Getopt optional arguments?

Question

I have a program where you enter an option -d and then whether or not you supply a non-optional argument after the option, do something.
Heres my code:

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

#define OPT_LIST "d::" 

int main (int argc, char *argv[])
{
    int c;
    char string[] = "blah";

    while ((c = getopt (argc, argv, OPT_LIST)) != -1)
    {
        switch (c)
        {
            case 'd':
                    printf("%s\n", optarg);
                    break;

            case '?':
                fprintf(stderr, "invalid option\n");
                exit(EXIT_FAILURE);
        }   
    }
}

So if you enter a non-optional argument after the option, it prints the argument. But I want it to print out the char "string" if the user doesn't supply a non-optional argument (this is why I put the double colon in the OPT_LIST). But I'm not sure how to do this so any help would be greatly appreciated.

Heres what happens when I run the program:

user:desktop shaun$ ./arg -d hello
hello
user:desktop shaun$ ./arg -d 
./arg: option requires an argument -- d
invalid option

I'm running a Mac with OS X using C language.

Answer 1

The "optional value of an option" feature is only a GNU libc extension, not required by POSIX, and is probably simply unimplemented by the libc shipped with Mac OS X.

The options argument is a string that specifies the option characters that are valid for this program. An option character in this string can be followed by a colon (‘:’) to indicate that it takes a required argument. If an option character is followed by two colons (‘::’), its argument is optional; this is a GNU extension.

https://www.gnu.org/software/libc/manual/html_node/Using-Getopt.html

In fact, POSIX.1-2008, section 12.2, "Utility Syntax Guidelines", explicitly forbids this feature:

Guideline 7: Option-arguments should not be optional.

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html#tag_12_02