Is returning zero from main necessary, and how can the return value from main be useful?

Question

I know it's been the convention in C89 to always return a 0 integer value from main in a C program, like this:

int main() {

    /* do something useful here */

    return 0;
}

This is to return a "successful" result to the operating system. I still consider myself a novice (or an intermediate programmer at best) in C, but to date I've never fully understood why this is important.

My guess is, this is a useful return result if you're tying the output of this program into the input of another, but I'm not sure. I've never found it useful, or maybe I just don't understand what the intention is.

My questions:

1. Is returning zero always necessary from a C program?
2. How is the return value from main() useful?

Answer 1

When writing scripts (like in Bash, or CMD.exe on Windows) you can chain some commands with the && and || operators.

Canonically, a && b will run b if the result of a is zero, and a || b will run b if a returned nonzero.

This is useful if you wish to conditionally run a command if the previous one succeeded. For example, you would like to delete a file if it contains word foo. Then you will use :

grep foo myfile && rm myfile

grep returns 0 when there was a match, else nonzero.

Answer 2

Returning 0 is a convention. When a program returns 0, it can be assumed that it worked OK, without actually looking at what the program did (ahem :D).

As a widely used convention, that assumption is in a lot of places. As Benoit points out that's the case of the shell (UNIX and Windows) and other parts of the Operating system.

So answering your questions:

1. For a C program you must return either EXIT_SUCCESS or EXIT_FAILURE. But you can return EXIT_FAILURE even if your program worked OK.
2. If you don't return a 0 (EXIT_SUCCESS), it's quite possible that other programs will assume your program failed.

There's a related question with C++ and C great responses

What should main() return in C and C++?