91 lines
5.5 KiB
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91 lines
5.5 KiB
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<title>Packet Acknowledgment - Debugging with GDB</title>
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<link rel="up" href="Remote-Protocol.html#Remote-Protocol" title="Remote Protocol">
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<link rel="prev" href="Remote-Non_002dStop.html#Remote-Non_002dStop" title="Remote Non-Stop">
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<div class="node">
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<a name="Packet-Acknowledgment"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Examples.html#Examples">Examples</a>,
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Previous: <a rel="previous" accesskey="p" href="Remote-Non_002dStop.html#Remote-Non_002dStop">Remote Non-Stop</a>,
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Up: <a rel="up" accesskey="u" href="Remote-Protocol.html#Remote-Protocol">Remote Protocol</a>
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</div>
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<h3 class="section">E.11 Packet Acknowledgment</h3>
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<p><a name="index-acknowledgment_002c-for-_0040value_007bGDBN_007d-remote-3592"></a><a name="index-packet-acknowledgment_002c-for-_0040value_007bGDBN_007d-remote-3593"></a>By default, when either the host or the target machine receives a packet,
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the first response expected is an acknowledgment: either ‘<samp><span class="samp">+</span></samp>’ (to indicate
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the package was received correctly) or ‘<samp><span class="samp">-</span></samp>’ (to request retransmission).
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This mechanism allows the <span class="sc">gdb</span> remote protocol to operate over
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unreliable transport mechanisms, such as a serial line.
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<p>In cases where the transport mechanism is itself reliable (such as a pipe or
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TCP connection), the ‘<samp><span class="samp">+</span></samp>’/‘<samp><span class="samp">-</span></samp>’ acknowledgments are redundant.
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It may be desirable to disable them in that case to reduce communication
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overhead, or for other reasons. This can be accomplished by means of the
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‘<samp><span class="samp">QStartNoAckMode</span></samp>’ packet; see <a href="QStartNoAckMode.html#QStartNoAckMode">QStartNoAckMode</a>.
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<p>When in no-acknowledgment mode, neither the stub nor <span class="sc">gdb</span> shall send or
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expect ‘<samp><span class="samp">+</span></samp>’/‘<samp><span class="samp">-</span></samp>’ protocol acknowledgments. The packet
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and response format still includes the normal checksum, as described in
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<a href="Overview.html#Overview">Overview</a>, but the checksum may be ignored by the receiver.
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<p>If the stub supports ‘<samp><span class="samp">QStartNoAckMode</span></samp>’ and prefers to operate in
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no-acknowledgment mode, it should report that to <span class="sc">gdb</span>
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by including ‘<samp><span class="samp">QStartNoAckMode+</span></samp>’ in its response to ‘<samp><span class="samp">qSupported</span></samp>’;
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see <a href="qSupported.html#qSupported">qSupported</a>.
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If <span class="sc">gdb</span> also supports ‘<samp><span class="samp">QStartNoAckMode</span></samp>’ and it has not been
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disabled via the <code>set remote noack-packet off</code> command
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(see <a href="Remote-Configuration.html#Remote-Configuration">Remote Configuration</a>),
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<span class="sc">gdb</span> may then send a ‘<samp><span class="samp">QStartNoAckMode</span></samp>’ packet to the stub.
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Only then may the stub actually turn off packet acknowledgments.
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<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>’
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response, which can be safely ignored by the stub.
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<p>Note that <code>set remote noack-packet</code> command only affects negotiation
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between <span class="sc">gdb</span> and the stub when subsequent connections are made;
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it does not affect the protocol acknowledgment state for any current
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connection.
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Since ‘<samp><span class="samp">+</span></samp>’/‘<samp><span class="samp">-</span></samp>’ acknowledgments are enabled by default when a
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new connection is established,
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there is also no protocol request to re-enable the acknowledgments
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for the current connection, once disabled.
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</body></html>
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