Introduction to argc and argv

You may be familiar with the good ’ol int main(void), but surely, you think, there’s more to it than that… and you’d be right! This lesson will introduce you to argc and argv, and how exactly they are used.

Preface: The Command Line

When running programs in the command line, you may run programs like this:

prompt> gcc myprogram.c
prompt> ./a.out

gcc is simply a program that compiles (converts) C code to the language of zeroes and ones that computers can understand. But how exactly does the program gcc know that it should compile myprogram.c once it was specified in the command line? The answer is different depending on each programming language, but for the case of C, it’s the magic of argc and argv!

How main() normally looks

You are probably familiar with the int main(void) function:

#include <stdio.h>

int main(void)
{
  printf("Hello, world!\n");

  return 0;
}

But why does main() have void as its function parameters? Surely main() could take in some type of input, maybe even command line input?

Side note: Some courses use int main() (without the void), and while this does achieve the same goal, it is always good practice to exactly specify the parameters of a function. Using empty parentheses leaves the function with undefined parameters, meaning you could actually call it with any arrangement of arguments, regardless of whether or not the function would actually be able to use them. Generally, specifying a function’s parameters is better practice since it strictly enforces how that function is called.

What you actually came here for: argc and argv

There are multiple valid function definitions for main() in the C standard. Other than int main(void), the next one is int main(int argc, char *argv[]). This allows you to receive input from the command line arguments!

There are two parts to this, argc and argv.

argc is short for “argument count,” and tells how many elements are in the following argv array.

argv is short for “argument vector,” and is a list of arguments (in the form of strings) retrieved by the command line!

Here is some example code of this in action:

#include <stdio.h>

int main(int argc, char *argv[])
{
  printf("The number of arguments passed in from the command line is %d.\n", argc);

  printf("These are the contents of argv: [");
  for (int i = 0; i < argc; i++)
  {
    printf("\"%s\"", argv[i]);

    if (i < argc - 1)
    {
      printf(", ");
    }
  }
  printf("]\n");

  return 0;
}

Here’s an example of running the program with some arguments, then getting some output:

prompt> gcc myprogram.c
prompt> ./a.out hello!
The number of arguments passed in from the command line is 2.
These are the contents of argv: ["./a.out", "hello!"]
prompt>

Notice that "./a.out" is included in the output! The program name itself is always the first argument.