# Resolve arguments from command line
###### tags: `C` `C++`

## getopt and getopt_long
The getopt() function parses the command-line arguments. Its arguments argc and argv are the argument count and array as passed to the main() function on program invocation.

 #include <unistd.h>
 int getopt(int argc, char *const argv[],
                  const char *optstring);

 extern char *optarg;
 extern int optind, opterr, optopt;

 #include <getopt.h>

 int getopt_long(int argc, char *const argv[],
                  const char *optstring,
                  const struct option *longopts, int *longindex);
int getopt_long_only(int argc, char *const argv[],
                  const char *optstring,
                  const struct option *longopts, int *longindex);

### arguments
1. **argc, argv**
argc and argv are the argument count and array as passed to the main() function on program invocation. An element of argv that starts with '-' is an option element. The characters of this element(aside from '-') are option characters.
Ex:

./test.out -v  // -v is a option element, v is a option character

2. **optind**
The variable optind is the index of the next element to be processed of argv. The initial value of optind is 1 and it can be reset when rescanning the same argv is needed.

3. **optstring**
optstring is a string containing legitimate **option characters**. If such character is **followed by a colon**, the corresponding option **requires an argument** which will be assigned to variable **optarg**. If such character is followed by two colons, the corresponding argument of the option is optional. The special argument **"--" forces an end** of option-scanning regardless of the scanning mode.

optstring EX:

    "a:bc"  // optstring

Code:

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

    int main(int argc, char **argv) {
        int ch;
        while((ch = getopt(argc, argv, "a:b")) != -1) {
            switch(ch) {
                case 'a':
                    printf("option a: %s\n", optarg);
                    break;
                case 'b':
                    printf("option b \n");
                    break;
                case '?':  //when option error
                    printf("unknown option \n");
                    break;
                default:
                    printf("default \n");
            }
        }
    }

Input:

    ./test -a aa -b -c

Output:

    option a: aa
    option b 
    unknown option

By default, getopt() prints an error message on standard error, places the erroneous option character in optopt, and returns '?' as the function result.

### getopt_long
The getopt_long() function works like getopt() except that it also **accepts long options**, started with **two dashes**. (If the program accepts only long options, then optstring should be specified as an empty string (""), not NULL.)

**longopts** is a pointer to the first element of an array of struct option declared in **<getopt.h>** as

  struct option {
        const char *name;  //name of the long option
        int         has_arg; //0: no argument, 1: require argument, 2: optionalo
        int        *flag;
        int         val;
           };