pthreads mutex vs semaphore

semaphores have a synchronized counter and mutex's are just binary (true / false).

A semaphore is often used as a definitive mechanism for answering how many elements of a resource are in use -- e.g., an object that represents n worker threads might use a semaphore to count how many worker threads are available.

Truth is you can represent a semaphore by an INT that is synchronized by a mutex.

mutex is used to avoid race condition between multiple threads.

whereas semaphore is used as synchronizing element used across multiple process.

mutex can't be replaced with binary semaphore since, one process waits for semaphore while other process releases semaphore. In case mutex both acquisition and release is handled by same.

Mutex is like sempaphore with with S=1.

You can control number of concurrent accesses with semaphore but with mutex only one process at a time can access it.

See the implemenation of these two below: (all functions are atomic)

Semaphore:

wait(S) {
      while (S <= 0 )
         ; // busy wait
      S--;
}

signal(S) {
      S++;
}

Mutex:

acquire() {
      while (!available)
            ; // busy wait
      available = false;
}

release() {
      available = true;
}