https://old.reddit.com/r/C_Programming/comments/1mcxo1t/producerconsumer_with_semaphores_shared_mutex/

test.c

// $ cc -g3 -fsanitize=thread,undefined test.c
//
// No data races detected because the mutex is unnecessary. To verify TSan
// is indeed working, modify newqueue to initialize improperly, such as
// setting a zero field to non-zero. This will cause a data race.

#define lenof(a)    (int)(sizeof(a) / sizeof(*(a)))

typedef struct {
    int count;
} Sem;

static void post(Sem *s)
{
    __atomic_add_fetch(&s->count, 1, __ATOMIC_RELEASE);
}

static void wait(Sem *s)
{
    for (;;) {
        int old = __atomic_load_n(&s->count, __ATOMIC_ACQUIRE);
        while (old) {
            bool r = __atomic_compare_exchange_n(
                &s->count, &old, old-1, false,
                __ATOMIC_ACQUIRE,
                __ATOMIC_RELAXED
            );
            if (r) return;
        }
    }
}

typedef struct {
    Sem  filled;
    Sem  empty;
    int  read;
    int  write;
    long slots[1<<5];
} Queue;

static Queue newqueue()
{
    Queue q = {};
    q.empty = (Sem){lenof(q.slots)};
    return q;
}

static void push(Queue *q, long v)
{
    int mask = lenof(q->slots) - 1;
    wait(&q->empty);
    q->slots[(q->write += 1u)&mask] = v;
    post(&q->filled);
}

static long pop(Queue *q)
{
    int mask = lenof(q->slots) - 1;
    wait(&q->filled);
    long v = q->slots[(q->read += 1u)&mask];
    post(&q->empty);
    return v;
}


// Test
#include <assert.h>
#include <pthread.h>

enum { N = 1'000'000 };

static void *producer(void *arg)
{
    Queue *q = arg;
    for (int i = 1; i <= N; i++) {
        push(q, i);
    }
    push(q, 0);
    return 0;
}

int main()
{
    Queue q = newqueue();

    pthread_t th;
    pthread_create(&th, 0, producer, &q);
    pthread_detach(th);

    long long sum = 0;
    for (long v; (v = pop(&q));) {
        sum += v;
    }
    assert(sum == (N * (N + 1LL))/2);
}