getopt() function in C to parse command line arguments

The getopt() function is a builtin function in C and is used to parse command line arguments.

Syntax:

getopt(int argc, char *const argv[], const char *optstring)

optstring is simply  a list of characters, 
each representing a single character option.

Return Value: The getopt() function returns different values:

• If the option takes a value, that value is pointer to the external variable optarg.
• ‘-1’ if there are no more options to process.
• ‘?’ when there is an unrecognized option and it stores into external variable optopt.
• If an option requires a value (such as -f in our example) and no value is given, getopt normally returns ?.
  By placing a colon as the first character of the options string, getopt returns: instead of ? when no value is given.

Generally, the getopt() function is called from inside of a loop’s conditional statement. The loop terminates when the getopt() function returns -1. A switch statement is then executed with the value returned by getopt() function.

A second loop is used to process the remaining extra arguments that cannot be processed in the first loop.

Below program illustrate the getopt() function in C:

// Program to illustrate the getopt()
// function in C

#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
    int opt;

    // put ':' in the starting of the
    // string so that program can
    //distinguish between '?' and ':'
    while((opt = getopt(argc, argv, “:if:lrx”)) != -1)
    {
        switch(opt)
        {
            case ‘i’:
            case ‘l’:
            case ‘r’:
                printf(“option: %c\n”, opt);
                break;
            case ‘f’:
                printf(“filename: %s\n”, optarg);
                break;
            case ‘:’:
                printf(“option needs a value\n”);
                break;
            case ‘?’:
                printf(“unknown option: %c\n”, optopt);
                break;
        }
    }

    // optind is for the extra arguments
    // which are not parsed
    for(; optind < argc; optind++){
        printf(“extra arguments: %s\n”, argv[optind]);
    }

    return 0;
}

Output: