So what does "return 0" actually mean?

Question

I'm pretty proficient in PHP, but I've started dabbling with C. I've seen the code

return 0;

at the end of functions that don't return a value. This isn't used in PHP, because if a function is doesn't have a return, a value NULL is automatically returned.

All I'm asking is, in simple English, what does the return 0 actually do? Is it like PHP, where it returns its argument as the value of the function call? Is it just good practice?

I know this question has been asked many times before, but I'm asking it from the point of view of a PHP developer. The answers google throws up haven't been that concise.

Answer 1

Is it like php, where it returns its argument as the value of the function call? Is it just good practise?

Yes, PHP and many other languages borrowed the return keyword from 'C'. And in all the languages, the return keyword has the same function - to return from the function. Anything that follows return keyword is the value that is returned to the caller.

Is it a good practise? Yes and No. Not all functions should return a value. And quite a few in the standard library even, do not return any value. Hence their return type is void.

But main function should return 0(also EXIT_SUCCESS) to identify that the program has executed successfully. And -1 otherwise (also EXIT_FAILURE)

EDIT: (Thanks to @KeithThompson):

EXIT_FAILURE is implementation defined. 1 is a common value of EXIT_FAILURE but the whole point is, you need not know.

Answer 2

For historic reasons, it is possible to write return 0; to return from a function that has been declared as void, like so:

void foo( /* arguments */ )
{
  /* do things */
  return 0;
}

This does nothing, and the 0 (or whatever you put there) is thrown away. Also, sensible compilers will give you a warning message if you do this. So don't do this.