libc/stdlib/getopt.3

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.\"     from: @(#)getopt.3	6.16 (Berkeley) 4/19/91
.\"	$Id: getopt.3,v 1.3 1993/09/14 22:37:24 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 <stdlib.h>
.Vt extern char *optarg
.Vt extern int   optind
.Vt extern int   opterr
.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
.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, 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 \-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 a character not found in the string
.Va optarg
or detects
a missing option argument
it writes error message
.Ql ?
to the
.Em stderr .
Setting
.Va opterr
to a zero will disable these error messages.
.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: unable to read file %s.\en", optarg);
			exit(1) ;
		}
		break;
	case '?':
	default:
		usage();
}
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