getopt fails to detect missing argument for option

Question

I have a program which takes various command line arguments. For the sake of simplification, we will say it takes 3 flags, -a, -b, and -c, and use the following code to parse my arguments:

    int c;
    while((c =  getopt(argc, argv, ":a:b:c")) != EOF)
    {
        switch (c)
        {
             case 'a':
                 cout << optarg << endl;
                 break;
             case 'b':
                 cout << optarg << endl;
                 break;
             case ':':
                 cerr << "Missing option." << endl;
                 exit(1);
                 break;
        }
    }

note: a, and b take parameters after the flag.

But I run into an issue if I invoke my program say with

./myprog -a -b parameterForB

where I forgot parameterForA, the parameterForA (represented by optarg) is returned as -b and parameterForB is considered an option with no parameter and optind is set to the index of parameterForB in argv.

The desired behavior in this situation would be that ':' is returned after no argument is found for -a, and Missing option. is printed to standard error. However, that only occurs in the event that -a is the last parameter passed into the program.

I guess the question is: is there a way to make getopt() assume that no options will begin with -?

Answer 1

See the POSIX standard definition for getopt. It says that

If it [getopt] detects a missing option-argument, it shall return the colon character ( ':' ) if the first character of optstring was a colon, or a question-mark character ( '?' ) otherwise.

As for that detection,

1. If the option was the last character in the string pointed to by an element of argv, then optarg shall contain the next element of argv, and optind shall be incremented by 2. If the resulting value of optind is greater than argc, this indicates a missing option-argument, and getopt() shall return an error indication.
2. Otherwise, optarg shall point to the string following the option character in that element of argv, and optind shall be incremented by 1.

It looks like getopt is defined not to do what you want, so you have to implement the check yourself. Fortunately, you can do that by inspecting *optarg and changing optind yourself.

int c, prev_ind;
while(prev_ind = optind, (c =  getopt(argc, argv, ":a:b:c")) != EOF)
{
    if ( optind == prev_ind + 2 && *optarg == '-' ) {
        c = ':';
        -- optind;
    }
    switch ( …