Spec defines any remaining elements to be initialized to 0.

You can even do sparse initialization if we're talking C99 here, like so:

int array[] = {1, 2, 3, [99] = -1}

And you'll get an array of length 100 {1, 2, 3, 0, 0, 0, ..., 0, -1}

Any missing initialized element is always 0 if initialization is done at all, though.