getopt.h / getopt.c, from http://www.raspberryginger.com/jbailey/minix/html/lib_2posix_2getopt_8c-source.html

getopt.c

// Put this in a separate .h file (called "getopt.h").
// The prototype for the header file is:
/*
#ifndef GETOPT_H
#define GETOPT_H

int getopt(int nargc, char * const nargv[], const char *ostr) ;

#endif
*/

#include "getopt.h" // make sure you construct the header file as dictated above

/*
* Copyright (c) 1987, 1993, 1994
*      The Regents of the University of California.  All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
*    notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
*    notice, this list of conditions and the following disclaimer in the
*    documentation and/or other materials provided with the distribution.
* 3. All advertising materials mentioning features or use of this software
*    must display the following acknowledgement:
*      This product includes software developed by the University of
*      California, Berkeley and its contributors.
* 4. Neither the name of the University nor the names of its contributors
*    may be used to endorse or promote products derived from this software
*    without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/

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

int     opterr = 1,             /* if error message should be printed */
        optind = 1,             /* index into parent argv vector */
        optopt,                 /* character checked for validity */
        optreset;               /* reset getopt */
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 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 */
}

I think you need to add extern char *optarg; in getopt.h as well to get that portion of the getopt interface to work.

Actually, according to how http://pubs.opengroup.org/onlinepubs/9699919799/functions/getopt.html describe the functionality, the header should look like this, perhaps optreset not but I guess it doesn't hurt to have it there:

ifndef GETOPT_H

define GETOPT_H

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

int getopt(int nargc, char * const nargv[], const char *ostr);

endif

Exactly what Marraz says!

#ifndef GETOPT_H

#define GETOPT_H

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

int getopt(int nargc, char * const nargv[], const char *ostr);

#endif