Parser for argv that works similarly to getopt
C CMake
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Most command-line programs have to parse options, so there are a lot of different solutions to this problem. Some offer many features, while others are more basic.

One of the simpler solutions for C is the getopt function, and its extension getopt_long. They iterate over the options in argv, returning them one at a time on successive calls.

One nice thing about them is that they are available on most Unix-like operating systems (and usually accompany GCC elsewhere, like Windows). Unfortunately, some implementation details vary between platforms.

A potential question is what license the version you get when you include them is available under. Some are GPL, others LGPL. There are also ports of getopt that use more liberal licenses.

parg is a parser for argv that works similarly to getopt, but does not aim to be a direct replacement. It attempts to make some choices about how to handle the extensions and idiosyncrasies of other getopt implementations, and document them.

It consists of a single source and include file, written in portable ANSI C. It is made available under CC0.


The include file parg.h contains documentation in the form of doxygen comments. A configuration file is included, so you can simply run doxygen to generate documentation in HTML format.

parg uses CMake to generate build systems. To create one for the tools on your platform, and build parg, use something along the lines of:

mkdir build
cd build
cmake ..
cmake --build .

You can also simply compile the source files and link them into your project. CMake just provides an easy way to build and test across various platforms and toolsets.


Here is an example that parses command-line options using parg_getopt():

#include <stdio.h>
#include <stdlib.h>

#include "parg.h"

int main(int argc, char *argv[])
	struct parg_state ps;
	int c;


	while ((c = parg_getopt(&ps, argc, argv, "hs:v")) != -1) {
		switch (c) {
		case 1:
			printf("nonoption '%s'\n", ps.optarg);
		case 'h':
			printf("Usage: testparg [-h] [-v] [-s STRING]\n");
			return EXIT_SUCCESS;
		case 's':
			printf("option -s with argument '%s'\n", ps.optarg);
		case 'v':
			printf("testparg 1.0.0\n");
			return EXIT_SUCCESS;
		case '?':
			if (ps.optopt == 's') {
				printf("option -s requires an argument\n");
			else {
				printf("unknown option -%c\n", ps.optopt);
			return EXIT_FAILURE;
			printf("error: unhandled option -%c\n", c);
			return EXIT_FAILURE;

	for (c = ps.optind; c < argc; ++c) {
		printf("nonoption '%s'\n", argv[c]);


Comparison to getopt

Use of global variables

getopt uses global variables to store its state between calls. parg uses a struct parg_state, which you must pass with each call.

Handling of nonoptions

POSIX and BSD getopt return -1 on the first nonoption argument. GNU getopt by default reorders argv (even though it is passed as const), so all options come first.

parg does not change argv, and returns each nonoption as the option argument of an option with value 1 (like GNU getopt, if optstring were prefixed by '-').

If you wish to process all options first, and have the nonoptions ordered at the end of argv, you can use parg_reorder():

	optend = parg_reorder(argc, argv, optstring, NULL);

	while ((c = parg_getopt(&ps, optend, argv, optstring)) != -1) {
		/* ... */

	/* elements of argv[] from optend to argc are nonoptions */

Value of optind on error

When there are multiple short options in one argument, getopt does not increment optind until the last one is processed. This makes it harder to tell which argument an unknown option came from (if a is an unknown option, -a and -ab will return '?' with different values in optind).

parg always increments the optind value in it's state so it points to the next argv element to be processed. So when parg returns '?' (or ':'), the element that contains the error is argv[optind - 1].

Value of optopt on error

With getopt_long, it varies what the values of optopt and longindex are when an error is found with option arguments of long options. Sometimes these values are not documented.

parg sets optopt to val if flag is NULL, and 0 otherwise (which equals the return value on successful match), and longindex is set to the index of the entry in longopts that matched.

Return value on option argument error

When the first character of optstring is ':', it varies what getopt returns on extraneous option arguments.

In this case, parg returns '?' if no option match is found, and ':' if a match is found, but is missing a required argument, or has an extraneous argument.


Some ports of getopt:

Other command-line parsing libraries that support C:

A few C++ command-line parsing libraries: