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