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A message is delivered to a given destination port with the PutMsg()
function.  The message is queued to the port, and that port's arrival
action is invoked.  If the action specifies a task signal or a software
interrupt, the originating task may temporarily lose the processor while
the destination processes the message.  If a reply to the message is
required, the mn_ReplyPort field must be set up prior to the call to

Here is a code fragment for putting a message to a public port.  A
complete example is printed at the end of the chapter.

#Include <exec/types.h>
#include <exec/memory.h>
#include <exec/ports.h>
#include <libraries/dos.h>

VOID main(VOID);
BOOL SafePutToPort(struct Message *, STRPTR);

struct XYMessage {
    struct Message xy_Msg;
    UWORD          xy_X;
    UWORD          xy_Y;

    struct MsgPort *xyport, *xyreplyport;
    struct XYMessage *xymsg, *msg;
    BOOL   foundport;

    /* Allocate memory for the message we're going to send. */
    if (xymsg = (struct XYMessage *) AllocMem(sizeof(struct XYMessage),
                                             MEMF_PUBLIC | MEMF_CLEAR))

        /* The replyport we'll use to get response */
        if (xyreplyport = CreateMsgPort()) {
                                         /* or use CreatePort(0,0) */

            xymsg->xy_Msg.mn_Node.ln_Type = NT_MESSAGE;
                                         /* Compose the message    */
            xymsg->xy_Msg.mn_Length = sizeof(struct XYMessage);
            xymsg->xy_Msg.mn_ReplyPort = xyreplyport;
            xymsg->xy_X = 10;
            xymsg->xy_Y = 20;

            /* Now try to send that message to a public port named
             *  "xyport". If foundport eq 0, the port isn't out there.
            if (foundport = SafePutToPort((struct Message *)xymsg,

            . . .   /* Now let's wait till the someone responds... */

            else printf("Couldn't find 'xyport'\n");

            DeleteMsgPort(xyreplyport);  /* Use DeletePort() if    */
                                         /* the port was created   */
        }                                /* with CreatePort().     */
        else printf("Couldn't create message port\n");
        FreeMem(xymsg, sizeof(struct XYMessage));
    else printf("Couldn't get memory for xymessage\n");

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