I have a library for managing child processes that relies on passing the POSIX WNOHANG option to waitpid to perform a non-blocking wait on a process. It is said that not all systems support this option, but it has been a while since I have worked on any of those systems. What systems don't support this option? I'd like to know so that either I can try to find workarounds for those systems or so I can decide not to target those systems.
waitpid was specified in POSIX.1 I believe, so you'll have a tough time finding a system that isn't compliant. Depending on what sort of library you're building, you might want to take a look at this list of POSIX-Compliant systems: http://en.wikipedia.org/wiki/POSIX#Fully_POSIX-compliant
A quick search on Google turned up non-compliant systems, which leads me to think that you should be safe with your library on anything post-1988.
If you do come across a non-compliant system, it might not be a bad idea to post it on SO so it's archived. Good luck!