nachos.threads
Class Semaphore

java.lang.Object
  nachos.threads.Semaphore

public class Semaphore
extends Object

A Semaphore is a synchronization primitive with an unsigned value. A semaphore has only two operations:

• P(): waits until the semaphore's value is greater than zero, then decrements it.
• V(): increments the semaphore's value, and wakes up one thread waiting in P() if possible.

Note that this API does not allow a thread to read the value of the semaphore directly. Even if you did read the value, the only thing you would know is what the value used to be. You don't know what the value is now, because by the time you get the value, a context switch might have occurred, and some other thread might have called P() or V(), so the true value might now be different.

Constructor Summary

Semaphore(int initialValue)
Allocate a new semaphore.

Method Summary

void	P() Atomically wait for this semaphore to become non-zero and decrement it.
static void	selfTest() Test if this module is working.
void	V() Atomically increment this semaphore and wake up at most one other thread sleeping on this semaphore.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Semaphore

public Semaphore(int initialValue)

Allocate a new semaphore.

Parameters:

initialValue - the initial value of this semaphore.

Method Detail

P

public void P()

Atomically wait for this semaphore to become non-zero and decrement it.

V

public void V()

Atomically increment this semaphore and wake up at most one other thread sleeping on this semaphore.

selfTest

public static void selfTest()

Test if this module is working.