all the non-options are at the end.
          This allows options to be given in any order, even with
          programs that were not written to expect this.

        * If the OPTIONS argument string begins with a hyphen (`-'),
          this is treated specially.  It permits arguments that are not
          options to be returned as if they were associated with option
          character `\1'.

        * POSIX demands the following behavior: The first non-option
          stops option processing.  This mode is selected by either
          setting the environment variable `POSIXLY_CORRECT' or
          beginning the OPTIONS argument string with a plus sign (`+').

     The `getopt' function returns the option character for the next
     command line option.  When no more option arguments are available,
     it returns `-1'.  There may still be more non-option arguments; you
     must compare the external variable `optind' against the ARGC
     parameter to check this.

     If the option has an argument, `getopt' returns the argument by
     storing it in the variable OPTARG.  You don't ordinarily need to
     copy the `optarg' string, since it is a pointer into the original
     ARGV array, not into a static area that might be overwritten.

     If `getopt' finds an option character in ARGV that was not
     included in OPTIONS, or a missing option argument, it returns `?'
     and sets the external variable `optopt' to the actual option
     character.  If the first character of OPTIONS is a colon (`:'),
     then `getopt' returns `:' instead of `?' to indicate a missing
     option argument.  In addition, if the external variable `opterr'
     is nonzero (which is the default), `getopt' prints an error
     message.


File: libc.info,  Node: Example of Getopt,  Next: Getopt Long Options,  Prev: Using Getopt,  Up: Getopt

25.2.2 Example of Parsing Arguments with `getopt'
-------------------------------------------------

Here is an example showing how `getopt' is typically used.  The key
points to notice are:

   * Normally, `getopt' is called in a loop.  When `getopt' returns
     `-1', indicating no more options are present, the loop terminates.

   * A `switch' statement is used to dispatch on the return value from
     `getopt'.  In typical use, each case just sets a variable that is
     used later in the program.

   * A second loop is used to process the remaining non-option
     arguments.

     #include <ctype.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <unistd.h>

     int
     main (int argc, char **argv)
     {
       int aflag = 0;
       int bflag = 0;
       char *cvalue = NULL;
       int index;
       int c;

       opterr = 0;

       while ((c = getopt (argc, argv, "abc:")) != -1)
         switch (c)
           {
           case 'a':
             aflag = 1;
             break;
           case 'b':
             bflag = 1;
             break;
           case 'c':
             cvalue = optarg;
             break;
           case '?':
             if (optopt == 'c')
               fprintf (stderr, "Option -%c requires an argument.\n", optopt);
             else if (isprint (optopt))
               fprintf (stderr, "Unknown option `-%c'.\n", optopt);
             else
               fprintf (stderr,
                        "Unknown option character `\\x%x'.\n",
                        optopt);
             return 1;
           default:
             abort ();
           }

       printf ("aflag = %d, bflag = %d, cvalue = %s\n",
               aflag, bflag, cvalue);

       for (index = optind; index < argc; index++)
         printf ("Non-option argument %s\n", argv[index]);
       return 0;
     }

   Here are some examples showing what this program prints with
different combinations of arguments:

     % testopt
     aflag = 0, bflag = 0, cvalue = (null)

     % testopt -a -b
     aflag = 1, bflag = 1, cvalue = (null)

     % testopt -ab
     aflag = 1, bflag = 1, cvalue = (null)

     % testopt -c foo
     aflag = 0, bflag = 0, cvalue = foo

     % testopt -cfoo
     aflag = 0, bflag = 0, cvalue = foo

     % testopt arg1
     aflag = 0, bflag = 0, cvalue = (null)
     Non-option argument arg1

     % testopt -a arg1
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument arg1

     % testopt -c foo arg1
     aflag = 0, bflag = 0, cvalue = foo
     Non-option argument arg1

     % testopt -a -- -b
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument -b

     % testopt -a -
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument -


File: libc.info,  Node: Getopt Long Options,  Next: Getopt Long Option Example,  Prev: Example of Getopt,  Up: Getopt

25.2.3 Parsing Long Options with `getopt_long'
----------------------------------------------

To accept GNU-style long options as well as single-character options,
use `getopt_long' instead of `getopt'.  This function is declared in
`getopt.h', not `unistd.h'.  You should make every program accept long
options if it uses any options, for this takes little extra work and
helps beginners remember how to use the program.

 -- Data Type: struct option
     This structure describes a single long option name for the sake of
     `getopt_long'.  The argument LONGOPTS must be an array of these
     structures, one for each long option.  Terminate the array with an
     element containing all zeros.

     The `struct option' structure has these fields:

    `const char *name'
          This field is the name of the option.  It is a string.

    `int has_arg'
          This field says whether the option takes an argument.  It is
          an integer, and there are three legitimate values:
          `no_argument', `required_argument' and `optional_argument'.

    `int *flag'
    `int val'
          These fields control how to report or act on the option when
          it occurs.

          If `flag' is a null pointer, then the `val' is a value which
          identifies this option.  Often these values are chosen to
          uniquely identify particular long options.

          If `flag' is not a null pointer, it should be the address of
          an `int' variable which is the flag for this option.  The
          value in `val' is the value to store in the flag to indicate
          that the option was seen.

 -- Function: int getopt_long (int ARGC, char *const *ARGV, const char
          *SHORTOPTS, const struct option *LONGOPTS, int *INDEXPTR)
     Decode options from the vector ARGV (whose length is ARGC).  The
     argument SHORTOPTS describes the short options to accept, just as
     it does in `getopt'.  The argument LONGOPTS describes the long
     options to accept (see above).

     When `getopt_long' encounters a short option, it does the same
     thing that `getopt' would do: it returns the character code for the
     option, and stores the options argument (if it has one) in
     `optarg'.

     When `getopt_long' encounters a long option, it takes actions based
     on the `flag' and `val' fields of the definition of that option.

     If `flag' is a null pointer, then `getopt_long' returns the
     contents of `val' to indicate which option it found.  You should
     arrange distinct values in the `val' field for options with
     different meanings, so you can decode these values after
     `getopt_long' returns.  If the long option is equivalent to a short
     option, you can use the short option's character code in `val'.

     If `flag' is not a null pointer, that means this option should just
     set a flag in the program.  The flag is a variable of type `int'
     that you define.  Put the address of the flag in the `flag' field.
     Put in the `val' field the value you would like this option to
     store in the flag.  In this case, `getopt_long' returns `0'.

     For any long option, `getopt_long' tells you the index in the array
     LONGOPTS of the options definition, by storing it into
     `*INDEXPTR'.  You can get the name of the option with
     `LONGOPTS[*INDEXPTR].name'.  So you can distinguish among long
     options either by the values in their `val' fields or by their
     indices.  You can also distinguish in this way among long options
     that set flags.

     When a long option has an argument, `getopt_long' puts the argument
     value in the variable `optarg' before returning.  When the option
     has no argument, the value in `optarg' is a null pointer.  This is
     how you can tell whether an optional argument was supplied.

     When `getopt_long' has no more options to handle, it returns `-1',
     and leaves in the variable `optind' the index in ARGV of the next
     remaining argument.

   Since long option names were used before the `getopt_long' options
was invented there are program interfaces which require programs to
recognize options like `-option value' instead of `--option value'.  To
enable these programs to use the GNU getopt functionality there is one
more function available.

 -- Function: int getopt_long_only (int ARGC, char *const *ARGV, const
          char *SHORTOPTS, const struct option *LONGOPTS, int *INDEXPTR)
     The `getopt_long_only' function is equivalent to the `getopt_long'
     function but it allows to specify the user of the application to
     pass long options with only `-' instead of `--'.  The `--' prefix
     is still recognized but instead of looking through the short
     options if a `-' is seen it is first tried whether this parameter
     names a long option.  If not, it is parsed as a short option.

     Assuming `getopt_long_only' is used starting an application with

            app -foo

     the `getopt_long_only' will first look for a long option named
     `foo'.  If this is not found, the short options `f', `o', and
     again `o' are recognized.


File: libc.info,  Node: Getopt Long Option Example,  Prev: Getopt Long Options,  Up: Getopt

25.2.4 Example of Parsing Long Options with `getopt_long'
---------------------------------------------------------

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

     /* Flag set by `--verbose'. */
     static int verbose_flag;

     int
     main (argc, argv)
          int argc;
          char **argv;
     {
       int c;

       while (1)
         {
           static struct option long_options[] =
             {
               /* These options set a flag. */
               {"verbose", no_argument,       &verbose_flag, 1},
               {"brief",   no_argument,       &verbose_flag, 0},
               /* These options don't set a flag.
                  We distinguish them by their indices. */
               {"add",     no_argument,       0, 'a'},
               {"append",  no_argument,       0, 'b'},
               {"delete",  required_argument, 0, 'd'},
               {"create",  required_argument, 0, 'c'},
               {"file",    required_argument, 0, 'f'},
               {0, 0, 0, 0}
             };
           /* `getopt_long' stores the option index here. */
           int option_index = 0;

           c = getopt_long (argc, argv, "abc:d:f:",
                            long_options, &option_index);

           /* Detect the end of the options. */
           if (c == -1)
             break;

           switch (c)
             {
             case 0:
               /* If this option set a flag, do nothing else now. */
               if (long_options[option_index].flag != 0)
                 break;
               printf ("option %s", long_options[option_index].name);
               if (optarg)
                 printf (" with arg %s", optarg);
               printf ("\n");
               break;

             case 'a':
               puts ("option -a\n");
               break;

             case 'b':
               puts ("option -b\n");
               break;

             case 'c':
               printf ("option -c with value `%s'\n", optarg);
               break;

             case 'd':
               printf ("option -d with value `%s'\n", optarg);
               break;

             case 'f':
               printf ("option -f with value `%s'\n", optarg);
               break;

             case '?':
               /* `getopt_long' already printed an error message. */
               break;

             default:
               abort ();
             }
         }

       /* Instead of reporting `--verbose'
          and `--brief' as they are encountered,
          we report the final status resulting from them. */
       if (verbose_flag)
         puts ("verbose flag is set");

       /* Print any remaining command line arguments (not options). */
       if (optind < argc)
         {
           printf ("non-option ARGV-elements: ");
           while (optind < argc)
             printf ("%s ", argv[optind++]);
           putchar ('\n');
         }

       exit (0);
     }


File: libc.info,  Node: Argp,  Next: Suboptions,  Prev: Getopt,  Up: Parsing Program Arguments

25.3 Parsing Program Options with Argp
======================================

"Argp" is an interface for parsing unix-style argument vectors.  *Note
Program Arguments::.

   Argp provides features unavailable in the more commonly used
`getopt' interface.  These features include automatically producing
output in response to the `--help' and `--version' options, as
described in the GNU coding standards.  Using argp makes it less likely
that programmers will neglect to implement these additional options or
keep them up to date.

   Argp also provides the ability to merge several independently defined
option parsers into one, mediating conflicts between them and making the
result appear seamless.  A library can export an argp option parser that
user programs might employ in conjunction with their own option parsers,
resulting in less work for the user programs.  Some programs may use
only argument parsers exported by libraries, thereby achieving
consistent and efficient option-parsing for abstractions implemented by
the libraries.

   The header file `<argp.h>' should be included to use argp.

25.3.1 The `argp_parse' Function
--------------------------------

The main interface to argp is the `argp_parse' function.  In many
cases, calling `argp_parse' is the only argument-parsing code needed in
`main'.  *Note Program Arguments::.

 -- Function: error_t argp_parse (const struct argp *ARGP, int ARGC,
          char **ARGV, unsigned FLAGS, int *ARG_INDEX, void *INPUT)
     The `argp_parse' function parses the arguments in ARGV, of length
     ARGC, using the argp parser ARGP.  *Note Argp Parsers::.

     A value of zero is the same as a `struct argp'containing all
     zeros.  FLAGS is a set of flag bits that modify the parsing
     behavior.  *Note Argp Flags::.  INPUT is passed through to the argp
     parser ARGP, and has meaning defined by ARGP.  A typical usage is
     to pass a pointer to a structure which is used for specifying
     parameters to the parser and passing back the results.

     Unless the `ARGP_NO_EXIT' or `ARGP_NO_HELP' flags are included in
     FLAGS, calling `argp_parse' may result in the program exiting.
     This behavior is true if an error is detected, or when an unknown
     option is encountered.  *Note Program Termination::.

     If ARG_INDEX is non-null, the index of the first unparsed option
     in ARGV is returned as a value.

     The return value is zero for successful parsing, or an error code
     (*note Error Codes::) if an error is detected.  Different argp
     parsers may return arbitrary error codes, but the standard error
     codes are: `ENOMEM' if a memory allocation error occurred, or
     `EINVAL' if an unknown option or option argument is encountered.

* Menu:

* Globals: Argp Global Variables.  Global argp parameters.
* Parsers: Argp Parsers.        Defining parsers for use with `argp_parse'.
* Flags: Argp Flags.            Flags that modify the behavior of `argp_parse'.
* Help: Argp Help.              Printing help messages when not parsing.
* Examples: Argp Examples.      Simple examples of programs using argp.
* Customization: Argp User Customization.
                                Users may control the `--help' output format.


File: libc.info,  Node: Argp Global Variables,  Next: Argp Parsers,  Up: Argp

25.3.2 Argp Global Variables
----------------------------

These variables make it easy for user programs to implement the
`--version' option and provide a bug-reporting address in the `--help'
output.  These are implemented in argp by default.

 -- Variable: const char * argp_program_version
     If defined or set by the user program to a non-zero value, then a
     `--version' option is added when parsing with `argp_parse', which
     will print the `--version' string followed by a newline and exit.
     The exception to this is if the `ARGP_NO_EXIT' flag is used.

 -- Variable: const char * argp_program_bug_address
     If defined or set by the user program to a non-zero value,
     `argp_program_bug_address' should point to a string that will be
     printed at the end of the standard output for the `--help' option,
     embedded in a sentence that says `Report bugs to ADDRESS.'.

 -- Variable: argp_program_version_hook
     If defined or set by the user program to a non-zero value, a
     `--version' option is added when parsing with `arg_parse', which
     prints the program version and exits with a status of zero.  This
     is not the case if the `ARGP_NO_HELP' flag is used.  If the
     `ARGP_NO_EXIT' flag is set, the exit behavior of the program is
     suppressed or modified, as when the argp parser is going to be
     used by other programs.

     It should point to a function with this type of signature:

          void PRINT-VERSION (FILE *STREAM, struct argp_state *STATE)

     *Note Argp Parsing State::, for an explanation of STATE.

     This variable takes precedence over `argp_program_version', and is
     useful if a program has version information not easily expressed
     in a simple string.

 -- Variable: error_t argp_err_exit_status
     This is the exit status used when argp exits due to a parsing
     error.  If not defined or set by the user program, this defaults
     to: `EX_USAGE' from `<sysexits.h>'.


File: libc.info,  Node: Argp Parsers,  Next: Argp Flags,  Prev: Argp Global Variables,  Up: Argp

25.3.3 Specifying Argp Parsers
------------------------------

The first argument to the `argp_parse' function is a pointer to a
`struct argp', which is known as an "argp parser":

 -- Data Type: struct argp
     This structure specifies how to parse a given set of options and
     arguments, perhaps in conjunction with other argp parsers.  It has
     the following fields:

    `const struct argp_option *options'
          A pointer to a vector of `argp_option' structures specifying
          which options this argp parser understands; it may be zero if
          there are no options at all.  *Note Argp Option Vectors::.

    `argp_parser_t parser'
          A pointer to a function that defines actions for this parser;
          it is called for each option parsed, and at other
          well-defined points in the parsing process.  A value of zero
          is the same as a pointer to a function that always returns
          `ARGP_ERR_UNKNOWN'.  *Note Argp Parser Functions::.

    `const char *args_doc'
          If non-zero, a string describing what non-option arguments
          are called by this parser.  This is only used to print the
          `Usage:' message.  If it contains newlines, the strings
          separated by them are considered alternative usage patterns
          and printed on separate lines.  Lines after the first are
          prefixed by ` or: ' instead of `Usage:'.

    `const char *doc'
          If non-zero, a string containing extra text to be printed
          before and after the options in a long help message, with the
          two sections separated by a vertical tab (`'\v'', `'\013'')
          character.  By convention, the documentation before the
          options is just a short string explaining what the program
          does.  Documentation printed after the options describe
          behavior in more detail.

    `const struct argp_child *children'
          A pointer to a vector of `argp_children' structures.  This
          pointer specifies which additional argp parsers should be
          combined with this one.  *Note Argp Children::.

    `char *(*help_filter)(int KEY, const char *TEXT, void *INPUT)'
          If non-zero, a pointer to a function that filters the output
          of help messages.  *Note Argp Help Filtering::.

