cnd_wait blocks a thread until its condition variable is signaled, temporarily releasing the caller-held mutex and reacquiring it before returning. Test the shared condition in a loop, because returning does not by itself establish that the condition is true. section 18.5
Waiting until accounting work exists
mtx_lock(&L->mtx); while (!L->finished && (L->accounted == L->iteration)) cnd_wait(&L->acco, &L->mtx); life_account(L); mtx_unlock(&L->mtx); The mutex protects the condition and the data used after the wait. Another thread signals the condition after changing shared state. section 18.5
Use cnd_wait with these rules:
- Hold the mutex before calling cnd_wait. section 18.5
- Keep one mutex associated with a condition variable; one mutex may serve several condition variables. section 18.5
- Use cnd_signal to wake one waiter or cnd_broadcast to wake all waiters. section 18.5
Listing 18.2. The account thread function of B9
99 int account_thread(void* Lv) {
100 life*restrict L = Lv;
101 while (!L->finished) {
102 // Blocks until there is work
103 mtx_lock(&L->mtx);
104 while (!L->finished && (L->accounted == L->iteration))
105 life_wait(&L->acco, &L->mtx);
106
107 // VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV
108 life_account(L);
109 if ((L->last + repetition) < L->accounted) {
110 L->finished = true;
111 }
112 // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
113
114 cnd_signal(&L->upda);
115 mtx_unlock(&L->mtx);
116 }
117 return 0;
118 }
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Browse the complete account-thread example showing mutex acquisition, predicate checking, cnd_wait, work, signaling, and unlock. section 18.5
