C Command Line Arguments | argc and argv Tutorial

What are argc and argv?

Up until now you have written int main(void), but main can also be written to accept arguments.

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

argc

Number of arguments (including the program name)

argv[]

Each argument as a string (array of char*)

For example, running ./myprogram hello world:

Variable	Value	Meaning
argc	3	3 arguments
argv[0]	"./myprogram"	program name
argv[1]	"hello"	1st argument
argv[2]	"world"	2nd argument

Important: argv[0] is the program's own name. User-supplied arguments start from argv[1].

Basic Usage

A program that prints all arguments:

#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 (char*). To use one as a number, you must convert it.

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

Validating arguments is essential. Check argc and if there are not enough arguments, print usage and exit.

Practical Example: Passing a File Name

Real programs often take the name of the 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 UNIX cat command.

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 = 3.
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, atoi("20") → 20. 10 + 20 = 30.
Without atoi you would get string concatenation ("1020"), so be careful.