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.\"	$OpenBSD: getopt.3,v 1.47 2021/10/24 21:24:20 deraadt Exp $
.\"
.Dd $Mdocdate: October 24 2021 $
.Dt GETOPT 3
.Os
.Sh NAME
.Nm getopt
.Nd get option character from command line argument list
.Sh SYNOPSIS
.In unistd.h
.Vt extern char *optarg;
.Vt extern int   opterr;
.Vt extern int   optind;
.Vt extern int   optopt;
.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,
characters followed by a colon, and characters followed by two colons.
A character followed by a single colon indicates that an argument
is to follow the option on the command line.
Two colons indicates that the argument is optional \- this is an
extension not covered by POSIX.
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; except in the case where
the argument is optional, denoted with two colons, no leading whitespace
is permitted.
.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 variables
.Va opterr
and
.Va optind
are both initialized to 1.
The
.Va optind
variable may be set to another value larger than 0 before a set of calls to
.Fn getopt
in order to skip over more or less
.Fa argv
entries.
An
.Va optind
value of 0 is reserved for compatibility with GNU
.Fn getopt .
.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
.Sq \&?
(question mark).
If
.Fa optstring
has a leading
.Sq \&:
then a missing option argument causes
.Sq \&:
to be returned instead of
.Sq \&? .
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
The following code accepts the options
.Fl b
and
.Fl f Ar argument
and adjusts
.Va argc
and
.Va argv
after option argument processing has completed.
.Bd -literal -offset indent
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)) == -1)
			err(1, "%s", optarg);
		break;
	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
.Em 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
.Ql - ;
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
.Fn getopt
function implements a superset of the functionality specified by
.St -p1003.1 .
.Pp
The following extensions are supported:
.Bl -bullet
.It
The
.Va optreset
variable was added to make it possible to call the
.Fn getopt
function multiple times.
.It
If the
.Va optind
variable is set to 0,
.Fn getopt
will behave as if the
.Va optreset
variable has been set.
This is for compatibility with
.Tn GNU
.Fn getopt .
New code should use
.Va optreset
instead.
.It
If the first character of
.Fa optstring
is a plus sign
.Pq Ql + ,
it will be ignored.
This is for compatibility with
.Tn GNU
.Fn getopt .
.It
If the first character of
.Fa optstring
is a dash
.Pq Ql - ,
non-options will be returned as arguments to the option character
.Ql \e1 .
This is for compatibility with
.Tn GNU
.Fn getopt .
.It
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 .
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
semantics (see above).
By default, a single dash causes
.Fn getopt
to return \-1.
.El
.Pp
Historic
.Bx
versions of
.Fn getopt
set
.Fa optopt
to the last option character processed.
However, this conflicts with
.St -p1003.1
which stipulates that
.Fa optopt
be set to the last character that caused an error.
.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
It is 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 and can handle mixed
number and letter arguments.
.Bd -literal -offset indent
int aflag = 0, bflag = 0, ch, lastch = '\e0';
int length = -1, newarg = 1, prevoptind = 1;

while ((ch = getopt(argc, argv, "0123456789ab")) != -1) {
	switch (ch) {
	case '0': case '1': case '2': case '3': case '4':
	case '5': case '6': case '7': case '8': case '9':
		if (newarg || !isdigit(lastch))
			length = 0;
		else if (length > INT_MAX / 10)
			usage();
		length = (length * 10) + (ch - '0');
		break;
	case 'a':
		aflag = 1;
		break;
	case 'b':
		bflag = 1;
		break;
	default:
		usage();
	}
	lastch = ch;
	newarg = optind != prevoptind;
	prevoptind = optind;
}
.Ed