Implementing semaphore by using mutex operations and primitives

Question

Some time ago had an interview and was asked to implement Semaphore by using mutex operations and primitives only (he allowed int to be considered as atomic). I came with soluti

Answer 1

EDIT - use a second mutex for queuing intstead of threads

Since a mutex already have proper thread-support, it can be used to queue the threads (instead of doing it explicitly as I first had tried to do). Unless the mutex is restricted to only let the owner unlock it (a lock?), then this solution doesn't work.

I found the solution in Anthony Howe's pdf that I came across when searching. There are two more solutions given there as well. I changed the names to make more sense for this example.

more or less pseudo code:

Semaphore{
    int n;
    mutex* m_count;         //unlocked initially
    mutex* m_queue;         //locked initially
};

void wait(){
    m_count.lock();
    n = n-1;
    if(n < 0){
        m_count.unlock();
        m_queue.lock();     //wait
    }
    m_count.unlock();       //unlock signal's lock
}

void signal(){
    m_count.lock();
    n = n+1;

    if(n <= 0){
        m_queue.unlock();   //leave m_count locked
    }
    else{
        m_count.unlock();
    }
}