This C function should always return false, but it doesn’t?

Question

I stumbled over an interesting question in a forum a long time ago and I want to know the answer.Consider the following C function:f1.c-- #include <stdbool.h> bool f1() {  int var1 = 1000;  int var2 = 2000;  int var3 = var1 + var2;  return (var3 == 0) ? true : false; }This should always return `false` since `var3 == 3000`. The `main` function looks like this:main.c-- #include <stdio.h> #include <stdbool.h> int main() {  printf( f1() == true ? "true\\n" : "false\\n");  if( f1() )  {   printf("executed\\n");  }  return 0; }Since `f1()` should always return `false`, one would expect the program to print only one *false* to the screen. But after compiling and running it, *executed* is also displayed:<!-- language: lang-none --> $ gcc main.c f1.c -o test $ ./test false executedWhy is that? Does this code have some sort of undefined behavior?Note: I compiled it with `gcc (Ubuntu 4.9.2-10ubuntu13) 4.9.2`.

Answer 1

When testing your code here

https://onlinegdb.com/ByiLx7UoV

The results were not the same as what you describe. You can try it and see it's "false" with no "execute" seen

Can you give it a shot with a newer compiler? Not to pull you away from your 4.9.2 target specifically (you might have reasons for staying on that version) but purely to isolate and diagnose.