semaphore solution for OS exam

semaphores.c

#define N             // number of buffers
int in = 0, out = 0;  // Reader and writer pointers to buffer spaces

// Semaphores
resources = 0;        // number of resources that we are producing
buffer = N;           // buffer space
mutex = 1;            // Mutual exclusion to make sure we aren't reading and writing at the same time
criticalA = 1;        // critical A and B are used to enforce the requirement that we produce A, then B, then A, etc...
criticalB = 1;

producerA(res A) {
  down(buffer);
  down(mutex);
  down(criticalB);
  // write resource A to buffer
  up(criticalA);
  up(mutex);
  up(resources);
}

producerB(res B) {
  down(buffer);
  down(mutex);
  down(criticalA);
  // write resource B to buffer
  up(criticalB);
  up(mutex);
  up(resources);
}

res consumer() {
  down(mutex);
  down(resources);
  // read index, index + 1 from buffer
  up(buffer);
  up(buffer); // up buffer twice since we consume two resources
}