# getopt() function in C to parse command line arguments

The getopt() is one of the built-in C function that are used for taking the command line options. The syntax of this function is like below −

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

The opstring is a list of characters. Each of them representing a single character option.

This function returns many values. These are like below −

• If the option takes a value, then that value will be pointed by optarg.
• It will return -1, when no more options to proces
• Returns ‘?’ to show that this is an unrecognized option, it stores it to optopt.
• Sometimes some options need some value, If the option is present but the values are not there, then also it will return ‘?’. We can use ‘:’ as the first character of the optstring, so in that time, it will return ‘:’ instead of ‘?’ if no value is given.

## Example

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

main(int argc, char *argv[]) {
int option;
// put ':' at the starting of the string so compiler can distinguish between '?' and ':'
while((option = getopt(argc, argv, ":if:lrx")) != -1){ //get option from the getopt() method
switch(option){
//For option i, r, l, print that these are options
case 'i':
case 'l':
case 'r':
printf("Given Option: %c\n", option);
break;
case 'f': //here f is used for some file name
printf("Given File: %s\n", optarg);
break;
case ':':
printf("option needs a value\n");
break;
case '?': //used for some unknown options
printf("unknown option: %c\n", optopt);
break;
}
}
for(; optind < argc; optind++){ //when some extra arguments are passed
printf("Given extra arguments: %s\n", argv[optind]);
}
}

## Output

Given Option: i
Given File: test_file.c
Given Option: l
Given Option: r
Given extra arguments: hello