    `const char *argp_domain'
          If non-zero, the strings used in the argp library are
          translated using the domain described by this string.  If
          zero, the current default domain is used.


   Of the above group, `options', `parser', `args_doc', and the `doc'
fields are usually all that are needed.  If an argp parser is defined
as an initialized C variable, only the fields used need be specified in
the initializer.  The rest will default to zero due to the way C
structure initialization works.  This design is exploited in most argp
structures; the most-used fields are grouped near the beginning, the
unused fields left unspecified.

* Menu:

* Options: Argp Option Vectors.   Specifying options in an argp parser.
* Argp Parser Functions::         Defining actions for an argp parser.
* Children: Argp Children.        Combining multiple argp parsers.
* Help Filtering: Argp Help Filtering.  Customizing help output for an argp parser.


File: libc.info,  Node: Argp Option Vectors,  Next: Argp Parser Functions,  Prev: Argp Parsers,  Up: Argp Parsers

25.3.4 Specifying Options in an Argp Parser
-------------------------------------------

The `options' field in a `struct argp' points to a vector of `struct
argp_option' structures, each of which specifies an option that the
argp parser supports.  Multiple entries may be used for a single option
provided it has multiple names.  This should be terminated by an entry
with zero in all fields.  Note that when using an initialized C array
for options, writing `{ 0 }' is enough to achieve this.

 -- Data Type: struct argp_option
     This structure specifies a single option that an argp parser
     understands, as well as how to parse and document that option.  It
     has the following fields:

    `const char *name'
          The long name for this option, corresponding to the long
          option `--NAME'; this field may be zero if this option _only_
          has a short name.  To specify multiple names for an option,
          additional entries may follow this one, with the
          `OPTION_ALIAS' flag set.  *Note Argp Option Flags::.

    `int key'
          The integer key provided by the current option to the option
          parser.  If KEY has a value that is a printable ASCII
          character (i.e., `isascii (KEY)' is true), it _also_
          specifies a short option `-CHAR', where CHAR is the ASCII
          character with the code KEY.

    `const char *arg'
          If non-zero, this is the name of an argument associated with
          this option, which must be provided (e.g., with the
          `--NAME=VALUE' or `-CHAR VALUE' syntaxes), unless the
          `OPTION_ARG_OPTIONAL' flag (*note Argp Option Flags::) is
          set, in which case it _may_ be provided.

    `int flags'
          Flags associated with this option, some of which are referred
          to above.  *Note Argp Option Flags::.

    `const char *doc'
          A documentation string for this option, for printing in help
          messages.

          If both the `name' and `key' fields are zero, this string
          will be printed tabbed left from the normal option column,
          making it useful as a group header.  This will be the first
          thing printed in its group.  In this usage, it's conventional
          to end the string with a `:' character.

    `int group'
          Group identity for this option.

          In a long help message, options are sorted alphabetically
          within each group, and the groups presented in the order 0,
          1, 2, ..., N, -M, ..., -2, -1.

          Every entry in an options array with this field 0 will
          inherit the group number of the previous entry, or zero if
          it's the first one.  If it's a group header with `name' and
          `key' fields both zero, the previous entry + 1 is the
          default.  Automagic options such as `--help' are put into
          group -1.

          Note that because of C structure initialization rules, this
          field often need not be specified, because 0 is the correct
          value.

* Menu:

* Flags: Argp Option Flags.     Flags for options.


File: libc.info,  Node: Argp Option Flags,  Up: Argp Option Vectors

25.3.4.1 Flags for Argp Options
...............................

The following flags may be or'd together in the `flags' field of a
`struct argp_option'.  These flags control various aspects of how that
option is parsed or displayed in help messages:

`OPTION_ARG_OPTIONAL'
     The argument associated with this option is optional.

`OPTION_HIDDEN'
     This option isn't displayed in any help messages.

`OPTION_ALIAS'
     This option is an alias for the closest previous non-alias option.
     This means that it will be displayed in the same help entry, and
     will inherit fields other than `name' and `key' from the option
     being aliased.

`OPTION_DOC'
     This option isn't actually an option and should be ignored by the
     actual option parser.  It is an arbitrary section of documentation
     that should be displayed in much the same manner as the options.
     This is known as a "documentation option".

     If this flag is set, then the option `name' field is displayed
     unmodified (e.g., no `--' prefix is added) at the left-margin where
     a _short_ option would normally be displayed, and this
     documentation string is left in it's usual place.  For purposes of
     sorting, any leading whitespace and punctuation is ignored, unless
     the first non-whitespace character is `-'.  This entry is displayed
     after all options, after `OPTION_DOC' entries with a leading `-',
     in the same group.

`OPTION_NO_USAGE'
     This option shouldn't be included in `long' usage messages, but
     should still be included in other help messages.  This is intended
     for options that are completely documented in an argp's `args_doc'
     field.  *Note Argp Parsers::.  Including this option in the
     generic usage list would be redundant, and should be avoided.

     For instance, if `args_doc' is `"FOO BAR\n-x BLAH"', and the `-x'
     option's purpose is to distinguish these two cases, `-x' should
     probably be marked `OPTION_NO_USAGE'.


File: libc.info,  Node: Argp Parser Functions,  Next: Argp Children,  Prev: Argp Option Vectors,  Up: Argp Parsers

25.3.5 Argp Parser Functions
----------------------------

The function pointed to by the `parser' field in a `struct argp' (*note
Argp Parsers::) defines what actions take place in response to each
option or argument parsed.  It is also used as a hook, allowing a
parser to perform tasks at certain other points during parsing.

   Argp parser functions have the following type signature:

     error_t PARSER (int KEY, char *ARG, struct argp_state *STATE)

where the arguments are as follows:

KEY
     For each option that is parsed, PARSER is called with a value of
     KEY from that option's `key' field in the option vector.  *Note
     Argp Option Vectors::.  PARSER is also called at other times with
     special reserved keys, such as `ARGP_KEY_ARG' for non-option
     arguments.  *Note Argp Special Keys::.

ARG
     If KEY is an option, ARG is its given value.  This defaults to
     zero if no value is specified.  Only options that have a non-zero
     `arg' field can ever have a value.  These must _always_ have a
     value unless the `OPTION_ARG_OPTIONAL' flag is specified.  If the
     input being parsed specifies a value for an option that doesn't
     allow one, an error results before PARSER ever gets called.

     If KEY is `ARGP_KEY_ARG', ARG is a non-option argument.  Other
     special keys always have a zero ARG.

STATE
     STATE points to a `struct argp_state', containing useful
     information about the current parsing state for use by PARSER.
     *Note Argp Parsing State::.

   When PARSER is called, it should perform whatever action is
appropriate for KEY, and return `0' for success, `ARGP_ERR_UNKNOWN' if
the value of KEY is not handled by this parser function, or a unix
error code if a real error occurred.  *Note Error Codes::.

 -- Macro: int ARGP_ERR_UNKNOWN
     Argp parser functions should return `ARGP_ERR_UNKNOWN' for any KEY
     value they do not recognize, or for non-option arguments (`KEY ==
     ARGP_KEY_ARG') that they are not equipped to handle.

   A typical parser function uses a switch statement on KEY:

     error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       switch (key)
         {
         case OPTION_KEY:
           ACTION
           break;
         ...
         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }

* Menu:

* Keys: Argp Special Keys.           Special values for the KEY argument.
* State: Argp Parsing State.         What the STATE argument refers to.
* Functions: Argp Helper Functions.  Functions to help during argp parsing.


File: libc.info,  Node: Argp Special Keys,  Next: Argp Parsing State,  Up: Argp Parser Functions

25.3.5.1 Special Keys for Argp Parser Functions
...............................................

In addition to key values corresponding to user options, the KEY
argument to argp parser functions may have a number of other special
values.  In the following example ARG and STATE refer to parser
function arguments.  *Note Argp Parser Functions::.

`ARGP_KEY_ARG'
     This is not an option at all, but rather a command line argument,
     whose value is pointed to by ARG.

     When there are multiple parser functions in play due to argp
     parsers being combined, it's impossible to know which one will
     handle a specific argument.  Each is called until one returns 0 or
     an error other than `ARGP_ERR_UNKNOWN'; if an argument is not
     handled, `argp_parse' immediately returns success, without parsing
     any more arguments.

     Once a parser function returns success for this key, that fact is
     recorded, and the `ARGP_KEY_NO_ARGS' case won't be used.
     _However_, if while processing the argument a parser function
     decrements the `next' field of its STATE argument, the option
     won't be considered processed; this is to allow you to actually
     modify the argument, perhaps into an option, and have it processed
     again.

`ARGP_KEY_ARGS'
     If a parser function returns `ARGP_ERR_UNKNOWN' for
     `ARGP_KEY_ARG', it is immediately called again with the key
     `ARGP_KEY_ARGS', which has a similar meaning, but is slightly more
     convenient for consuming all remaining arguments.  ARG is 0, and
     the tail of the argument vector may be found at `STATE->argv +
     STATE->next'.  If success is returned for this key, and
     `STATE->next' is unchanged, all remaining arguments are considered
     to have been consumed.  Otherwise, the amount by which
     `STATE->next' has been adjusted indicates how many were used.
     Here's an example that uses both, for different args:

          ...
          case ARGP_KEY_ARG:
            if (STATE->arg_num == 0)
              /* First argument */
              first_arg = ARG;
            else
              /* Let the next case parse it.  */
              return ARGP_KEY_UNKNOWN;
            break;
          case ARGP_KEY_ARGS:
            remaining_args = STATE->argv + STATE->next;
            num_remaining_args = STATE->argc - STATE->next;
            break;

`ARGP_KEY_END'
     This indicates that there are no more command line arguments.
     Parser functions are called in a different order, children first.
     This allows each parser to clean up its state for the parent.

`ARGP_KEY_NO_ARGS'
     Because it's common to do some special processing if there aren't
     any non-option args, parser functions are called with this key if
     they didn't successfully process any non-option arguments.  This
     is called just before `ARGP_KEY_END', where more general validity
     checks on previously parsed arguments take place.

`ARGP_KEY_INIT'
     This is passed in before any parsing is done.  Afterwards, the
     values of each element of the `child_input' field of STATE, if
     any, are copied to each child's state to be the initial value of
     the `input' when _their_ parsers are called.

`ARGP_KEY_SUCCESS'
     Passed in when parsing has successfully been completed, even if
     arguments remain.

`ARGP_KEY_ERROR'
     Passed in if an error has occurred and parsing is terminated.  In
     this case a call with a key of `ARGP_KEY_SUCCESS' is never made.

`ARGP_KEY_FINI'
     The final key ever seen by any parser, even after
     `ARGP_KEY_SUCCESS' and `ARGP_KEY_ERROR'.  Any resources allocated
     by `ARGP_KEY_INIT' may be freed here.  At times, certain resources
     allocated are to be returned to the caller after a successful
     parse.  In that case, those particular resources can be freed in
     the `ARGP_KEY_ERROR' case.

   In all cases, `ARGP_KEY_INIT' is the first key seen by parser
functions, and `ARGP_KEY_FINI' the last, unless an error was returned
by the parser for `ARGP_KEY_INIT'.  Other keys can occur in one the
following orders.  OPT refers to an arbitrary option key:

OPT... `ARGP_KEY_NO_ARGS' `ARGP_KEY_END' `ARGP_KEY_SUCCESS'
     The arguments being parsed did not contain any non-option
     arguments.

( OPT | `ARGP_KEY_ARG' )... `ARGP_KEY_END' `ARGP_KEY_SUCCESS'
     All non-option arguments were successfully handled by a parser
     function.  There may be multiple parser functions if multiple argp
     parsers were combined.

( OPT | `ARGP_KEY_ARG' )... `ARGP_KEY_SUCCESS'
     Some non-option argument went unrecognized.

     This occurs when every parser function returns `ARGP_KEY_UNKNOWN'
     for an argument, in which case parsing stops at that argument if
     ARG_INDEX is a null pointer.  Otherwise an error occurs.

   In all cases, if a non-null value for ARG_INDEX gets passed to
`argp_parse', the index of the first unparsed command-line argument is
passed back in that value.

   If an error occurs and is either detected by argp or because a parser
function returned an error value, each parser is called with
`ARGP_KEY_ERROR'.  No further calls are made, except the final call
with `ARGP_KEY_FINI'.


File: libc.info,  Node: Argp Helper Functions,  Prev: Argp Parsing State,  Up: Argp Parser Functions

25.3.5.2 Functions For Use in Argp Parsers
..........................................

Argp provides a number of functions available to the user of argp
(*note Argp Parser Functions::), mostly for producing error messages.
These take as their first argument the STATE argument to the parser
function.  *Note Argp Parsing State::.

 -- Function: void argp_usage (const struct argp_state *STATE)
     Outputs the standard usage message for the argp parser referred to
     by STATE to `STATE->err_stream' and terminate the program with
     `exit (argp_err_exit_status)'.  *Note Argp Global Variables::.

 -- Function: void argp_error (const struct argp_state *STATE, const
          char *FMT, ...)
     Prints the printf format string FMT and following args, preceded
     by the program name and `:', and followed by a `Try ... --help'
     message, and terminates the program with an exit status of
     `argp_err_exit_status'.  *Note Argp Global Variables::.

 -- Function: void argp_failure (const struct argp_state *STATE, int
          STATUS, int ERRNUM, const char *FMT, ...)
     Similar to the standard gnu error-reporting function `error', this
     prints the program name and `:', the printf format string FMT, and
     the appropriate following args.  If it is non-zero, the standard
     unix error text for ERRNUM is printed.  If STATUS is non-zero, it
     terminates the program with that value as its exit status.

     The difference between `argp_failure' and `argp_error' is that
     `argp_error' is for _parsing errors_, whereas `argp_failure' is
     for other problems that occur during parsing but don't reflect a
     syntactic problem with the input, such as illegal values for
     options, bad phase of the moon, etc.

 -- Function: void argp_state_help (const struct argp_state *STATE,
          FILE *STREAM, unsigned FLAGS)
     Outputs a help message for the argp parser referred to by STATE,
     to STREAM.  The FLAGS argument determines what sort of help
     message is produced.  *Note Argp Help Flags::.

   Error output is sent to `STATE->err_stream', and the program name
printed is `STATE->name'.

   The output or program termination behavior of these functions may be
suppressed if the `ARGP_NO_EXIT' or `ARGP_NO_ERRS' flags are passed to
`argp_parse'.  *Note Argp Flags::.

   This behavior is useful if an argp parser is exported for use by
other programs (e.g., by a library), and may be used in a context where
it is not desirable to terminate the program in response to parsing
errors.  In argp parsers intended for such general use, and for the
case where the program _doesn't_ terminate, calls to any of these
functions should be followed by code that returns the appropriate error
code:

     if (BAD ARGUMENT SYNTAX)
       {
          argp_usage (STATE);
          return EINVAL;
       }

If a parser function will _only_ be used when `ARGP_NO_EXIT' is not
set, the return may be omitted.


File: libc.info,  Node: Argp Parsing State,  Next: Argp Helper Functions,  Prev: Argp Special Keys,  Up: Argp Parser Functions

25.3.5.3 Argp Parsing State
...........................

The third argument to argp parser functions (*note Argp Parser
Functions::) is a pointer to a `struct argp_state', which contains
information about the state of the option parsing.

 -- Data Type: struct argp_state
     This structure has the following fields, which may be modified as
     noted:

    `const struct argp *const root_argp'
          The top level argp parser being parsed.  Note that this is
          often _not_ the same `struct argp' passed into `argp_parse' by
          the invoking program.  *Note Argp::.  It is an internal argp
          parser that contains options implemented by `argp_parse'
          itself, such as `--help'.

    `int argc'
    `char **argv'
          The argument vector being parsed.  This may be modified.

    `int next'
          The index in `argv' of the next argument to be parsed.  This
          may be modified.

          One way to consume all remaining arguments in the input is to
          set `STATE->next = STATE->argc', perhaps after recording the
          value of the `next' field to find the consumed arguments.  The
          current option can be re-parsed immediately by decrementing
          this field, then modifying `STATE->argv[STATE->next]' to
          reflect the option that should be reexamined.

    `unsigned flags'
          The flags supplied to `argp_parse'.  These may be modified,
          although some flags may only take effect when `argp_parse' is
          first invoked.  *Note Argp Flags::.

    `unsigned arg_num'
          While calling a parsing function with the KEY argument
          `ARGP_KEY_ARG', this represents the number of the current arg,
          starting at 0.  It is incremented after each `ARGP_KEY_ARG'
          call returns.  At all other times, this is the number of
          `ARGP_KEY_ARG' arguments that have been processed.

