getopt_long long options work but not short options

Question

Possible Duplicate:
getopt_long() — proper way to use it?

I'm struggling with getopt_long in my C program. Code:

const struct option long_options[] = { 
    { "help", 0, NULL, 'h' },
    { "num", 1, NULL, 'n' },
    { NULL, 0, NULL, 0 } 
};  
do {
    next_option = getopt_long(argc, argv, short_options, 
        long_options, NULL);
    switch(next_option) {
        case 'h':
            print_usage(stdout, 0); 
        case 'n':
            printf("num %s\n", optarg);
            break;
        case '?':
            print_usage(stderr, 1); 
            break;
        default:
            abort();
    }   
} while(next_option != -1);

This works:

./a.out --num 3
num 3

This works (why?!):

./a.out --n 3            
num 3

This does not:

./a.out -n 3  
num (null)

so long option works, short does with two '-'s (why?) and the short option doesn't work (printf prints NULL), why is this? Many thanks.

Answer 1

you need to pass a short options string too, something like this:

const char *short_options ="hn:";

Note the : means -n accepts an argument.