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PostOffice CRC

Name:

 PostOfficeHeader

Comment:

 The following class defines a "Post Office", or a collection of mailboxes. The Post Office is a synchronization object that provides two main operations: Send -- send a message to a mailbox on a remote machine, and Receive -- wait until a message is in the mailbox, then remove and return it.

Incoming messages are put by the PostOffice into the appropriate mailbox, waking up any threads waiting on Receive.

Collaborators:

Responsibilities: 

class PostOffice {
  public:
    PostOffice(NetworkAddress addr, double reliability, int nBoxes);
                                // Allocate and initialize Post Office
                                //   "reliability" is how many packets
                                //   get dropped by the underlying network
    ~PostOffice();              // De-allocate Post Office data

    void Send(PacketHeader pktHdr, MailHeader mailHdr, char *data);
                                // Send a message to a mailbox on a remote
                                // machine.  The fromBox in the MailHeader is
                                // the return box for ack's.

    void Receive(int box, PacketHeader *pktHdr,
                MailHeader *mailHdr, char *data);
                                // Retrieve a message from "box".  Wait if
                                // there is no message in the box.

    void PostalDelivery();      // Wait for incoming messages,
                                // and then put them in the correct mailbox

    void PacketSent();          // Interrupt handler, called when outgoing
                                // packet has been put on network; next
                                // packet can now be sent
    void IncomingPacket();      // Interrupt handler, called when incoming
                                // packet has arrived and can be pulled
                                // off of network (i.e., time to call
                                // PostalDelivery)

  private:
    Network *network;           // Physical network connection
    NetworkAddress netAddr;     // Network address of this machine
    MailBox *boxes;             // Table of mail boxes to hold incoming mail
    int numBoxes;               // Number of mail boxes
    Semaphore *messageAvailable;// V'ed when message has arrived from network
    Semaphore *messageSent;     // V'ed when next message can be sent to network
    Lock *sendLock;             // Only one outgoing message at a time
};