Question

Many are familiar with the hello world program in C:

#include <stdio.h>

main ()
{
    printf ("hello world");
    return 0;
}

Why do some precede the main() function with int as in:

int main()

Also, I’ve seen the word void entered inside the () as in:

int main(void)

It seems like extra typing for nothing, but maybe it’s a best practice that pays dividends in other situations?

Also, why precede main() with an int if you’re returning a character string? If anything, one would expect:

char main(void)

I’m also foggy about why we return 0 at the end of the function.

Answer 1

The following has been valid in C89

main() {
  return 0;
}

But in modern C (C99), this isn’t allowed anymore because you need to explicitly tell the type of variables and return type of functions, so it becomes

int main() {
  return 0;
}

Also, it’s legal to omit the return 0 in modern C, so it is legal to write

int main() {

}

And the behavior is as if it returned 0.

People put void between the parentheses because it ensures proper typechecking for function calls. An empty set of parentheses in C mean that no information about the amount and type of the parameters are exposed outside of the function, and the caller has to exactly know these.

void f();
 /* defined as  void f(int a) { } later in a different unit */

int main() {
  f("foo");
}

The call to f causes undefined behavior, because the compiler can’t verify the type of the argument against what f expects in the other modules. If you were to write it with void or with int, the compiler would know

void f(int); /* only int arguments accepted! */

int main(void) {
  f("foo"); /* 'char*' isn't 'int'! */
}

So for main it’s just a good habit to put void there since it’s good to do it elsewhere. In C you are allowed to recursively call main in which case such differences may even matter.

Sadly, only a few compilers support modern C, so on many C compilers you may still get warnings for using modern C features.

Also, you may see programs to declare main to return different things than int. Such programs can do that if they use a freestanding C implementation. Such C implementations do not impose any restrictions on main since they don’t even know or require such a function in the first place. But to have a common and portable interface to the program’s entry point, the C specification requires strictly conforming programs to declare main with return type int, and require hosted C implementations to accept such programs.