    `int quoted'
          If non-zero, the index in `argv' of the first argument
          following a special `--' argument.  This prevents anything
          that follows from being interpreted as an option.  It is only
          set after argument parsing has proceeded past this point.

    `void *input'
          An arbitrary pointer passed in from the caller of
          `argp_parse', in the INPUT argument.

    `void **child_inputs'
          These are values that will be passed to child parsers.  This
          vector will be the same length as the number of children in
          the current parser.  Each child parser will be given the
          value of `STATE->child_inputs[I]' as _its_ `STATE->input'
          field, where I is the index of the child in the this parser's
          `children' field.  *Note Argp Children::.

    `void *hook'
          For the parser function's use.  Initialized to 0, but
          otherwise ignored by argp.

    `char *name'
          The name used when printing messages.  This is initialized to
          `argv[0]', or `program_invocation_name' if `argv[0]' is
          unavailable.

    `FILE *err_stream'
    `FILE *out_stream'
          The stdio streams used when argp prints.  Error messages are
          printed to `err_stream', all other output, such as `--help'
          output) to `out_stream'.  These are initialized to `stderr'
          and `stdout' respectively.  *Note Standard Streams::.

    `void *pstate'
          Private, for use by the argp implementation.


File: libc.info,  Node: Argp Children,  Next: Argp Help Filtering,  Prev: Argp Parser Functions,  Up: Argp Parsers

25.3.6 Combining Multiple Argp Parsers
--------------------------------------

The `children' field in a `struct argp' enables other argp parsers to
be combined with the referencing one for the parsing of a single set of
arguments.  This field should point to a vector of `struct argp_child',
which is terminated by an entry having a value of zero in the `argp'
field.

   Where conflicts between combined parsers arise, as when two specify
an option with the same name, the parser conflicts are resolved in
favor of the parent argp parser(s), or the earlier of the argp parsers
in the list of children.

 -- Data Type: struct argp_child
     An entry in the list of subsidiary argp parsers pointed to by the
     `children' field in a `struct argp'.  The fields are as follows:

    `const struct argp *argp'
          The child argp parser, or zero to end of the list.

    `int flags'
          Flags for this child.

    `const char *header'
          If non-zero, this is an optional header to be printed within
          help output before the child options.  As a side-effect, a
          non-zero value forces the child options to be grouped
          together.  To achieve this effect without actually printing a
          header string, use a value of `""'.  As with header strings
          specified in an option entry, the conventional value of the
          last character is `:'.  *Note Argp Option Vectors::.

    `int group'
          This is where the child options are grouped relative to the
          other `consolidated' options in the parent argp parser.  The
          values are the same as the `group' field in `struct
          argp_option'.  *Note Argp Option Vectors::.  All
          child-groupings follow parent options at a particular group
          level.  If both this field and `header' are zero, then the
          child's options aren't grouped together, they are merged with
          parent options at the parent option group level.



File: libc.info,  Node: Argp Flags,  Next: Argp Help,  Prev: Argp Parsers,  Up: Argp

25.3.7 Flags for `argp_parse'
-----------------------------

The default behavior of `argp_parse' is designed to be convenient for
the most common case of parsing program command line argument.  To
modify these defaults, the following flags may be or'd together in the
FLAGS argument to `argp_parse':

`ARGP_PARSE_ARGV0'
     Don't ignore the first element of the ARGV argument to
     `argp_parse'.  Unless `ARGP_NO_ERRS' is set, the first element of
     the argument vector is skipped for option parsing purposes, as it
     corresponds to the program name in a command line.

`ARGP_NO_ERRS'
     Don't print error messages for unknown options to `stderr'; unless
     this flag is set, `ARGP_PARSE_ARGV0' is ignored, as `argv[0]' is
     used as the program name in the error messages.  This flag implies
     `ARGP_NO_EXIT'.  This is based on the assumption that silent
     exiting upon errors is bad behavior.

`ARGP_NO_ARGS'
     Don't parse any non-option args.  Normally these are parsed by
     calling the parse functions with a key of `ARGP_KEY_ARG', the
     actual argument being the value.  This flag needn't normally be
     set, as the default behavior is to stop parsing as soon as an
     argument fails to be parsed.  *Note Argp Parser Functions::.

`ARGP_IN_ORDER'
     Parse options and arguments in the same order they occur on the
     command line.  Normally they're rearranged so that all options
     come first.

`ARGP_NO_HELP'
     Don't provide the standard long option `--help', which ordinarily
     causes usage and option help information to be output to `stdout'
     and `exit (0)'.

`ARGP_NO_EXIT'
     Don't exit on errors, although they may still result in error
     messages.

`ARGP_LONG_ONLY'
     Use the gnu getopt `long-only' rules for parsing arguments.  This
     allows long-options to be recognized with only a single `-' (i.e.,
     `-help').  This results in a less useful interface, and its use is
     discouraged as it conflicts with the way most GNU programs work as
     well as the GNU coding standards.

`ARGP_SILENT'
     Turns off any message-printing/exiting options, specifically
     `ARGP_NO_EXIT', `ARGP_NO_ERRS', and `ARGP_NO_HELP'.


File: libc.info,  Node: Argp Help Filtering,  Prev: Argp Children,  Up: Argp Parsers

25.3.8 Customizing Argp Help Output
-----------------------------------

The `help_filter' field in a `struct argp' is a pointer to a function
that filters the text of help messages before displaying them.  They
have a function signature like:

     char *HELP-FILTER (int KEY, const char *TEXT, void *INPUT)

Where KEY is either a key from an option, in which case TEXT is that
option's help text.  *Note Argp Option Vectors::.  Alternately, one of
the special keys with names beginning with `ARGP_KEY_HELP_' might be
used, describing which other help text TEXT will contain.  *Note Argp
Help Filter Keys::.

   The function should return either TEXT if it remains as-is, or a
replacement string allocated using `malloc'.  This will be either be
freed by argp or zero, which prints nothing.  The value of TEXT is
supplied _after_ any translation has been done, so if any of the
replacement text needs translation, it will be done by the filter
function.  INPUT is either the input supplied to `argp_parse' or it is
zero, if `argp_help' was called directly by the user.

* Menu:

* Keys: Argp Help Filter Keys.  Special KEY values for help filter functions.


File: libc.info,  Node: Argp Help Filter Keys,  Up: Argp Help Filtering

25.3.8.1 Special Keys for Argp Help Filter Functions
....................................................

The following special values may be passed to an argp help filter
function as the first argument in addition to key values for user
options.  They specify which help text the TEXT argument contains:

`ARGP_KEY_HELP_PRE_DOC'
     The help text preceding options.

`ARGP_KEY_HELP_POST_DOC'
     The help text following options.

`ARGP_KEY_HELP_HEADER'
     The option header string.

`ARGP_KEY_HELP_EXTRA'
     This is used after all other documentation; TEXT is zero for this
     key.

`ARGP_KEY_HELP_DUP_ARGS_NOTE'
     The explanatory note printed when duplicate option arguments have
     been suppressed.

`ARGP_KEY_HELP_ARGS_DOC'
     The argument doc string; formally the `args_doc' field from the
     argp parser.  *Note Argp Parsers::.


File: libc.info,  Node: Argp Help,  Next: Argp Examples,  Prev: Argp Flags,  Up: Argp

25.3.9 The `argp_help' Function
-------------------------------

Normally programs using argp need not be written with particular
printing argument-usage-type help messages in mind as the standard
`--help' option is handled automatically by argp.  Typical error cases
can be handled using `argp_usage' and `argp_error'.  *Note Argp Helper
Functions::.  However, if it's desirable to print a help message in
some context other than parsing the program options, argp offers the
`argp_help' interface.

 -- Function: void argp_help (const struct argp *ARGP, FILE *STREAM,
          unsigned FLAGS, char *NAME)
     This outputs a help message for the argp parser ARGP to STREAM.
     The type of messages printed will be determined by FLAGS.

     Any options such as `--help' that are implemented automatically by
     argp itself will _not_ be present in the help output; for this
     reason it is best to use `argp_state_help' if calling from within
     an argp parser function.  *Note Argp Helper Functions::.

* Menu:

* Flags: Argp Help Flags.       Specifying what sort of help message to print.


File: libc.info,  Node: Argp Help Flags,  Up: Argp Help

25.3.10 Flags for the `argp_help' Function
------------------------------------------

When calling `argp_help' (*note Argp Help::) or `argp_state_help'
(*note Argp Helper Functions::) the exact output is determined by the
FLAGS argument.  This should consist of any of the following flags,
or'd together:

`ARGP_HELP_USAGE'
     A unix `Usage:' message that explicitly lists all options.

`ARGP_HELP_SHORT_USAGE'
     A unix `Usage:' message that displays an appropriate placeholder to
     indicate where the options go; useful for showing the non-option
     argument syntax.

`ARGP_HELP_SEE'
     A `Try ... for more help' message; `...' contains the program name
     and `--help'.

`ARGP_HELP_LONG'
     A verbose option help message that gives each option available
     along with its documentation string.

`ARGP_HELP_PRE_DOC'
     The part of the argp parser doc string preceding the verbose
     option help.

`ARGP_HELP_POST_DOC'
     The part of the argp parser doc string that following the verbose
     option help.

`ARGP_HELP_DOC'
     `(ARGP_HELP_PRE_DOC | ARGP_HELP_POST_DOC)'

`ARGP_HELP_BUG_ADDR'
     A message that prints where to report bugs for this program, if the
     `argp_program_bug_address' variable contains this information.

`ARGP_HELP_LONG_ONLY'
     This will modify any output to reflect the `ARGP_LONG_ONLY' mode.

   The following flags are only understood when used with
`argp_state_help'.  They control whether the function returns after
printing its output, or terminates the program:

`ARGP_HELP_EXIT_ERR'
     This will terminate the program with `exit (argp_err_exit_status)'.

`ARGP_HELP_EXIT_OK'
     This will terminate the program with `exit (0)'.

   The following flags are combinations of the basic flags for printing
standard messages:

`ARGP_HELP_STD_ERR'
     Assuming that an error message for a parsing error has printed,
     this prints a message on how to get help, and terminates the
     program with an error.

`ARGP_HELP_STD_USAGE'
     This prints a standard usage message and terminates the program
     with an error.  This is used when no other specific error messages
     are appropriate or available.

`ARGP_HELP_STD_HELP'
     This prints the standard response for a `--help' option, and
     terminates the program successfully.


File: libc.info,  Node: Argp Examples,  Next: Argp User Customization,  Prev: Argp Help,  Up: Argp

25.3.11 Argp Examples
---------------------

These example programs demonstrate the basic usage of argp.

* Menu:

* 1: Argp Example 1.            A minimal program using argp.
* 2: Argp Example 2.            A program using only default options.
* 3: Argp Example 3.            A simple program with user options.
* 4: Argp Example 4.            Combining multiple argp parsers.


File: libc.info,  Node: Argp Example 1,  Next: Argp Example 2,  Up: Argp Examples

25.3.11.1 A Minimal Program Using Argp
......................................

This is perhaps the smallest program possible that uses argp.  It won't
do much except give an error messages and exit when there are any
arguments, and prints a rather pointless message for `--help'.

     /* Argp example #1 - a minimal program using argp */

     /* This is (probably) the smallest possible program that
        uses argp.  It won't do much except give an error
        messages and exit when there are any arguments, and print
        a (rather pointless) messages for -help. */

     #include <argp.h>

     int main (int argc, char **argv)
     {
       argp_parse (0, argc, argv, 0, 0, 0);
       exit (0);
     }


File: libc.info,  Node: Argp Example 2,  Next: Argp Example 3,  Prev: Argp Example 1,  Up: Argp Examples

25.3.11.2 A Program Using Argp with Only Default Options
........................................................

This program doesn't use any options or arguments, it uses argp to be
compliant with the GNU standard command line format.

   In addition to giving no arguments and implementing a `--help'
option, this example has a `--version' option, which will put the given
documentation string and bug address in the `--help' output, as per GNU
standards.

   The variable `argp' contains the argument parser specification.
Adding fields to this structure is the way most parameters are passed
to `argp_parse'.  The first three fields are normally used, but they
are not in this small program.  There are also two global variables
that argp can use defined here, `argp_program_version' and
`argp_program_bug_address'.  They are considered global variables
because they will almost always be constant for a given program, even
if they use different argument parsers for various tasks.

     /* Argp example #2 - a pretty minimal program using argp */

     /* This program doesn't use any options or arguments, but uses
        argp to be compliant with the GNU standard command line
        format.

        In addition to making sure no arguments are given, and
        implementing a -help option, this example will have a
        -version option, and will put the given documentation string
        and bug address in the -help output, as per GNU standards.

        The variable ARGP contains the argument parser specification;
        adding fields to this structure is the way most parameters are
        passed to argp_parse (the first three fields are usually used,
        but not in this small program).  There are also two global
        variables that argp knows about defined here,
        ARGP_PROGRAM_VERSION and ARGP_PROGRAM_BUG_ADDRESS (they are
        global variables because they will almost always be constant
        for a given program, even if it uses different argument
        parsers for various tasks). */

     #include <argp.h>

     const char *argp_program_version =
       "argp-ex2 1.0";
     const char *argp_program_bug_address =
       "<bug-gnu-utils@gnu.org>";

     /* Program documentation. */
     static char doc[] =
       "Argp example #2 -- a pretty minimal program using argp";

     /* Our argument parser.  The `options', `parser', and
        `args_doc' fields are zero because we have neither options or
        arguments; `doc' and `argp_program_bug_address' will be
        used in the output for `--help', and the `--version'
        option will print out `argp_program_version'. */
     static struct argp argp = { 0, 0, 0, doc };

     int main (int argc, char **argv)
     {
       argp_parse (&argp, argc, argv, 0, 0, 0);
       exit (0);
     }


File: libc.info,  Node: Argp Example 3,  Next: Argp Example 4,  Prev: Argp Example 2,  Up: Argp Examples

25.3.11.3 A Program Using Argp with User Options
................................................

This program uses the same features as example 2, adding user options
and arguments.

   We now use the first four fields in `argp' (*note Argp Parsers::)
and specify `parse_opt' as the parser function.  *Note Argp Parser
Functions::.

   Note that in this example, `main' uses a structure to communicate
with the `parse_opt' function, a pointer to which it passes in the
`input' argument to `argp_parse'.  *Note Argp::.  It is retrieved by
`parse_opt' through the `input' field in its `state' argument.  *Note
Argp Parsing State::.  Of course, it's also possible to use global
variables instead, but using a structure like this is somewhat more
flexible and clean.

     /* Argp example #3 - a program with options and arguments using argp */

     /* This program uses the same features as example 2, and uses options and
        arguments.

        We now use the first four fields in ARGP, so here's a description of them:
          OPTIONS  - A pointer to a vector of struct argp_option (see below)
          PARSER   - A function to parse a single option, called by argp
          ARGS_DOC - A string describing how the non-option arguments should look
          DOC      - A descriptive string about this program; if it contains a
                      vertical tab character (\v), the part after it will be
                      printed *following* the options

        The function PARSER takes the following arguments:
          KEY  - An integer specifying which option this is (taken
                  from the KEY field in each struct argp_option), or
                  a special key specifying something else; the only
                  special keys we use here are ARGP_KEY_ARG, meaning
                  a non-option argument, and ARGP_KEY_END, meaning
                  that all arguments have been parsed
          ARG  - For an option KEY, the string value of its
                  argument, or NULL if it has none
          STATE- A pointer to a struct argp_state, containing
                  various useful information about the parsing state; used here
                  are the INPUT field, which reflects the INPUT argument to
                  argp_parse, and the ARG_NUM field, which is the number of the
                  current non-option argument being parsed
        It should return either 0, meaning success, ARGP_ERR_UNKNOWN, meaning the
        given KEY wasn't recognized, or an errno value indicating some other
        error.

        Note that in this example, main uses a structure to communicate with the
        parse_opt function, a pointer to which it passes in the INPUT argument to
        argp_parse.  Of course, it's also possible to use global variables
        instead, but this is somewhat more flexible.

