@node Getopt, Argp, , Parsing Program Arguments
@section Parsing program options using @code{getopt}

The @code{getopt} and @code{getopt_long} functions automate some of the
chore involved in parsing typical unix command line options.

@menu
* Using Getopt::                Using the @code{getopt} function.
* Example of Getopt::           An example of parsing options with @code{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 @code{getopt_long}.
@end menu

@node Using Getopt, Example of Getopt, , Getopt
@subsection Using the @code{getopt} function

Here are the details about how to call the @code{getopt} function.  To
use this facility, your program must include the header file
@file{unistd.h}.
@pindex unistd.h

@comment unistd.h
@comment POSIX.2
@deftypevar int opterr
If the value of this variable is nonzero, then @code{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, @code{getopt}
does not print any messages, but it still returns the character @code{?}
to indicate an error.
@end deftypevar

@comment unistd.h
@comment POSIX.2
@deftypevar int optopt
When @code{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.
@end deftypevar

@comment unistd.h
@comment POSIX.2
@deftypevar int optind
This variable is set by @code{getopt} to the index of the next element
of the @var{argv} array to be processed.  Once @code{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 @code{1}.
@end deftypevar

@comment unistd.h
@comment POSIX.2
@deftypevar {char *} optarg
This variable is set by @code{getopt} to point at the value of the
option argument, for those options that accept arguments.
@end deftypevar

@comment unistd.h
@comment POSIX.2
@deftypefun int getopt (int @var{argc}, char **@var{argv}, const char *@var{options})
The @code{getopt} function gets the next option argument from the
argument list specified by the @var{argv} and @var{argc} arguments.
Normally these values come directly from the arguments received by
@code{main}.

The @var{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 (@samp{:}) to indicate that it takes a
required argument.  If an option character is followed by two colons
(@samp{::}), its argument is optional; this is a GNU extension.

@code{getopt} has three ways to deal with options that follow
non-options @var{argv} elements.  The special argument @samp{--} forces
in all cases the end of option scanning.

@itemize @bullet
@item
The default is to permute the contents of @var{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.

@item
If the @var{options} argument string begins with a hyphen (@samp{-}), this
is treated specially.  It permits arguments that are not options to be
returned as if they were associated with option character @samp{\1}.

@item
POSIX demands the following behavior: The first non-option stops option
processing.  This mode is selected by either setting the environment
variable @code{POSIXLY_CORRECT} or beginning the @var{options} argument
string with a plus sign (@samp{+}).
@end itemize

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

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

If @code{getopt} finds an option character in @var{argv} that was not
included in @var{options}, or a missing option argument, it returns
@samp{?} and sets the external variable @code{optopt} to the actual
option character.  If the first character of @var{options} is a colon
(@samp{:}), then @code{getopt} returns @samp{:} instead of @samp{?} to
indicate a missing option argument.  In addition, if the external
variable @code{opterr} is nonzero (which is the default), @code{getopt}
prints an error message.
@end deftypefun

@node Example of Getopt
@subsection Example of Parsing Arguments with @code{getopt}

Here is an example showing how @code{getopt} is typically used.  The
key points to notice are:

@itemize @bullet
@item
Normally, @code{getopt} is called in a loop.  When @code{getopt} returns
@code{-1}, indicating no more options are present, the loop terminates.

@item
A @code{switch} statement is used to dispatch on the return value from
@code{getopt}.  In typical use, each case just sets a variable that
is used later in the program.

@item
A second loop is used to process the remaining non-option arguments.
@end itemize

@smallexample
@include testopt.c.texi
@end smallexample

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

@smallexample
% 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 -
@end smallexample

@node Getopt Long Options
@subsection Parsing Long Options with @code{getopt_long}

To accept GNU-style long options as well as single-character options,
use @code{getopt_long} instead of @code{getopt}.  This function is
declared in @file{getopt.h}, not @file{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.

@comment getopt.h
@comment GNU
@deftp {Data Type} {struct option}
This structure describes a single long option name for the sake of
@code{getopt_long}.  The argument @var{longopts} must be an array of
these structures, one for each long option.  Terminate the array with an
element containing all zeros.

The @code{struct option} structure has these fields:

@table @code
@item const char *name
This field is the name of the option.  It is a string.

@item int has_arg
This field says whether the option takes an argument.  It is an integer,
and there are three legitimate values: @w{@code{no_argument}},
@code{required_argument} and @code{optional_argument}.

@item int *flag
@itemx int val
These fields control how to report or act on the option when it occurs.

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

If @code{flag} is not a null pointer, it should be the address of an
@code{int} variable which is the flag for this option.  The value in
@code{val} is the value to store in the flag to indicate that the option
was seen.
@end table
@end deftp

@comment getopt.h
@comment GNU
@deftypefun int getopt_long (int @var{argc}, char *const *@var{argv}, const char *@var{shortopts}, const struct option *@var{longopts}, int *@var{indexptr})
Decode options from the vector @var{argv} (whose length is @var{argc}).
The argument @var{shortopts} describes the short options to accept, just as
it does in @code{getopt}.  The argument @var{longopts} describes the long
options to accept (see above).

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

When @code{getopt_long} encounters a long option, it takes actions based
on the @code{flag} and @code{val} fields of the definition of that
option.

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

If @code{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 @code{int}
that you define.  Put the address of the flag in the @code{flag} field.
Put in the @code{val} field the value you would like this option to
store in the flag.  In this case, @code{getopt_long} returns @code{0}.

For any long option, @code{getopt_long} tells you the index in the array
@var{longopts} of the options definition, by storing it into
@code{*@var{indexptr}}.  You can get the name of the option with
@code{@var{longopts}[*@var{indexptr}].name}.  So you can distinguish among
long options either by the values in their @code{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, @code{getopt_long} puts the argument
value in the variable @code{optarg} before returning.  When the option
has no argument, the value in @code{optarg} is a null pointer.  This is
how you can tell whether an optional argument was supplied.

When @code{getopt_long} has no more options to handle, it returns
@code{-1}, and leaves in the variable @code{optind} the index in
@var{argv} of the next remaining argument.
@end deftypefun

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

@comment getopt.h
@comment GNU
@deftypefun int getopt_long_only (int @var{argc}, char *const *@var{argv}, const char *@var{shortopts}, const struct option *@var{longopts}, int *@var{indexptr})

The @code{getopt_long_only} function is equivalent to the
@code{getopt_long} function but it allows to specify the user of the
application to pass long options with only @samp{-} instead of
@samp{--}.  The @samp{--} prefix is still recognized but instead of
looking through the short options if a @samp{-} is seen it is first
tried whether this parameter names a long option.  If not, it is parsed
as a short option.

Assuming @code{getopt_long_only} is used starting an application with

@smallexample
  app -foo
@end smallexample

@noindent
the @code{getopt_long_only} will first look for a long option named
@samp{foo}.  If this is not found, the short options @samp{f}, @samp{o},
and again @samp{o} are recognized.
@end deftypefun

@node Getopt Long Option Example
@subsection Example of Parsing Long Options with @code{getopt_long}

@smallexample
@include longopt.c.texi
@end smallexample