Difference between gcc5 and gcc8 with respect to return values of a function

Question

I have two tools: gcc5 and gcc8. Following is the snippet code

bool foo() {
  int var;
  var = 1;
  printf("var=%d\n", var);
}

int calling_foo() {
  foo();
}

If I compile and run w/ gcc5, foo() call returns. But, if I compile and run w/ gcc8, foo() call doesn't return.

I understand, there is no return value in foo(), but at least the function should return. I assume that gcc8 is far more stricter. But why the call is not returning.

Answer 1

Your program (assuming it is calling calling_foo or foo directly) has undefined behavior, because execution flow will reach the closing brace of the foo function body, which is declared to return non-void. That, by itself, causes undefined behavior in C++.

Therefore both versions of GCC are correct. They can emit or not emit whatever code they want. The program is not guaranteed to behave in any particular way.