Question

Implement synchronous send and receive of one word messages (also known as Ada-style rendezvous), using condition variables (don't use semaphores!). Implement the Communicator class with operations, void speak(int word) and int listen().

speak() atomically waits until listen() is called on the same Communicator object, and then transfers the word over to listen(). Once the transfer is made, both can return. Similarly, listen() waits until speak() is called, at which point the transfer is made, and both can return (listen() returns the word). Your solution should work even if there are multiple speakers and listeners for the same Communicator (note: this is equivalent to a zero-length bounded buffer; since the buffer has no room, the producer and consumer must interact directly, requiring that they wait for one another). Each communicator should only use exactly one lock. If you're using more than one lock, you're making things too complicated.

Answer 1

package nachos.threads;
import nachos.machine.*;

/**
* A communicator allows threads to synchronously exchange 32-bit
* messages. Multiple threads can be waiting to speak,
* and multiple threads can be waiting to listen. But there should never
* be a time when both a speaker and a listener are waiting, because the two
* threads can be paired off at this point.
*/

public class Communicator {

/**
* Allocate a new communicator.
*/

    public Communicator() {
      mutex = new Lock();
    listenReady = new Condition(mutex);
    speakReady = new Condition(mutex);
    }

/**
* Wait for a thread to listen through this communicator, and then transfer
* <i>word</i> to the listener.
*
* <p>
* Does not return until this thread is paired up with a listening thread.
* Exactly one listener should receive <i>word</i>.
*
* @param　word　the integer to transfer.
*/

    public void speak(int word) {

        mutex.acquire();
　　speaker++;
      while(listener==0){
            listenReady.sleep();
        }
    listener--;
　messenger = word;
    speakReady.wake();

    mutex.release();
    }

/**
* Wait for a thread to speak through this communicator, and then return
* the word that thread passed to speak().
*
* @return　the integer transferred.
*/

    public int listen() {

        mutex.acquire();
　　listener++;
　　listenReady.wake();
　　while(speaker==0) {
            speakReady.sleep();
        }
　　speaker--;
　　mutex.release();
　　return messenger;
    }
    private Condition speakReady;
　private Condition listenReady;
　private int speaker=0, listener=0;
　private int messenger;
　private Lock mutex;
}

package nachos.threads;
import nachos.machine.*;

/**
* A communicator allows threads to synchronously exchange 32-bit
* messages. Multiple threads can be waiting to speak,
* and multiple threads can be waiting to listen. But there should never
* be a time when both a speaker and a listener are waiting, because the two
* threads can be paired off at this point.
*/

public class Communicator {

/**
* Allocate a new communicator.
*/

    public Communicator() {
      mutex = new Lock();
    listenReady = new Condition(mutex);
    speakReady = new Condition(mutex);
    }

/**
* Wait for a thread to listen through this communicator, and then transfer
* <i>word</i> to the listener.
*
* <p>
* Does not return until this thread is paired up with a listening thread.
* Exactly one listener should receive <i>word</i>.
*
* @param　word　the integer to transfer.
*/

    public void speak(int word) {

        mutex.acquire();
　　speaker++;
      while(listener==0){
            listenReady.sleep();
        }
    listener--;
　messenger = word;
    speakReady.wake();

    mutex.release();
    }

/**
* Wait for a thread to speak through this communicator, and then return
* the word that thread passed to speak().
*
* @return　the integer transferred.
*/

    public int listen() {

        mutex.acquire();
　　listener++;
　　listenReady.wake();
　　while(speaker==0) {
            speakReady.sleep();
        }
　　speaker--;
　　mutex.release();
　　return messenger;
    }
    private Condition speakReady;
　private Condition listenReady;
　private int speaker=0, listener=0;
　private int messenger;
　private Lock mutex;
}

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