// two threads work on a single variable

int i = 0;
par
  { i = i+1; } // critical region
  { i = 2*i; } // critical region
end par
print(i)

// Expected results: 2 or 1 (non-determinism, race condition)
// but can be also 0 without safe critical regions!


// Solution: semaphore [Dijkstra, 1968]

// Semaphor atomic(!) operations:
P(s) {
  if s.value > 0
  then s.value = s.value - 1;
  else <suspend current process and add to s.queue>
}

V(s) {
  if <s.queue not empty>
  then <delete some thread from s.queue and start it>
  else s.value = s.value + 1;
}


// same example with semaphore:
int i = 0;
Sempahore s = 1; // binary
par
  { P(s); i = i+1; V(s); } // critical region
  { P(s); i = 2*i; V(s); } // critical region
end par
print(i)