        The OPTIONS field contains a pointer to a vector of struct argp_option's;
        that structure has the following fields (if you assign your option
        structures using array initialization like this example, unspecified
        fields will be defaulted to 0, and need not be specified):
          NAME   - The name of this option's long option (may be zero)
          KEY    - The KEY to pass to the PARSER function when parsing this option,
                    *and* the name of this option's short option, if it is a
                    printable ascii character
          ARG    - The name of this option's argument, if any
          FLAGS  - Flags describing this option; some of them are:
                      OPTION_ARG_OPTIONAL - The argument to this option is optional
                      OPTION_ALIAS        - This option is an alias for the
                                             previous option
                      OPTION_HIDDEN       - Don't show this option in -help output
          DOC    - A documentation string for this option, shown in -help output

        An options vector should be terminated by an option with all fields zero. */

     #include <argp.h>

     const char *argp_program_version =
       "argp-ex3 1.0";
     const char *argp_program_bug_address =
       "<bug-gnu-utils@gnu.org>";

     /* Program documentation. */
     static char doc[] =
       "Argp example #3 -- a program with options and arguments using argp";

     /* A description of the arguments we accept. */
     static char args_doc[] = "ARG1 ARG2";

     /* The options we understand. */
     static struct argp_option options[] = {
       {"verbose",  'v', 0,      0,  "Produce verbose output" },
       {"quiet",    'q', 0,      0,  "Don't produce any output" },
       {"silent",   's', 0,      OPTION_ALIAS },
       {"output",   'o', "FILE", 0,
        "Output to FILE instead of standard output" },
       { 0 }
     };

     /* Used by `main' to communicate with `parse_opt'. */
     struct arguments
     {
       char *args[2];                /* ARG1 & ARG2 */
       int silent, verbose;
       char *output_file;
     };

     /* Parse a single option. */
     static error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       /* Get the INPUT argument from `argp_parse', which we
          know is a pointer to our arguments structure. */
       struct arguments *arguments = state->input;

       switch (key)
         {
         case 'q': case 's':
           arguments->silent = 1;
           break;
         case 'v':
           arguments->verbose = 1;
           break;
         case 'o':
           arguments->output_file = arg;
           break;

         case ARGP_KEY_ARG:
           if (state->arg_num >= 2)
             /* Too many arguments. */
             argp_usage (state);

           arguments->args[state->arg_num] = arg;

           break;

         case ARGP_KEY_END:
           if (state->arg_num < 2)
             /* Not enough arguments. */
             argp_usage (state);
           break;

         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }

     /* Our argp parser. */
     static struct argp argp = { options, parse_opt, args_doc, doc };

     int main (int argc, char **argv)
     {
       struct arguments arguments;

       /* Default values. */
       arguments.silent = 0;
       arguments.verbose = 0;
       arguments.output_file = "-";

       /* Parse our arguments; every option seen by `parse_opt' will
          be reflected in `arguments'. */
       argp_parse (&argp, argc, argv, 0, 0, &arguments);

       printf ("ARG1 = %s\nARG2 = %s\nOUTPUT_FILE = %s\n"
               "VERBOSE = %s\nSILENT = %s\n",
               arguments.args[0], arguments.args[1],
               arguments.output_file,
               arguments.verbose ? "yes" : "no",
               arguments.silent ? "yes" : "no");

       exit (0);
     }


File: libc.info,  Node: Argp Example 4,  Prev: Argp Example 3,  Up: Argp Examples

25.3.11.4 A Program Using Multiple Combined Argp Parsers
........................................................

This program uses the same features as example 3, but has more options,
and presents more structure in the `--help' output.  It also
illustrates how you can `steal' the remainder of the input arguments
past a certain point for programs that accept a list of items.  It also
illustrates the KEY value `ARGP_KEY_NO_ARGS', which is only given if no
non-option arguments were supplied to the program.  *Note Argp Special
Keys::.

   For structuring help output, two features are used: _headers_ and a
two part option string.  The _headers_ are entries in the options
vector.  *Note Argp Option Vectors::.  The first four fields are zero.
The two part documentation string are in the variable `doc', which
allows documentation both before and after the options.  *Note Argp
Parsers::, the two parts of `doc' are separated by a vertical-tab
character (`'\v'', or `'\013'').  By convention, the documentation
before the options is a short string stating what the program does, and
after any options it is longer, describing the behavior in more detail.
All documentation strings are automatically filled for output, although
newlines may be included to force a line break at a particular point.
In addition, documentation strings are passed to the `gettext'
function, for possible translation into the current locale.

     /* Argp example #4 - a program with somewhat more complicated options */

     /* This program uses the same features as example 3, but has more
        options, and somewhat more structure in the -help output.  It
        also shows how you can `steal' the remainder of the input
        arguments past a certain point, for programs that accept a
        list of items.  It also shows the special argp KEY value
        ARGP_KEY_NO_ARGS, which is only given if no non-option
        arguments were supplied to the program.

        For structuring the help output, two features are used,
        *headers* which are entries in the options vector with the
        first four fields being zero, and a two part documentation
        string (in the variable DOC), which allows documentation both
        before and after the options; the two parts of DOC are
        separated by a vertical-tab character ('\v', or '\013').  By
        convention, the documentation before the options is just a
        short string saying what the program does, and that afterwards
        is longer, describing the behavior in more detail.  All
        documentation strings are automatically filled for output,
        although newlines may be included to force a line break at a
        particular point.  All documentation strings are also passed to
        the `gettext' function, for possible translation into the
        current locale. */

     #include <stdlib.h>
     #include <error.h>
     #include <argp.h>

     const char *argp_program_version =
       "argp-ex4 1.0";
     const char *argp_program_bug_address =
       "<bug-gnu-utils@prep.ai.mit.edu>";

     /* Program documentation. */
     static char doc[] =
       "Argp example #4 -- a program with somewhat more complicated\
     options\
     \vThis part of the documentation comes *after* the options;\
      note that the text is automatically filled, but it's possible\
      to force a line-break, e.g.\n<-- here.";

     /* A description of the arguments we accept. */
     static char args_doc[] = "ARG1 [STRING...]";

     /* Keys for options without short-options. */
     #define OPT_ABORT  1            /* -abort */

     /* The options we understand. */
     static struct argp_option options[] = {
       {"verbose",  'v', 0,       0, "Produce verbose output" },
       {"quiet",    'q', 0,       0, "Don't produce any output" },
       {"silent",   's', 0,       OPTION_ALIAS },
       {"output",   'o', "FILE",  0,
        "Output to FILE instead of standard output" },

       {0,0,0,0, "The following options should be grouped together:" },
       {"repeat",   'r', "COUNT", OPTION_ARG_OPTIONAL,
        "Repeat the output COUNT (default 10) times"},
       {"abort",    OPT_ABORT, 0, 0, "Abort before showing any output"},

       { 0 }
     };

     /* Used by `main' to communicate with `parse_opt'. */
     struct arguments
     {
       char *arg1;                   /* ARG1 */
       char **strings;               /* [STRING...] */
       int silent, verbose, abort;   /* `-s', `-v', `--abort' */
       char *output_file;            /* FILE arg to `--output' */
       int repeat_count;             /* COUNT arg to `--repeat' */
     };

     /* Parse a single option. */
     static error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       /* Get the `input' argument from `argp_parse', which we
          know is a pointer to our arguments structure. */
       struct arguments *arguments = state->input;

       switch (key)
         {
         case 'q': case 's':
           arguments->silent = 1;
           break;
         case 'v':
           arguments->verbose = 1;
           break;
         case 'o':
           arguments->output_file = arg;
           break;
         case 'r':
           arguments->repeat_count = arg ? atoi (arg) : 10;
           break;
         case OPT_ABORT:
           arguments->abort = 1;
           break;

         case ARGP_KEY_NO_ARGS:
           argp_usage (state);

         case ARGP_KEY_ARG:
           /* Here we know that `state->arg_num == 0', since we
              force argument parsing to end before any more arguments can
              get here. */
           arguments->arg1 = arg;

           /* Now we consume all the rest of the arguments.
              `state->next' is the index in `state->argv' of the
              next argument to be parsed, which is the first STRING
              we're interested in, so we can just use
              `&state->argv[state->next]' as the value for
              arguments->strings.

              _In addition_, by setting `state->next' to the end
              of the arguments, we can force argp to stop parsing here and
              return. */
           arguments->strings = &state->argv[state->next];
           state->next = state->argc;

           break;

         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }

     /* Our argp parser. */
     static struct argp argp = { options, parse_opt, args_doc, doc };

     int main (int argc, char **argv)
     {
       int i, j;
       struct arguments arguments;

       /* Default values. */
       arguments.silent = 0;
       arguments.verbose = 0;
       arguments.output_file = "-";
       arguments.repeat_count = 1;
       arguments.abort = 0;

       /* Parse our arguments; every option seen by `parse_opt' will be
          reflected in `arguments'. */
       argp_parse (&argp, argc, argv, 0, 0, &arguments);

       if (arguments.abort)
         error (10, 0, "ABORTED");

       for (i = 0; i < arguments.repeat_count; i++)
         {
           printf ("ARG1 = %s\n", arguments.arg1);
           printf ("STRINGS = ");
           for (j = 0; arguments.strings[j]; j++)
             printf (j == 0 ? "%s" : ", %s", arguments.strings[j]);
           printf ("\n");
           printf ("OUTPUT_FILE = %s\nVERBOSE = %s\nSILENT = %s\n",
                   arguments.output_file,
                   arguments.verbose ? "yes" : "no",
                   arguments.silent ? "yes" : "no");
         }

       exit (0);
     }


File: libc.info,  Node: Argp User Customization,  Prev: Argp Examples,  Up: Argp

25.3.12 Argp User Customization
-------------------------------

The formatting of argp `--help' output may be controlled to some extent
by a program's users, by setting the `ARGP_HELP_FMT' environment
variable to a comma-separated list of tokens.  Whitespace is ignored:

`dup-args'
`no-dup-args'
     These turn "duplicate-argument-mode" on or off.  In duplicate
     argument mode, if an option that accepts an argument has multiple
     names, the argument is shown for each name.  Otherwise, it is only
     shown for the first long option.  A note is subsequently printed
     so the user knows that it applies to other names as well.  The
     default is `no-dup-args', which is less consistent, but prettier.

`dup-args-note'

`no-dup-args-note'
     These will enable or disable the note informing the user of
     suppressed option argument duplication.  The default is
     `dup-args-note'.

`short-opt-col=N'
     This prints the first short option in column N.  The default is 2.

`long-opt-col=N'
     This prints the first long option in column N.  The default is 6.

`doc-opt-col=N'
     This prints `documentation options' (*note Argp Option Flags::) in
     column N.  The default is 2.

`opt-doc-col=N'
     This prints the documentation for options starting in column N.
     The default is 29.

`header-col=N'
     This will indent the group headers that document groups of options
     to column N.  The default is 1.

`usage-indent=N'
     This will indent continuation lines in `Usage:' messages to column
     N.  The default is 12.

`rmargin=N'
     This will word wrap help output at or before column N.  The default
     is 79.


File: libc.info,  Node: Suboptions,  Next: Suboptions Example,  Prev: Argp,  Up: Parsing Program Arguments

25.3.12.1 Parsing of Suboptions
...............................

Having a single level of options is sometimes not enough.  There might
be too many options which have to be available or a set of options is
closely related.

   For this case some programs use suboptions.  One of the most
prominent programs is certainly `mount'(8).  The `-o' option take one
argument which itself is a comma separated list of options.  To ease the
programming of code like this the function `getsubopt' is available.

 -- Function: int getsubopt (char **OPTIONP, const char* const *TOKENS,
          char **VALUEP)
     The OPTIONP parameter must be a pointer to a variable containing
     the address of the string to process.  When the function returns
     the reference is updated to point to the next suboption or to the
     terminating `\0' character if there is no more suboption available.

     The TOKENS parameter references an array of strings containing the
     known suboptions.  All strings must be `\0' terminated and to mark
     the end a null pointer must be stored.  When `getsubopt' finds a
     possible legal suboption it compares it with all strings available
     in the TOKENS array and returns the index in the string as the
     indicator.

     In case the suboption has an associated value introduced by a `='
     character, a pointer to the value is returned in VALUEP.  The
     string is `\0' terminated.  If no argument is available VALUEP is
     set to the null pointer.  By doing this the caller can check
     whether a necessary value is given or whether no unexpected value
     is present.

     In case the next suboption in the string is not mentioned in the
     TOKENS array the starting address of the suboption including a
     possible value is returned in VALUEP and the return value of the
     function is `-1'.


File: libc.info,  Node: Suboptions Example,  Prev: Suboptions,  Up: Parsing Program Arguments

25.3.13 Parsing of Suboptions Example
-------------------------------------

The code which might appear in the `mount'(8) program is a perfect
example of the use of `getsubopt':

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

     int do_all;
     const char *type;
     int read_size;
     int write_size;
     int read_only;

     enum
     {
       RO_OPTION = 0,
       RW_OPTION,
       READ_SIZE_OPTION,
       WRITE_SIZE_OPTION,
       THE_END
     };

     const char *mount_opts[] =
     {
       [RO_OPTION] = "ro",
       [RW_OPTION] = "rw",
       [READ_SIZE_OPTION] = "rsize",
       [WRITE_SIZE_OPTION] = "wsize",
       [THE_END] = NULL
     };

     int
     main (int argc, char *argv[])
     {
       char *subopts, *value;
       int opt;

       while ((opt = getopt (argc, argv, "at:o:")) != -1)
         switch (opt)
           {
           case 'a':
             do_all = 1;
             break;
           case 't':
             type = optarg;
             break;
           case 'o':
             subopts = optarg;
             while (*subopts != '\0')
               switch (getsubopt (&subopts, mount_opts, &value))
                 {
                 case RO_OPTION:
                   read_only = 1;
                   break;
                 case RW_OPTION:
                   read_only = 0;
                   break;
                 case READ_SIZE_OPTION:
                   if (value == NULL)
                     abort ();
                   read_size = atoi (value);
                   break;
                 case WRITE_SIZE_OPTION:
                   if (value == NULL)
                     abort ();
                   write_size = atoi (value);
                   break;
                 default:
                   /* Unknown suboption. */
                   printf ("Unknown suboption `%s'\n", value);
                   break;
                 }
             break;
           default:
             abort ();
           }

       /* Do the real work. */

       return 0;
     }


File: libc.info,  Node: Environment Variables,  Next: System Calls,  Prev: Program Arguments,  Up: Program Basics

25.4 Environment Variables
==========================

When a program is executed, it receives information about the context in
which it was invoked in two ways.  The first mechanism uses the ARGV
and ARGC arguments to its `main' function, and is discussed in *note
Program Arguments::.  The second mechanism uses "environment variables"
and is discussed in this section.

   The ARGV mechanism is typically used to pass command-line arguments
specific to the particular program being invoked.  The environment, on
the other hand, keeps track of information that is shared by many
programs, changes infrequently, and that is less frequently used.

   The environment variables discussed in this section are the same
environment variables that you set using assignments and the `export'
command in the shell.  Programs executed from the shell inherit all of
the environment variables from the shell.

   Standard environment variables are used for information about the
user's home directory, terminal type, current locale, and so on; you
can define additional variables for other purposes.  The set of all
environment variables that have values is collectively known as the
"environment".

   Names of environment variables are case-sensitive and must not
contain the character `='.  System-defined environment variables are
invariably uppercase.

   The values of environment variables can be anything that can be
represented as a string.  A value must not contain an embedded null
character, since this is assumed to terminate the string.

* Menu:

* Environment Access::          How to get and set the values of
				 environment variables.
* Standard Environment::        These environment variables have
                		 standard interpretations.


File: libc.info,  Node: Environment Access,  Next: Standard Environment,  Up: Environment Variables

25.4.1 Environment Access
-------------------------

The value of an environment variable can be accessed with the `getenv'
function.  This is declared in the header file `stdlib.h'.
Modifications of enviroment variables are not allowed in Multi-threaded
programs.  The `getenv' function can be safely used in multi-threaded
programs 

 -- Function: char * getenv (const char *NAME)
     This function returns a string that is the value of the environment
     variable NAME.  You must not modify this string.  In some non-Unix
     systems not using the GNU library, it might be overwritten by
     subsequent calls to `getenv' (but not by any other library
     function).  If the environment variable NAME is not defined, the
     value is a null pointer.

 -- Function: int putenv (char *STRING)
     The `putenv' function adds or removes definitions from the
     environment.  If the STRING is of the form `NAME=VALUE', the
     definition is added to the environment.  Otherwise, the STRING is
     interpreted as the name of an environment variable, and any
     definition for this variable in the environment is removed.

     The difference to the `setenv' function is that the exact string
     given as the parameter STRING is put into the environment.  If the
     user should change the string after the `putenv' call this will
     reflect in automatically in the environment.  This also requires
     that STRING is no automatic variable which scope is left before the
     variable is removed from the environment.  The same applies of
     course to dynamically allocated variables which are freed later.

     This function is part of the extended Unix interface.  Since it
     was also available in old SVID libraries you should define either
     _XOPEN_SOURCE or _SVID_SOURCE before including any header.

