timedwait cond mtx timeout waits on condition variable cond
with mutex mtx until either the condition is signalled, or until
timeout expires. Note that timeout is an absolute Unix-time to
prevent time-related race conditions.
false iff the timer expired, but this does not mean that
the condition is not true due to an unavoidable race condition in
the system call.man pthread_cond_timedwait for details.