@ignore
   Documentation for the argp argument parser

   Copyright (C) 1995, 1996, 1997 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Written by Miles Bader <miles@gnu.ai.mit.edu>.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Library General Public License as
   published by the Free Software Foundation; either version 2 of the
   License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Library General Public License for more details.

   You should have received a copy of the GNU Library General Public
   License along with the GNU C Library; see the file COPYING.LIB.  If not,
   write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
   Boston, MA 02111-1307, USA.  */
@end ignore

@node Argp, Suboptions, Getopt, Parsing Program Arguments
@need 5000
@section Parsing Program Options with Argp
@cindex argp (program argument parser)
@cindex argument parsing with argp
@cindex option parsing with argp

@dfn{Argp} is an interface for parsing unix-style argument vectors
(@pxref{Program Arguments}).

Unlike the more common @code{getopt} interface, it provides many related
convenience features in addition to parsing options, such as
automatically producing output in response to @samp{--help} and
@samp{--version} options (as defined by the GNU coding standards).
Doing these things in argp results in a more consistent look for
programs that use it, and makes less likely that implementors will
neglect to implement them 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,
which programs can easily use in conjunction with their own option
parser.  This results in less work for user programs (indeed, some may
use only argument parsers exported by libraries, and have no options of
their own), and more consistent option-parsing for the abstractions
implemented by the library.

@pindex argp.h
The header file @file{<argp.h>} should be included to use argp.

@subsection The @code{argp_parse} Function

The main interface to argp is the @code{argp_parse} function; often, a
call to @code{argp_parse} is the only argument-parsing code needed in
@code{main} (@pxref{Program Arguments}).

@comment argp.h
@comment GNU
@deftypefun {error_t} argp_parse (const struct argp *@var{argp}, @w{int @var{argc}, char **@var{argv}}, @w{unsigned @var{flags}}, @w{int *@var{arg_index}}, @w{void *@var{input}})
The @code{argp_parse} function parses the arguments in in @var{argv}, of
length @var{argc}, using the argp parser @var{argp} (@pxref{Argp
Parsers}); a value of zero is the same as a @code{struct argp}
containing all zeros.  @var{flags} is a set of flag bits that modify the
parsing behavior (@pxref{Argp Flags}).  @var{input} is passed through to
the argp parser @var{argp}, and has meaning defined by it; a typical
usage is to pass a pointer to a structure which can be used for
specifying parameters to the parser and passing back results from it.

Unless the @code{ARGP_NO_EXIT} or @code{ARGP_NO_HELP} flags are included
in @var{flags}, calling @code{argp_parse} may result in the program
exiting---for instance when an unknown option is encountered.
@xref{Program Termination}.

The return value is zero for successful parsing, or a unix error code
(@pxref{Error Codes}) if an error was detected.  Different argp parsers
may return arbitrary error codes, but standard ones are @code{ENOMEM} if
a memory allocation error occurred, or @code{EINVAL} if an unknown option
or option argument was encountered.
@end deftypefun

@menu
* Globals: Argp Global Variables.  Global argp parameters.
* Parsers: Argp Parsers.        Defining parsers for use with @code{argp_parse}.
* Flags: Argp Flags.            Flags that modify the behavior of @code{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 @samp{--help} output format.
@end menu

@node Argp Global Variables, Argp Parsers, , Argp
@subsection Argp Global Variables

These variables make it very easy for every user program to implement
the @samp{--version} option and provide a bug-reporting address in the
@samp{--help} output (which is implemented by argp regardless).

@comment argp.h
@comment GNU
@deftypevar {const char *} argp_program_version
If defined or set by the user program to a non-zero value, then a
@samp{--version} option is added when parsing with @code{argp_parse}
(unless the @code{ARGP_NO_HELP} flag is used), which will print this
string followed by a newline and exit (unless the @code{ARGP_NO_EXIT}
flag is used).
@end deftypevar

@comment argp.h
@comment GNU
@deftypevar {const char *} argp_program_bug_address
If defined or set by the user program to a non-zero value,
@code{argp_program_bug_address} should point to string that is the
bug-reporting address for the program.  It will be printed at the end of
the standard output for the @samp{--help} option, embedded in a sentence
that says something like @samp{Report bugs to @var{address}.}.
@end deftypevar

@need 1500
@comment argp.h
@comment GNU
@defvar argp_program_version_hook
If defined or set by the user program to a non-zero value, then a
@samp{--version} option is added when parsing with @code{argp_parse}
(unless the @code{ARGP_NO_HELP} flag is used), which calls this function
to print the version, and then exits with a status of 0 (unless the
@code{ARGP_NO_EXIT} flag is used).  It should point to a function with
the following type signature:

@smallexample
void @var{print-version} (FILE *@var{stream}, struct argp_state *@var{state})
@end smallexample

@noindent
@xref{Argp Parsing State}, for an explanation of @var{state}.

