xref: /freebsd/lib/libc/stdlib/getopt.3 (revision dc36d6f9bb1753f3808552f3afd30eda9a7b206a)

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.Dd June 5, 2014
.Dt GETOPT 3
.Os
.Sh NAME
.Nm getopt
.Nd get option character from command line argument list
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In 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
.Em known
option character.
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 elements: individual characters, and
characters followed by a colon to indicate an option argument
is to follow.
If an individual character is followed by two colons, then the
option argument is optional;
.Va optarg
is set to the rest of the current
.Va argv
word, or
.Dv NULL
if there were no more characters in the current word.
This is a
.Tn GNU
extension.
For example, an option string
.Li \&"x"
recognizes an option
.Dq Fl x ,
and an option string
.Li \&"x:"
recognizes an option and argument
.Dq Fl x Ar argument .
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 .
The variable
.Va optopt
saves the last
.Em 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 return \-1.
When all options have been processed (i.e., up to the first non-option
argument),
.Fn getopt
returns \-1.
.Sh RETURN VALUES
The
.Fn getopt
function returns the next known option character in
.Fa optstring .
If
.Fn getopt
encounters a character not found in
.Fa optstring
or if it detects a missing option argument,
it returns
.Ql \&?
(question mark).
If
.Fa optstring
has a leading
.Ql \&:
then a missing option argument causes
.Ql \&:
to be returned instead of
.Ql \&? .
In either case, the variable
.Va optopt
is set to the character that caused the error.
The
.Fn getopt
function returns \-1 when the argument list is exhausted.
.Sh EXAMPLES
.Bd -literal -compact
#include <unistd.h>
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)) \*[Lt] 0) {
			(void)fprintf(stderr,
			    "myname: %s: %s\en", optarg, strerror(errno));
			exit(1);
		}
		break;
	case '?':
	default:
		usage();
	}
}
argc -= optind;
argv += optind;
.Ed
.Sh DIAGNOSTICS
If the
.Fn getopt
function encounters a character not found in the string
.Fa optstring
or detects
a missing option argument it writes an error message to the
.Dv stderr
and returns
.Ql \&? .
Setting
.Va opterr
to a zero will disable these error messages.
If
.Fa 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
.Dq Li \- ;
this is reasonable but reduces the amount of error checking possible.
.Sh SEE ALSO
.Xr getopt 1 ,
.Xr getopt_long 3 ,
.Xr getsubopt 3
.Sh STANDARDS
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 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
.In stdio.h .
.Pp
A single dash
.Dq Li -
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
.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 .
Care should be taken not to use
.Ql \&-
as the first character in
.Fa optstring
to avoid a semantic conflict with
.Tn GNU
.Fn getopt ,
which assigns different meaning to an
.Fa optstring
that begins with a
.Ql \&- .
By default, a single dash causes
.Fn getopt
to return \-1.
.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 ch;
long length;
char *p, *ep;

while ((ch = getopt(argc, argv, "0123456789")) != -1)
	switch (ch) {
	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] == '-' \*[Am]\*[Am] p[1] == ch \*[Am]\*[Am] !p[2]) {
			length = ch - '0';
			ep = "";
		} else if (argv[optind] \*[Am]\*[Am] argv[optind][1] == ch) {
			length = strtol((p = argv[optind] + 1),
			    \*[Am]ep, 10);
			optind++;
			optreset = 1;
		} else
			usage();
		if (*ep != '\e0')
			errx(EX_USAGE, "illegal number -- %s", p);
		break;
	}
.Ed