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<title>Debugging with GDB: Stop Reply Packets</title>
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<a name="Stop-Reply-Packets"></a>
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<div class="header">
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<p>
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Next: <a href="General-Query-Packets.html#General-Query-Packets" accesskey="n" rel="next">General Query Packets</a>, Previous: <a href="Packets.html#Packets" accesskey="p" rel="previous">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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</div>
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<hr>
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<a name="Stop-Reply-Packets-1"></a>
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<h3 class="section">E.3 Stop Reply Packets</h3>
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<a name="index-stop-reply-packets"></a>
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<p>The ‘<samp>C</samp>’, ‘<samp>c</samp>’, ‘<samp>S</samp>’, ‘<samp>s</samp>’, ‘<samp>vCont</samp>’,
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‘<samp>vAttach</samp>’, ‘<samp>vRun</samp>’, ‘<samp>vStopped</samp>’, and ‘<samp>?</samp>’ packets can
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receive any of the below as a reply. Except for ‘<samp>?</samp>’
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and ‘<samp>vStopped</samp>’, that reply is only returned
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when the target halts. In the below the exact meaning of <em>signal
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number</em> is defined by the header <samp>include/gdb/signals.h</samp> in the
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<small>GDB</small> source code.
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</p>
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<p>In non-stop mode, the server will simply reply ‘<samp>OK</samp>’ to commands
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such as ‘<samp>vCont</samp>’; any stop will be the subject of a future
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notification. See <a href="Remote-Non_002dStop.html#Remote-Non_002dStop">Remote Non-Stop</a>.
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</p>
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<p>As in the description of request packets, we include spaces in the
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reply templates for clarity; these are not part of the reply packet’s
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syntax. No <small>GDB</small> stop reply packet uses spaces to separate its
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components.
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</p>
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<dl compact="compact">
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<dt>‘<samp>S <var>AA</var></samp>’</dt>
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<dd><p>The program received signal number <var>AA</var> (a two-digit hexadecimal
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number). This is equivalent to a ‘<samp>T</samp>’ response with no
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<var>n</var>:<var>r</var> pairs.
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</p>
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</dd>
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<dt>‘<samp>T <var>AA</var> <var>n1</var>:<var>r1</var>;<var>n2</var>:<var>r2</var>;…</samp>’</dt>
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<dd><a name="index-T-packet-reply"></a>
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<p>The program received signal number <var>AA</var> (a two-digit hexadecimal
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number). This is equivalent to an ‘<samp>S</samp>’ response, except that the
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‘<samp><var>n</var>:<var>r</var></samp>’ pairs can carry values of important registers
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and other information directly in the stop reply packet, reducing
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round-trip latency. Single-step and breakpoint traps are reported
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this way. Each ‘<samp><var>n</var>:<var>r</var></samp>’ pair is interpreted as follows:
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</p>
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<ul>
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<li> If <var>n</var> is a hexadecimal number, it is a register number, and the
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corresponding <var>r</var> gives that register’s value. The data <var>r</var> is a
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series of bytes in target byte order, with each byte given by a
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two-digit hex number.
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</li><li> If <var>n</var> is ‘<samp>thread</samp>’, then <var>r</var> is the <var>thread-id</var> of
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the stopped thread, as specified in <a href="Packets.html#thread_002did-syntax">thread-id syntax</a>.
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</li><li> If <var>n</var> is ‘<samp>core</samp>’, then <var>r</var> is the hexadecimal number of
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the core on which the stop event was detected.
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</li><li> If <var>n</var> is a recognized <em>stop reason</em>, it describes a more
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specific event that stopped the target. The currently defined stop
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reasons are listed below. The <var>aa</var> should be ‘<samp>05</samp>’, the trap
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signal. At most one stop reason should be present.
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</li><li> Otherwise, <small>GDB</small> should ignore this ‘<samp><var>n</var>:<var>r</var></samp>’ pair
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and go on to the next; this allows us to extend the protocol in the
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future.
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</li></ul>
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<p>The currently defined stop reasons are:
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</p>
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<dl compact="compact">
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<dt>‘<samp>watch</samp>’</dt>
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<dt>‘<samp>rwatch</samp>’</dt>
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<dt>‘<samp>awatch</samp>’</dt>
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<dd><p>The packet indicates a watchpoint hit, and <var>r</var> is the data address, in
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hex.
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</p>
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</dd>
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<dt>‘<samp>syscall_entry</samp>’</dt>
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<dt>‘<samp>syscall_return</samp>’</dt>
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<dd><p>The packet indicates a syscall entry or return, and <var>r</var> is the
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syscall number, in hex.