This variable takes precedent over @code{argp_program_version}, and is
useful if a program has version information that cannot be easily
specified as a simple string.
@end defvar

@comment argp.h
@comment GNU
@deftypevar error_t argp_err_exit_status
The exit status that argp will use when exiting due to a parsing error.
If not defined or set by the user program, this defaults to
@code{EX_USAGE} from @file{<sysexits.h>}.
@end deftypevar

@node Argp Parsers, Argp Flags, Argp Global Variables, Argp
@subsection Specifying Argp Parsers

The first argument to the @code{argp_parse} function is a pointer to a
@code{struct argp}, which known as an @dfn{argp parser}:

@comment argp.h
@comment GNU
@deftp {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:

@table @code
@item const struct argp_option *options
A pointer to a vector of @code{argp_option} structures specifying which
options this argp parser understands; it may be zero if there are no
options at all.  @xref{Argp Option Vectors}.

@item 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 @code{ARGP_ERR_UNKNOWN}.
@xref{Argp Parser Functions}.

@item const char *args_doc
If non-zero, a string describing what non-option arguments are wanted by
this parser; it is only used to print the @samp{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 prefix by @samp{ or: } instead of @samp{Usage:}).

@item 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 (@code{'\v'}, @code{'\013'}) character.  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.

@item const struct argp_child *children
A pointer to a vector of @code{argp_children} structures specifying
additional argp parsers that should be combined with this one.
@xref{Argp Children}.

@item char *(*help_filter)(int @var{key}, const char *@var{text}, void *@var{input})
If non-zero, a pointer to a function to filter the output of help
messages.  @xref{Argp Help Filtering}.
@end table
@end deftp

The @code{options}, @code{parser}, @code{args_doc}, and @code{doc}
fields are usually all that are needed.  If an argp parser is defined as
an initialized C variable, only the used fields need be specified in in
the initializer---the rest will default to zero due to the way C
structure initialization works (this fact is exploited for most argp
structures, grouping the most-used fields near the beginning, so that
unused fields can simply be 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.
@end menu

@node Argp Option Vectors, Argp Parser Functions, Argp Parsers, Argp Parsers
@subsection Specifying Options in an Argp Parser

The @code{options} field in a @code{struct argp} points to a vector of
@code{struct argp_option} structures, each of which specifies an option
that argp parser supports (actually, sometimes multiple entries may used
for a single option if it has many names).  It should be terminated by
an entry with zero in all fields (note that when using an initialized C
array for options, writing @code{@{ 0 @}} is enough to achieve this).

@comment argp.h
@comment GNU
@deftp {Data Type} {struct argp_option}
This structure specifies a single option that an argp parser
understands, and how to parse and document it.  It has the following fields:

@table @code
@item const char *name
The long name for this option, corresponding to the long option
@samp{--@var{name}}; this field can 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 @code{OPTION_ALIAS} flag set (@pxref{Argp
Option Flags}).

@item int key
The integer key that is provided to the argp parser's parsing function
when this option is being parsed.  Also, if @var{key} has a value that
is a printable @sc{ascii} character (i.e., @code{isascii (@var{key})} is
true), it @emph{also} specifies a short option @samp{-@var{char}}, where
@var{char} is the @sc{ascii} character with the code @var{key}.

@item 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
@samp{--@var{name}=@var{value}} or @samp{-@var{char} @var{value}}
syntaxes) unless the @code{OPTION_ARG_OPTIONAL} flag (@pxref{Argp Option
Flags}) is set, in which case it @emph{may} be provided.

@item int flags
Flags associated with this option (some of which are referred to above).
@xref{Argp Option Flags}.

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

If both the @code{name} and @code{key} fields are zero, this string
will be printed out-dented from the normal option column, making it
useful as a group header (it will be the first thing printed in its
group); in this usage, it's conventional to end the string with a
@samp{:} character.

@item int group
The group this option is in.

In a long help message, options are sorted alphabetically within each
group, and the groups presented in the order @math{0, 1, 2,} @dots{},
@math{@var{n}, -@var{m},} @dots{}, @math{-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, unless its a group header (@code{name} and
@code{key} fields both zero), in which case, the previous entry
@math{@w{} + 1} is
the default.  Automagic options such as @samp{--help} are put into group
@math{-1}.

Note that because of C structure initialization rules, this field
often need not be specified, because 0 is the right value.
@end table
@end deftp

@menu
* Flags: Argp Option Flags.     Flags for options.
@end menu

@node Argp Option Flags, , , Argp Option Vectors
@subsubsection Flags for Argp Options

The following flags may be or'd together in the @code{flags} field of a
@code{struct argp_option}, and control various aspects of how that
option is parsed or displayed in help messages:

@vtable @code
@comment argp.h
@comment GNU
@item OPTION_ARG_OPTIONAL
The argument associated with this option is optional.

