From Linux Man Pages
getopt - Parse command-line options
SYNOPSIS
#include <unistd.h>
int getopt(int argc, char * const argv[],
const char *optstring);
extern char *optarg;
extern int optind, opterr, optopt;
#define _GNU_SOURCE
#include <getopt.h>
int getopt_long(int argc, char * const argv[],
const char *optstring,
const struct option *longopts, int *longindex);
int getopt_long_only(int argc, char * const argv[],
const char *optstring,
const struct option *longopts, int *longindex);
DESCRIPTION
The getopt() function parses the command-line arguments. Its arguments argc and argv are the argument count and
array as passed to the main() function on program invocation. An element of argv that starts with '-' (and is
not exactly "-" or "--") is an option element. The characters of this element (aside from the initial '-') are
option characters. If getopt() is called repeatedly, it returns successively each of the option characters from
each of the option elements.
If getopt() finds another option character, it returns that character, updating the external variable optind and
a static variable nextchar so that the next call to getopt() can resume the scan with the following option char-
acter or argv-element.
If there are no more option characters, getopt() returns -1. Then optind is the index in argv of the first argv-
element that is not an option.
optstring is a string containing the legitimate option characters. If such a character is followed by a colon,
the option requires an argument, so getopt() places a pointer to the following text in the same argv-element, or
the text of the following argv-element, in optarg. Two colons mean an option takes an optional arg; if there is
text in the current argv-element, it is returned in optarg, otherwise optarg is set to zero. This is a GNU
extension. If optstring contains W followed by a semicolon, then -W foo is treated as the long option --foo.
(The -W option is reserved by POSIX.2 for implementation extensions.) This behaviour is a GNU extension, not
available with libraries before GNU libc 2.
By default, getopt() permutes the contents of argv as it scans, so that eventually all the non-options are at the
end. Two other modes are also implemented. If the first character of optstring is '+' or the environment vari-
able POSIXLY_CORRECT is set, then option processing stops as soon as a non-option argument is encountered. If
the first character of optstring is '-', then each non-option argv-element is handled as if it were the argument
of an option with character code 1. (This is used by programs that were written to expect options and other
argv-elements in any order and that care about the ordering of the two.) The special argument "--" forces an end
of option-scanning regardless of the scanning mode.
If getopt() does not recognize an option character, it prints an error message to stderr, stores the character in
optopt, and returns '?'. The calling program may prevent the error message by setting opterr to 0.
If getopt() finds an option character in argv that was not included in optstring, or if it detects a missing
option argument, it returns '?' and sets the external variable optopt to the actual option character. If the
first character (following any optional '+' or '-' described above) of optstring is a colon (':'), then getopt()
returns ':' instead of '?' to indicate a missing option argument. If an error was detected, and the first char-
acter of optstring is not a colon, and the external variable opterr is non-zero (which is the default), getopt()
prints an error message.
The getopt_long() function works like getopt() except that it also accepts long options, started out by two
dashes. (If the program accepts only long options, then optstring should be specified as an empty string (""),
not NULL.) Long option names may be abbreviated if the abbreviation is unique or is an exact match for some
defined option. A long option may take a parameter, of the form --arg=param or --arg param.
longopts is a pointer to the first element of an array of struct option declared in <getopt.h> as
struct option {
const char *name;
int has_arg;
int *flag;
int val;
};
The meanings of the different fields are:
name is the name of the long option.
has_arg
is: no_argument (or 0) if the option does not take an argument; required_argument (or 1) if the option
requires an argument; or optional_argument (or 2) if the option takes an optional argument.
flag specifies how results are returned for a long option. If flag is NULL, then getopt_long() returns val.
(For example, the calling program may set val to the equivalent short option character.) Otherwise,
getopt_long() returns 0, and flag points to a variable which is set to val if the option is found, but
left unchanged if the option is not found.
val is the value to return, or to load into the variable pointed to by flag.
The last element of the array has to be filled with zeroes.
If longindex is not NULL, it points to a variable which is set to the index of the long option relative to lon-
gopts.
getopt_long_only() is like getopt_long(), but '-' as well as '--' can indicate a long option. If an option that
starts with '-' (not '--') doesn't match a long option, but does match a short option, it is parsed as a short
option instead.
RETURN VALUE
If an option was successfully found, then getopt() returns the option character. If all command-line options
have been parsed, then getopt() returns -1. If getopt() encounters an option character that was not in opt-
string, then '?' is returned. If getopt() encounters an option with a missing argument, then the return value
depends on the first character in optstring: if it is ':', then ':' is returned; otherwise '?' is returned.
getopt_long() and getopt_long_only() also return the option character when a short option is recognized. For a
long option, they return val if flag is NULL, and 0 otherwise. Error and -1 returns are the same as for
getopt(), plus '?' for an ambiguous match or an extraneous parameter.
ENVIRONMENT VARIABLES
POSIXLY_CORRECT
If this is set, then option processing stops as soon as a non-option argument is encountered.
_<PID>_GNU_nonoption_argv_flags_
This variable was used by bash 2.0 to communicate to GNU libc which arguments are the results of wildcard
expansion and so should not be considered as options. This behaviour was removed in bash version 2.01,
but the support remains in GNU libc.
EXAMPLE
The following example program illustrates the use of getopt_long() with most of its features.
#include <stdio.h> /* for printf */
#include <stdlib.h> /* for exit */
#include <getopt.h>
int
main (int argc, char **argv) {
int c;
int digit_optind = 0;
while(1) {
int this_option_optind = optind ? optind : 1;
int option_index = 0;
static struct option long_options[] = {
{"add", 1, 0, 0},
{"append", 0, 0, 0},
{"delete", 1, 0, 0},
{"verbose", 0, 0, 0},
{"create", 1, 0, 'c'},
{"file", 1, 0, 0},
{0, 0, 0, 0}
};
c = getopt_long (argc, argv, "abc:d:012",
long_options, &option_index);
if (c == -1)
break;
switch (c) {
case 0:
printf ("option %s", long_options[option_index].name);
if (optarg)
printf (" with arg %s", optarg);
printf ("\n");
break;
case '0':
case '1':
case '2':
if (digit_optind != 0 && digit_optind != this_option_optind)
printf ("digits occur in two different argv-elements.\n");
digit_optind = this_option_optind;
printf ("option %c\n", c);
break;
case 'a':
printf ("option a\n");
break;
case 'b':
printf ("option b\n");
break;
case 'c':
printf ("option c with value '%s'\n", optarg);
break;
case 'd':
printf ("option d with value '%s'\n", optarg);
break;
case '?':
break;
default:
printf ("?? getopt returned character code 0%o ??\n", c);
}
}
if (optind < argc) {
printf ("non-option ARGV-elements: ");
while (optind < argc)
printf ("%s ", argv[optind++]);
printf ("\n");
}
exit(0);
}
BUGS
The POSIX.2 specification of getopt() has a technical error described in POSIX.2 Interpretation 150. The GNU
implementation (and probably all other implementations) implements the correct behaviour rather than that speci-
fied.
CONFORMING TO
getopt():
POSIX.2 and POSIX.1-2001, provided the environment variable POSIXLY_CORRECT is set. Otherwise, the ele-
ments of argv aren't really const, because we permute them. We pretend they're const in the prototype to
be compatible with other systems.
On some older implementations, getopt() was declared in <stdio.h>. SUSv1 permitted the declaration to
appear in either <unistd.h> or <stdio.h>. POSIX.1-2001 marked the use of <stdio.h> for this purpose as
LEGACY. POSIX.1-2001 does not allow the declaration to appear in <stdio.h>.
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