That isn't what is meant by "side effect" here. A function with no side effects only modifies its local variables and returns a result. So, if we have int foo(

That isn't what is meant by "side effect" here. A function with no side effects only modifies its local variables and returns a result. So, if we have int foo(int a, int b) { int c = bar(a); return b; } and we can see bar's body, and therefore we can determine that it has no side effects, we can optimize the call to bar away, because we can assume that it terminates and it doesn't affect the result. But if bar writes to global variables, that's a side effect. If we can't see the body of bar we have to assume that it might.