@comment argp.h
@comment GNU
@item OPTION_HIDDEN
This option isn't displayed in any help messages.

@comment argp.h
@comment GNU
@item 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 @code{name} and @code{key} from the aliased option.

@comment argp.h
@comment GNU
@item OPTION_DOC
This option isn't actually an option (and so should be ignored by the
actual option parser), but rather an arbitrary piece of documentation
that should be displayed in much the same manner as the options (known
as a @dfn{documentation option}).

If this flag is set, then the option @code{name} field is displayed
unmodified (e.g., no @samp{--} prefix is added) at the left-margin
(where a @emph{short} option would normally be displayed), and the
documentation string in the normal place.  For purposes of sorting, any
leading whitespace and punctuation is ignored, except that if the first
non-whitespace character is not @samp{-}, this entry is displayed after
all options (and @code{OPTION_DOC} entries with a leading @samp{-}) in
the same group.

@comment argp.h
@comment GNU
@item OPTION_NO_USAGE
This option shouldn't be included in `long' usage messages (but is still
included in help messages).  This is mainly intended for options that
are completely documented in an argp's @code{args_doc} field
(@pxref{Argp Parsers}), in which case including the option
in the generic usage list would be redundant.

For instance, if @code{args_doc} is @code{"FOO BAR\n-x BLAH"}, and the
@samp{-x} option's purpose is to distinguish these two cases, @samp{-x}
should probably be marked @code{OPTION_NO_USAGE}.
@end vtable

@node Argp Parser Functions, Argp Children, Argp Option Vectors, Argp Parsers
@subsection Argp Parser Functions

The function pointed to by the @code{parser} field in a @code{struct
argp} (@pxref{Argp Parsers}) defines what actions take place in response
to each option or argument that is parsed, and is also used as a hook,
to allow a parser to do something at certain other points during
parsing.

@need 2000
Argp parser functions have the following type signature:

@cindex argp parser functions
@smallexample
error_t @var{parser} (int @var{key}, char *@var{arg}, struct argp_state *@var{state})
@end smallexample

@noindent
where the arguments are as follows:

@table @var
@item key
For each option that is parsed, @var{parser} is called with a value of
@var{key} from that option's @code{key} field in the option vector
(@pxref{Argp Option Vectors}).  @var{parser} is also called at other
times with special reserved keys, such as @code{ARGP_KEY_ARG} for
non-option arguments.  @xref{Argp Special Keys}.

@item arg
If @var{key} is an option, @var{arg} is the value given for it, or zero
if no value was specified.  Only options that have a non-zero @code{arg}
field can ever have a value, and those must @emph{always} have a value,
unless the @code{OPTION_ARG_OPTIONAL} flag was specified (if the input
being parsed specifies a value for an option that doesn't allow one, an
error results before @var{parser} ever gets called).

If @var{key} is @code{ARGP_KEY_ARG}, @var{arg} is a non-option argument;
other special keys always have a zero @var{arg}.

@item state
@var{state} points to a @code{struct argp_state}, containing useful
information about the current parsing state for use by @var{parser}.
@xref{Argp Parsing State}.
@end table

When @var{parser} is called, it should perform whatever action is
appropriate for @var{key}, and return either @code{0} for success,
@code{ARGP_ERR_UNKNOWN}, if the value of @var{key} is not handled by
this parser function, or a unix error code if a real error occurred
(@pxref{Error Codes}).

@comment argp.h
@comment GNU
@deftypevr Macro int ARGP_ERR_UNKNOWN
Argp parser functions should return @code{ARGP_ERR_UNKNOWN} for any
@var{key} value they do not recognize, or for non-option arguments
(@code{@var{key} == ARGP_KEY_ARG}) that they do not which to handle.
@end deftypevr

@need 3000
A typical parser function uses a switch statement on @var{key}:

@smallexample
error_t
parse_opt (int key, char *arg, struct argp_state *state)
@{
  switch (key)
    @{
    case @var{option_key}:
      @var{action}
      break;
    @dots{}
    default:
      return ARGP_ERR_UNKNOWN;
    @}
  return 0;
@}
@end smallexample

@menu
* Keys: Argp Special Keys.           Special values for the @var{key} argument.
* State: Argp Parsing State.         What the @var{state} argument refers to.
* Functions: Argp Helper Functions.  Functions to help during argp parsing.
@end menu

@node Argp Special Keys, Argp Parsing State, , Argp Parser Functions
@subsubsection Special Keys for Argp Parser Functions

In addition to key values corresponding to user options, the @var{key}
argument to argp parser functions may have a number of other special
values (@var{arg} and @var{state} refer to parser function arguments;
@pxref{Argp Parser Functions}):

@vtable @code
@comment argp.h
@comment GNU
@item ARGP_KEY_ARG
This is not an option at all, but rather a command line argument, whose
value is pointed to by @var{arg}.

