SDL2

SDL_sem* SDL_CreateSemaphore(Uint32 initial_value)

SDL_atomic_t done;
SDL_sem *sem;

SDL_AtomicSet(&done, 0);
sem = SDL_CreateSemaphore(0);
.
.
Thread A:
    while (!SDL_AtomicGet(&done))
    {
        add_data_to_queue();
        SDL_SemPost(sem);
    }

Thread B:
    while (!SDL_AtomicGet(&done))
    {
        SDL_SemWait(sem);
        if (data_available())
        {
            get_data_from_queue();
        }
    }
.
.
SDL_AtomicSet(&done, 1);
SDL_SemPost(sem);
wait_for_threads();
SDL_DestroySemaphore(sem);