.\" Copyright (c) 1988, 1991 Regents of the University of California.
.\" All rights reserved.
.\"
.\" %sccs.include.redist.man%
.\"
.\"     @(#)getopt.3	6.17 (Berkeley) 07/06/92
.\"
.Dd 
.Dt GETOPT 3
.Os BSD 4.3
.Sh NAME
.Nm getopt
.Nd get option letter from argv
.Sh SYNOPSIS
.Fd #include <stdlib.h>
.Vt extern char *optarg
.Vt extern int   optind
.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 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 the function
.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.
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 an
.Dv EOF
when the argument list is exhausted, or a non-recognized
option is encountered.
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 an
.Dv EOF . 
When all options have been processed (i.e., up to the first non-option
argument),
.Fn getopt
returns
.Dv EOF .
.Sh DIAGNOSTICS
If the
.Fn getopt
function encounters a character not found in the string
.Va optarg
or detects
a missing option argument it writes an error message and returns
.Ql ?
to the
.Em stderr .
Setting
.Va opterr
to a zero will disable these error messages.
If
.Va optstring 
has a leading 
.Ql \&:
then then a missing option argumet causes a
.Ql \&:
to be returned in addition to supressing any error messages.
option argument
.Pp
Option arguments are allowed to begin with
.Dq Li \- ;
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:")) != EOF)
	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
.Bx 4.3 .
.Sh BUGS
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 return
.Dv EOF .
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")) != EOF)
	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