C Command Line Arguments | argc and argv Tutorial

What are argc and argv?

Until now, you've been writing int main(void). But main can also be written to accept arguments from the command line.

int main(int argc, char *argv[]) {
    // argc: number of arguments (Argument Count)
    // argv: array of argument strings (Argument Vector)
}

argc

The number of arguments (including the program name itself).

argv[]

Each argument as a string — an array of char*.

For example, if you run ./myprogram hello world:

Variable	Value	Meaning
argc	3	3 arguments total
argv[0]	"./myprogram"	the program name
argv[1]	"hello"	first argument
argv[2]	"world"	second argument

Important: argv[0] holds the program's own name. The arguments supplied by the user start at argv[1].

Basic Usage

Here's a program that prints every argument it receives:

#include <stdio.h>

int main(int argc, char *argv[]) {
    printf("Number of arguments: %d\n", argc);
    for (int i = 0; i < argc; i++) {
        printf("argv[%d] = %s\n", i, argv[i]);
    }
    return 0;
}

$ gcc args.c -o args $ ./args hello world 123 Number of arguments: 4 argv[0] = ./args argv[1] = hello argv[2] = world argv[3] = 123

Using Arguments as Numbers

Every element of argv is a string (a char*). To use one as a number, you have to convert it first.

Function	Target type	Header	Example
`atoi(s)`	int	stdlib.h	`int n = atoi(argv[1]);`
`atof(s)`	double	stdlib.h	`double d = atof(argv[1]);`
`atol(s)`	long	stdlib.h	`long l = atol(argv[1]);`

Example: command-line calculator

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

int main(int argc, char *argv[]) {
    if (argc != 3) {
        printf("Usage: %s num1 num2\n", argv[0]);
        return 1;
    }
    int a = atoi(argv[1]);
    int b = atoi(argv[2]);
    printf("%d + %d = %d\n", a, b, a + b);
    return 0;
}

$ ./calc 15 27 15 + 27 = 42

Always validate your arguments. Check argc first, and if the count is wrong, print a usage message and exit.

Practical Example: Passing a File Name

Real-world programs often take the name of a file to process as a command-line argument.

#include <stdio.h>

int main(int argc, char *argv[]) {
    if (argc != 2) {
        printf("Usage: %s filename\n", argv[0]);
        return 1;
    }
    FILE *fp = fopen(argv[1], "r");
    if (fp == NULL) {
        printf("Cannot open file '%s'\n", argv[1]);
        return 1;
    }
    char line[256];
    while (fgets(line, 256, fp) != NULL) {
        printf("%s", line);
    }
    fclose(fp);
    return 0;
}

$ ./cat scores.txt Taro 80 Hanako 95 Jiro 70

This is a simplified version of the classic UNIX cat command.

Review Quiz

// When you run the following command
// $ ./prog hello world

What is the value of argc?

Explanation: argc counts the arguments including the program name: ./prog, hello, world — that's 3.
So argv[0]="./prog", argv[1]="hello", argv[2]="world".

// Running $ ./calc 10 20
int a = atoi(argv[1]);
int b = atoi(argv[2]);
printf("%d\n", a + b);

What does it print?

1020

error

Explanation: atoi("10") → 10 and atoi("20") → 20, so 10 + 20 = 30.
Without atoi, you might end up with string concatenation ("1020") — watch out for that.