CIS241

System-Level Programming and Utilities

C Command Line Arguments

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

Let's get argumentative

We've not done anything with comand line args yet, we should fix that

Go from:

int main()
{
    return 0;
}

to

int main(int argc, char* argv[])
{
    return 0;
}

The main function prototype:

We will pass command line args an array of strings

What is the actual type of a string in C?

• char[] or char*

Thus, we pass two arguments to main:

• The number of arguments: int argc
• The array of strings: char* argv[]

Using command line arguments

First argument (index 0) is always the name of the program (just like in bash)

For example: ./a.out 100 foo

argv[0] = the string "./a.out"
argv[1] = the string "100"
argv[2] = the string "foo"

Sometimes we need to convert to numeric types:

• int x = atoi(argv[1]); // need stdlib.h for this!

List of other conversions in stdlib.h:

• https://cplusplus.com/reference/cstdlib/

A sample - get and print command line args

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char* argv[]) {
    if (argc > 1) {
        for (int i = 0; i < argc; i++) {
            printf("Argument %d: %s\n", i, argv[i]);
        }
    } else {
        printf("Arguments required\n");
    }
    return 0;
}