One thread at the shared state
An accounting thread calls `mtx_lock(&L->mtx)` before checking whether a new iteration needs accounting. When work is ready, it calls `life_account(L)`, signals the update thread, and calls `mtx_unlock(&L->mtx)`. The update and draw threads use the same mutex around their own work on the shared state. 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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Follow the accounting thread from lock acquisition through its wait loop, computation, signal, and unlock. section 18.5
A mutex provides mutual exclusion for a critical section: code that accesses critical shared data. In C, `mtx_t` is a mutex type from `<threads.h>`. Calling `mtx_lock` blocks until no other thread is inside a critical section protected by that same mutex; `mtx_unlock` releases it. The protection depends on the threads using the same mutex. section 18.4
A game board is an example of data that needs this protection: arrays cannot be specified with `_Atomic`, and an atomic workaround would not be efficient. Smaller shared values can instead be `_Atomic`. TAKEAWAY: Mutex operations also provide linearizability. After acquiring a mutex, a thread can rely on operations performed before another thread released that mutex having taken effect. section 18.4 section 18.7
Related interfaces and rules:
- `mtx_trylock` tests whether the lock can be acquired without waiting; `mtx_timedlock` limits the wait. `mtx_init` and `mtx_destroy` are the initialization and destruction interfaces. section 18.4
- A condition-variable wait takes a mutex and may return even when the condition expression has not changed. Keep the wait inside a loop that checks the expression again. section 18.5
- A thread blocked on a mutex or condition variable frees processing resources for other threads. section 18.6