When there are multiple parser functions (due to argp parsers being
combined), it's impossible to know which one wants to handle an
argument, so each is called in turn, until one returns 0 or an error
other than @code{ARGP_ERR_UNKNOWN}; if an argument is handled by no one,
@code{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 @code{ARGP_KEY_NO_ARGS} case won't be used.
@emph{However}, if while processing the argument, a parser function
decrements the @code{next} field of its @var{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.

@comment argp.h
@comment GNU
@item ARGP_KEY_ARGS
If a parser function returns @code{ARGP_ERR_UNKNOWN} for
@code{ARGP_KEY_ARG}, it is immediately called again with the key
@code{ARGP_KEY_ARGS}, which has a similar meaning, but is slightly more
convenient for consuming all remaining arguments.  @var{arg} is 0, and
the tail of the argument vector may be found at @code{@var{state}->argv
+ @var{state}->next}.  If success is returned for this key, and
@code{@var{state}->next} is unchanged, then all remaining arguments are
considered to have been consumed, otherwise, the amount by which
@code{@var{state}->next} has been adjust indicates how many were used.
For instance, here's an example that uses both, for different args:

@smallexample
...
case ARGP_KEY_ARG:
  if (@var{state}->arg_num == 0)
    /* First argument */
    first_arg = @var{arg};
  else
    return ARGP_KEY_UNKNOWN;    /* Let the next case parse it.  */
  break;
case ARGP_KEY_ARGS:
  remaining_args = @var{state}->argv + @var{state}->next;
  num_remaining_args = @var{state}->argc - @var{state}->next;
  break;
@end smallexample

@comment argp.h
@comment GNU
@item ARGP_KEY_END
There are no more command line arguments at all.

@comment argp.h
@comment GNU
@item ARGP_KEY_NO_ARGS
Because it's common to want 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.  Called just
before @code{ARGP_KEY_END} (where more general validity checks on
previously parsed arguments can take place).

@comment argp.h
@comment GNU
@item ARGP_KEY_INIT
Passed in before any parsing is done.  Afterwards, the values of each
element of the @code{child_input} field of @var{state}, if any, are
copied to each child's state to be the initial value of the @code{input}
when @emph{their} parsers are called.

@comment argp.h
@comment GNU
@item ARGP_KEY_SUCCESS
Passed in when parsing has successfully been completed (even if there are
still arguments remaining).

@comment argp.h
@comment GNU
@item ARGP_KEY_ERROR
Passed in if an error has occurred, and parsing terminated (in which case
a call with a key of @code{ARGP_KEY_SUCCESS} is never made).

@comment argp.h
@comment GNU
@item ARGP_KEY_FINI
The final key ever seen by any parser (even after
@code{ARGP_KEY_SUCCESS} and @code{ARGP_KEY_ERROR}).  Any resources
allocated by @code{ARGP_KEY_INIT} may be freed here (although sometimes
certain resources allocated there are to be returned to the caller after
a successful parse; in that case, those particular resources can be
freed in the @code{ARGP_KEY_ERROR} case).
@end vtable

In all cases, @code{ARGP_KEY_INIT} is the first key seen by parser
functions, and @code{ARGP_KEY_FINI} the last (unless an error was
returned by the parser for @code{ARGP_KEY_INIT}).  Other keys can occur
in one the following orders (@var{opt} refers to an arbitrary option
key):

@table @asis
@item @var{opt}@dots{} @code{ARGP_KEY_NO_ARGS} @code{ARGP_KEY_END} @code{ARGP_KEY_SUCCESS}
The arguments being parsed contained no non-option arguments at all.

@item ( @var{opt} | @code{ARGP_KEY_ARG} )@dots{} @code{ARGP_KEY_END} @code{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).

@item ( @var{opt} | @code{ARGP_KEY_ARG} )@dots{} @code{ARGP_KEY_SUCCESS}
Some non-option argument was unrecognized.

This occurs when every parser function returns @code{ARGP_KEY_UNKNOWN}
for an argument, in which case parsing stops at that argument.  If a
non-zero value for @var{arg_index} was passed to @code{argp_parse}, the
index of this argument is returned in it, otherwise an error occurs.
@end table

If an error occurs (either detected by argp, or because a parser
function returned an error value), then each parser is called with
@code{ARGP_KEY_ERROR}, and no further calls are made except the final
call with @code{ARGP_KEY_FINI}.