 -- Function: int setenv (const char *NAME, const char *VALUE, int
          REPLACE)
     The `setenv' function can be used to add a new definition to the
     environment.  The entry with the name NAME is replaced by the
     value `NAME=VALUE'.  Please note that this is also true if VALUE
     is the empty string.  To do this a new string is created and the
     strings NAME and VALUE are copied.  A null pointer for the VALUE
     parameter is illegal.  If the environment already contains an
     entry with key NAME the REPLACE parameter controls the action.  If
     replace is zero, nothing happens.  Otherwise the old entry is
     replaced by the new one.

     Please note that you cannot remove an entry completely using this
     function.

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

 -- Function: int unsetenv (const char *NAME)
     Using this function one can remove an entry completely from the
     environment.  If the environment contains an entry with the key
     NAME this whole entry is removed.  A call to this function is
     equivalent to a call to `putenv' when the VALUE part of the string
     is empty.

     The function return `-1' if NAME is a null pointer, points to an
     empty string, or points to a string containing a `=' character.
     It returns `0' if the call succeeded.

     This function was originally part of the BSD library but is now
     part of the Unix standard.  The BSD version had no return value,
     though.

   There is one more function to modify the whole environment.  This
function is said to be used in the POSIX.9 (POSIX bindings for Fortran
77) and so one should expect it did made it into POSIX.1.  But this
never happened.  But we still provide this function as a GNU extension
to enable writing standard compliant Fortran environments.

 -- Function: int clearenv (void)
     The `clearenv' function removes all entries from the environment.
     Using `putenv' and `setenv' new entries can be added again later.

     If the function is successful it returns `0'.  Otherwise the return
     value is nonzero.

   You can deal directly with the underlying representation of
environment objects to add more variables to the environment (for
example, to communicate with another program you are about to execute;
*note Executing a File::).

 -- Variable: char ** environ
     The environment is represented as an array of strings.  Each
     string is of the format `NAME=VALUE'.  The order in which strings
     appear in the environment is not significant, but the same NAME
     must not appear more than once.  The last element of the array is
     a null pointer.

     This variable is declared in the header file `unistd.h'.

     If you just want to get the value of an environment variable, use
     `getenv'.

   Unix systems, and the GNU system, pass the initial value of
`environ' as the third argument to `main'.  *Note Program Arguments::.


File: libc.info,  Node: Standard Environment,  Prev: Environment Access,  Up: Environment Variables

25.4.2 Standard Environment Variables
-------------------------------------

These environment variables have standard meanings.  This doesn't mean
that they are always present in the environment; but if these variables
_are_ present, they have these meanings.  You shouldn't try to use
these environment variable names for some other purpose.

`HOME'
     This is a string representing the user's "home directory", or
     initial default working directory.

     The user can set `HOME' to any value.  If you need to make sure to
     obtain the proper home directory for a particular user, you should
     not use `HOME'; instead, look up the user's name in the user
     database (*note User Database::).

     For most purposes, it is better to use `HOME', precisely because
     this lets the user specify the value.

`LOGNAME'
     This is the name that the user used to log in.  Since the value in
     the environment can be tweaked arbitrarily, this is not a reliable
     way to identify the user who is running a program; a function like
     `getlogin' (*note Who Logged In::) is better for that purpose.

     For most purposes, it is better to use `LOGNAME', precisely because
     this lets the user specify the value.

`PATH'
     A "path" is a sequence of directory names which is used for
     searching for a file.  The variable `PATH' holds a path used for
     searching for programs to be run.

     The `execlp' and `execvp' functions (*note Executing a File::) use
     this environment variable, as do many shells and other utilities
     which are implemented in terms of those functions.

     The syntax of a path is a sequence of directory names separated by
     colons.  An empty string instead of a directory name stands for the
     current directory (*note Working Directory::).

     A typical value for this environment variable might be a string
     like:

          :/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local/bin

     This means that if the user tries to execute a program named `foo',
     the system will look for files named `foo', `/bin/foo',
     `/etc/foo', and so on.  The first of these files that exists is
     the one that is executed.

`TERM'
     This specifies the kind of terminal that is receiving program
     output.  Some programs can make use of this information to take
     advantage of special escape sequences or terminal modes supported
     by particular kinds of terminals.  Many programs which use the
     termcap library (*note Find: (termcap)Finding a Terminal
     Description.) use the `TERM' environment variable, for example.

`TZ'
     This specifies the time zone.  *Note TZ Variable::, for
     information about the format of this string and how it is used.

`LANG'
     This specifies the default locale to use for attribute categories
     where neither `LC_ALL' nor the specific environment variable for
     that category is set.  *Note Locales::, for more information about
     locales.

`LC_ALL'
     If this environment variable is set it overrides the selection for
     all the locales done using the other `LC_*' environment variables.
     The value of the other `LC_*' environment variables is simply
     ignored in this case.

`LC_COLLATE'
     This specifies what locale to use for string sorting.

`LC_CTYPE'
     This specifies what locale to use for character sets and character
     classification.

`LC_MESSAGES'
     This specifies what locale to use for printing messages and to
     parse responses.

`LC_MONETARY'
     This specifies what locale to use for formatting monetary values.

`LC_NUMERIC'
     This specifies what locale to use for formatting numbers.

`LC_TIME'
     This specifies what locale to use for formatting date/time values.

`NLSPATH'
     This specifies the directories in which the `catopen' function
     looks for message translation catalogs.

`_POSIX_OPTION_ORDER'
     If this environment variable is defined, it suppresses the usual
     reordering of command line arguments by `getopt' and `argp_parse'.
     *Note Argument Syntax::.



File: libc.info,  Node: System Calls,  Next: Program Termination,  Prev: Environment Variables,  Up: Program Basics

25.5 System Calls
=================

A system call is a request for service that a program makes of the
kernel.  The service is generally something that only the kernel has
the privilege to do, such as doing I/O.  Programmers don't normally
need to be concerned with system calls because there are functions in
the GNU C library to do virtually everything that system calls do.
These functions work by making system calls themselves.  For example,
there is a system call that changes the permissions of a file, but you
don't need to know about it because you can just use the GNU C
library's `chmod' function.

   System calls are sometimes called kernel calls.

   However, there are times when you want to make a system call
explicitly, and for that, the GNU C library provides the `syscall'
function.  `syscall' is harder to use and less portable than functions
like `chmod', but easier and more portable than coding the system call
in assembler instructions.

   `syscall' is most useful when you are working with a system call
which is special to your system or is newer than the GNU C library you
are using.  `syscall' is implemented in an entirely generic way; the
function does not know anything about what a particular system call
does or even if it is valid.

   The description of `syscall' in this section assumes a certain
protocol for system calls on the various platforms on which the GNU C
library runs.  That protocol is not defined by any strong authority, but
we won't describe it here either because anyone who is coding `syscall'
probably won't accept anything less than kernel and C library source
code as a specification of the interface between them anyway.

   `syscall' is declared in `unistd.h'.

 -- Function: long int syscall (long int SYSNO, ...)
     `syscall' performs a generic system call.

     SYSNO is the system call number.  Each kind of system call is
     identified by a number.  Macros for all the possible system call
     numbers are defined in `sys/syscall.h'

     The remaining arguments are the arguments for the system call, in
     order, and their meanings depend on the kind of system call.  Each
     kind of system call has a definite number of arguments, from zero
     to five.  If you code more arguments than the system call takes,
     the extra ones to the right are ignored.

     The return value is the return value from the system call, unless
     the system call failed.  In that case, `syscall' returns `-1' and
     sets `errno' to an error code that the system call returned.  Note
     that system calls do not return `-1' when they succeed.  

     If you specify an invalid SYSNO, `syscall' returns `-1' with
     `errno' = `ENOSYS'.

     Example:


          #include <unistd.h>
          #include <sys/syscall.h>
          #include <errno.h>

          ...

          int rc;

          rc = syscall(SYS_chmod, "/etc/passwd", 0444);

          if (rc == -1)
             fprintf(stderr, "chmod failed, errno = %d\n", errno);

     This, if all the compatibility stars are aligned, is equivalent to
     the following preferable code:


          #include <sys/types.h>
          #include <sys/stat.h>
          #include <errno.h>

          ...

          int rc;

          rc = chmod("/etc/passwd", 0444);
          if (rc == -1)
             fprintf(stderr, "chmod failed, errno = %d\n", errno);



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

25.6 Program Termination
========================

The usual way for a program to terminate is simply for its `main'
function to return.  The "exit status value" returned from the `main'
function is used to report information back to the process's parent
process or shell.

   A program can also terminate normally by calling the `exit' function.

   In addition, programs can be terminated by signals; this is
discussed in more detail in *note Signal Handling::.  The `abort'
function causes a signal that kills the program.

* Menu:

* Normal Termination::          If a program calls `exit', a
                                 process terminates normally.
* Exit Status::                 The `exit status' provides information
                                 about why the process terminated.
* Cleanups on Exit::            A process can run its own cleanup
                                 functions upon normal termination.
* Aborting a Program::          The `abort' function causes
                                 abnormal program termination.
* Termination Internals::       What happens when a process terminates.


File: libc.info,  Node: Normal Termination,  Next: Exit Status,  Up: Program Termination

25.6.1 Normal Termination
-------------------------

A process terminates normally when its program signals it is done by
calling `exit'.  Returning from `main' is equivalent to calling `exit',
and the value that `main' returns is used as the argument to `exit'.

 -- Function: void exit (int STATUS)
     The `exit' function tells the system that the program is done,
     which causes it to terminate the process.

     STATUS is the program's exit status, which becomes part of the
     process' termination status.  This function does not return.

   Normal termination causes the following actions:

  1. Functions that were registered with the `atexit' or `on_exit'
     functions are called in the reverse order of their registration.
     This mechanism allows your application to specify its own
     "cleanup" actions to be performed at program termination.
     Typically, this is used to do things like saving program state
     information in a file, or unlocking locks in shared data bases.

  2. All open streams are closed, writing out any buffered output data.
     See *note Closing Streams::.  In addition, temporary files opened
     with the `tmpfile' function are removed; see *note Temporary
     Files::.

  3. `_exit' is called, terminating the program.  *Note Termination
     Internals::.


File: libc.info,  Node: Exit Status,  Next: Cleanups on Exit,  Prev: Normal Termination,  Up: Program Termination

25.6.2 Exit Status
------------------

When a program exits, it can return to the parent process a small
amount of information about the cause of termination, using the "exit
status".  This is a value between 0 and 255 that the exiting process
passes as an argument to `exit'.

   Normally you should use the exit status to report very broad
information about success or failure.  You can't provide a lot of
detail about the reasons for the failure, and most parent processes
would not want much detail anyway.

   There are conventions for what sorts of status values certain
programs should return.  The most common convention is simply 0 for
success and 1 for failure.  Programs that perform comparison use a
different convention: they use status 1 to indicate a mismatch, and
status 2 to indicate an inability to compare.  Your program should
follow an existing convention if an existing convention makes sense for
it.

   A general convention reserves status values 128 and up for special
purposes.  In particular, the value 128 is used to indicate failure to
execute another program in a subprocess.  This convention is not
universally obeyed, but it is a good idea to follow it in your programs.

   *Warning:* Don't try to use the number of errors as the exit status.
This is actually not very useful; a parent process would generally not
care how many errors occurred.  Worse than that, it does not work,
because the status value is truncated to eight bits.  Thus, if the
program tried to report 256 errors, the parent would receive a report
of 0 errors--that is, success.

   For the same reason, it does not work to use the value of `errno' as
the exit status--these can exceed 255.

   *Portability note:* Some non-POSIX systems use different conventions
for exit status values.  For greater portability, you can use the
macros `EXIT_SUCCESS' and `EXIT_FAILURE' for the conventional status
value for success and failure, respectively.  They are declared in the
file `stdlib.h'.  

 -- Macro: int EXIT_SUCCESS
     This macro can be used with the `exit' function to indicate
     successful program completion.

     On POSIX systems, the value of this macro is `0'.  On other
     systems, the value might be some other (possibly non-constant)
     integer expression.

 -- Macro: int EXIT_FAILURE
     This macro can be used with the `exit' function to indicate
     unsuccessful program completion in a general sense.

     On POSIX systems, the value of this macro is `1'.  On other
     systems, the value might be some other (possibly non-constant)
     integer expression.  Other nonzero status values also indicate
     failures.  Certain programs use different nonzero status values to
     indicate particular kinds of "non-success".  For example, `diff'
     uses status value `1' to mean that the files are different, and
     `2' or more to mean that there was difficulty in opening the files.

   Don't confuse a program's exit status with a process' termination
status.  There are lots of ways a process can terminate besides having
it's program finish.  In the event that the process termination _is_
caused by program termination (i.e., `exit'), though, the program's
exit status becomes part of the process' termination status.


File: libc.info,  Node: Cleanups on Exit,  Next: Aborting a Program,  Prev: Exit Status,  Up: Program Termination

25.6.3 Cleanups on Exit
-----------------------

Your program can arrange to run its own cleanup functions if normal
termination happens.  If you are writing a library for use in various
application programs, then it is unreliable to insist that all
applications call the library's cleanup functions explicitly before
exiting.  It is much more robust to make the cleanup invisible to the
application, by setting up a cleanup function in the library itself
using `atexit' or `on_exit'.

 -- Function: int atexit (void (*FUNCTION) (void))
     The `atexit' function registers the function FUNCTION to be called
     at normal program termination.  The FUNCTION is called with no
     arguments.

     The return value from `atexit' is zero on success and nonzero if
     the function cannot be registered.

 -- Function: int on_exit (void (*FUNCTION)(int STATUS, void *ARG),
          void *ARG)
     This function is a somewhat more powerful variant of `atexit'.  It
     accepts two arguments, a function FUNCTION and an arbitrary
     pointer ARG.  At normal program termination, the FUNCTION is
     called with two arguments:  the STATUS value passed to `exit', and
     the ARG.

     This function is included in the GNU C library only for
     compatibility for SunOS, and may not be supported by other
     implementations.

   Here's a trivial program that illustrates the use of `exit' and
`atexit':

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

     void
     bye (void)
     {
       puts ("Goodbye, cruel world....");
     }

     int
     main (void)
     {
       atexit (bye);
       exit (EXIT_SUCCESS);
     }

When this program is executed, it just prints the message and exits.


File: libc.info,  Node: Aborting a Program,  Next: Termination Internals,  Prev: Cleanups on Exit,  Up: Program Termination

25.6.4 Aborting a Program
-------------------------

You can abort your program using the `abort' function.  The prototype
for this function is in `stdlib.h'.  

 -- Function: void abort (void)
     The `abort' function causes abnormal program termination.  This
     does not execute cleanup functions registered with `atexit' or
     `on_exit'.

     This function actually terminates the process by raising a
     `SIGABRT' signal, and your program can include a handler to
     intercept this signal; see *note Signal Handling::.

   *Future Change Warning:* Proposed Federal censorship regulations may
prohibit us from giving you information about the possibility of
calling this function.  We would be required to say that this is not an
acceptable way of terminating a program.


File: libc.info,  Node: Termination Internals,  Prev: Aborting a Program,  Up: Program Termination

25.6.5 Termination Internals
----------------------------

The `_exit' function is the primitive used for process termination by
`exit'.  It is declared in the header file `unistd.h'.  

 -- Function: void _exit (int STATUS)
     The `_exit' function is the primitive for causing a process to
     terminate with status STATUS.  Calling this function does not
     execute cleanup functions registered with `atexit' or `on_exit'.

 -- Function: void _Exit (int STATUS)
     The `_Exit' function is the ISO C equivalent to `_exit'.  The
     ISO C committee members were not sure whether the definitions of
     `_exit' and `_Exit' were compatible so they have not used the
     POSIX name.

     This function was introduced in ISO C99 and is declared in
     `stdlib.h'.

   When a process terminates for any reason--either because the program
terminates, or as a result of a signal--the following things happen:

   * All open file descriptors in the process are closed.  *Note
     Low-Level I/O::.  Note that streams are not flushed automatically
     when the process terminates; see *note I/O on Streams::.

   * A process exit status is saved to be reported back to the parent
     process via `wait' or `waitpid'; see *note Process Completion::.
     If the program exited, this status includes as its low-order 8
     bits the program exit status.

   * Any child processes of the process being terminated are assigned a
     new parent process.  (On most systems, including GNU, this is the
     `init' process, with process ID 1.)

   * A `SIGCHLD' signal is sent to the parent process.

   * If the process is a session leader that has a controlling
     terminal, then a `SIGHUP' signal is sent to each process in the
     foreground job, and the controlling terminal is disassociated from
     that session.  *Note Job Control::.

   * If termination of a process causes a process group to become
     orphaned, and any member of that process group is stopped, then a
     `SIGHUP' signal and a `SIGCONT' signal are sent to each process in
     the group.  *Note Job Control::.


