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E.9 Notification Packets

The gdb remote serial protocol includes notifications, packets that require no acknowledgment. Both the GDB and the stub may send notifications (although the only notifications defined at present are sent by the stub). Notifications carry information without incurring the round-trip latency of an acknowledgment, and so are useful for low-impact communications where occasional packet loss is not a problem.

A notification packet has the form ‘% data # checksum’, where data is the content of the notification, and checksum is a checksum of data, computed and formatted as for ordinary gdb packets. A notification's data never contains ‘$’, ‘%’ or ‘#’ characters. Upon receiving a notification, the recipient sends no ‘+’ or ‘-’ to acknowledge the notification's receipt or to report its corruption.

Every notification's data begins with a name, which contains no colon characters, followed by a colon character.

Recipients should silently ignore corrupted notifications and notifications they do not understand. Recipients should restart timeout periods on receipt of a well-formed notification, whether or not they understand it.

Senders should only send the notifications described here when this protocol description specifies that they are permitted. In the future, we may extend the protocol to permit existing notifications in new contexts; this rule helps older senders avoid confusing newer recipients.

(Older versions of gdb ignore bytes received until they see the ‘$’ byte that begins an ordinary packet, so new stubs may transmit notifications without fear of confusing older clients. There are no notifications defined for gdb to send at the moment, but we assume that most older stubs would ignore them, as well.)

Each notification is comprised of three parts:

The notification packet is sent by the side that initiates the exchange (currently, only the stub does that), with event carrying the specific information about the notification. name is the name of the notification.
The acknowledge sent by the other side, usually gdb, to acknowledge the exchange and request the event.

The purpose of an asynchronous notification mechanism is to report to gdb that something interesting happened in the remote stub.

The remote stub may send notification name:event at any time, but gdb acknowledges the notification when appropriate. The notification event is pending before gdb acknowledges. Only one notification at a time may be pending; if additional events occur before gdb has acknowledged the previous notification, they must be queued by the stub for later synchronous transmission in response to ack packets from gdb. Because the notification mechanism is unreliable, the stub is permitted to resend a notification if it believes gdb may not have received it.

Specifically, notifications may appear when gdb is not otherwise reading input from the stub, or when gdb is expecting to read a normal synchronous response or a ‘+’/‘-’ acknowledgment to a packet it has sent. Notification packets are distinct from any other communication from the stub so there is no ambiguity.

After receiving a notification, gdb shall acknowledge it by sending a ack packet as a regular, synchronous request to the stub. Such acknowledgment is not required to happen immediately, as gdb is permitted to send other, unrelated packets to the stub first, which the stub should process normally.

Upon receiving a ack packet, if the stub has other queued events to report to gdb, it shall respond by sending a normal event. gdb shall then send another ack packet to solicit further responses; again, it is permitted to send other, unrelated packets as well which the stub should process normally.

If the stub receives a ack packet and there are no additional event to report, the stub shall return an ‘OK’ response. At this point, gdb has finished processing a notification and the stub has completed sending any queued events. gdb won't accept any new notifications until the final ‘OK’ is received . If further notification events occur, the stub shall send a new notification, gdb shall accept the notification, and the process shall be repeated.

The process of asynchronous notification can be illustrated by the following example:

     <- %%Stop:T0505:98e7ffbf;04:4ce6ffbf;08:b1b6e54c;thread:p7526.7526;core:0;
     -> vStopped
     <- T0505:68f37db7;04:40f37db7;08:63850408;thread:p7526.7528;core:0;
     -> vStopped
     <- T0505:68e3fdb6;04:40e3fdb6;08:63850408;thread:p7526.7529;core:0;
     -> vStopped
     <- OK

The following notifications are defined:

Notification Ack Event Description

Stop vStopped reply. The reply has the form of a stop reply, as described in Stop Reply Packets. Refer to Remote Non-Stop, for information on how these notifications are acknowledged by gdb. Report an asynchronous stop event in non-stop mode.