Semaphores using Monitors

Monitor Semaphore

int s;

condition s_c;

init(value) { s = value }

P () { s--; if (s<0) s_c.wait() }

V() { S++; if (s<=0) c_s.signal() }

End Monitor

Monitors using Semaphores

Monitor M; // the monitor

Entry func(..) {…

… ..

c.wait()

… ..

c.signal() ..

}

End Monitor;

semaphore mutex, next, x_sem;

int next_count, x_count;

func()

{

P(mutex) // entry section

….

….

x_count ++ // “wait call”

if (next_count > 0) V(next)

else V(mutex)

P(x_sem)

X_count--;

…..

…..

if(x_count > 0) { // “signal” call

next_count++;

V(x_sem)

P(next)

next_count--;

}

… ..

… ..

if (next_count > 0) V(next) // “exit” section

else V(mutex)

}