File: libc.info,  Node: Processes,  Next: Job Control,  Prev: Program Basics,  Up: Top

26 Processes
************

"Processes" are the primitive units for allocation of system resources.
Each process has its own address space and (usually) one thread of
control.  A process executes a program; you can have multiple processes
executing the same program, but each process has its own copy of the
program within its own address space and executes it independently of
the other copies.

   Processes are organized hierarchically.  Each process has a "parent
process" which explicitly arranged to create it.  The processes created
by a given parent are called its "child processes".  A child inherits
many of its attributes from the parent process.

   This chapter describes how a program can create, terminate, and
control child processes.  Actually, there are three distinct operations
involved: creating a new child process, causing the new process to
execute a program, and coordinating the completion of the child process
with the original program.

   The `system' function provides a simple, portable mechanism for
running another program; it does all three steps automatically.  If you
need more control over the details of how this is done, you can use the
primitive functions to do each step individually instead.

* Menu:

* Running a Command::           The easy way to run another program.
* Process Creation Concepts::   An overview of the hard way to do it.
* Process Identification::      How to get the process ID of a process.
* Creating a Process::          How to fork a child process.
* Executing a File::            How to make a process execute another program.
* Process Completion::          How to tell when a child process has completed.
* Process Completion Status::   How to interpret the status value
                                 returned from a child process.
* BSD Wait Functions::  	More functions, for backward compatibility.
* Process Creation Example::    A complete example program.


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

26.1 Running a Command
======================

The easy way to run another program is to use the `system' function.
This function does all the work of running a subprogram, but it doesn't
give you much control over the details: you have to wait until the
subprogram terminates before you can do anything else.

 -- Function: int system (const char *COMMAND)
     This function executes COMMAND as a shell command.  In the GNU C
     library, it always uses the default shell `sh' to run the command.
     In particular, it searches the directories in `PATH' to find
     programs to execute.  The return value is `-1' if it wasn't
     possible to create the shell process, and otherwise is the status
     of the shell process.  *Note Process Completion::, for details on
     how this status code can be interpreted.

     If the COMMAND argument is a null pointer, a return value of zero
     indicates that no command processor is available.

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

     The `system' function is declared in the header file `stdlib.h'.

   *Portability Note:* Some C implementations may not have any notion
of a command processor that can execute other programs.  You can
determine whether a command processor exists by executing
`system (NULL)'; if the return value is nonzero, a command processor is
available.

   The `popen' and `pclose' functions (*note Pipe to a Subprocess::)
are closely related to the `system' function.  They allow the parent
process to communicate with the standard input and output channels of
the command being executed.


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

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

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

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

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

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

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

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


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

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

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

 -- Data Type: pid_t
     The `pid_t' data type is a signed integer type which is capable of
     representing a process ID.  In the GNU library, this is an `int'.

 -- Function: pid_t getpid (void)
     The `getpid' function returns the process ID of the current
     process.

 -- Function: pid_t getppid (void)
     The `getppid' function returns the process ID of the parent of the
     current process.


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

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

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

 -- Function: pid_t fork (void)
     The `fork' function creates a new process.

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

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

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

    `ENOMEM'
          The process requires more space than the system can supply.

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

   * The child process has its own unique process ID.

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

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

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

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

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

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

 -- Function: pid_t vfork (void)
     The `vfork' function is similar to `fork' but on some systems it
     is more efficient; however, there are restrictions you must follow
     to use it safely.

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

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

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


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

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

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

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

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

 -- Function: int execv (const char *FILENAME, char *const ARGV[])
     The `execv' function executes the file named by FILENAME as a new
     process image.

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

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

 -- Function: int execl (const char *FILENAME, const char *ARG0, ...)
     This is similar to `execv', but the ARGV strings are specified
     individually instead of as an array.  A null pointer must be
     passed as the last such argument.

 -- Function: int execve (const char *FILENAME, char *const ARGV[],
          char *const ENV[])
     This is similar to `execv', but permits you to specify the
     environment for the new program explicitly as the ENV argument.
     This should be an array of strings in the same format as for the
     `environ' variable; see *note Environment Access::.

 -- Function: int execle (const char *FILENAME, const char *ARG0, char
          *const ENV[], ...)
     This is similar to `execl', but permits you to specify the
     environment for the new program explicitly.  The environment
     argument is passed following the null pointer that marks the last
     ARGV argument, and should be an array of strings in the same
     format as for the `environ' variable.

 -- Function: int execvp (const char *FILENAME, char *const ARGV[])
     The `execvp' function is similar to `execv', except that it
     searches the directories listed in the `PATH' environment variable
     (*note Standard Environment::) to find the full file name of a
     file from FILENAME if FILENAME does not contain a slash.

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

 -- Function: int execlp (const char *FILENAME, const char *ARG0, ...)
     This function is like `execl', except that it performs the same
     file name searching as the `execvp' function.

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

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

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

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

`ENOMEM'
     Executing the specified file requires more storage than is
     available.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

 -- Function: pid_t waitpid (pid_t PID, int *STATUS-PTR, int OPTIONS)
     The `waitpid' function is used to request status information from a
     child process whose process ID is PID.  Normally, the calling
     process is suspended until the child process makes status
     information available by terminating.

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

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

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

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

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

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

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

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

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

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

    `EINVAL'
          An invalid value was provided for the OPTIONS argument.

   These symbolic constants are defined as values for the PID argument
to the `waitpid' function.

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

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

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

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

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

 -- Function: pid_t wait (int *STATUS-PTR)
     This is a simplified version of `waitpid', and is used to wait
     until any one child process terminates.  The call:

          wait (&status)

     is exactly equivalent to:

          waitpid (-1, &status, 0)

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

 -- Function: pid_t wait4 (pid_t PID, int *STATUS-PTR, int OPTIONS,
          struct rusage *USAGE)
     If USAGE is a null pointer, `wait4' is equivalent to `waitpid
     (PID, STATUS-PTR, OPTIONS)'.

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

     This function is a BSD extension.

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

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


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

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

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

 -- Macro: int WIFEXITED (int STATUS)
     This macro returns a nonzero value if the child process terminated
     normally with `exit' or `_exit'.

 -- Macro: int WEXITSTATUS (int STATUS)
     If `WIFEXITED' is true of STATUS, this macro returns the low-order
     8 bits of the exit status value from the child process.  *Note
     Exit Status::.

 -- Macro: int WIFSIGNALED (int STATUS)
     This macro returns a nonzero value if the child process terminated
     because it received a signal that was not handled.  *Note Signal
     Handling::.

 -- Macro: int WTERMSIG (int STATUS)
     If `WIFSIGNALED' is true of STATUS, this macro returns the signal
     number of the signal that terminated the child process.

 -- Macro: int WCOREDUMP (int STATUS)
     This macro returns a nonzero value if the child process terminated
     and produced a core dump.

 -- Macro: int WIFSTOPPED (int STATUS)
     This macro returns a nonzero value if the child process is stopped.

 -- Macro: int WSTOPSIG (int STATUS)
     If `WIFSTOPPED' is true of STATUS, this macro returns the signal
     number of the signal that caused the child process to stop.


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

26.8 BSD Process Wait Functions
===============================

The GNU library also provides these related facilities for compatibility
with BSD Unix.  BSD uses the `union wait' data type to represent status
values rather than an `int'.  The two representations are actually
interchangeable; they describe the same bit patterns.  The GNU C
Library defines macros such as `WEXITSTATUS' so that they will work on
either kind of object, and the `wait' function is defined to accept
either type of pointer as its STATUS-PTR argument.

   These functions are declared in `sys/wait.h'.  

 -- Data Type: union wait
     This data type represents program termination status values.  It
     has the following members:

    `int w_termsig'
          The value of this member is the same as that of the
          `WTERMSIG' macro.

    `int w_coredump'
          The value of this member is the same as that of the
          `WCOREDUMP' macro.

    `int w_retcode'
          The value of this member is the same as that of the
          `WEXITSTATUS' macro.

    `int w_stopsig'
          The value of this member is the same as that of the
          `WSTOPSIG' macro.

     Instead of accessing these members directly, you should use the
     equivalent macros.

   The `wait3' function is the predecessor to `wait4', which is more
flexible.  `wait3' is now obsolete.

 -- Function: pid_t wait3 (union wait *STATUS-PTR, int OPTIONS, struct
          rusage *USAGE)
     If USAGE is a null pointer, `wait3' is equivalent to `waitpid (-1,
     STATUS-PTR, OPTIONS)'.

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


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

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

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

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

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

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

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

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

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

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

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


File: libc.info,  Node: Job Control,  Next: Name Service Switch,  Prev: Processes,  Up: Top

27 Job Control
**************

"Job control" refers to the protocol for allowing a user to move
between multiple "process groups" (or "jobs") within a single "login
session".  The job control facilities are set up so that appropriate
behavior for most programs happens automatically and they need not do
anything special about job control.  So you can probably ignore the
material in this chapter unless you are writing a shell or login
program.

   You need to be familiar with concepts relating to process creation
(*note Process Creation Concepts::) and signal handling (*note Signal
Handling::) in order to understand this material presented in this
chapter.

* Menu:

* Concepts of Job Control::     Jobs can be controlled by a shell.
* Job Control is Optional::     Not all POSIX systems support job control.
* Controlling Terminal::        How a process gets its controlling terminal.
* Access to the Terminal::      How processes share the controlling terminal.
* Orphaned Process Groups::     Jobs left after the user logs out.
* Implementing a Shell::        What a shell must do to implement job control.
* Functions for Job Control::   Functions to control process groups.


File: libc.info,  Node: Concepts of Job Control,  Next: Job Control is Optional,  Up: Job Control

27.1 Concepts of Job Control
============================

The fundamental purpose of an interactive shell is to read commands
from the user's terminal and create processes to execute the programs
specified by those commands.  It can do this using the `fork' (*note
Creating a Process::) and `exec' (*note Executing a File::) functions.

   A single command may run just one process--but often one command uses
several processes.  If you use the `|' operator in a shell command, you
explicitly request several programs in their own processes.  But even
if you run just one program, it can use multiple processes internally.
For example, a single compilation command such as `cc -c foo.c'
typically uses four processes (though normally only two at any given
time).  If you run `make', its job is to run other programs in separate
processes.

   The processes belonging to a single command are called a "process
group" or "job".  This is so that you can operate on all of them at
once.  For example, typing `C-c' sends the signal `SIGINT' to terminate
all the processes in the foreground process group.

   A "session" is a larger group of processes.  Normally all the
processes that stem from a single login belong to the same session.

   Every process belongs to a process group.  When a process is
created, it becomes a member of the same process group and session as
its parent process.  You can put it in another process group using the
`setpgid' function, provided the process group belongs to the same
session.

   The only way to put a process in a different session is to make it
the initial process of a new session, or a "session leader", using the
`setsid' function.  This also puts the session leader into a new
process group, and you can't move it out of that process group again.

   Usually, new sessions are created by the system login program, and
the session leader is the process running the user's login shell.

   A shell that supports job control must arrange to control which job
can use the terminal at any time.  Otherwise there might be multiple
jobs trying to read from the terminal at once, and confusion about which
process should receive the input typed by the user.  To prevent this,
the shell must cooperate with the terminal driver using the protocol
described in this chapter.

   The shell can give unlimited access to the controlling terminal to
only one process group at a time.  This is called the "foreground job"
on that controlling terminal.  Other process groups managed by the shell
that are executing without such access to the terminal are called
"background jobs".

   If a background job needs to read from its controlling terminal, it
is "stopped" by the terminal driver; if the `TOSTOP' mode is set,
likewise for writing.  The user can stop a foreground job by typing the
SUSP character (*note Special Characters::) and a program can stop any
job by sending it a `SIGSTOP' signal.  It's the responsibility of the
shell to notice when jobs stop, to notify the user about them, and to
provide mechanisms for allowing the user to interactively continue
stopped jobs and switch jobs between foreground and background.

   *Note Access to the Terminal::, for more information about I/O to the
controlling terminal,


File: libc.info,  Node: Job Control is Optional,  Next: Controlling Terminal,  Prev: Concepts of Job Control,  Up: Job Control

27.2 Job Control is Optional
============================

Not all operating systems support job control.  The GNU system does
support job control, but if you are using the GNU library on some other
system, that system may not support job control itself.

   You can use the `_POSIX_JOB_CONTROL' macro to test at compile-time
whether the system supports job control.  *Note System Options::.

   If job control is not supported, then there can be only one process
group per session, which behaves as if it were always in the foreground.
The functions for creating additional process groups simply fail with
the error code `ENOSYS'.

   The macros naming the various job control signals (*note Job Control
Signals::) are defined even if job control is not supported.  However,
the system never generates these signals, and attempts to send a job
control signal or examine or specify their actions report errors or do
nothing.


File: libc.info,  Node: Controlling Terminal,  Next: Access to the Terminal,  Prev: Job Control is Optional,  Up: Job Control

27.3 Controlling Terminal of a Process
======================================

One of the attributes of a process is its controlling terminal.  Child
processes created with `fork' inherit the controlling terminal from
their parent process.  In this way, all the processes in a session
inherit the controlling terminal from the session leader.  A session
leader that has control of a terminal is called the "controlling
process" of that terminal.

   You generally do not need to worry about the exact mechanism used to
allocate a controlling terminal to a session, since it is done for you
by the system when you log in.

   An individual process disconnects from its controlling terminal when
it calls `setsid' to become the leader of a new session.  *Note Process
Group Functions::.


File: libc.info,  Node: Access to the Terminal,  Next: Orphaned Process Groups,  Prev: Controlling Terminal,  Up: Job Control

27.4 Access to the Controlling Terminal
=======================================

Processes in the foreground job of a controlling terminal have
unrestricted access to that terminal; background processes do not.  This
section describes in more detail what happens when a process in a
background job tries to access its controlling terminal.

   When a process in a background job tries to read from its controlling
terminal, the process group is usually sent a `SIGTTIN' signal.  This
normally causes all of the processes in that group to stop (unless they
handle the signal and don't stop themselves).  However, if the reading
process is ignoring or blocking this signal, then `read' fails with an
`EIO' error instead.

   Similarly, when a process in a background job tries to write to its
controlling terminal, the default behavior is to send a `SIGTTOU'
signal to the process group.  However, the behavior is modified by the
`TOSTOP' bit of the local modes flags (*note Local Modes::).  If this
bit is not set (which is the default), then writing to the controlling
terminal is always permitted without sending a signal.  Writing is also
permitted if the `SIGTTOU' signal is being ignored or blocked by the
writing process.

   Most other terminal operations that a program can do are treated as
reading or as writing.  (The description of each operation should say
which.)

   For more information about the primitive `read' and `write'
functions, see *note I/O Primitives::.


File: libc.info,  Node: Orphaned Process Groups,  Next: Implementing a Shell,  Prev: Access to the Terminal,  Up: Job Control

27.5 Orphaned Process Groups
============================

When a controlling process terminates, its terminal becomes free and a
new session can be established on it.  (In fact, another user could log
in on the terminal.)  This could cause a problem if any processes from
the old session are still trying to use that terminal.

   To prevent problems, process groups that continue running even after
the session leader has terminated are marked as "orphaned process
groups".

   When a process group becomes an orphan, its processes are sent a
`SIGHUP' signal.  Ordinarily, this causes the processes to terminate.
However, if a program ignores this signal or establishes a handler for
it (*note Signal Handling::), it can continue running as in the orphan
process group even after its controlling process terminates; but it
still cannot access the terminal any more.


File: libc.info,  Node: Implementing a Shell,  Next: Functions for Job Control,  Prev: Orphaned Process Groups,  Up: Job Control

27.6 Implementing a Job Control Shell
=====================================

This section describes what a shell must do to implement job control, by
presenting an extensive sample program to illustrate the concepts
involved.

* Menu:

* Data Structures::             Introduction to the sample shell.
* Initializing the Shell::      What the shell must do to take
				 responsibility for job control.
* Launching Jobs::              Creating jobs to execute commands.
* Foreground and Background::   Putting a job in foreground of background.
* Stopped and Terminated Jobs::  Reporting job status.
* Continuing Stopped Jobs::     How to continue a stopped job in
				 the foreground or background.
* Missing Pieces::              Other parts of the shell.


File: libc.info,  Node: Data Structures,  Next: Initializing the Shell,  Up: Implementing a Shell

27.6.1 Data Structures for the Shell
------------------------------------

All of the program examples included in this chapter are part of a
simple shell program.  This section presents data structures and
utility functions which are used throughout the example.

