Warnings like
comparison of unsigned expression < 0 is always false
are reported with -Wextra when enum types that are compiled to an
unsigned type (which is up to the compiler) are checked for negativity.
During destruction the main thread locks the mutex in processor_t and
waits on a condvar for threads to have terminated. Because the mutex
has also to be locked to decrement the thread count the condvar cannot
be signaled before doing that as otherwise the main thread might already
be waiting to join the threads while locking the mutex and thus causing
a deadlock.
Tobias Brunner