Command Line Arguments in C

When a C program runs, it can receive information directly from the command line. This information is known as command line arguments.

Command line arguments allow programs to behave dynamically without changing the source code.

Why Command Line Arguments Are Useful

They are commonly used to:

• Pass filenames to a program
• Provide configuration values
• Control program behavior
• Automate tasks

Main Function with Arguments

To receive command line arguments, the main() function is written as:

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

Here:

• argc → Argument count
• argv → Argument vector (array of strings)

Simple Example

#include <stdio.h>

int main(int argc, char *argv[]) {
    printf("Total arguments: %d\n", argc);

    for(int i = 0; i < argc; i++) {
        printf("Argument %d: %s\n", i, argv[i]);
    }
    return 0;
}

The first argument argv[0] always stores the program name.

How to Run the Program

Compile the program:

gcc args.c -o args

Run with arguments:

./args input.txt 5 debug

Real-World Example

A backup program might receive:

• Source directory
• Destination directory
• Backup mode

This avoids hard-coding paths inside the program.

Using Arguments as Numbers

Since arguments are strings, conversion is required.

#include <stdlib.h>

int value = atoi(argv[1]);

Functions like atoi, atof and strtol are commonly used.

Quiz

Q1: What does argc represent?

Number of command line arguments

Q2: What is stored in argv[0]?

Program name

Q3: Are command line arguments strings by default?

Yes

Q4: Which function converts string to integer?

atoi()

Q5: Can we pass arguments without recompiling?

Yes

Mini Practice

• Write a program that adds two numbers from command line
• Pass a filename and display it
• Validate number of arguments using argc