   The sample shell deals mainly with two data structures.  The `job'
type contains information about a job, which is a set of subprocesses
linked together with pipes.  The `process' type holds information about
a single subprocess.  Here are the relevant data structure declarations:

     /* A process is a single process.  */
     typedef struct process
     {
       struct process *next;       /* next process in pipeline */
       char **argv;                /* for exec */
       pid_t pid;                  /* process ID */
       char completed;             /* true if process has completed */
       char stopped;               /* true if process has stopped */
       int status;                 /* reported status value */
     } process;

     /* A job is a pipeline of processes.  */
     typedef struct job
     {
       struct job *next;           /* next active job */
       char *command;              /* command line, used for messages */
       process *first_process;     /* list of processes in this job */
       pid_t pgid;                 /* process group ID */
       char notified;              /* true if user told about stopped job */
       struct termios tmodes;      /* saved terminal modes */
       int stdin, stdout, stderr;  /* standard i/o channels */
     } job;

     /* The active jobs are linked into a list.  This is its head.   */
     job *first_job = NULL;

   Here are some utility functions that are used for operating on `job'
objects.

     /* Find the active job with the indicated PGID.  */
     job *
     find_job (pid_t pgid)
     {
       job *j;

       for (j = first_job; j; j = j->next)
         if (j->pgid == pgid)
           return j;
       return NULL;
     }

     /* Return true if all processes in the job have stopped or completed.  */
     int
     job_is_stopped (job *j)
     {
       process *p;

       for (p = j->first_process; p; p = p->next)
         if (!p->completed && !p->stopped)
           return 0;
       return 1;
     }

     /* Return true if all processes in the job have completed.  */
     int
     job_is_completed (job *j)
     {
       process *p;

       for (p = j->first_process; p; p = p->next)
         if (!p->completed)
           return 0;
       return 1;
     }


File: libc.info,  Node: Initializing the Shell,  Next: Launching Jobs,  Prev: Data Structures,  Up: Implementing a Shell

27.6.2 Initializing the Shell
-----------------------------

When a shell program that normally performs job control is started, it
has to be careful in case it has been invoked from another shell that is
already doing its own job control.

   A subshell that runs interactively has to ensure that it has been
placed in the foreground by its parent shell before it can enable job
control itself.  It does this by getting its initial process group ID
with the `getpgrp' function, and comparing it to the process group ID
of the current foreground job associated with its controlling terminal
(which can be retrieved using the `tcgetpgrp' function).

   If the subshell is not running as a foreground job, it must stop
itself by sending a `SIGTTIN' signal to its own process group.  It may
not arbitrarily put itself into the foreground; it must wait for the
user to tell the parent shell to do this.  If the subshell is continued
again, it should repeat the check and stop itself again if it is still
not in the foreground.

   Once the subshell has been placed into the foreground by its parent
shell, it can enable its own job control.  It does this by calling
`setpgid' to put itself into its own process group, and then calling
`tcsetpgrp' to place this process group into the foreground.

   When a shell enables job control, it should set itself to ignore all
the job control stop signals so that it doesn't accidentally stop
itself.  You can do this by setting the action for all the stop signals
to `SIG_IGN'.

   A subshell that runs non-interactively cannot and should not support
job control.  It must leave all processes it creates in the same process
group as the shell itself; this allows the non-interactive shell and its
child processes to be treated as a single job by the parent shell.  This
is easy to do--just don't use any of the job control primitives--but
you must remember to make the shell do it.

   Here is the initialization code for the sample shell that shows how
to do all of this.

     /* Keep track of attributes of the shell.  */

     #include <sys/types.h>
     #include <termios.h>
     #include <unistd.h>

     pid_t shell_pgid;
     struct termios shell_tmodes;
     int shell_terminal;
     int shell_is_interactive;


     /* Make sure the shell is running interactively as the foreground job
        before proceeding. */

     void
     init_shell ()
     {

       /* See if we are running interactively.  */
       shell_terminal = STDIN_FILENO;
       shell_is_interactive = isatty (shell_terminal);

       if (shell_is_interactive)
         {
           /* Loop until we are in the foreground.  */
           while (tcgetpgrp (shell_terminal) != (shell_pgid = getpgrp ()))
             kill (- shell_pgid, SIGTTIN);

           /* Ignore interactive and job-control signals.  */
           signal (SIGINT, SIG_IGN);
           signal (SIGQUIT, SIG_IGN);
           signal (SIGTSTP, SIG_IGN);
           signal (SIGTTIN, SIG_IGN);
           signal (SIGTTOU, SIG_IGN);
           signal (SIGCHLD, SIG_IGN);

           /* Put ourselves in our own process group.  */
           shell_pgid = getpid ();
           if (setpgid (shell_pgid, shell_pgid) < 0)
             {
               perror ("Couldn't put the shell in its own process group");
               exit (1);
             }

           /* Grab control of the terminal.  */
           tcsetpgrp (shell_terminal, shell_pgid);

           /* Save default terminal attributes for shell.  */
           tcgetattr (shell_terminal, &shell_tmodes);
         }
     }


File: libc.info,  Node: Launching Jobs,  Next: Foreground and Background,  Prev: Initializing the Shell,  Up: Implementing a Shell

27.6.3 Launching Jobs
---------------------

Once the shell has taken responsibility for performing job control on
its controlling terminal, it can launch jobs in response to commands
typed by the user.

   To create the processes in a process group, you use the same `fork'
and `exec' functions described in *note Process Creation Concepts::.
Since there are multiple child processes involved, though, things are a
little more complicated and you must be careful to do things in the
right order.  Otherwise, nasty race conditions can result.

   You have two choices for how to structure the tree of parent-child
relationships among the processes.  You can either make all the
processes in the process group be children of the shell process, or you
can make one process in group be the ancestor of all the other processes
in that group.  The sample shell program presented in this chapter uses
the first approach because it makes bookkeeping somewhat simpler.

   As each process is forked, it should put itself in the new process
group by calling `setpgid'; see *note Process Group Functions::.  The
first process in the new group becomes its "process group leader", and
its process ID becomes the "process group ID" for the group.

   The shell should also call `setpgid' to put each of its child
processes into the new process group.  This is because there is a
potential timing problem: each child process must be put in the process
group before it begins executing a new program, and the shell depends on
having all the child processes in the group before it continues
executing.  If both the child processes and the shell call `setpgid',
this ensures that the right things happen no matter which process gets
to it first.

   If the job is being launched as a foreground job, the new process
group also needs to be put into the foreground on the controlling
terminal using `tcsetpgrp'.  Again, this should be done by the shell as
well as by each of its child processes, to avoid race conditions.

   The next thing each child process should do is to reset its signal
actions.

   During initialization, the shell process set itself to ignore job
control signals; see *note Initializing the Shell::.  As a result, any
child processes it creates also ignore these signals by inheritance.
This is definitely undesirable, so each child process should explicitly
set the actions for these signals back to `SIG_DFL' just after it is
forked.

   Since shells follow this convention, applications can assume that
they inherit the correct handling of these signals from the parent
process.  But every application has a responsibility not to mess up the
handling of stop signals.  Applications that disable the normal
interpretation of the SUSP character should provide some other
mechanism for the user to stop the job.  When the user invokes this
mechanism, the program should send a `SIGTSTP' signal to the process
group of the process, not just to the process itself.  *Note Signaling
Another Process::.

   Finally, each child process should call `exec' in the normal way.
This is also the point at which redirection of the standard input and
output channels should be handled.  *Note Duplicating Descriptors::,
for an explanation of how to do this.

   Here is the function from the sample shell program that is
responsible for launching a program.  The function is executed by each
child process immediately after it has been forked by the shell, and
never returns.

     void
     launch_process (process *p, pid_t pgid,
                     int infile, int outfile, int errfile,
                     int foreground)
     {
       pid_t pid;

       if (shell_is_interactive)
         {
           /* Put the process into the process group and give the process group
              the terminal, if appropriate.
              This has to be done both by the shell and in the individual
              child processes because of potential race conditions.  */
           pid = getpid ();
           if (pgid == 0) pgid = pid;
           setpgid (pid, pgid);
           if (foreground)
             tcsetpgrp (shell_terminal, pgid);

           /* Set the handling for job control signals back to the default.  */
           signal (SIGINT, SIG_DFL);
           signal (SIGQUIT, SIG_DFL);
           signal (SIGTSTP, SIG_DFL);
           signal (SIGTTIN, SIG_DFL);
           signal (SIGTTOU, SIG_DFL);
           signal (SIGCHLD, SIG_DFL);
         }

       /* Set the standard input/output channels of the new process.  */
       if (infile != STDIN_FILENO)
         {
           dup2 (infile, STDIN_FILENO);
           close (infile);
         }
       if (outfile != STDOUT_FILENO)
         {
           dup2 (outfile, STDOUT_FILENO);
           close (outfile);
         }
       if (errfile != STDERR_FILENO)
         {
           dup2 (errfile, STDERR_FILENO);
           close (errfile);
         }

       /* Exec the new process.  Make sure we exit.  */
       execvp (p->argv[0], p->argv);
       perror ("execvp");
       exit (1);
     }

   If the shell is not running interactively, this function does not do
anything with process groups or signals.  Remember that a shell not
performing job control must keep all of its subprocesses in the same
process group as the shell itself.

   Next, here is the function that actually launches a complete job.
After creating the child processes, this function calls some other
functions to put the newly created job into the foreground or
background; these are discussed in *note Foreground and Background::.

     void
     launch_job (job *j, int foreground)
     {
       process *p;
       pid_t pid;
       int mypipe[2], infile, outfile;

       infile = j->stdin;
       for (p = j->first_process; p; p = p->next)
         {
           /* Set up pipes, if necessary.  */
           if (p->next)
             {
               if (pipe (mypipe) < 0)
                 {
                   perror ("pipe");
                   exit (1);
                 }
               outfile = mypipe[1];
             }
           else
             outfile = j->stdout;

           /* Fork the child processes.  */
           pid = fork ();
           if (pid == 0)
             /* This is the child process.  */
             launch_process (p, j->pgid, infile,
                             outfile, j->stderr, foreground);
           else if (pid < 0)
             {
               /* The fork failed.  */
               perror ("fork");
               exit (1);
             }
           else
             {
               /* This is the parent process.  */
               p->pid = pid;
               if (shell_is_interactive)
                 {
                   if (!j->pgid)
                     j->pgid = pid;
                   setpgid (pid, j->pgid);
                 }
             }

           /* Clean up after pipes.  */
           if (infile != j->stdin)
             close (infile);
           if (outfile != j->stdout)
             close (outfile);
           infile = mypipe[0];
         }

       format_job_info (j, "launched");

       if (!shell_is_interactive)
         wait_for_job (j);
       else if (foreground)
         put_job_in_foreground (j, 0);
       else
         put_job_in_background (j, 0);
     }


File: libc.info,  Node: Foreground and Background,  Next: Stopped and Terminated Jobs,  Prev: Launching Jobs,  Up: Implementing a Shell

27.6.4 Foreground and Background
--------------------------------

Now let's consider what actions must be taken by the shell when it
launches a job into the foreground, and how this differs from what must
be done when a background job is launched.

   When a foreground job is launched, the shell must first give it
access to the controlling terminal by calling `tcsetpgrp'.  Then, the
shell should wait for processes in that process group to terminate or
stop.  This is discussed in more detail in *note Stopped and Terminated
Jobs::.

   When all of the processes in the group have either completed or
stopped, the shell should regain control of the terminal for its own
process group by calling `tcsetpgrp' again.  Since stop signals caused
by I/O from a background process or a SUSP character typed by the user
are sent to the process group, normally all the processes in the job
stop together.

   The foreground job may have left the terminal in a strange state, so
the shell should restore its own saved terminal modes before
continuing.  In case the job is merely stopped, the shell should first
save the current terminal modes so that it can restore them later if
the job is continued.  The functions for dealing with terminal modes are
`tcgetattr' and `tcsetattr'; these are described in *note Terminal
Modes::.

   Here is the sample shell's function for doing all of this.

     /* Put job J in the foreground.  If CONT is nonzero,
        restore the saved terminal modes and send the process group a
        `SIGCONT' signal to wake it up before we block.  */

     void
     put_job_in_foreground (job *j, int cont)
     {
       /* Put the job into the foreground.  */
       tcsetpgrp (shell_terminal, j->pgid);

       /* Send the job a continue signal, if necessary.  */
       if (cont)
         {
           tcsetattr (shell_terminal, TCSADRAIN, &j->tmodes);
           if (kill (- j->pgid, SIGCONT) < 0)
             perror ("kill (SIGCONT)");
         }

       /* Wait for it to report.  */
       wait_for_job (j);

       /* Put the shell back in the foreground.  */
       tcsetpgrp (shell_terminal, shell_pgid);

       /* Restore the shell's terminal modes.  */
       tcgetattr (shell_terminal, &j->tmodes);
       tcsetattr (shell_terminal, TCSADRAIN, &shell_tmodes);
     }

   If the process group is launched as a background job, the shell
should remain in the foreground itself and continue to read commands
from the terminal.

   In the sample shell, there is not much that needs to be done to put
a job into the background.  Here is the function it uses:

     /* Put a job in the background.  If the cont argument is true, send
        the process group a `SIGCONT' signal to wake it up.  */

     void
     put_job_in_background (job *j, int cont)
     {
       /* Send the job a continue signal, if necessary.  */
       if (cont)
         if (kill (-j->pgid, SIGCONT) < 0)
           perror ("kill (SIGCONT)");
     }


File: libc.info,  Node: Stopped and Terminated Jobs,  Next: Continuing Stopped Jobs,  Prev: Foreground and Background,  Up: Implementing a Shell

27.6.5 Stopped and Terminated Jobs
----------------------------------

When a foreground process is launched, the shell must block until all of
the processes in that job have either terminated or stopped.  It can do
this by calling the `waitpid' function; see *note Process Completion::.
Use the `WUNTRACED' option so that status is reported for processes
that stop as well as processes that terminate.

   The shell must also check on the status of background jobs so that it
can report terminated and stopped jobs to the user; this can be done by
calling `waitpid' with the `WNOHANG' option.  A good place to put a
such a check for terminated and stopped jobs is just before prompting
for a new command.

   The shell can also receive asynchronous notification that there is
status information available for a child process by establishing a
handler for `SIGCHLD' signals.  *Note Signal Handling::.

   In the sample shell program, the `SIGCHLD' signal is normally
ignored.  This is to avoid reentrancy problems involving the global data
structures the shell manipulates.  But at specific times when the shell
is not using these data structures--such as when it is waiting for
input on the terminal--it makes sense to enable a handler for
`SIGCHLD'.  The same function that is used to do the synchronous status
checks (`do_job_notification', in this case) can also be called from
within this handler.

   Here are the parts of the sample shell program that deal with
checking the status of jobs and reporting the information to the user.

     /* Store the status of the process PID that was returned by waitpid.
        Return 0 if all went well, nonzero otherwise.  */

     int
     mark_process_status (pid_t pid, int status)
     {
       job *j;
       process *p;

       if (pid > 0)
         {
           /* Update the record for the process.  */
           for (j = first_job; j; j = j->next)
             for (p = j->first_process; p; p = p->next)
               if (p->pid == pid)
                 {
                   p->status = status;
                   if (WIFSTOPPED (status))
                     p->stopped = 1;
                   else
                     {
                       p->completed = 1;
                       if (WIFSIGNALED (status))
                         fprintf (stderr, "%d: Terminated by signal %d.\n",
                                  (int) pid, WTERMSIG (p->status));
                     }
                   return 0;
                  }
           fprintf (stderr, "No child process %d.\n", pid);
           return -1;
         }
       else if (pid == 0 || errno == ECHILD)
         /* No processes ready to report.  */
         return -1;
       else {
         /* Other weird errors.  */
         perror ("waitpid");
         return -1;
       }
     }

     /* Check for processes that have status information available,
        without blocking.  */

     void
     update_status (void)
     {
       int status;
       pid_t pid;

       do
         pid = waitpid (WAIT_ANY, &status, WUNTRACED|WNOHANG);
       while (!mark_process_status (pid, status));
     }

     /* Check for processes that have status information available,
        blocking until all processes in the given job have reported.  */

     void
     wait_for_job (job *j)
     {
       int status;
       pid_t pid;

       do
         pid = waitpid (WAIT_ANY, &status, WUNTRACED);
       while (!mark_process_status (pid, status)
              && !job_is_stopped (j)
              && !job_is_completed (j));
     }

     /* Format information about job status for the user to look at.  */

     void
     format_job_info (job *j, const char *status)
     {
       fprintf (stderr, "%ld (%s): %s\n", (long)j->pgid, status, j->command);
     }

     /* Notify the user about stopped or terminated jobs.
        Delete terminated jobs from the active job list.  */

     void
     do_job_notification (void)
     {
       job *j, *jlast, *jnext;
       process *p;

       /* Update status information for child processes.  */
       update_status ();

       jlast = NULL;
       for (j = first_job; j; j = jnext)
         {
           jnext = j->next;

           /* If all processes have completed, tell the user the job has
              completed and delete it from the list of active jobs.  */
           if (job_is_completed (j)) {
             format_job_info (j, "completed");
             if (jlast)
               jlast->next = jnext;
             else
               first_job = jnext;
             free_job (j);
           }

           /* Notify the user about stopped jobs,
              marking them so that we won't do this more than once.  */
           else if (job_is_stopped (j) && !j->notified) {
             format_job_info (j, "stopped");
             j->notified = 1;
             jlast = j;
           }

           /* Don't say anything about jobs that are still running.  */
           else
             jlast = j;
         }
     }


File: libc.info,  Node: Continuing Stopped Jobs,  Next: Missing Pieces,  Prev: Stopped and Terminated Jobs,  Up: Implementing a Shell

27.6.6 Continuing Stopped Jobs
------------------------------

The shell can continue a stopped job by sending a `SIGCONT' signal to
its process group.  If the job is being continued in the foreground,
the shell should first invoke `tcsetpgrp' to give the job access to the
terminal, and restore the saved terminal settings.  After continuing a
job in the foreground, the shell should wait for the job to stop or
complete, as if the job had just been launched in the foreground.

