Chapter 11 Receiving Messages

This chapter describes how to retrieve messages delivered to your application and how to handle the message once you have examined it. It also shows you how to send replies to requests that you receive.

To retrieve and handle ToolTalk messages, your application must perform several operations: it must be able to retrieve ToolTalk messages; it must be able to examine messages; it must provide callback routines; it must be able to respond to requests; and it must be able to destroy the message when it is no longer needed.

Retrieving Messages

When a message arrives for your process, the ToolTalk-supplied file descriptor becomes active. When notified of the active state of the file descriptor, your process must call tt_message_receive to get a handle for the incoming message.

Example 11–1 illustrates how to receive a message.

Example 11–1 Receiving a Message

/*
 * When a ToolTalk message is available, receive it; if it's a
 * ttsample1_value message, update the gauge with the new value.
 */
void
receive_tt_message()
{
	Tt_message msg_in;
	int mark;
	int val_in;

	msg_in = tt_message_receive();

	/*
	 * It's possible that the file descriptor would become active
	 * even though ToolTalk doesn't really have a message for us.
	 * The returned message handle is NULL in this case.
	 */

	if (msg_in == NULL) return;

Handles for messages remain constant. For example, when a process sends a message, both the message and any replies to the message have the same handle as the sent message. Example 11–2 is an example of how you can check the message state for TT_HANDLED.

Example 11–2 Code Checking the Message State

Tt_message m, n;
m = tt_message_create();
...
tt_message_send(m);

... wait around for tt_fd to become active

n = tt_message_receive();
if (m == n) {
	/* This is the reply to the message we sent */
	if (TT_HANDLED == tt_message_state(m) ) {
			/* The receiver has handled the message so we can go
on */
			...
	}
} else {
	/* This is a new message coming in */
)

Identifying and Processing Messages Easily

To easily identify and process messages received by you:

• Add a callback to a dynamic pattern with tt_pattern_callback_add. When you retrieve the message, the ToolTalk service will invoke any message or pattern callbacks. See Chapter 9, Dynamic Message Patterns for more information on placing callbacks on patterns.

• Retrieve the message's opnum if you are receiving messages that match your ptype message patterns.

Recognizing and Handling Replies Easily

To easily recognize and handle replies to messages sent by you:

• Place specific callbacks on requests before you send them with tt_message_callback_add. See Chapter 8, Sending Messages for more information on placing callbacks on messages.

• Compare the handle of the message you sent with the message you just received. The handles will be the same if the message is a reply.

• Add information meaningful to your application on the request with the tt_message_user_set call.

Checking Message Status

When you receive a message, you must check its status. If the status is TT_WRN_START_MESSAGE, you must either reply, reject, or fail the message even if the message is a notice, or issue a tt_message_accept call. Programs started using the ToolTalk service that receive a status of TT_WRN_START_MESSAGE should check tt_message_uid() and tt_message_gid(). You may want to fail the request with TT_DESKTOP_EACCES if the UNIX UID and/or GID do not agree with the request. Similarly, applications already running may want to reject requests with TT_DESKTOP_EACCES if there is UID or GID disagreement. This will cause serial rejection of the message until either a matching-ID handler is found, or an autostarted handler fails the request.

Examining Messages

When your process receives a message, you examine the message and take appropriate action.

Before you start to retrieve values, obtain a mark on the ToolTalk API stack so that you can release the information the ToolTalk service returns to you all at once. Example 11–3 allocates storage, examines message contents, and releases the storage.

Example 11–3 Allocating, Examining, and Releasing Storage

        /*
         * Get a storage mark so we can easily free all the data
         * ToolTalk returns to us.
         */

        mark = tt_mark();

        if (0==strcmp(“ttsample1_value”, tt_message_op(msg_in))) {
                tt_message_arg_ival(msg_in, 0, &val_in);
                xv_set(gauge, PANEL_VALUE, val_in, NULL);
        }

        tt_message_destroy(msg_in);
        tt_release(mark);
        return;

Table 11–1 lists the ToolTalk functions you use to examine the attributes of a message you have received.

Callback Routines

You can tell the ToolTalk service to invoke a callback when a message arrives because a pattern has been matched.

p = tt_pattern_create();
   tt_pattern_op_add(p, "EDIT");
   ... other pattern attributes
   tt_pattern_callback_add(p, do_edit_message);
   tt_pattern_register(p);

Callbacks are called in reverse order of registration (for example, the most recently added callback is called first).

Figure 11–1 illustrates how the ToolTalk service invokes message and pattern callbacks when tt_message_receive is called to retrieve a new message.

Figure 11–1 How Callbacks Are Invoked

Callbacks for Messages Addressed to Handlers

After the ToolTalk service determines the receiver for a message addressed to a handler, it matches the message against any patterns registered by the receiver. (Messages explicitly addressed to handlers are point-to-point messages and do not use pattern matching.)