@node Argp Helper Functions, , Argp Parsing State, Argp Parser Functions
@subsubsection Functions For Use in Argp Parsers

Argp provides a number of functions for the user of argp parser
functions (@pxref{Argp Parser Functions}), mostly for producing error
messages.  These take as their first argument the @var{state} argument
to the parser function (@pxref{Argp Parsing State}).

@cindex usage messages, in argp
@comment argp.h
@comment GNU
@deftypefun void argp_usage (const struct argp_state *@var{state})
Output the standard usage message for the argp parser referred to by
@var{state} to @code{@var{state}->err_stream} and terminate the program
with @code{exit (argp_err_exit_status)} (@pxref{Argp Global Variables}).
@end deftypefun

@cindex syntax error messages, in argp
@comment argp.h
@comment GNU
@deftypefun void argp_error (const struct argp_state *@var{state}, @w{const char *@var{fmt}, @dots{}})
Print the printf format string @var{fmt} and following args, preceded by
the program name and @samp{:}, and followed by a @w{@samp{Try @dots{}
--help}} message, and terminate the program with an exit status of
@code{argp_err_exit_status} (@pxref{Argp Global Variables}).
@end deftypefun

@cindex error messages, in argp
@comment argp.h
@comment GNU
@deftypefun void argp_failure (const struct argp_state *@var{state}, @w{int @var{status}, int @var{errnum},} @w{const char *@var{fmt}, @dots{}})
Similarly to the standard gnu error-reporting function @code{error},
print the printf format string @var{fmt} and following args, preceded by
the program name and @samp{:}, and followed by the standard unix error
text for @var{errnum} if it is non-zero; then if @var{status} is
non-zero, terminate the program with that as its exit status.

The difference between this function and @code{argp_error} is that
@code{argp_error} is for @emph{parsing errors}, whereas
@code{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.
@end deftypefun

@comment argp.h
@comment GNU
@deftypefun void argp_state_help (const struct argp_state *@var{state}, @w{FILE *@var{stream}}, @w{unsigned @var{flags}})
Output a help message for the argp parser referred to by @var{state} to
@var{stream}.  The @var{flags} argument determines what sort of help
message is produced.  @xref{Argp Help Flags}.
@end deftypefun

Error output is sent to @code{@var{state}->err_stream}, and the program
name printed is @code{@var{state}->name}.

The output or program termination behavior of these functions may be
suppressed if the @code{ARGP_NO_EXIT} or @code{ARGP_NO_ERRS} flags,
respectively, were passed to @code{argp_parse}.  @xref{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, calls to any of these
functions should be followed by code return of an appropriate error code
for the case where the program @emph{doesn't} terminate; for example:

@smallexample
if (@var{bad argument syntax})
  @{
     argp_usage (@var{state});
     return EINVAL;
  @}
@end smallexample

@noindent
If it's known that a parser function will only be used when
@code{ARGP_NO_EXIT} is not set, the return may be omitted.

@node Argp Parsing State, Argp Helper Functions, Argp Special Keys, Argp Parser Functions
@subsubsection Argp Parsing State

The third argument to argp parser functions (@pxref{Argp Parser
Functions}) is a pointer to a @code{struct argp_state}, which contains
information about the state of the option parsing.

@comment argp.h
@comment GNU
@deftp {Data Type} {struct argp_state}
This structure has the following fields, which may be modified as noted:

@table @code
@item const struct argp *const root_argp
The top level argp parser being parsed.  Note that this is often
@emph{not} the same @code{struct argp} passed into @code{argp_parse} by
the invoking program (@pxref{Argp}), but instead an internal argp parser
that contains options implemented by @code{argp_parse} itself (such as
@samp{--help}).

@item int argc
@itemx char **argv
The argument vector being parsed.  May be modified.

@item int next
The index in @code{argv} of the next argument to be parsed.  May be modified.

One way to consume all remaining arguments in the input is to set
@code{@var{state}->next = @var{state}->argc} (perhaps after recording
the value of the @code{next} field to find the consumed arguments).
Also, you can cause the current option to be re-parsed by decrementing
this field, and then modifying
@code{@var{state}->argv[@var{state}->next]} to be the option that should
be reexamined.

@item unsigned flags
The flags supplied to @code{argp_parse}.  May be modified, although some
flags may only take effect when @code{argp_parse} is first invoked.
@xref{Argp Flags}.

@item unsigned arg_num
While calling a parsing function with the @var{key} argument
@code{ARGP_KEY_ARG}, this is the number of the current arg, starting at
0, and incremented after each such call returns.  At all other times,
this is the number of such arguments that have been processed.

@item int quoted
If non-zero, the index in @code{argv} of the first argument following a
special @samp{--} argument (which prevents anything following being
interpreted as an option).  Only set once argument parsing has proceeded
past this point.

