getopt does not parse optional arguments to parameters

Question

In C, getopt_long does not parse the optional arguments to command line parameters parameters.

When I run the program, the optional argument is not recognized like the example run below.

$ ./respond --praise John
Kudos to John
$ ./respond --blame John
You suck !
$ ./respond --blame
You suck !

Here is the test code.

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

int main(int argc, char ** argv )
{
    int getopt_ret, option_index;
    static struct option long_options[] = {
               {"praise",  required_argument, 0, 'p'},
               {"blame",  optional_argument, 0, 'b'},
               {0, 0, 0, 0}       };
    while (1) {
        getopt_ret = getopt_long( argc, argv, "p:b::",
                                  long_options,  &option_index);
        if (getopt_ret == -1) break;

        switch(getopt_ret)
        {
            case 0: break;
            case 'p':
                printf("Kudos to %s\n", optarg); break;
            case 'b':
                printf("You suck ");
                if (optarg)
                    printf (", %s!\n", optarg);
                else
                    printf ("!\n", optarg);
                break;
            case '?':
                printf("Unknown option\n"); break;
        }
    } 
    return 0;
}

Solution

I recently came across this issue myself. I arrived at a similar solution to the one Brian Vandenberg and Haystack suggested. But to improve readability and avoid code duplication, you can wrap it all up in a macro like below:

#define OPTIONAL_ARGUMENT_IS_PRESENT \
    ((optarg == NULL && optind < argc && argv[optind][0] != '-') \
     ? (bool) (optarg = argv[optind++]) \
     : (optarg != NULL))

The macro can be used like this:

case 'o': // option with optional argument
    if (OPTIONAL_ARGUMENT_IS_PRESENT)
    {
        // Handle is present
    }
    else
    {
        // Handle is not present
    }
    break;

If you are interested, you can read more about how this solution works in a blog post I wrote: https://cfengine.com/blog/2021/optional-arguments-with-getopt-long/

This solution is tested and is – at the time of this writing – currently used in CFEngine.