How does returning values from a function work?

Question

I recently had a serious bug, where I forgot to return a value in a function. The problem was that even though nothing was returned it worked fine under Linux/Windows and on

Answer 1

Probably by luck, 'a' left in a register that happens to be used for returning single pointer results, something like that.

The calling/ conventions and function result returns are architecture-dependent, so it's not surprising that your code works on Windows/Linux but not on a Mac.