Using getopt to parse program arguments in c++

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

There are quite a few different versions of getopt around, so even if you can get it to work for one version, there will probably be at least five others for which your workaround will break. Unless you have an overwhelming reason to use getopt, I'd consider something else, such as Boost.Program_options.

Answer 2

If you are working in C++, boost::program_option is my recommendation to parse command line argument:

• Boost::program_options library

Answer 3

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 ( …