On Wed, 17 Feb 2021, 21:12 Thanos Makatos via Gcc, <gcc@gcc.gnu.org> wrote:
> I run into a problem that I'm not sure whether it's a bug in my program > (most likely) or something wrong with GCC (highly unlikely, I know, hence > why I haven't sent this to gcc-bugs). This is the wrong list, please use gcc-help for questions like this. The problem is using a function that returns a bool, defined in another > source file without a declaration, and the program compiled with -O. In my > test, function foo1 returns true while it should be returning false. > As Martin explained, the compiler assumes that an implicitly-declared function returns an int. Your function initializes a single byte with its return value and the caller reads 4 bytes, including 3 bytes of uninitialized garbage. If that garbage happens to contains any non-zero bits, you get a non-zero int and converting that to bool yields true. Declare your functions. Don't ignore warnings.