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</p>
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<a name="index-shared-library-events_002c-remote-reply"></a>
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</dd>
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<dt>‘<samp>library</samp>’</dt>
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<dd><p>The packet indicates that the loaded libraries have changed.
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<small>GDB</small> should use ‘<samp>qXfer:libraries:read</samp>’ to fetch a new
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list of loaded libraries. The <var>r</var> part is ignored.
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</p>
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<a name="index-replay-log-events_002c-remote-reply"></a>
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</dd>
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<dt>‘<samp>replaylog</samp>’</dt>
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<dd><p>The packet indicates that the target cannot continue replaying
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logged execution events, because it has reached the end (or the
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beginning when executing backward) of the log. The value of <var>r</var>
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will be either ‘<samp>begin</samp>’ or ‘<samp>end</samp>’. See <a href="Reverse-Execution.html#Reverse-Execution">Reverse Execution</a>,
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for more information.
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</p>
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</dd>
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<dt>‘<samp>swbreak</samp>’</dt>
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<dd><a name="swbreak-stop-reason"></a><p>The packet indicates a software breakpoint instruction was executed,
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irrespective of whether it was <small>GDB</small> that planted the
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breakpoint or the breakpoint is hardcoded in the program. The <var>r</var>
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part must be left empty.
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</p>
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<p>On some architectures, such as x86, at the architecture level, when a
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breakpoint instruction executes the program counter points at the
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breakpoint address plus an offset. On such targets, the stub is
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responsible for adjusting the PC to point back at the breakpoint
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address.
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</p>
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<p>This packet should not be sent by default; older <small>GDB</small> versions
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did not support it. <small>GDB</small> requests it, by supplying an
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appropriate ‘<samp>qSupported</samp>’ feature (see <a href="General-Query-Packets.html#qSupported">qSupported</a>). The
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remote stub must also supply the appropriate ‘<samp>qSupported</samp>’ feature
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indicating support.
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</p>
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<p>This packet is required for correct non-stop mode operation.
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</p>
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</dd>
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<dt>‘<samp>hwbreak</samp>’</dt>
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<dd><p>The packet indicates the target stopped for a hardware breakpoint.
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The <var>r</var> part must be left empty.
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</p>
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<p>The same remarks about ‘<samp>qSupported</samp>’ and non-stop mode above
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apply.
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</p>
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<a name="index-fork-events_002c-remote-reply"></a>
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</dd>
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<dt>‘<samp>fork</samp>’</dt>
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<dd><p>The packet indicates that <code>fork</code> was called, and <var>r</var>
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is the thread ID of the new child process. Refer to
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<a href="Packets.html#thread_002did-syntax">thread-id syntax</a> for the format of the <var>thread-id</var>
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field. This packet is only applicable to targets that support
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fork events.
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</p>
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<p>This packet should not be sent by default; older <small>GDB</small> versions
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did not support it. <small>GDB</small> requests it, by supplying an
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appropriate ‘<samp>qSupported</samp>’ feature (see <a href="General-Query-Packets.html#qSupported">qSupported</a>). The
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remote stub must also supply the appropriate ‘<samp>qSupported</samp>’ feature
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indicating support.
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</p>
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<a name="index-vfork-events_002c-remote-reply"></a>
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</dd>
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<dt>‘<samp>vfork</samp>’</dt>
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<dd><p>The packet indicates that <code>vfork</code> was called, and <var>r</var>
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is the thread ID of the new child process. Refer to
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<a href="Packets.html#thread_002did-syntax">thread-id syntax</a> for the format of the <var>thread-id</var>
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field. This packet is only applicable to targets that support
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vfork events.
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</p>
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<p>This packet should not be sent by default; older <small>GDB</small> versions
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did not support it. <small>GDB</small> requests it, by supplying an
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appropriate ‘<samp>qSupported</samp>’ feature (see <a href="General-Query-Packets.html#qSupported">qSupported</a>). The
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remote stub must also supply the appropriate ‘<samp>qSupported</samp>’ feature
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indicating support.
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</p>
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<a name="index-vforkdone-events_002c-remote-reply"></a>
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</dd>
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<dt>‘<samp>vforkdone</samp>’</dt>
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<dd><p>The packet indicates that a child process created by a vfork
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has either called <code>exec</code> or terminated, so that the
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address spaces of the parent and child process are no longer
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shared. The <var>r</var> part is ignored. This packet is only
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applicable to targets that support vforkdone events.
