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

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:

$ gcc main.c f1.c -o test
$ ./test
false
executed

Why 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.

Solution

As noted in other answers, the problem is that you use gcc with no compiler options set. If you do this, it defaults to what is called "gnu90", which is a non-standard implementation of the old, withdrawn C90 standard from 1990.

In the old C90 standard there was a major flaw in the C language: if you didn't declare a prototype before using a function, it would default to int func () (where ( ) means "accept any parameter"). This changes the calling convention of the function func, but it doesn't change the actual function definition. Since the size of bool and int are different, your code invokes undefined behavior when the function is called.

This dangerous nonsense behavior was fixed in the year 1999, with the release of the C99 standard. Implicit function declarations were banned.

Unfortunately, GCC up to version 5.x.x still uses the old C standard by default. There is probably no reason why you should want to compile your code as anything but standard C. So you have to explicitly tell GCC that it should compile your code as modern C code, instead of some 25+ years old, non-standard GNU crap.

Fix the problem by always compiling your program as:

gcc -std=c11 -pedantic-errors -Wall -Wextra

-std=c11 tells it to make a half-hearted attempt to compile according the (current) C standard (informally known as C11).
-pedantic-errors tells it to whole-heartedly do the above, and give compiler errors when you write incorrect code which violates the C standard.
-Wall means give me some extra warnings that might be good to have.
-Wextra means give me some other extra warnings that might be good to have.