Getopt in C

References

C Programming using getopt Tutorial in Linux

https://man7.org/linux/man-pages/man3/getopt.3.html

If there are no more option characters, getopt() returns -1. Then optind is the index in argv of the first argv-element that is not an option.

If an option was successfully found, then getopt() returns the option character. If all command-line options have been parsed, then getopt() returns -1. If getopt() encounters an option character that was not in optstring, then '?' is returned. If getopt() encounters an option with a missing argument, then the return value depends on the first character in optstring: if it is ':', then ':' is returned; otherwise '?' is returned.

Includes

In addition to your other includes, add in #include <getopt.h>:

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

Example Program

Here’s an examply program with an option with a required argument, and one without.

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

int main(int argc, char **argv)
{
  int option;
  while ((option = getopt(argc, argv, "a:b")) != -1) {
    printf("debug: option=%d, ('%c')\n", option, option);
    switch (option) {
      case 'a':
        printf("Specified -%c option with required optarg=%s\n", option, optarg);
        break;
      case 'b':
        printf("Specified -%c option, no required optarg\n", option);
        break;
      default:
        printf("unrecognized option\n");
    }
  }
  printf("debug: getopt() returned -1\n");

  return 0;
}

Compile it:

gcc -o options ./options.c

Running it a few different ways:

➜  getopt ./options -a 1234 -b
debug: option=97, ('a')
Specified -a option with required optarg=1234
debug: option=98, ('b')
Specified -b option, no required optarg
debug: getopt() returned -1

➜  getopt ./options hello -b
debug: option=98, ('b')
Specified -b option, no required optarg
debug: getopt() returned -1

➜  getopt ./options hello -a
./options: option requires an argument -- 'a'
debug: option=63, ('?')
unrecognized option
debug: getopt() returned -1

➜  getopt ./options hello -x
./options: invalid option -- 'x'
debug: option=63, ('?')
unrecognized option
debug: getopt() returned -1

➜  getopt ./options
debug: getopt() returned -1