NAME
        GetMsg -- get next message from a message port

   SYNOPSIS
        message = GetMsg(port)
        D0               A0

        struct message *getmsg(struct msgport *);

   FUNCTION
        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().

   INPUT
        port - a pointer to the receiver message port

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

   SEE ALSO
        putmsg(), replymsg(), waitport(), wait(), exec/ports.h