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Semaphore CRC

Name:

 SemaphoreHeader

Comment:

 The following class defines a "semaphore" whose value is a non-negative integer. The semaphore has only two operations P() and V():

P() -- waits until value > 0, then decrement

V() -- increment, waking up a thread waiting in P() if necessary

Note that the interface 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 into a register, a context switch might have occurred, and some other thread might have called P or V, so the true value might now be different.

Collaborators:

Responsibilities: 

class Semaphore {
  public:
    Semaphore(char* debugName, int initialValue);       // set initial value
    ~Semaphore();                                       // de-allocate semaphore
    char* getName() { return name;}                     // debugging assist

    void P();    // these are the only operations on a semaphore
    void V();    // they are both *atomic*

  private:
    char* name;        // useful for debugging
    int value;         // semaphore value, always >= 0
    List *queue;       // threads waiting in P() for the value to be > 0
};