Dealing with args using getopt() [Part.1]

3 July 2014 – Shanghai

Everyone start to learn coding by helloworld.c :) Yes, it’s the basic of digital daylife:
Command line

When dealing commandline style coding, every one need to manage the skill of controlling different kind of argument. It is really a dirty job by analyzing

int main(int argc, char *argv[])

But if you are familiar with POSIX style console command, you’ll find it’s argument definitions and help are so elegent and stylish.

Luckily we have a standard GNU C Library function called getopt

In short, we just need to add the include:

#include <getopt.h>

Then start coding with something like:

int c;
opterr = 0;
while ((c = getopt (argc, argv, "abc:")) != -1)
{
    switch (c)
    {
        case 'a':
            aflag = 1;
            break;
        case 'b':
            bflag = 1;
            break;
        case 'c':
            cvalue = atoi(optarg);
            break;
        case '?':
        case ':':
            if (optopt == 'c')
                fprintf (stderr, "Option -%c requires an argument.\n", optopt);
            else if (isprint (optopt))
                fprintf (stderr, "Unknown option `-%c'.\n", optopt);
            else
                fprintf (stderr, "Unknown option character `\\x%x'.\n",optopt);
            return 1;
        default:
            abort ();
    }
}

The third argument in funtion getopt defines the commandline option you need; single charactor means it is a switch with no extra argument is needed, on the other hand, if you need extra argument, put a ‘:’ after your option charactor.

Within the switch statment, you may notice the following case:

case ‘?’: If getopt finds an option character in argv that was not included in options, or a missing option argument, it returns ‘?’ and sets the external variable optopt to the actual option character.
case ‘:’: getopt returns ‘:’ instead of ‘?’ to indicate a missing option argument.

If you carefully read through the document, you will know the library will expose the following global variables:

optopt: When getopt encounters an unknown option character or an option with a missing required argument, it stores that option character in this variable. You can use this for providing your own diagnostic messages.
optind: This variable is set by getopt to the index of the next element of the argv array to be processed. Once getopt has found all of the option arguments, you can use this variable to determine where the remaining non-option arguments begin. The initial value of this variable is 1.
opterr: If the value of this variable is nonzero, then getopt prints an error message to the standard error stream if it encounters an unknown option character or an option with a missing required argument. This is the default behavior. If you set this variable to zero, getopt does not print any messages, but it still returns the character ? to indicate an error.
optarg: This variable is set by getopt to point at the value of the option argument, for those options that accept arguments.

Sounds like a beautiful story huh? But it do not have a happy ending.
I will discuss the matter and workaround in the next thread: Dealing with args using getopt() [Part.2]

Dealing with args using getopt() [Part.1]

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