@item void *input
An arbitrary pointer passed in from the caller of @code{argp_parse}, in
the @var{input} argument.

@item void **child_inputs
Values to pass to child parsers.  This vector will be the same length as
the number of children in the current parser, and each child parser will
be given the value of @code{@var{state}->child_inputs[@var{i}]} as
@emph{its} @code{@var{state}->input} field, where @var{i} is the index
of the child in the this parser's @code{children} field.  @xref{Argp
Children}.

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

@item char *name
The name used when printing messages.  This is initialized to
@code{argv[0]}, or @code{program_invocation_name} if that is
unavailable.

@item FILE *err_stream
@itemx FILE *out_stream
Stdio streams used when argp prints something; error messages are
printed to @code{err_stream}, and all other output (such as
@samp{--help} output) to @code{out_stream}.  These are initialized to
@code{stderr} and @code{stdout} respectively (@pxref{Standard Streams}).

@item void *pstate
Private, for use by the argp implementation.
@end table
@end deftp

@node Argp Children, Argp Help Filtering, Argp Parser Functions, Argp Parsers
@subsection Combining Multiple Argp Parsers

The @code{children} field in a @code{struct argp} allows other argp
parsers to be combined with the referencing one to parse a single set of
arguments.  It should point to a vector of @code{struct argp_child},
terminated by an entry having a value of zero in the @code{argp} field.

Where conflicts between combined parsers arise (for instance, if two
specify an option with the same name), they are resolved in favor of
the parent argp parsers, or earlier argp parsers in the list of children.

@comment argp.h
@comment GNU
@deftp {Data Type} {struct argp_child}
An entry in the list of subsidiary argp parsers pointed to by the
@code{children} field in a @code{struct argp}.  The fields are as follows:

@table @code
@item const struct argp *argp
The child argp parser, or zero to end the list.

@item int flags
Flags for this child.

@item const char *header
If non-zero, an optional header to be printed in 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 @code{""}.  As with header
strings specified in an option entry, the value conventionally has
@samp{:} as the last character.  @xref{Argp Option Vectors}.

@item int group
Where to group the child options relative to the other (`consolidated')
options in the parent argp parser.  The values are the same as the
@code{group} field in @code{struct argp_option} (@pxref{Argp Option
Vectors}), but all child-groupings follow parent options at a particular
group level.  If both this field and @code{header} are zero, then the
child's options aren't grouped together at all, but rather merged with
the parent options (merging the child's grouping levels with the
parents).
@end table
@end deftp

@node Argp Flags, Argp Help, Argp Parsers, Argp
@subsection Flags for @code{argp_parse}

The default behavior of @code{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
@var{flags} argument to @code{argp_parse}:

@vtable @code
@comment argp.h
@comment GNU
@item ARGP_PARSE_ARGV0
Don't ignore the first element of the @var{argv} argument to
@code{argp_parse}.  Normally (and always unless @code{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.

@comment argp.h
@comment GNU
@item ARGP_NO_ERRS
Don't print error messages for unknown options to @code{stderr}; unless
this flag is set, @code{ARGP_PARSE_ARGV0} is ignored, as @code{argv[0]}
is used as the program name in the error messages.  This flag implies
@code{ARGP_NO_EXIT} (on the assumption that silent exiting upon errors
is bad behaviour).

@comment argp.h
@comment GNU
@item ARGP_NO_ARGS
Don't parse any non-option args.  Normally non-option args are parsed by
calling the parse functions with a key of @code{ARGP_KEY_ARG}, and the
actual arg as the value.  This flag needn't normally be set, as the
normal behavior is to stop parsing as soon as some argument isn't
accepted by a parsing function.  @xref{Argp Parser Functions}.

@comment argp.h
@comment GNU
@item 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

@comment argp.h
@comment GNU
@item ARGP_NO_HELP
Don't provide the standard long option @samp{--help}, which ordinarily
causes usage and option help information to be output to @code{stdout},
and @code{exit (0)} called.

@comment argp.h
@comment GNU
@item ARGP_NO_EXIT
Don't exit on errors (they may still result in error messages).

@comment argp.h
@comment GNU
@item ARGP_LONG_ONLY
Use the gnu getopt `long-only' rules for parsing arguments.  This
allows long-options to be recognized with only a single @samp{-} (for
instances, @samp{-help}), but results in a generally somewhat less
useful interface, that conflicts with the way most GNU programs work.
For this reason, its use is discouraged.

