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| <title>Debugging with GDB: Notification Packets</title> |
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| <a name="Notification-Packets"></a> |
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| Next: <a href="Remote-Non_002dStop.html#Remote-Non_002dStop" accesskey="n" rel="next">Remote Non-Stop</a>, Previous: <a href="Interrupts.html#Interrupts" accesskey="p" rel="prev">Interrupts</a>, Up: <a href="Remote-Protocol.html#Remote-Protocol" accesskey="u" rel="up">Remote Protocol</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Notification-Packets-1"></a> |
| <h3 class="section">E.9 Notification Packets</h3> |
| <a name="index-notification-packets"></a> |
| <a name="index-packets_002c-notification"></a> |
| |
| <p>The <small>GDB</small> remote serial protocol includes <em>notifications</em>, |
| 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. |
| </p> |
| <p>A notification packet has the form ‘<samp>% <var>data</var> # |
| <var>checksum</var></samp>’, where <var>data</var> is the content of the notification, |
| and <var>checksum</var> is a checksum of <var>data</var>, computed and formatted |
| as for ordinary <small>GDB</small> packets. A notification’s <var>data</var> |
| never contains ‘<samp>$</samp>’, ‘<samp>%</samp>’ or ‘<samp>#</samp>’ characters. Upon |
| receiving a notification, the recipient sends no ‘<samp>+</samp>’ or ‘<samp>-</samp>’ |
| to acknowledge the notification’s receipt or to report its corruption. |
| </p> |
| <p>Every notification’s <var>data</var> begins with a name, which contains no |
| colon characters, followed by a colon character. |
| </p> |
| <p>Recipients should silently ignore corrupted notifications and |
| notifications they do not understand. Recipients should restart |
| timeout periods on receipt of a well-formed notification, whether or |
| not they understand it. |
| </p> |
| <p>Senders should only send the notifications described here when this |
| protocol description specifies that they are permitted. In the |
| future, we may extend the protocol to permit existing notifications in |
| new contexts; this rule helps older senders avoid confusing newer |
| recipients. |
| </p> |
| <p>(Older versions of <small>GDB</small> ignore bytes received until they see |
| the ‘<samp>$</samp>’ byte that begins an ordinary packet, so new stubs may |
| transmit notifications without fear of confusing older clients. There |
| are no notifications defined for <small>GDB</small> to send at the moment, but we |
| assume that most older stubs would ignore them, as well.) |
| </p> |
| <p>Each notification is comprised of three parts: |
| </p><dl compact="compact"> |
| <dt>‘<samp><var>name</var>:<var>event</var></samp>’</dt> |
| <dd><p>The notification packet is sent by the side that initiates the |
| exchange (currently, only the stub does that), with <var>event</var> |
| carrying the specific information about the notification, and |
| <var>name</var> specifying the name of the notification. |
| </p></dd> |
| <dt>‘<samp><var>ack</var></samp>’</dt> |
| <dd><p>The acknowledge sent by the other side, usually <small>GDB</small>, to |
| acknowledge the exchange and request the event. |
| </p></dd> |
| </dl> |
| |
| <p>The purpose of an asynchronous notification mechanism is to report to |
| <small>GDB</small> that something interesting happened in the remote stub. |
| </p> |
| <p>The remote stub may send notification <var>name</var>:<var>event</var> |
| at any time, but <small>GDB</small> acknowledges the notification when |
| appropriate. The notification event is pending before <small>GDB</small> |
| acknowledges. Only one notification at a time may be pending; if |
| additional events occur before <small>GDB</small> has acknowledged the |
| previous notification, they must be queued by the stub for later |
| synchronous transmission in response to <var>ack</var> packets from |
| <small>GDB</small>. Because the notification mechanism is unreliable, |
| the stub is permitted to resend a notification if it believes |
| <small>GDB</small> may not have received it. |
| </p> |
| <p>Specifically, notifications may appear when <small>GDB</small> is not |
| otherwise reading input from the stub, or when <small>GDB</small> is |
| expecting to read a normal synchronous response or a |
| ‘<samp>+</samp>’/‘<samp>-</samp>’ acknowledgment to a packet it has sent. |
| Notification packets are distinct from any other communication from |
| the stub so there is no ambiguity. |
| </p> |
| <p>After receiving a notification, <small>GDB</small> shall acknowledge it by |
| sending a <var>ack</var> packet as a regular, synchronous request to the |
| stub. Such acknowledgment is not required to happen immediately, as |
| <small>GDB</small> is permitted to send other, unrelated packets to the |
| stub first, which the stub should process normally. |
| </p> |
| <p>Upon receiving a <var>ack</var> packet, if the stub has other queued |
| events to report to <small>GDB</small>, it shall respond by sending a |
| normal <var>event</var>. <small>GDB</small> shall then send another <var>ack</var> |
| packet to solicit further responses; again, it is permitted to send |
| other, unrelated packets as well which the stub should process |
| normally. |
| </p> |
| <p>If the stub receives a <var>ack</var> packet and there are no additional |
| <var>event</var> to report, the stub shall return an ‘<samp>OK</samp>’ response. |
| At this point, <small>GDB</small> has finished processing a notification |
| and the stub has completed sending any queued events. <small>GDB</small> |
| won’t accept any new notifications until the final ‘<samp>OK</samp>’ is |
| received . If further notification events occur, the stub shall send |
| a new notification, <small>GDB</small> shall accept the notification, and |
| the process shall be repeated. |
| </p> |
| <p>The process of asynchronous notification can be illustrated by the |
| following example: |
| </p><div class="smallexample"> |
| <pre class="smallexample"><- <code>%%Stop:T0505:98e7ffbf;04:4ce6ffbf;08:b1b6e54c;thread:p7526.7526;core:0;</code> |
| <code>...</code> |
| -> <code>vStopped</code> |
| <- <code>T0505:68f37db7;04:40f37db7;08:63850408;thread:p7526.7528;core:0;</code> |
| -> <code>vStopped</code> |
| <- <code>T0505:68e3fdb6;04:40e3fdb6;08:63850408;thread:p7526.7529;core:0;</code> |
| -> <code>vStopped</code> |
| <- <code>OK</code> |
| </pre></div> |
| |
| <p>The following notifications are defined: |
| </p><table> |
| <tr><td width="12%">Notification</td><td width="12%">Ack</td><td width="38%">Event</td><td width="38%">Description</td></tr> |
| <tr><td width="12%">Stop</td><td width="12%">vStopped</td><td width="38%"><var>reply</var>. The <var>reply</var> has the form of a stop reply, as |
| described in <a href="Stop-Reply-Packets.html#Stop-Reply-Packets">Stop Reply Packets</a>. Refer to <a href="Remote-Non_002dStop.html#Remote-Non_002dStop">Remote Non-Stop</a>, |
| for information on how these notifications are acknowledged by |
| <small>GDB</small>.</td><td width="38%">Report an asynchronous stop event in non-stop mode.</td></tr> |
| </table> |
| |
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