Command Line Arguments in C (argc and argv)

Introduction

When you run a program from the terminal, the shell passes command line arguments. In C, main can receive them through argc (argument count) and argv (argument vector).

Syntax

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

Parameter	Meaning
argc	Number of arguments (including program name)
argv[0]	Program name
argv[1] … argv[argc-1]	User-supplied arguments
argv[argc]	Always NULL

Example Program

#include <stdio.h>

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

    printf("Program name: %s\n", argv[0]);
    printf("Total arguments: %d\n", argc - 1);

    for (i = 1; i < argc; i++)
        printf("Argument %d: %s\n", i, argv[i]);

    return 0;
}

How to Run

./args Hello World 123

Sample Output

Program name: ./args
Total arguments: 3
Argument 1: Hello
Argument 2: World
Argument 3: 123

Converting String Arguments to Numbers

Use <stdlib.h>:

int value = atoi(argv[1]);       /* string to int */
long lv = strtol(argv[1], NULL, 10);
double dv = atof(argv[1]);

Best Practices

• Always check argc before reading argv[1].
• Validate user input — atoi returns 0 on failure with no error indication; prefer strtol.
• Quote arguments with spaces when passing from shell.

Common Mistakes

• Accessing argv[1] when no arguments were passed (undefined behavior).
• Forgetting that argv elements are strings, not numbers.
• Modifying argv strings on some platforms may be unsafe — treat as read-only.

Related Pages

Continue learning with these related tutorials and programs:

• C Tutorials — Browse all C Tutorials.
• C Introduction — Start here — what C is and why it matters.
• C History — More in c basics.
• C Specific Properties and Implementation — More in c basics.