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.\"     @(#)getopt.3	6.16 (Berkeley) 4/19/91
.\"
.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 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 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:")) != EOF)
	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 HISTORY
The
.Fn getopt
function appeared
.Bx 4.3 .
.Sh BUGS
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 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 fairly well.
.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