getopt() function in C to parse command line arguments

Last Updated : 10 Sep, 2018

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; 
} 