@comment argp.h
@comment GNU
@item ARGP_SILENT
Turns off any message-printing/exiting options, specifically
@code{ARGP_NO_EXIT}, @code{ARGP_NO_ERRS}, and @code{ARGP_NO_HELP}.
@end vtable

@node Argp Help Filtering, , Argp Children, Argp Parsers
@need 2000
@subsection Customizing Argp Help Output

The @code{help_filter} field in a a @code{struct argp} is a pointer to a
function to filter the text of help messages before displaying them.
They have a function signature like:

@smallexample
char *@var{help-filter} (int @var{key}, const char *@var{text}, void *@var{input})
@end smallexample

@noindent
where @var{key} is either a key from an option, in which case @var{text}
is that option's help text (@pxref{Argp Option Vectors}), or one of the
special keys with names beginning with @samp{ARGP_KEY_HELP_}, describing
which other help text @var{text} is (@pxref{Argp Help Filter Keys}).

The function should return either @var{text}, if it should be used
as-is, a replacement string, which should be allocated using
@code{malloc}, and will be freed by argp, or zero, meaning `print
nothing'.  The value of @var{text} supplied is @emph{after} any
translation has been done, so if any of the replacement text also needs
translation, that should be done by the filter function.  @var{input} is
either the input supplied to @code{argp_parse}, or zero, if
@code{argp_help} was called directly by the user.

@menu
* Keys: Argp Help Filter Keys.  Special @var{key} values for help filter functions.
@end menu

@node Argp Help Filter Keys, , , Argp Help Filtering
@subsubsection 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, and specify which help text the @var{text} argument contains:

@vtable @code
@comment argp.h
@comment GNU
@item ARGP_KEY_HELP_PRE_DOC
Help text preceding options.

@comment argp.h
@comment GNU
@item ARGP_KEY_HELP_POST_DOC
Help text following options.

@comment argp.h
@comment GNU
@item ARGP_KEY_HELP_HEADER
Option header string.

@comment argp.h
@comment GNU
@item ARGP_KEY_HELP_EXTRA
After all other documentation; @var{text} is zero for this key.

@comment argp.h
@comment GNU
@item ARGP_KEY_HELP_DUP_ARGS_NOTE
The explanatory note emitted when duplicate option arguments have been
suppressed.

@comment argp.h
@comment GNU
@item ARGP_KEY_HELP_ARGS_DOC
The argument doc string (the @code{args_doc} field from the argp parser;
@pxref{Argp Parsers}).
@end vtable

@node Argp Help, Argp Examples, Argp Flags, Argp
@subsection The @code{argp_help} Function

Normally programs using argp need not worry too much about printing
argument-usage-type help messages, because the standard @samp{--help}
option is handled automatically by argp, and the typical error cases can
be handled using @code{argp_usage} and @code{argp_error} (@pxref{Argp
Helper Functions}).

However, if it's desirable to print a standard help message in some
context other than parsing the program options, argp offers the
@code{argp_help} interface.

@comment argp.h
@comment GNU
@deftypefun void argp_help (const struct argp *@var{argp}, @w{FILE *@var{stream}}, @w{unsigned @var{flags}}, @w{char *@var{name}})
Output a help message for the argp parser @var{argp} to @var{stream}.
What sort of messages is printed is determined by @var{flags}.

Any options such as @samp{--help} that are implemented automatically by
argp itself will @emph{not} be present in the help output; for this
reason, it is better to use @code{argp_state_help} if calling from
within an argp parser function.  @xref{Argp Helper Functions}.
@end deftypefun

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

@node Argp Help Flags, , , Argp Help
@subsection Flags for the @code{argp_help} Function

When calling @code{argp_help} (@pxref{Argp Help}), or
@code{argp_state_help} (@pxref{Argp Helper Functions}), exactly what is
output is determined by the @var{flags} argument, which should consist
of any of the following flags, or'd together:

@vtable @code
@item ARGP_HELP_USAGE
A unix @samp{Usage:} message that explicitly lists all options.

@item ARGP_HELP_SHORT_USAGE
A unix @samp{Usage:} message that displays only an appropriate
placeholder to indicate where the options go; useful for showing
the non-option argument syntax.

@item ARGP_HELP_SEE
A @samp{Try @dots{} for more help} message; @samp{@dots{}} contains the
program name and @samp{--help}.

@item ARGP_HELP_LONG
A verbose option help message that gives each option understood along
with its documentation string.

@item ARGP_HELP_PRE_DOC
The part of the argp parser doc string that precedes the verbose option help.

@item ARGP_HELP_POST_DOC
The part of the argp parser doc string that follows the verbose option help.

@item ARGP_HELP_DOC
@code{(ARGP_HELP_PRE_DOC | ARGP_HELP_POST_DOC)}

@item ARGP_HELP_BUG_ADDR
A message saying where to report bugs for this program, if the
@code{argp_program_bug_address} variable contains one.

@item ARGP_HELP_LONG_ONLY
Modify any output appropriately to reflect @code{ARGP_LONG_ONLY} mode.
@end vtable

The following flags are only understood when used with
@code{argp_state_help}, and control whether the function returns after
printing its output, or terminates the program:

@vtable @code
@item ARGP_HELP_EXIT_ERR
Terminate the program with @code{exit (argp_err_exit_status)}.

