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# Critical sections (OpenMP)
In POSIX threads, mutual exclusion is explicit: you call `pthread_mutex_lock` before a shared update and `pthread_mutex_unlock` after.

**Critical sections** in OpenMP do the same thing with a pragma. The enclosed block is protected by an implicit mutex: only one thread at a time executes it, and threads that arrive while another is inside are blocked until it exits. Unlike [[atomic-openmp|`atomic`]], which is limited to a single read-modify-write operation, `critical` can protect an arbitrary sequence of statements.

```c
#pragma omp parallel for
for (int i = 0; i < N; i++) {
    int val = expensive_compute(i);
    #pragma omp critical
    {
        if (val > best) {
            best = val;
            best_idx = i;
        }
    }
}
```

Critical sections can be named (`#pragma omp critical(name)`) so that threads only contend with sections sharing the same name. Two differently-named critical sections can be entered simultaneously by different threads. Unnamed sections all share a single implicit lock, so mixing named and unnamed sections in the same program can cause unexpected serialisation.