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    GetMsg -- get next message from a message port

    message = GetMsg(port)
    D0               A0

    struct Message *GetMsg(struct MsgPort *);

    This function receives a message from a given message port. It
    provides a fast, non-copying message receiving mechanism. The
    received message is removed from the message port.

    This function will not wait.  If a message is not present this
    function will return zero.  If a program must wait for a message,
    it can Wait() on the signal specified for the port or use the
    WaitPort() function.  There can only be one task waiting for any
    given port.

    Getting a message does not imply to the sender that the message is
    free to be reused by the sender.  When the receiver is finished
    with the message, it may ReplyMsg() it back to the sender.

    Getting a signal does NOT always imply a message is ready.  More
    than one message may arrive per signal, and signals may show up
    without messages.  Typically you must loop to GetMsg() until it
    returns zero, then Wait() or WaitPort().

    port - a pointer to the receiver message port

    message - a pointer to the first message available.  If
              there are no messages, return zero.
              Callers must be prepared for zero at any time.

    PutMsg, ReplyMsg, WaitPort, Wait, exec/ports.h

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