@node getopt
@section @code{getopt}
@findex getopt

POSIX specification:@* @url{https://pubs.opengroup.org/onlinepubs/9699919799/functions/getopt.html}

Gnulib module: getopt-posix or getopt-gnu

The module @code{getopt-gnu} has support for ``long options'' and for
``options that take optional arguments''.  Compared to the API defined by POSIX,
it adds a header file @code{<getopt.h>} and a function @code{getopt_long}.

Portability problems fixed by either Gnulib module @code{getopt-posix} or @code{getopt-gnu}:
@itemize
@item
This function is missing on some platforms:
MSVC 14.
@item
The value of @code{optind} after a missing required argument is wrong
on some platforms:
Mac OS X 10.13, AIX 7.1, mingw.
@end itemize

Portability problems fixed by Gnulib module @code{getopt-gnu}:
@itemize
@item
The function @code{getopt} does not support the @samp{+} flag in the options
string on some platforms:
Mac OS X 10.13, AIX 5.2, HP-UX 11, IRIX 6.5, Solaris 11 2010-11.
@item
The function @code{getopt} does not obey the combination of @samp{+}
and @samp{:} flags in the options string on some platforms:
glibc 2.11.
@item
The function @code{getopt} does not obey the @samp{-} flag in the options
string when @env{POSIXLY_CORRECT} is set on some platforms:
Cygwin 1.7.0.
@item
The function @code{getopt} does not support options with optional arguments
on some platforms:
Mac OS X 10.13, OpenBSD 4.0, AIX 5.2, HP-UX 11, IRIX 6.5, Solaris 11 2010-11, Cygwin 1.5.x.
@item
The function @code{getopt_long} is missing on some platforms:
AIX 5.1, HP-UX 11, IRIX 6.5, Solaris 9, MSVC 14.
@item
The function @code{getopt_long} does not support abbreviated long options
where all disambiguations are equivalent on some platforms:
OpenBSD 5.0.
@item
The function @code{getopt_long_only} is missing on some platforms:
FreeBSD 5.2.1, NetBSD 9.0, AIX 5.1, HP-UX 11, IRIX 6.5, Solaris 9, mingw, MSVC 14.
@item
This function crashes if the option string includes @code{W;} on some
platforms:
glibc 2.14.
@end itemize

Portability problems not fixed by Gnulib:
@itemize
@item
The default behavior of the glibc implementation of @code{getopt} allows
mixing option and non-option arguments on the command line in any order.
Other implementations, such as the one in Cygwin, enforce strict POSIX
compliance: they require that the option arguments precede the non-option
arguments.  This is something to watch out in your program's
testsuite.
@item
The glibc implementation allows a complete reset of the environment,
including re-checking for @env{POSIXLY_CORRECT}, by setting
@code{optind} to 0.  Several BSD implementations provide @code{optreset},
causing a reset by setting it non-zero, although it does not
necessarily re-read @env{POSIXLY_CORRECT}.  Solaris @code{getopt} does
not support either reset method, but does not maintain state that
needs the extra level of reset.
@item
On some platforms, this function does not set the stream error
indicator on attempts to write to a read-only stream:
glibc 2.13, Cygwin 1.7.9.
@end itemize