   The sample shell program handles both newly created and continued
jobs with the same pair of functions, `put_job_in_foreground' and
`put_job_in_background'.  The definitions of these functions were given
in *note Foreground and Background::.  When continuing a stopped job, a
nonzero value is passed as the CONT argument to ensure that the
`SIGCONT' signal is sent and the terminal modes reset, as appropriate.

   This leaves only a function for updating the shell's internal
bookkeeping about the job being continued:

     /* Mark a stopped job J as being running again.  */

     void
     mark_job_as_running (job *j)
     {
       Process *p;

       for (p = j->first_process; p; p = p->next)
         p->stopped = 0;
       j->notified = 0;
     }

     /* Continue the job J.  */

     void
     continue_job (job *j, int foreground)
     {
       mark_job_as_running (j);
       if (foreground)
         put_job_in_foreground (j, 1);
       else
         put_job_in_background (j, 1);
     }


File: libc.info,  Node: Missing Pieces,  Prev: Continuing Stopped Jobs,  Up: Implementing a Shell

27.6.7 The Missing Pieces
-------------------------

The code extracts for the sample shell included in this chapter are only
a part of the entire shell program.  In particular, nothing at all has
been said about how `job' and `program' data structures are allocated
and initialized.

   Most real shells provide a complex user interface that has support
for a command language; variables; abbreviations, substitutions, and
pattern matching on file names; and the like.  All of this is far too
complicated to explain here!  Instead, we have concentrated on showing
how to implement the core process creation and job control functions
that can be called from such a shell.

   Here is a table summarizing the major entry points we have presented:

`void init_shell (void)'
     Initialize the shell's internal state.  *Note Initializing the
     Shell::.

`void launch_job (job *J, int FOREGROUND)'
     Launch the job J as either a foreground or background job.  *Note
     Launching Jobs::.

`void do_job_notification (void)'
     Check for and report any jobs that have terminated or stopped.
     Can be called synchronously or within a handler for `SIGCHLD'
     signals.  *Note Stopped and Terminated Jobs::.

`void continue_job (job *J, int FOREGROUND)'
     Continue the job J.  *Note Continuing Stopped Jobs::.

   Of course, a real shell would also want to provide other functions
for managing jobs.  For example, it would be useful to have commands to
list all active jobs or to send a signal (such as `SIGKILL') to a job.


File: libc.info,  Node: Functions for Job Control,  Prev: Implementing a Shell,  Up: Job Control

27.7 Functions for Job Control
==============================

This section contains detailed descriptions of the functions relating
to job control.

* Menu:

* Identifying the Terminal::    Determining the controlling terminal's name.
* Process Group Functions::     Functions for manipulating process groups.
* Terminal Access Functions::   Functions for controlling terminal access.


File: libc.info,  Node: Identifying the Terminal,  Next: Process Group Functions,  Up: Functions for Job Control

27.7.1 Identifying the Controlling Terminal
-------------------------------------------

You can use the `ctermid' function to get a file name that you can use
to open the controlling terminal.  In the GNU library, it returns the
same string all the time: `"/dev/tty"'.  That is a special "magic" file
name that refers to the controlling terminal of the current process (if
it has one).  To find the name of the specific terminal device, use
`ttyname'; *note Is It a Terminal::.

   The function `ctermid' is declared in the header file `stdio.h'.  

 -- Function: char * ctermid (char *STRING)
     The `ctermid' function returns a string containing the file name of
     the controlling terminal for the current process.  If STRING is
     not a null pointer, it should be an array that can hold at least
     `L_ctermid' characters; the string is returned in this array.
     Otherwise, a pointer to a string in a static area is returned,
     which might get overwritten on subsequent calls to this function.

     An empty string is returned if the file name cannot be determined
     for any reason.  Even if a file name is returned, access to the
     file it represents is not guaranteed.

 -- Macro: int L_ctermid
     The value of this macro is an integer constant expression that
     represents the size of a string large enough to hold the file name
     returned by `ctermid'.

   See also the `isatty' and `ttyname' functions, in *note Is It a
Terminal::.


File: libc.info,  Node: Process Group Functions,  Next: Terminal Access Functions,  Prev: Identifying the Terminal,  Up: Functions for Job Control

27.7.2 Process Group Functions
------------------------------

Here are descriptions of the functions for manipulating process groups.
Your program should include the header files `sys/types.h' and
`unistd.h' to use these functions.  

 -- Function: pid_t setsid (void)
     The `setsid' function creates a new session.  The calling process
     becomes the session leader, and is put in a new process group whose
     process group ID is the same as the process ID of that process.
     There are initially no other processes in the new process group,
     and no other process groups in the new session.

     This function also makes the calling process have no controlling
     terminal.

     The `setsid' function returns the new process group ID of the
     calling process if successful.  A return value of `-1' indicates an
     error.  The following `errno' error conditions are defined for this
     function:

    `EPERM'
          The calling process is already a process group leader, or
          there is already another process group around that has the
          same process group ID.

 -- Function: pid_t getsid (pid_t PID)
     The `getsid' function returns the process group ID of the session
     leader of the specified process.  If a PID is `0', the process
     group ID of the session leader of the current process is returned.

     In case of error `-1' is returned and `errno' is set.  The
     following `errno' error conditions are defined for this function:

    `ESRCH'
          There is no process with the given process ID PID.

    `EPERM'
          The calling process and the process specified by PID are in
          different sessions, and the implementation doesn't allow to
          access the process group ID of the session leader of the
          process with ID PID from the calling process.

   The `getpgrp' function has two definitions: one derived from BSD
Unix, and one from the POSIX.1 standard.  The feature test macros you
have selected (*note Feature Test Macros::) determine which definition
you get.  Specifically, you get the BSD version if you define
`_BSD_SOURCE'; otherwise, you get the POSIX version if you define
`_POSIX_SOURCE' or `_GNU_SOURCE'.  Programs written for old BSD systems
will not include `unistd.h', which defines `getpgrp' specially under
`_BSD_SOURCE'.  You must link such programs with the `-lbsd-compat'
option to get the BSD definition.  

 -- POSIX.1 Function: pid_t getpgrp (void)
     The POSIX.1 definition of `getpgrp' returns the process group ID of
     the calling process.

 -- BSD Function: pid_t getpgrp (pid_t PID)
     The BSD definition of `getpgrp' returns the process group ID of the
     process PID.  You can supply a value of `0' for the PID argument
     to get information about the calling process.

 -- System V Function: int getpgid (pid_t PID)
     `getpgid' is the same as the BSD function `getpgrp'.  It returns
     the process group ID of the process PID.  You can supply a value
     of `0' for the PID argument to get information about the calling
     process.

     In case of error `-1' is returned and `errno' is set.  The
     following `errno' error conditions are defined for this function:

    `ESRCH'
          There is no process with the given process ID PID.  The
          calling process and the process specified by PID are in
          different sessions, and the implementation doesn't allow to
          access the process group ID of the process with ID PID from
          the calling process.

 -- Function: int setpgid (pid_t PID, pid_t PGID)
     The `setpgid' function puts the process PID into the process group
     PGID.  As a special case, either PID or PGID can be zero to
     indicate the process ID of the calling process.

     This function fails on a system that does not support job control.
     *Note Job Control is Optional::, for more information.

     If the operation is successful, `setpgid' returns zero.  Otherwise
     it returns `-1'.  The following `errno' error conditions are
     defined for this function:

    `EACCES'
          The child process named by PID has executed an `exec'
          function since it was forked.

    `EINVAL'
          The value of the PGID is not valid.

    `ENOSYS'
          The system doesn't support job control.

    `EPERM'
          The process indicated by the PID argument is a session leader,
          or is not in the same session as the calling process, or the
          value of the PGID argument doesn't match a process group ID
          in the same session as the calling process.

    `ESRCH'
          The process indicated by the PID argument is not the calling
          process or a child of the calling process.

 -- Function: int setpgrp (pid_t PID, pid_t PGID)
     This is the BSD Unix name for `setpgid'.  Both functions do exactly
     the same thing.


File: libc.info,  Node: Terminal Access Functions,  Prev: Process Group Functions,  Up: Functions for Job Control

27.7.3 Functions for Controlling Terminal Access
------------------------------------------------

These are the functions for reading or setting the foreground process
group of a terminal.  You should include the header files `sys/types.h'
and `unistd.h' in your application to use these functions.  

   Although these functions take a file descriptor argument to specify
the terminal device, the foreground job is associated with the terminal
file itself and not a particular open file descriptor.

 -- Function: pid_t tcgetpgrp (int FILEDES)
     This function returns the process group ID of the foreground
     process group associated with the terminal open on descriptor
     FILEDES.

     If there is no foreground process group, the return value is a
     number greater than `1' that does not match the process group ID
     of any existing process group.  This can happen if all of the
     processes in the job that was formerly the foreground job have
     terminated, and no other job has yet been moved into the
     foreground.

     In case of an error, a value of `-1' is returned.  The following
     `errno' error conditions are defined for this function:

    `EBADF'
          The FILEDES argument is not a valid file descriptor.

    `ENOSYS'
          The system doesn't support job control.

    `ENOTTY'
          The terminal file associated with the FILEDES argument isn't
          the controlling terminal of the calling process.

 -- Function: int tcsetpgrp (int FILEDES, pid_t PGID)
     This function is used to set a terminal's foreground process group
     ID.  The argument FILEDES is a descriptor which specifies the
     terminal; PGID specifies the process group.  The calling process
     must be a member of the same session as PGID and must have the same
     controlling terminal.

     For terminal access purposes, this function is treated as output.
     If it is called from a background process on its controlling
     terminal, normally all processes in the process group are sent a
     `SIGTTOU' signal.  The exception is if the calling process itself
     is ignoring or blocking `SIGTTOU' signals, in which case the
     operation is performed and no signal is sent.

     If successful, `tcsetpgrp' returns `0'.  A return value of `-1'
     indicates an error.  The following `errno' error conditions are
     defined for this function:

    `EBADF'
          The FILEDES argument is not a valid file descriptor.

    `EINVAL'
          The PGID argument is not valid.

    `ENOSYS'
          The system doesn't support job control.

    `ENOTTY'
          The FILEDES isn't the controlling terminal of the calling
          process.

    `EPERM'
          The PGID isn't a process group in the same session as the
          calling process.

 -- Function: pid_t tcgetsid (int FILDES)
     This function is used to obtain the process group ID of the session
     for which the terminal specified by FILDES is the controlling
     terminal.  If the call is successful the group ID is returned.
     Otherwise the return value is `(pid_t) -1' and the global variable
     ERRNO is set to the following value:
    `EBADF'
          The FILEDES argument is not a valid file descriptor.

    `ENOTTY'
          The calling process does not have a controlling terminal, or
          the file is not the controlling terminal.


File: libc.info,  Node: Name Service Switch,  Next: Users and Groups,  Prev: Job Control,  Up: Top

28 System Databases and Name Service Switch
*******************************************

Various functions in the C Library need to be configured to work
correctly in the local environment.  Traditionally, this was done by
using files (e.g., `/etc/passwd'), but other nameservices (like the
Network Information Service (NIS) and the Domain Name Service (DNS))
became popular, and were hacked into the C library, usually with a fixed
search order (*note frobnicate: (jargon)frobnicate.).

   The GNU C Library contains a cleaner solution of this problem.  It is
designed after a method used by Sun Microsystems in the C library of
Solaris 2.  GNU C Library follows their name and calls this scheme
"Name Service Switch" (NSS).

   Though the interface might be similar to Sun's version there is no
common code.  We never saw any source code of Sun's implementation and
so the internal interface is incompatible.  This also manifests in the
file names we use as we will see later.

* Menu:

* NSS Basics::                  What is this NSS good for.
* NSS Configuration File::      Configuring NSS.
* NSS Module Internals::        How does it work internally.
* Extending NSS::               What to do to add services or databases.


File: libc.info,  Node: NSS Basics,  Next: NSS Configuration File,  Prev: Name Service Switch,  Up: Name Service Switch

28.1 NSS Basics
===============

The basic idea is to put the implementation of the different services
offered to access the databases in separate modules.  This has some
advantages:

  1. Contributors can add new services without adding them to GNU C
     Library.

  2. The modules can be updated separately.

  3. The C library image is smaller.

   To fulfill the first goal above the ABI of the modules will be
described below.  For getting the implementation of a new service right
it is important to understand how the functions in the modules get
called.  They are in no way designed to be used by the programmer
directly.  Instead the programmer should only use the documented and
standardized functions to access the databases.

The databases available in the NSS are

`aliases'
     Mail aliases

`ethers'
     Ethernet numbers,

`group'
     Groups of users, *note Group Database::.

`hosts'
     Host names and numbers, *note Host Names::.

`netgroup'
     Network wide list of host and users, *note Netgroup Database::.

`networks'
     Network names and numbers, *note Networks Database::.

`protocols'
     Network protocols, *note Protocols Database::.

`passwd'
     User passwords, *note User Database::.

`rpc'
     Remote procedure call names and numbers,

`services'
     Network services, *note Services Database::.

`shadow'
     Shadow user passwords,

There will be some more added later (`automount', `bootparams',
`netmasks', and `publickey').


File: libc.info,  Node: NSS Configuration File,  Next: NSS Module Internals,  Prev: NSS Basics,  Up: Name Service Switch

28.2 The NSS Configuration File
===============================

Somehow the NSS code must be told about the wishes of the user.  For
this reason there is the file `/etc/nsswitch.conf'.  For each database
this file contain a specification how the lookup process should work.
The file could look like this:

     # /etc/nsswitch.conf
     #
     # Name Service Switch configuration file.
     #

     passwd:     db files nis
     shadow:     files
     group:      db files nis

     hosts:      files nisplus nis dns
     networks:   nisplus [NOTFOUND=return] files

     ethers:     nisplus [NOTFOUND=return] db files
     protocols:  nisplus [NOTFOUND=return] db files
     rpc:        nisplus [NOTFOUND=return] db files
     services:   nisplus [NOTFOUND=return] db files

   The first column is the database as you can guess from the table
above.  The rest of the line specifies how the lookup process works.
Please note that you specify the way it works for each database
individually.  This cannot be done with the old way of a monolithic
implementation.

   The configuration specification for each database can contain two
different items:

   * the service specification like `files', `db', or `nis'.

   * the reaction on lookup result like `[NOTFOUND=return]'.

* Menu:

* Services in the NSS configuration::  Service names in the NSS configuration.
* Actions in the NSS configuration::  React appropriately to the lookup result.
* Notes on NSS Configuration File::  Things to take care about while
                                     configuring NSS.


File: libc.info,  Node: Services in the NSS configuration,  Next: Actions in the NSS configuration,  Prev: NSS Configuration File,  Up: NSS Configuration File

28.2.1 Services in the NSS configuration File
---------------------------------------------

The above example file mentions four different services: `files', `db',
`nis', and `nisplus'.  This does not mean these services are available
on all sites and it does also not mean these are all the services which
will ever be available.

   In fact, these names are simply strings which the NSS code uses to
find the implicitly addressed functions.  The internal interface will be
described later.  Visible to the user are the modules which implement an
individual service.

   Assume the service NAME shall be used for a lookup.  The code for
this service is implemented in a module called `libnss_NAME'.  On a
system supporting shared libraries this is in fact a shared library
with the name (for example) `libnss_NAME.so.2'.  The number at the end
is the currently used version of the interface which will not change
frequently.  Normally the user should not have to be cognizant of these
files since they should be placed in a directory where they are found
automatically.  Only the names of all available services are important.

