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.\"     from: @(#)getopt.3	6.16 (Berkeley) 4/19/91
.\"	$Id: getopt.3,v 1.5 1993/10/13 17:23:39 jtc Exp $
.\"
.Dd April 19, 1991
.Dt GETOPT 3
.Os BSD 4.3
.Sh NAME
.Nm getopt
.Nd get option letter from argv
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn getopt "int argc" "char * const *argv" "const char *optstring"
.Vt extern char *optarg;
.Vt extern int optind;
.Vt extern int opterr;
.Vt extern int optopt;
.Sh DESCRIPTION
The
.Fn getopt
function gets 
the next
.Em known
option character from
.Fa argv .
An option character is
.Em known
if it has been specified in the string of accepted option characters,
.Fa optstring .
.Pp
The option string
.Fa optstring
may contain the following characters; letters and
letters followed by a colon to indicate an option argument
is to follow. It does not matter to
.Fn getopt
if a following argument has leading white space.
.Pp
On return from
.Fn getopt ,
.Va optarg
points to an option argument, if it is anticipated,
and the variable
.Va optind
contains the index to the next
.Fa argv
argument for a subsequent call
to
.Fn getopt .
.Pp
The variable
.Va opterr
and
.Va optind
are both initialized to 1.
In order to use
.Fn getopt
to evaluate multiple sets of arguments, or to evaluate a single set of
arguments multiple times,
.Va optind
must be initialized to the number of argv entries to be skipped in each
evaluation.
.Pp
The
.Fn getopt
function
returns \-1
when the argument list is exhausted.
The interpretation of options in the argument list may be cancelled
by the option
.Ql --
(double dash) which causes
.Fn getopt
to signal the end of argument processing and return \-1.
When all options have been processed (i.e., up to the first non-option
argument),
.Fn getopt
returns \-1.
.Sh DIAGNOSTICS
If the 
.Fn getopt
function encounters an option character that is not contained in 
.Fa optstring ,
it returns a question mark 
.Pq ? 
character.
If it detects a missing option argument, it returns a colon 
.Pq \: 
character if the first character of
.Fa optstring 
is a colon, otherwise it returns a question mark.
In either case, a diagnostic message is written to 
.Em stderr
unless the application has set 
.Va opterr
to zero or the first character of
.Fa optstring
is a colon.
.Sh EXAMPLE
.\" The following example comes from section E.9.7 of the IEEE 1003.2-90
.\" standard (POSIX.2).
The following code fragment shows how one might process the arguments for 
a utility that can take the mutually exclusive options 
.Em a 
and 
.Em b 
and the options 
.Em f 
and 
.Em o ,
both of which require arguments:
.Pp
.Bd -literal -compact
#include <unistd.h>

int
main (argc, argv)
	int argc;
	char *argv[];
{
	int c, bflg, aflg, errflg = 0;
	char *ifile, *ofile;
	extern char *optarg;
	extern int optind, optopt;

	. . .

	while ((c = getopt(argc, argv, ":abf:o:")) != -1) {
		switch(ch) {
		case 'a':
			if (bflg)
				errflg = 1;
			else
				aflg = 1;
			break;
		case 'b':
			if (aflg)
				errflg = 1;
			else
				bflg = 1;
			break;
		case 'f':
			ifile = optarg;
			break;
		case 'o':
			ofile = optarg;
			break;
		case ':':	/* -f or -o without option-arg */
			fprintf (stderr,
				"Option -%c requires an option-argument\\n",
				optopt);
			errflg = 1;
			break;
		case '?':
			fprintf (stderr,
				"Unrecognized option: -%c\\n",
				optopt);
			errflg = 1;
			break;
		}
	}

	if (errflg) {
		fprintf (stderr, "usage: . . .\\n");
		exit (2);
	}

	argc -= optind;
	argv += optind;

	. . .

}
.Ed
.Sh STANDARDS
The
.Fn getopt
function conforms to
.St -p1003.2-92 .
.Sh HISTORY
The
.Fn getopt
function appeared
.Bx 4.3 .
.Sh BUGS
The
.Fn getopt
function was once specified to return
.Dv EOF 
instead of \-1.
This was changed by
.St -p1003.2-92
to decouple 
.Fn getopt
from 
.Pa <stdio.h> .
.Pp
Option arguments are allowed to begin with
.Dq Li \- ;
this is reasonable but
reduces the amount of error checking possible.
.Pp
A single dash
.Dq Li -
may be specified as an character in
.Fa optstring ,
however it should
.Em never
have an argument associated with it.
This allows
.Fn getopt
to be used with programs that expect
.Dq Li -
as an option flag.
This practice is wrong, and should not be used in any current development.
It is provided for backward compatibility
.Em only .
By default, a single dash causes
.Fn getopt
to returns \-1.
This is, we believe, compatible with System V.
.Pp
It is also possible to handle digits as option letters.
This allows
.Fn getopt
to be used with programs that expect a number
.Pq Dq Li \&-\&3
as an option.
This practice is wrong, and should not be used in any current development.
It is provided for backward compatibility
.Em only .
The following code fragment works fairly well.
.Bd -literal -offset indent
int length;
char *p;

while ((c = getopt(argc, argv, "0123456789")) != -1)
	switch (c) {
	case '0': case '1': case '2': case '3': case '4':
	case '5': case '6': case '7': case '8': case '9':
		p = argv[optind - 1];
		if (p[0] == '-' && p[1] == ch && !p[2])
			length = atoi(++p);
		else
			length = atoi(argv[optind] + 1);
		break;
	}
}
.Ed