source: bootcd/isolinux/syslinux-6.03/com32/lib/getopt.c

/*
 * getopt.c
 *
 * Simple POSIX getopt(), no GNU extensions...
 */

#include <stdint.h>
#include <unistd.h>
#include <string.h>

char *optarg;
int optind, opterr, optopt;
static struct getopt_private_state {
        const char *optptr;
        const char *last_optstring;
        char *const *last_argv;
} pvt;

int getopt(int argc, char *const *argv, const char *optstring)
{
        const char *carg;
        const char *osptr;
        int opt;

        /* getopt() relies on a number of different global state
           variables, which can make this really confusing if there is
           more than one use of getopt() in the same program.  This
           attempts to detect that situation by detecting if the
           "optstring" or "argv" argument have changed since last time
           we were called; if so, reinitialize the query state. */

        if (optstring != pvt.last_optstring || argv != pvt.last_argv ||
            optind < 1 || optind > argc) {
                /* optind doesn't match the current query */
                pvt.last_optstring = optstring;
                pvt.last_argv = argv;
                optind = 1;
                pvt.optptr = NULL;
        }

        carg = argv[optind];

        /* First, eliminate all non-option cases */

        if (!carg || carg[0] != '-' || !carg[1]) {
                return -1;
        }

        if (carg[1] == '-' && !carg[2]) {
                optind++;
                return -1;
        }

        if ((uintptr_t) (pvt.optptr - carg) > (uintptr_t) strlen(carg)) {
                /* Someone frobbed optind, change to new opt. */
                pvt.optptr = carg + 1;
        }

        opt = *pvt.optptr++;

        if (opt != ':' && (osptr = strchr(optstring, opt))) {
                if (osptr[1] == ':') {
                        if (*pvt.optptr) {
                                /* Argument-taking option with attached
                                   argument */
                                optarg = (char *)pvt.optptr;
                                optind++;
                        } else {
                                /* Argument-taking option with non-attached
                                   argument */
                                if (argv[optind + 1]) {
                                        optarg = (char *)argv[optind+1];
                                        optind += 2;
                                } else {
                                        /* Missing argument */
                                        optind++;
                                        return (optstring[0] == ':')
                                                ? ':' : '?';
                                }
                        }
                        return opt;
                } else {
                        /* Non-argument-taking option */
                        /* pvt.optptr will remember the exact position to
                           resume at */
                        if (!*pvt.optptr)
                                optind++;
                        return opt;
                }
        } else {
                /* Unknown option */
                optopt = opt;
                if (!*pvt.optptr)
                        optind++;
                return '?';
        }
}