This is libc.info, produced by makeinfo version 4.0 from libc.texinfo.

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

   This file documents the GNU C library.

   This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library
Reference Manual', for Version 2.2.x.

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

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

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

   A GNU Manual

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

* Menu:

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


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

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

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

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

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

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

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 <unistd.h>
     #include <stdio.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 (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

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

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

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

     The `struct option' structure has these fields:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

     Assuming `getopt_long_only' is used starting an application with

            app -foo

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


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

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

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.

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

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>'.

