cnd_signal is a C11 condition-variable operation that wakes one thread waiting on the supplied condition variable. The awakened thread returns from cnd_wait or cnd_timedwait and must recheck its condition expression. section 18.5
Signaling the next worker
After the account thread performs its work, it calls cnd_signal(&L->upda), notifying the update thread. The update thread similarly signals &L->acco and &L->draw after updating shared state. Each waiter remains inside a mutex-protected while loop, so wake-up does not replace checking whether work is actually available.
