Command Line Arguments

What are command line arguments?

Usually a program gets input with scanf while it runs. Command line arguments are different — you supply values as you launch the program, right after its name in the terminal:

./app hello 42

Here hello and 42 are arguments handed to the program. This is how tools like gcc file.c receive the file name. To catch them, main takes two parameters.

argc and argv

int main(int argc, char *argv[]) {
    // argc = how many arguments (including program name)
    // argv = array of the argument strings
}

For ./app hello 42, argc is 3, argv[0] is "./app", argv[1] is "hello", and argv[2] is "42".

Printing the arguments

#include <stdio.h>
int main(int argc, char *argv[]) {
    printf("You passed %d arguments.\n", argc - 1);
    for (int i = 1; i < argc; i++)   // start at 1, skip program name
        printf("argv[%d] = %s\n", i, argv[i]);
    return 0;
}

The loop starts at 1 because argv[0] is just the program's name.

Turning arguments into numbers

Every argument is a string, even "42". To use it as a number, convert it with atoi (to int) or atof (to double) from <stdlib.h>.

#include <stdlib.h>
int n = atoi(argv[1]);   // "42" -> 42

A sum program

Add up all the numbers passed on the command line.

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]) {
    if (argc < 2) {
        printf("Usage: ./app num1 num2 ...\n");
        return 1;
    }
    int sum = 0;
    for (int i = 1; i < argc; i++)
        sum += atoi(argv[i]);
    printf("Sum = %d\n", sum);
    return 0;
}

Common mistakes

• Starting the loop at 0 and accidentally including the program name.
• Using an argument as a number without converting it with atoi/atof.
• Reading argv[1] without checking argc — crashes if it is missing.
• Assuming arguments are validated — the user can type anything.

Summary

• Command line arguments are values passed when launching the program.
• main(int argc, char *argv[]) receives them.
• argc counts them; argv[0] is the program name, real values start at argv[1].
• Arguments are strings — convert with atoi or atof for numbers.
• Always check argc before reading an argument.