Portable, reentrant, getopt-like option parser
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README.md

Optparse

Optparse is a public domain, portable, reentrant, embeddable, getopt-like option parser. It's a single source and header file, so it can be trivially dropped into any project. It supports POSIX getopt optstrings, GNU-style long options, argument permutation, and subcommand processing.

Why not getopt?

The POSIX getopt option parser has three fatal flaws. These flaws are solved by Optparse.

  1. The getopt parser state is stored entirely in global variables, some of which are static and inaccessible. This means only one thread can use getopt. It also means it's not possible to recursively parse nested sub-arguments while in the middle of argument parsing. Optparse fixes this by storing all state on a local struct.

  2. The POSIX standard provides no way to properly reset the parser. For portable code this means getopt is only good for one run, over one argv with one optstring. It also means subcommand options cannot be reliably processed with getopt. Most implementations provide an implementation-specific method to reset the parser, but this is not portable. Optparse provides an optparse_arg() function for stepping through non-option arguments, and parsing of options can continue again at any time with a different optstring. The Optparse struct itself could be passed around to subcommand handlers for additional subcommand option parsing. If a full parser reset is needed, optparse_init() can be called again.

  3. In getopt, error messages are printed to stderr. This can be disabled with opterr, but the messages themselves are still inaccessible. Optparse solves this by writing the error message to its errmsg field, which can be printed to anywhere. The downside to Optparse is that this error message will always be in English rather than the current locale.

Permutation

By default, argv is permuted as it is parsed, moving non-option arguments to the end of the array. This can be disabled by setting the permute field to 0 after initialization.

Drop-in Replacement

Optparse's interface should be familiar with anyone accustomed to getopt. It's nearly a drop-in replacement. The optstring has the same format and the parser struct fields have the same names as the getopt global variables (optarg, optind, optopt).

The long option parser optparse_long() API is very similar to GNU's getopt_long() and can serve as a portable, embedded replacement.

Optparse does not allocate, so no cleanup is needed. Furthermore, Optparse has no dependencies, including libc itself, so it can be used in situations where the standard C library cannot.

See optparse.h for full API documentation.

Example Usage

Here's a traditional getopt setup.

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

int main(int argc, char **argv)
{
    bool amend = false;
    bool brief = false;
    const char *color = "white";
    int delay = 0;

    int option;
    while ((option = getopt(argc, argv, "abc:d::")) != -1) {
        switch (option) {
        case 'a':
            amend = true;
            break;
        case 'b':
            brief = true;
            break;
        case 'c':
            color = optarg;
            break;
        case 'd':
            delay = optarg ? atoi(optarg) : 1;
            break;
        case '?':
            exit(EXIT_FAILURE);
        }
    }

    /* Print remaining arguments. */
    for (; optind < argc; optind++)
        printf("%s\n", argv[optind]);
    return 0;
}

Here's the same thing translated to Optparse.

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include "optparse.h"

int main(int argc, char **argv)
{
    bool amend = false;
    bool brief = false;
    const char *color = "white";
    int delay = 0;

    struct optparse options;
    optparse_init(&options, argv);
    int option;
    while ((option = optparse(&options, "abc:d::")) != -1) {
        switch (option) {
        case 'a':
            amend = true;
            break;
        case 'b':
            brief = true;
            break;
        case 'c':
            color = options.optarg;
            break;
        case 'd':
            delay = options.optarg ? atoi(options.optarg) : 1;
            break;
        case '?':
            fprintf(stderr, "%s: %s\n", argv[0], options.errmsg);
            exit(EXIT_FAILURE);
        }
    }

    /* Print remaining arguments. */
    char *arg;
    while ((arg = optparse_arg(&options)))
        printf("%s\n", arg);
    return 0;
}

And here's a conversion to long options.

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include "optparse.h"

int main(int argc, char **argv)
{
    bool amend = false;
    bool brief = false;
    const char *color = "white";
    int delay = 0;

    struct optparse options;
    optparse_init(&options, argv);
    struct optparse_long longopts[] = {
        {"amend", 'a', OPTPARSE_NONE},
        {"brief", 'b', OPTPARSE_NONE},
        {"color", 'c', OPTPARSE_REQUIRED},
        {"delay", 'd', OPTPARSE_OPTIONAL},
        {0}
    };
    int option;
    while ((option = optparse_long(&options, longopts, NULL)) != -1) {
        switch (option) {
        case 'a':
            amend = true;
            break;
        case 'b':
            brief = true;
            break;
        case 'c':
            color = optarg;
            break;
        case 'd':
            delay = optarg ? atoi(options.optarg) : 1;
            break;
        case '?':
            fprintf(stderr, "%s: %s\n", argv[0], options.errmsg);
            exit(EXIT_FAILURE);
        }
    }

    /* Print remaining arguments. */
    char *arg;
    while ((arg = optparse_arg(&options)))
        printf("%s\n", arg);
    return 0;
}