C: OR operator in return line?

Question

Came across some code that has an OR operator (||) in the return line. Could someone explain what that does ?

---

Here's something that looks like it:

int main()
{
    ...

    return (1 || 0);
}

But really it was a function (that returned 1 or 0) instead of the 1 and 0:

int main()
{
    ...

    return (foo(x++, y) || foo(x, y++));
}

Answer 1

a || b evaluates to 1 if a is non-zero irrespective of the value of b, evaluates to 1 if a is 0 and b is non-zero, else it's 0.

So 1 || 0 is 1, as would be 2 || 0.

Note that b is not evaluated if a is non-zero: so if b was a function, it would not be called in such a case. So in your example, foo(x, y++) is not called including the evaluation of y++ if foo(x++, y) is non-zero.

Note that the type of a || b is an int irrespective of the type of the arguments. Cf. C++ where the type is a bool.

Answer 2

Logical OR has a short circuit property. The RHS is only evaluated if the LHS is evaluated to false.

In this case, since the operands are function call, so

• the LHS function call foo(x++, y) will be made.
• If the return value is 1 (TRUTHY), the RHS will not be evaluated and the value 1 will be returned.
• If the returned value is 0, the foo(x, y++) function call with made, and the return value will be based on teh return value of the function call.