@item ARGP_HELP_EXIT_OK
Terminate the program with @code{exit (0)}.
@end vtable

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

@vtable @code
@item ARGP_HELP_STD_ERR
Assuming an error message for a parsing error has already printed,
prints a note on how to get help, and terminates the program with an
error.

@item ARGP_HELP_STD_USAGE
Prints a standard usage message and terminates the program with an
error.  This is used when no more specific error message is appropriate.

@item ARGP_HELP_STD_HELP
Prints the standard response for a @samp{--help} option, and terminates
the program successfully.
@end vtable

@node Argp Examples, Argp User Customization, Argp Help, Argp
@subsection 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.
@end menu

@node Argp Example 1, Argp Example 2, , Argp Examples
@subsubsection 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) message for @samp{--help}.

@smallexample
@include argp-ex1.c.texi
@end smallexample

@node Argp Example 2, Argp Example 3, Argp Example 1, Argp Examples
@subsubsection A Program Using Argp with Only Default Options

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
@samp{--help} option, this example will have a @samp{--version} option,
and will put the given documentation string and bug address in the
@samp{--help} output, as per GNU standards.

The variable @code{argp} contains the argument parser specification;
adding fields to this structure is the way most parameters are passed to
@code{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, @code{argp_program_version} and
@code{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).

@smallexample
@include argp-ex2.c.texi
@end smallexample

@node Argp Example 3, Argp Example 4, Argp Example 2, Argp Examples
@subsubsection A Program Using Argp with User Options

This program uses the same features as example 2, and adds user options
and arguments.

We now use the first four fields in @code{argp} (@pxref{Argp Parsers}),
and specifies @code{parse_opt} as the parser function (@pxref{Argp
Parser Functions}).

Note that in this example, @code{main} uses a structure to communicate
with the @code{parse_opt} function, a pointer to which it passes in the
@code{input} argument to @code{argp_parse} (@pxref{Argp}), and is
retrieved by @code{parse_opt} through the @code{input} field in its
@code{state} argument (@pxref{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.

@smallexample
@include argp-ex3.c.texi
@end smallexample

@node Argp Example 4, , Argp Example 3, Argp Examples
@subsubsection A Program Using Multiple Combined Argp Parsers

This program uses the same features as example 3, but has more options,
and somewhat more structure in the @samp{--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, and the special
@var{key} value @code{ARGP_KEY_NO_ARGS}, which is only given if no
non-option arguments were supplied to the program (@pxref{Argp Special
Keys}).

For structuring the help output, two features are used: @emph{headers},
which are entries in the options vector (@pxref{Argp Option Vectors})
with the first four fields being zero, and a two part documentation
string (in the variable @code{doc}), which allows documentation both
before and after the options (@pxref{Argp Parsers}); the
two parts of @code{doc} are separated by a vertical-tab character
(@code{'\v'}, or @code{'\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 @code{gettext}
function, for possible translation into the current locale.

@smallexample
@include argp-ex4.c.texi
@end smallexample

@node Argp User Customization, , Argp Examples, Argp
@subsection Argp User Customization

@cindex ARGP_HELP_FMT environment variable
The way formatting of argp @samp{--help} output may be controlled to
some extent by a program's users, by setting the @code{ARGP_HELP_FMT}
environment variable to a comma-separated list (whitespace is ignored)
of the following tokens:

@table @samp
@item dup-args
@itemx no-dup-args
Turn @dfn{duplicate-argument-mode} on or off.  In duplicate argument
mode, if an option which accepts an argument has multiple names, the
argument is shown for each name; otherwise, it is only shown for the
first long option, and a note is emitted later so the user knows that it
applies to the other names as well.  The default is @samp{no-dup-args},
which is less consistent, but prettier.

@item dup-args-note
@item no-dup-args-note
Enable or disable the note informing the user of suppressed option
argument duplication.  The default is @samp{dup-args-note}.

@item short-opt-col=@var{n}
Show the first short option in column @var{n} (default 2).

@item long-opt-col=@var{n}
Show the first long option in column @var{n} (default 6).

@item doc-opt-col=@var{n}
Show `documentation options' (@pxref{Argp Option Flags}) in column
@var{n} (default 2).

@item opt-doc-col=@var{n}
Show the documentation for options starting in column @var{n} (default 29).

@item header-col=@var{n}
Indent group headers (which document groups of options) to column
@var{n} (default 1).

@item usage-indent=@var{n}
Indent continuation lines in @samp{Usage:} messages to column @var{n}
(default 12).

@item rmargin=@var{n}
Word wrap help output at or before column @var{n} (default 79).
@end table