How can I read arguments as options when I run a C program under Unix?

Question

I want to set arguments to c Program like this.

$./Program_name -a 100 -b 5 -c 30 

and I want to use this value a, b, c in the program.

For ex:

int count = a;
int number = b;
int limit_exec = c;

I don't know how it works. I couldn't find it in Google... I'd like to see some examples.

Also, in the same time, I want to make default and limits of values.

1. Set limits like this: Value a can only have a number from 1 to 10000.

2. Using default when I didn't put value like this:

   $./program -a 100 -c 30    // then value 'b' should use default number.
   

   I don't know how to set default. Is it okay to set default at c code like this?

      #define a 50   
      #define b 100   
      #define c 30
   

Answer 1

You can use getopt to parse the command line input.
Please see man page for a brief idea.

$ man 3 getopt

The following code works as per your requirement.

Code:

#include <stdio.h>
#include <unistd.h> /* Definition of getopt */
#include <stdlib.h> /* Definition of atoi */ 

int main (int argc, char ** argv)
{
    char option;

    /* Default Values */
    int count = 10, number = 20, limit = 30;

    while (-1 != (option = getopt (argc, argv, "a:b:c:")))
    {   
        switch (option)
        {
            case 'a':
                count = atoi(optarg);

                /* Check for limit */
                if (count <= 0 || count > 1000)
                {
                        printf("Usage <%s> [a] value, Range of value : 1-1000\n", argv[0]);
                        exit(EXIT_FAILURE);
                }
                break;

            case 'b':
                number = atoi(optarg);
                break;

            case 'c':
                limit = atoi(optarg);
                break;

            default:
                printf ("Usage: <%s> [a] value [b] value [c] value\n", argv[0]);
        }
    }

    printf("\nCount:[%d]\tNumber:[%d]\tlimit:[%d]\n\n", count, number, limit);
    return 0;
}

Compilation: gcc -o exe filename.c -Wall

Execution:

$ ./exe     /* Default values */
Count:[10]  Number:[20] limit:[30]

$ ./exe -a 1 -b 2 -c 3
Count:[1]   Number:[2]  limit:[3]

$ ./exe -a 1  -c 3
Count:[1]   Number:[20] limit:[3]

$ ./exe -a 9999  /* Same output for ./exe -a -1 */
Usage <./exe> [a] value, Range of Value : 1-1000