extern char *optarg; extern int optind, opterr, optopt;
#define _GNU_SOURCE #include <getopt.h>
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 ''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 character 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 variable 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.
The getopt_long() function works like getopt() except that it also accepts long options, started out by two dashes. 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 as
The meanings of the different fields are:
is the name of the long option.
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.
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.
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 longopts.
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.
The getopt() function returns the option character if the option was found successfully, `:' if there was a missing parameter for one of the options, `?' for an unknown option character, or -1 for the end of the option list.
If this is set, then option processing stops as soon as a non-option argument is encountered.
The following example program illustrates the use of getopt_long() with most of its features.
- include <getopt.h>
int c; int digit_optind = 0;
- printf ("option a\n"); break;
- printf ("option b\n"); break;
- printf ("option c with value `%s'\n", optarg); break;
POSIX.2, provided the environment variable POSIXLY_CORRECT is set. Otherwise, the elements of argv aren't really const, because we permute them. We pretend they're const in the prototype to be compatible with other systems.
If you want to use getopt more than once within a program, across different inputs, you'll need to reset its internal state counters before each use:
printf("%d %d %d\n",optind,opterr,optopt); ... do getopt stuff ... printf("%d %d %d\n",optind,opterr,optopt);
This outputs the following:
1 1 63 3 1 63
the '3' in my case is because I only had one option + parameter (+ program name == 3).
So, if you reset optind, opterr and optopt to 1, 1 and 63, before each new use of getopt, you should be fine.