pthreads mutex vs semaphore

Question

What is the difference between semaphores and mutex provided by pthread library ?

Answer 1

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;
}