Wait shall I do with you.

sem_futex.c

// Playing around with what's necessary to implement a semaphore on top of Linux
// futexes.
static inline int my_sem_wait(struct semaphore *sem)
{
    int new = __sync_sub_and_fetch(&sem->value, 1);
    if(new < 0)
    {
        // This incorrectly handles the failure case where value is no longer
        // new when the call executes. Not sure how to fix.
        // If the new value is >= 0 then we've missed a wakeup
        // essentially and should stop waiting. If it's under we should try the
        // wait again with the new value. Or something...
        while(syscall(SYS_futex, &sem->value, FUTEX_WAIT, new, NULL, NULL, 0))
        {
            if(errno == EINTR) continue;
            return -1;
        }
    }

    return 0;
}