Safe to use volatile bool to force another thread to wait? (C++)

Question

Everything I\'ve read about volatile says it\'s never safe, but I still feel inclined to try it, and I haven\'t seen this specific scenario declared unsafe.

I have a sep

Answer 1

It's completely unsafe, although it might work with some compilers. Basically, volatile only affects the variable it's attached to, so RendererThreadFunction, for example, could set stillRendering false before having finished renderer->render();. (This is true even if both stillRendering and programRunning were both volatile.) The probablility of a problem is very small, so testing probably won't reveal it. And finally, some versions of VC++ do give volatile the semantics of an atomic access under C++11, in which case, your code will work. (Until you compile with a different version of VC++, of course.)

Given that renderer->render() almost certainly takes a non-negligible amount of time, there's absolutely no reason for not using a conditional variable here. About the only time you'd use volatile for this sort of thing is if the shutdown mechanism were triggered by a signal (in which case, the type would be sig_atomic_t, and not bool, although in practice, it probably doesn't make any difference). In that case, there wouldn't be two threads, but just the renderer thread and a signal handler.