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<h3 class="section">E.10 Remote Protocol Support for Non-Stop Mode</h3>

<p><span class="sc">gdb</span>'s remote protocol supports non-stop debugging of
multi-threaded programs, as described in <a href="Non_002dStop-Mode.html#Non_002dStop-Mode">Non-Stop Mode</a>.  If the stub
supports non-stop mode, it should report that to <span class="sc">gdb</span> by including
&lsquo;<samp><span class="samp">QNonStop+</span></samp>&rsquo; in its &lsquo;<samp><span class="samp">qSupported</span></samp>&rsquo; response (see <a href="qSupported.html#qSupported">qSupported</a>).

   <p><span class="sc">gdb</span> typically sends a &lsquo;<samp><span class="samp">QNonStop</span></samp>&rsquo; packet only when
establishing a new connection with the stub.  Entering non-stop mode
does not alter the state of any currently-running threads, but targets
must stop all threads in any already-attached processes when entering
all-stop mode.  <span class="sc">gdb</span> uses the &lsquo;<samp><span class="samp">?</span></samp>&rsquo; packet as necessary to
probe the target state after a mode change.

   <p>In non-stop mode, when an attached process encounters an event that
would otherwise be reported with a stop reply, it uses the
asynchronous notification mechanism (see <a href="Notification-Packets.html#Notification-Packets">Notification Packets</a>) to
inform <span class="sc">gdb</span>.  In contrast to all-stop mode, where all threads
in all processes are stopped when a stop reply is sent, in non-stop
mode only the thread reporting the stop event is stopped.  That is,
when reporting a &lsquo;<samp><span class="samp">S</span></samp>&rsquo; or &lsquo;<samp><span class="samp">T</span></samp>&rsquo; response to indicate completion
of a step operation, hitting a breakpoint, or a fault, only the
affected thread is stopped; any other still-running threads continue
to run.  When reporting a &lsquo;<samp><span class="samp">W</span></samp>&rsquo; or &lsquo;<samp><span class="samp">X</span></samp>&rsquo; response, all running
threads belonging to other attached processes continue to run.

   <p>In non-stop mode, the target shall respond to the &lsquo;<samp><span class="samp">?</span></samp>&rsquo; packet as
follows.  First, any incomplete stop reply notification/&lsquo;<samp><span class="samp">vStopped</span></samp>&rsquo;
sequence in progress is abandoned.  The target must begin a new
sequence reporting stop events for all stopped threads, whether or not
it has previously reported those events to <span class="sc">gdb</span>.  The first
stop reply is sent as a synchronous reply to the &lsquo;<samp><span class="samp">?</span></samp>&rsquo; packet, and
subsequent stop replies are sent as responses to &lsquo;<samp><span class="samp">vStopped</span></samp>&rsquo; packets
using the mechanism described above.  The target must not send
asynchronous stop reply notifications until the sequence is complete. 
If all threads are running when the target receives the &lsquo;<samp><span class="samp">?</span></samp>&rsquo; packet,
or if the target is not attached to any process, it shall respond
&lsquo;<samp><span class="samp">OK</span></samp>&rsquo;.

   <p>If the stub supports non-stop mode, it should also support the
&lsquo;<samp><span class="samp">swbreak</span></samp>&rsquo; stop reason if software breakpoints are supported, and
the &lsquo;<samp><span class="samp">hwbreak</span></samp>&rsquo; stop reason if hardware breakpoints are supported
(see <a href="swbreak-stop-reason.html#swbreak-stop-reason">swbreak stop reason</a>).  This is because given the asynchronous
nature of non-stop mode, between the time a thread hits a breakpoint
and the time the event is finally processed by <span class="sc">gdb</span>, the
breakpoint may have already been removed from the target.  Due to
this, <span class="sc">gdb</span> needs to be able to tell whether a trap stop was
caused by a delayed breakpoint event, which should be ignored, as
opposed to a random trap signal, which should be reported to the user. 
Note the &lsquo;<samp><span class="samp">swbreak</span></samp>&rsquo; feature implies that the target is responsible
for adjusting the PC when a software breakpoint triggers, if
necessary, such as on the x86 architecture.

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