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171 lines
9.3 KiB
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<title>Notification Packets - Debugging with GDB</title>
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<div class="node">
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<a name="Notification-Packets"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Remote-Non_002dStop.html#Remote-Non_002dStop">Remote Non-Stop</a>,
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Previous: <a rel="previous" accesskey="p" href="Interrupts.html#Interrupts">Interrupts</a>,
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Up: <a rel="up" accesskey="u" href="Remote-Protocol.html#Remote-Protocol">Remote Protocol</a>
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<hr>
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</div>
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<h3 class="section">E.9 Notification Packets</h3>
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<p><a name="index-notification-packets-3590"></a><a name="index-packets_002c-notification-3591"></a>
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The <span class="sc">gdb</span> remote serial protocol includes <dfn>notifications</dfn>,
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packets that require no acknowledgment. Both the GDB and the stub
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may send notifications (although the only notifications defined at
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present are sent by the stub). Notifications carry information
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without incurring the round-trip latency of an acknowledgment, and so
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are useful for low-impact communications where occasional packet loss
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is not a problem.
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<p>A notification packet has the form ‘<samp><span class="samp">% </span><var>data</var><span class="samp"> #
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</span><var>checksum</var></samp>’, where <var>data</var> is the content of the notification,
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and <var>checksum</var> is a checksum of <var>data</var>, computed and formatted
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as for ordinary <span class="sc">gdb</span> packets. A notification's <var>data</var>
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never contains ‘<samp><span class="samp">$</span></samp>’, ‘<samp><span class="samp">%</span></samp>’ or ‘<samp><span class="samp">#</span></samp>’ characters. Upon
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receiving a notification, the recipient sends no ‘<samp><span class="samp">+</span></samp>’ or ‘<samp><span class="samp">-</span></samp>’
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to acknowledge the notification's receipt or to report its corruption.
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<p>Every notification's <var>data</var> begins with a name, which contains no
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colon characters, followed by a colon character.
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<p>Recipients should silently ignore corrupted notifications and
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notifications they do not understand. Recipients should restart
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timeout periods on receipt of a well-formed notification, whether or
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not they understand it.
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<p>Senders should only send the notifications described here when this
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protocol description specifies that they are permitted. In the
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future, we may extend the protocol to permit existing notifications in
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new contexts; this rule helps older senders avoid confusing newer
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recipients.
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<p>(Older versions of <span class="sc">gdb</span> ignore bytes received until they see
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the ‘<samp><span class="samp">$</span></samp>’ byte that begins an ordinary packet, so new stubs may
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transmit notifications without fear of confusing older clients. There
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are no notifications defined for <span class="sc">gdb</span> to send at the moment, but we
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assume that most older stubs would ignore them, as well.)
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<p>Each notification is comprised of three parts:
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<dl>
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<dt>‘<samp><var>name</var><span class="samp">:</span><var>event</var></samp>’<dd>The notification packet is sent by the side that initiates the
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exchange (currently, only the stub does that), with <var>event</var>
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carrying the specific information about the notification, and
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<var>name</var> specifying the name of the notification.
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<br><dt>‘<samp><var>ack</var></samp>’<dd>The acknowledge sent by the other side, usually <span class="sc">gdb</span>, to
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acknowledge the exchange and request the event.
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</dl>
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<p>The purpose of an asynchronous notification mechanism is to report to
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<span class="sc">gdb</span> that something interesting happened in the remote stub.
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<p>The remote stub may send notification <var>name</var>:<var>event</var>
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at any time, but <span class="sc">gdb</span> acknowledges the notification when
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appropriate. The notification event is pending before <span class="sc">gdb</span>
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acknowledges. Only one notification at a time may be pending; if
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additional events occur before <span class="sc">gdb</span> has acknowledged the
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previous notification, they must be queued by the stub for later
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synchronous transmission in response to <var>ack</var> packets from
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<span class="sc">gdb</span>. Because the notification mechanism is unreliable,
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the stub is permitted to resend a notification if it believes
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<span class="sc">gdb</span> may not have received it.
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<p>Specifically, notifications may appear when <span class="sc">gdb</span> is not
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otherwise reading input from the stub, or when <span class="sc">gdb</span> is
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expecting to read a normal synchronous response or a
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‘<samp><span class="samp">+</span></samp>’/‘<samp><span class="samp">-</span></samp>’ acknowledgment to a packet it has sent.
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Notification packets are distinct from any other communication from
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the stub so there is no ambiguity.
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<p>After receiving a notification, <span class="sc">gdb</span> shall acknowledge it by
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sending a <var>ack</var> packet as a regular, synchronous request to the
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stub. Such acknowledgment is not required to happen immediately, as
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<span class="sc">gdb</span> is permitted to send other, unrelated packets to the
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stub first, which the stub should process normally.
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<p>Upon receiving a <var>ack</var> packet, if the stub has other queued
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events to report to <span class="sc">gdb</span>, it shall respond by sending a
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normal <var>event</var>. <span class="sc">gdb</span> shall then send another <var>ack</var>
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packet to solicit further responses; again, it is permitted to send
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other, unrelated packets as well which the stub should process
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normally.
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<p>If the stub receives a <var>ack</var> packet and there are no additional
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<var>event</var> to report, the stub shall return an ‘<samp><span class="samp">OK</span></samp>’ response.
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At this point, <span class="sc">gdb</span> has finished processing a notification
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and the stub has completed sending any queued events. <span class="sc">gdb</span>
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won't accept any new notifications until the final ‘<samp><span class="samp">OK</span></samp>’ is
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received . If further notification events occur, the stub shall send
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a new notification, <span class="sc">gdb</span> shall accept the notification, and
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the process shall be repeated.
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<p>The process of asynchronous notification can be illustrated by the
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following example:
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<pre class="smallexample"> <- <code>%Stop:T0505:98e7ffbf;04:4ce6ffbf;08:b1b6e54c;thread:p7526.7526;core:0;</code>
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<code>...</code>
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-> <code>vStopped</code>
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<- <code>T0505:68f37db7;04:40f37db7;08:63850408;thread:p7526.7528;core:0;</code>
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-> <code>vStopped</code>
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<- <code>T0505:68e3fdb6;04:40e3fdb6;08:63850408;thread:p7526.7529;core:0;</code>
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-> <code>vStopped</code>
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<- <code>OK</code>
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</pre>
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<p>The following notifications are defined:
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<p><table summary="">
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<tr align="left"><td valign="top" width="12%">Notification
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</td><td valign="top" width="12%">Ack
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</td><td valign="top" width="38%">Event
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</td><td valign="top" width="38%">Description
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<p><br></td></tr><tr align="left"><td valign="top" width="12%">Stop
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</td><td valign="top" width="12%">vStopped
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</td><td valign="top" width="38%"><var>reply</var>. The <var>reply</var> has the form of a stop reply, as
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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>,
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for information on how these notifications are acknowledged by
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<span class="sc">gdb</span>.
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</td><td valign="top" width="38%">Report an asynchronous stop event in non-stop mode.
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<br></td></tr></table>
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</body></html>
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