You can pass an executable command line arguments when you run it like the following:

./my_prog 100 5.9 out.txt

A more advanced syntax (and likely using getopt library to parse) is the following:

./my_prog2 -n 100 -f 5.9 -o out.txt

This page discusses command line arguments and parsing the first format. Here is some information about using getopt to parse the second format.

Below is an example program that takes command line arguments using the first format. The first thing to note is main's function definition:

int main(int argc, char *argv[]) { ...

The first parameter to main, argc, is the number of command line arguments. For example, if the user enters:

./a.out 10 11 200

argc will be 4 (a.out counts as one of the command line arguments, and 10, 11, and 200 as three more).

Each command line argument is passed to main in the second parameter, argv, as a string (argv is an array of strings):

        -----
argv[0]:| *-|-----> "./a.out"
        -----
argv[1]:| *-|-----> "10"
        -----
argv[2]:| *-|-----> "11"
        -----
argv[3]:| *-|-----> "200"
        -----
argv[4]:| *-|-----|     (NULL pointer: no more command line strings)
        -----

C has functions that can convert strings of numeric characters to their int, float, and other basic types, values.

int x = atoi(argv[1]);  // x gets the int value 10

See the man page for atoi for more information (man atoi).

---

Example program:

/*
 * An example of command line arguments in C
 * (newhall, 2013)
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// argc: a count of the number of command line arguments, the executable 
//       name (command) is included so argc is at least 1 for every program
// argv: an array of strings, there is one string for each command line arg
//
int main(int argc, char *argv[]) {

  int age, i;
  float gpa;
  long long id;
  char *name;

  // print out the argv strings:
  for(i=0; i < argc; i++) {
    printf("Command line arg %2d: %s\n", i, argv[i]);
  }
  printf("\n");

  // often times we want to check the the user passed in the correct number
  // of command line arguments, if not, usually print an error message with
  // some information on how to call this execuatable (what command line
  // options it expects, and in which order), and then exit:
  if(argc != 5) {
    printf("usage: ./prog name age id_num gpa\n");
    exit(1);
  }

  // atoi, atol, atoll, atof convert a string of numeric characters to
  // the equivalent int, long, long long, or float value
  age = atoi(argv[2]);
  id = atoll(argv[3]);
  gpa = atof(argv[4]);

  // Let's copy the name argument to a dynamically allocated 
  // string and modify it  (just to illustrate malloc, strlen, strcpy)

  name = malloc( sizeof(char) * (strlen(argv[1])+1) );
  if(!name) { 
    printf("Error: malloc failed....\n");
    exit(1);
  } 

  if(strcpy(name, argv[1]) != NULL) { 
    name[3] = 'x';
    printf("name: %s age: %d id: %lld gpa: %3.2f\n", name, age, id, gpa);
  } else { 
    printf("Error: strcpy failed....\n");
  }

  free(name);
  name = NULL;

  return 0;
}