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| <h3 class="section">D.12 Packet Acknowledgment</h3> |
| |
| <p><a name="index-acknowledgment_002c-for-_0040value_007bGDBN_007d-remote-2492"></a><a name="index-packet-acknowledgment_002c-for-_0040value_007bGDBN_007d-remote-2493"></a>By default, when either the host or the target machine receives a packet, |
| the first response expected is an acknowledgment: either ‘<samp><span class="samp">+</span></samp>’ (to indicate |
| the package was received correctly) or ‘<samp><span class="samp">-</span></samp>’ (to request retransmission). |
| This mechanism allows the <span class="sc">gdb</span> remote protocol to operate over |
| unreliable transport mechanisms, such as a serial line. |
| |
| <p>In cases where the transport mechanism is itself reliable (such as a pipe or |
| TCP connection), the ‘<samp><span class="samp">+</span></samp>’/‘<samp><span class="samp">-</span></samp>’ acknowledgments are redundant. |
| It may be desirable to disable them in that case to reduce communication |
| overhead, or for other reasons. This can be accomplished by means of the |
| ‘<samp><span class="samp">QStartNoAckMode</span></samp>’ packet; see <a href="QStartNoAckMode.html#QStartNoAckMode">QStartNoAckMode</a>. |
| |
| <p>When in no-acknowledgment mode, neither the stub nor <span class="sc">gdb</span> shall send or |
| expect ‘<samp><span class="samp">+</span></samp>’/‘<samp><span class="samp">-</span></samp>’ protocol acknowledgments. The packet |
| and response format still includes the normal checksum, as described in |
| <a href="Overview.html#Overview">Overview</a>, but the checksum may be ignored by the receiver. |
| |
| <p>If the stub supports ‘<samp><span class="samp">QStartNoAckMode</span></samp>’ and prefers to operate in |
| no-acknowledgment mode, it should report that to <span class="sc">gdb</span> |
| by including ‘<samp><span class="samp">QStartNoAckMode+</span></samp>’ in its response to ‘<samp><span class="samp">qSupported</span></samp>’; |
| see <a href="qSupported.html#qSupported">qSupported</a>. |
| If <span class="sc">gdb</span> also supports ‘<samp><span class="samp">QStartNoAckMode</span></samp>’ and it has not been |
| disabled via the <code>set remote noack-packet off</code> command |
| (see <a href="Remote-Configuration.html#Remote-Configuration">Remote Configuration</a>), |
| <span class="sc">gdb</span> may then send a ‘<samp><span class="samp">QStartNoAckMode</span></samp>’ packet to the stub. |
| Only then may the stub actually turn off packet acknowledgments. |
| <span class="sc">gdb</span> sends a final ‘<samp><span class="samp">+</span></samp>’ acknowledgment of the stub's ‘<samp><span class="samp">OK</span></samp>’ |
| response, which can be safely ignored by the stub. |
| |
| <p>Note that <code>set remote noack-packet</code> command only affects negotiation |
| between <span class="sc">gdb</span> and the stub when subsequent connections are made; |
| it does not affect the protocol acknowledgment state for any current |
| connection. |
| Since ‘<samp><span class="samp">+</span></samp>’/‘<samp><span class="samp">-</span></samp>’ acknowledgments are enabled by default when a |
| new connection is established, |
| there is also no protocol request to re-enable the acknowledgments |
| for the current connection, once disabled. |
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