getopt.h

#include <string.h>
#include <stdio.h>

// global
int opterr = 1, /* if error message should be printed */
optind = 1, /* index into parent argv vector */
optopt, /* character checked for validity */
optreset; /* reset getopt */
const char *optarg; /* argument associated with option */

#define BADCH (int)'?'
#define BADARG (int)':'
#define EMSG ""

/*
 * getopt --
 *      Parse argc/argv argument vector.
 */
int
getopt (int nargc, char * const nargv[], const char *ostr)
{
	static const char *place = EMSG;              /* option letter processing */
	const char *oli;                        /* option letter list index */

	if (optreset || !*place) {              /* update scanning pointer */
		optreset = 0;
		if (optind >= nargc || *(place = nargv[optind]) != '-') {
			place = EMSG;
			return (-1);
		}
		if (place[1] && *++place == '-') {      /* found "--" */
			++optind;
			place = EMSG;
			return (-1);
		}
	}                                       /* option letter okay? */
	if ((optopt = (int)*place++) == (int)':' ||
		!(oli = strchr (ostr, optopt))) {
		/*
		* if the user didn't specify '-' as an option,
		* assume it means -1.
		*/
		if (optopt == (int)'-')
			return (-1);
		if (!*place)
			++optind;
		if (opterr && *ostr != ':')
			(void)printf ("illegal option -- %c\n", optopt);
		return (BADCH);
	}
	if (*++oli != ':') {                    /* don't need argument */
		optarg = NULL;
		if (!*place)
			++optind;
	}
	else {                                  /* need an argument */
		if (*place)                     /* no white space */
			optarg = place;
		else if (nargc <= ++optind) {   /* no arg */
			place = EMSG;
			if (*ostr == ':')
				return (BADARG);
			if (opterr)
				(void)printf ("option requires an argument -- %c\n", optopt);
			return (BADCH);
		}
		else                            /* white space */
			optarg = nargv[optind];
		place = EMSG;
		++optind;
	}
	return optopt;        /* dump back option letter */
}