Indeed, Dijkstra and colleagues invented the semaphore as a single primitive for all things related to synchronization; as you will see, one can use semaphores as both locks and condition variables.
Definition
A semaphore is an object with an integer value that we can manipulate with two routines; in the POSIX standard, these routines are sem_wait() P and sem_post() V.
int sem_wait(semt_t *s) {
decrement the value of semephore s by one
wait if value of semaphore s is negative
}
int sem_post(sem_t *s) {
increment the value of semphore s by one
if there are one or more threads waiting, wake one
}Implementation
void P(csem) {
while(1) {
mutex_acquire(csem.mx);
if (csem.value <= 0) {
mutex_release(csem.mx);
continue;
} else {
csem.value -= 1;
mutex_release(csem.mx);
break;
}
}
}
void V(csem) {
mutex_acquire(csem.mx);
csem.value += 1;
mutex_release(csem.mx);
}Re-think the P implementation. If the critical section is large, we could spend a great deal of time spinning.
Binary Semaphores (Locks)
@todo
reference
- cse170 semaphores and cv
- 信号量与mutex区别
- Goodbyes semaphores
- How are mutexes and semaphores different with respect to their implementation in a Linux kernel? PS - Ealrier mutex was implemented using binary semaphore but that is changed now. Please see below for more reference.