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<title>Debugging with GDB: Remote Non-Stop</title>
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<a name="Remote-Non_002dStop"></a>
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<div class="header">
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<p>
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Next: <a href="Packet-Acknowledgment.html#Packet-Acknowledgment" accesskey="n" rel="next">Packet Acknowledgment</a>, Previous: <a href="Notification-Packets.html#Notification-Packets" accesskey="p" rel="previous">Notification Packets</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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<hr>
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<a name="Remote-Protocol-Support-for-Non_002dStop-Mode"></a>
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<h3 class="section">E.10 Remote Protocol Support for Non-Stop Mode</h3>
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<p><small>GDB</small>’s remote protocol supports non-stop debugging of
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multi-threaded programs, as described in <a href="Non_002dStop-Mode.html#Non_002dStop-Mode">Non-Stop Mode</a>. If the stub
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supports non-stop mode, it should report that to <small>GDB</small> by including
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‘<samp>QNonStop+</samp>’ in its ‘<samp>qSupported</samp>’ response (see <a href="General-Query-Packets.html#qSupported">qSupported</a>).
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</p>
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<p><small>GDB</small> typically sends a ‘<samp>QNonStop</samp>’ packet only when
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establishing a new connection with the stub. Entering non-stop mode
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does not alter the state of any currently-running threads, but targets
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must stop all threads in any already-attached processes when entering
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all-stop mode. <small>GDB</small> uses the ‘<samp>?</samp>’ packet as necessary to
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probe the target state after a mode change.
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</p>
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<p>In non-stop mode, when an attached process encounters an event that
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would otherwise be reported with a stop reply, it uses the
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asynchronous notification mechanism (see <a href="Notification-Packets.html#Notification-Packets">Notification Packets</a>) to
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inform <small>GDB</small>. In contrast to all-stop mode, where all threads
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in all processes are stopped when a stop reply is sent, in non-stop
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mode only the thread reporting the stop event is stopped. That is,
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when reporting a ‘<samp>S</samp>’ or ‘<samp>T</samp>’ response to indicate completion
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of a step operation, hitting a breakpoint, or a fault, only the
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affected thread is stopped; any other still-running threads continue
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to run. When reporting a ‘<samp>W</samp>’ or ‘<samp>X</samp>’ response, all running
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threads belonging to other attached processes continue to run.
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</p>
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<p>In non-stop mode, the target shall respond to the ‘<samp>?</samp>’ packet as
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follows. First, any incomplete stop reply notification/‘<samp>vStopped</samp>’
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sequence in progress is abandoned. The target must begin a new
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sequence reporting stop events for all stopped threads, whether or not
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it has previously reported those events to <small>GDB</small>. The first
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stop reply is sent as a synchronous reply to the ‘<samp>?</samp>’ packet, and
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subsequent stop replies are sent as responses to ‘<samp>vStopped</samp>’ packets
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using the mechanism described above. The target must not send
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asynchronous stop reply notifications until the sequence is complete.
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If all threads are running when the target receives the ‘<samp>?</samp>’ packet,
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or if the target is not attached to any process, it shall respond
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‘<samp>OK</samp>’.
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</p>
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<p>If the stub supports non-stop mode, it should also support the
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‘<samp>swbreak</samp>’ stop reason if software breakpoints are supported, and
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the ‘<samp>hwbreak</samp>’ stop reason if hardware breakpoints are supported
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(see <a href="Stop-Reply-Packets.html#swbreak-stop-reason">swbreak stop reason</a>). This is because given the asynchronous
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nature of non-stop mode, between the time a thread hits a breakpoint
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and the time the event is finally processed by <small>GDB</small>, the
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breakpoint may have already been removed from the target. Due to
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this, <small>GDB</small> needs to be able to tell whether a trap stop was
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caused by a delayed breakpoint event, which should be ignored, as
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opposed to a random trap signal, which should be reported to the user.
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Note the ‘<samp>swbreak</samp>’ feature implies that the target is responsible
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for adjusting the PC when a software breakpoint triggers, if
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necessary, such as on the x86 architecture.
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</p>
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<div class="header">
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<p>
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Next: <a href="Packet-Acknowledgment.html#Packet-Acknowledgment" accesskey="n" rel="next">Packet Acknowledgment</a>, Previous: <a href="Notification-Packets.html#Notification-Packets" accesskey="p" rel="previous">Notification Packets</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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