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.\"	$OpenBSD: getopt.3,v 1.13 2000/04/20 13:50:02 aaron Exp $
.\"
.Dd April 19, 1994
.Dt GETOPT 3
.Os
.Sh NAME
.Nm getopt
.Nd get option character from command line argument list
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Vt extern char *optarg;
.Vt extern int   optind;
.Vt extern int   optopt;
.Vt extern int   opterr;
.Vt extern int   optreset;
.Ft int
.Fn getopt "int argc" "char * const *argv" "const char *optstring"
.Sh DESCRIPTION
The
.Fn getopt
function incrementally parses a command line argument list
.Fa argv
and returns the next known option character.
An option character is
.Dq 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 elements: individual characters and
characters followed by a colon to indicate an option argument
is to follow.
For example, an option string
.Qq x
recognizes an option
.Fl x ,
and an option string
.Qq Li x:
recognizes an option and argument
.Fl x Ar argument .
It does not matter to
.Fn getopt
if a following argument has leading whitespace.
.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 .
The variable
.Va optopt
saves the last known option character returned by
.Fn getopt .
.Pp
The variables
.Va opterr
and
.Va optind
are both initialized to 1.
The
.Va optind
variable may be set to another value before a set of calls to
.Fn getopt
in order to skip over more or less argv entries.
.Pp
In order to use
.Fn getopt
to evaluate multiple sets of arguments, or to evaluate a single set of
arguments multiple times,
the variable
.Va optreset
must be set to 1 before the second and each additional set of calls to
.Fn getopt ,
and the variable
.Va optind
must be reinitialized.
.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 returns \-1.
When all options have been processed (i.e., up to the first non-option
argument),
.Fn getopt
returns \-1.
.Sh SEE ALSO
.Xr getopt 1 ,
.Xr getsubopt 3
.Sh DIAGNOSTICS
If the
.Fn getopt
function encounters a character not found in the string
.Va optstring
or detects
a missing option argument it writes an error message to
.Em stderr
and returns
.Ql ? .
Setting
.Va opterr
to a zero will disable these error messages.
If
.Va optstring
has a leading
.Ql \&:
then a missing option argument causes a
.Ql \&:
to be returned in addition to suppressing any error messages.
.Pp
Option arguments are allowed to begin with
.Ql - ;
this is reasonable but reduces the amount of error checking possible.
.Sh EXTENSIONS
The
.Va optreset
variable was added to make it possible to call the
.Fn getopt
function multiple times.
This is an extension to the
.St -p1003.2
specification.
.Sh EXAMPLE
.Bd -literal -compact
extern char *optarg;
extern int optind;
int bflag, ch, fd;

bflag = 0;
while ((ch = getopt(argc, argv, "bf:")) != -1) {
	switch (ch) {
	case 'b':
		bflag = 1;
		break;
	case 'f':
		if ((fd = open(optarg, O_RDONLY, 0)) < 0) {
			(void)fprintf(stderr,
			    "myname: %s: %s\en", optarg, strerror(errno));
			exit(1);
		}
		break;
	case '?':
	default:
		usage();
	}
}
argc -= optind;
argv += optind;
.Ed
.Sh HISTORY
The
.Fn getopt
function appeared in
.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
A single dash
.Pq Ql -
may be specified as a 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
.Ql -
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 return \-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 in most cases.
.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