QNX Neutrino lets you do something else that's elegant. POSIX says that a mutex must operate between threads in the same process, and lets a conforming implementation extend that. QNX Neutrino extends this by allowing a mutex to operate between threads in different processes. To understand why this works, recall that there really are two parts to what's viewed as the “operating system”—the kernel, which deals with scheduling, and the process manager, which worries about memory protection and “processes” (among other things). A mutex is really just a synchronization object used between threads. Since the kernel worries only about threads, it really doesn't care that the threads are operating in different processes—this is an issue for the process manager.
So, if you've set up a shared memory area between two processes, and you've initialized a mutex in that shared memory, there's nothing stopping you from synchronizing multiple threads in those two (or more!) processes via the mutex. The same pthread_mutex_lock() and pthread_mutex_unlock() functions will still work.