• If the message does not match a pattern, the message is delivered in the normal manner.

• If the message is matched to a pattern, any callbacks attached to the pattern are run.

Attaching Callbacks to Static Patterns

Numeric tags (opnums) can be attached to each signature in a ptype when a static pattern is created. A callback can now be attached to the opnum. When a message is delivered because it matched a static pattern with an opnum, the ToolTalk service checks for any callbacks attached to the opnum and, if any exists, runs them.

Handling Requests

When your process receives a request (class = TT_REQUEST), you must either reply to the request, or reject or fail the request.

Replying to Requests

When you reply to a request, you need to:

1. Perform the requested operation.

2. Fill in any argument values with modes of TT_OUT or TT_INOUT.

3. Send the reply to the message.

   Table 11–2 lists the ToolTalk functions you use to reply to requests.

   Table 11–2 Functions to Reply to Requests

   ToolTalk Function	Description
   tt_message_arg_mode(Tt_message m, int n)	The argument mode (in, out, inout). Return type is Tt_mode.
   tt_message_arg_bval_set(Tt_message m, int n, const unsigned char *value, int len)	Sets an argument's value to the specified byte array. Return type is Tt_status.
   tt_message_arg_ival_set(Tt_message m, int n, int value)	Sets an argument's value to the specified integer. Return type is Tt_status.
   tt_message_arg_val_set(Tt_message m, int n, const char *value)	Sets an argument's value to the specified string. Return type is Tt_status.
   tt_message_arg_xval_set(Tt_message m, int n, xdrproc_t xdr_proc, void *value)	Return type is Tt_status.
   tt_message_context_set(Tt_message m, const char *slotname, const char *value);	Sets a context to the specified string. Return type is Tt_status.
   tt_message_bcontext_set(Tt_message m, const char *slotname, unsigned char *value, int length);	Sets a context to the specified byte array. Return type is Tt_status.
   tt_message_icontext_set(Tt_message m, const char *slotname, int value);	Sets a context to the specified integer. Return type is Tt_status.
   tt_message_xcontext_set(Tt_message m, const char *slotname, xdrproc_t xdr_proc, void *value)	Return type is Tt_status.
   tt_message_reply(Tt_message m)	Replies to message. Return type is Tt_status.

Rejecting or Failing a Request

If you have examined the request and your application is not currently able to handle the request, you can use the ToolTalk functions listed in Table 11–3 to reject or fail a request.

The return type for these requests is Tt_status.

Rejecting a Request

If you have examined the request and your application is not currently able to perform the operation but another application might be able to do so, use tt_message_reject to reject the request.

When you reject a request, the ToolTalk service attempts to find another receiver to handle it. If the ToolTalk service cannot find a handler that is currently running, it examines the disposition attribute, and either queues the message or attempts to start applications with ptypes that contain the appropriate message pattern.

Failing a Request

If you have examined the request and the requested operation cannot be performed by you or any other process with the same ptype as yours, use tt_message_fail to inform the ToolTalk service that the operation cannot be performed. The ToolTalk service will inform the sender that the request failed.

To inform the sender of the reason the request failed, use tt_message_status_set or tt_message_status_string_set before you call tt_message_fail.

The status code you specify with tt_message_status_set must be greater than TT_ERR_LAST.

Observing Offers

When your process receives an offer (class = TT_OFFER) in state TT_SENT, it must eventually do one of five things:

1. Accept the offer by calling tt_message_accept() on the message. This will tell the sending procid that the receiving procid has accepted the offer.

2. Reject the offer by calling tt_message_reject() on the message. This will tell the sending procid that the receiving procid has rejected the offer.

3. Abstain from the offer by calling tt_message_destroy() on the message without accepting or rejecting it first. This will tell the sending procid that the receiving procid has abstained from the offer.

4. Abstain from the offer by calling tt_message_receive() again without accepting or rejecting the offer first. This also will tell the sending procid that the receiving procid has abstained from the offer.

5. Disconnect from the ToolTalk service by calling tt_close(), or by exiting (normally or abnormally). In this case the ttsession process to which the client process is connected will mark the client process as abstaining from the offer.

When the handler (if any) and all the observers have accepted, rejected, or abstained from the message, the message state (Tt_state) will be set to TT_RETURNED. Intermediate states on an offer that will not be seen on other message classes are defined as:

1. TT_ACCEPTED—an Offer will enter this state whenever a receiver does a tt_message_accept() on it.

2. TT_REJECTED—an Offer will enter this state whenever a receiver does a tt_message_reject() on it.

3. TT_ABSTAINED—an Offer will enter this state whenever a receiver does choice 3, 4, or 5 above on it.

Destroying Messages

After you have processed a message and no longer need the information in the message, use tt_message_destroy to delete the message and free storage space.