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</p>
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<p>This packet should not be sent by default; older <small>GDB</small> versions
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did not support it. <small>GDB</small> requests it, by supplying an
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appropriate ‘<samp>qSupported</samp>’ feature (see <a href="General-Query-Packets.html#qSupported">qSupported</a>). The
|
remote stub must also supply the appropriate ‘<samp>qSupported</samp>’ feature
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indicating support.
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</p>
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<a name="index-exec-events_002c-remote-reply"></a>
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</dd>
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<dt>‘<samp>exec</samp>’</dt>
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<dd><p>The packet indicates that <code>execve</code> was called, and <var>r</var>
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is the absolute pathname of the file that was executed, in hex.
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This packet is only applicable to targets that support exec events.
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</p>
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<p>This packet should not be sent by default; older <small>GDB</small> versions
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did not support it. <small>GDB</small> requests it, by supplying an
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appropriate ‘<samp>qSupported</samp>’ feature (see <a href="General-Query-Packets.html#qSupported">qSupported</a>). The
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remote stub must also supply the appropriate ‘<samp>qSupported</samp>’ feature
|
indicating support.
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</p>
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<a name="index-thread-create-event_002c-remote-reply"></a>
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<a name="thread-create-event"></a></dd>
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<dt>‘<samp>create</samp>’</dt>
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<dd><p>The packet indicates that the thread was just created. The new thread
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is stopped until <small>GDB</small> sets it running with a resumption packet
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(see <a href="Packets.html#vCont-packet">vCont packet</a>). This packet should not be sent by default;
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<small>GDB</small> requests it with the <a href="General-Query-Packets.html#QThreadEvents">QThreadEvents</a> packet. See
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also the ‘<samp>w</samp>’ (see <a href="#thread-exit-event">thread exit event</a>) remote reply below. The
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<var>r</var> part is ignored.
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</p>
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</dd>
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</dl>
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</dd>
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<dt>‘<samp>W <var>AA</var></samp>’</dt>
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<dt>‘<samp>W <var>AA</var> ; process:<var>pid</var></samp>’</dt>
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<dd><p>The process exited, and <var>AA</var> is the exit status. This is only
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applicable to certain targets.
|
</p>
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<p>The second form of the response, including the process ID of the
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exited process, can be used only when <small>GDB</small> has reported
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support for multiprocess protocol extensions; see <a href="General-Query-Packets.html#multiprocess-extensions">multiprocess extensions</a>. Both <var>AA</var> and <var>pid</var> are formatted as big-endian
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hex strings.
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</p>
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</dd>
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<dt>‘<samp>X <var>AA</var></samp>’</dt>
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<dt>‘<samp>X <var>AA</var> ; process:<var>pid</var></samp>’</dt>
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<dd><p>The process terminated with signal <var>AA</var>.
|
</p>
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<p>The second form of the response, including the process ID of the
|
terminated process, can be used only when <small>GDB</small> has reported
|
support for multiprocess protocol extensions; see <a href="General-Query-Packets.html#multiprocess-extensions">multiprocess extensions</a>. Both <var>AA</var> and <var>pid</var> are formatted as big-endian
|
hex strings.
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</p>
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<a name="thread-exit-event"></a><a name="index-thread-exit-event_002c-remote-reply"></a>
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</dd>
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<dt>‘<samp>w <var>AA</var> ; <var>tid</var></samp>’</dt>
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<dd>
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<p>The thread exited, and <var>AA</var> is the exit status. This response
|
should not be sent by default; <small>GDB</small> requests it with the
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<a href="General-Query-Packets.html#QThreadEvents">QThreadEvents</a> packet. See also <a href="#thread-create-event">thread create event</a> above.
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<var>AA</var> is formatted as a big-endian hex string.
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</p>
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</dd>
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<dt>‘<samp>N</samp>’</dt>
|
<dd><p>There are no resumed threads left in the target. In other words, even
|
though the process is alive, the last resumed thread has exited. For
|
example, say the target process has two threads: thread 1 and thread
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2. The client leaves thread 1 stopped, and resumes thread 2, which
|
subsequently exits. At this point, even though the process is still
|
alive, and thus no ‘<samp>W</samp>’ stop reply is sent, no thread is actually
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executing either. The ‘<samp>N</samp>’ stop reply thus informs the client
|
that it can stop waiting for stop replies. This packet should not be
|
sent by default; older <small>GDB</small> versions did not support it.
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<small>GDB</small> requests it, by supplying an appropriate
|
‘<samp>qSupported</samp>’ feature (see <a href="General-Query-Packets.html#qSupported">qSupported</a>). The remote stub must
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also supply the appropriate ‘<samp>qSupported</samp>’ feature indicating
|
support.
|
</p>
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</dd>
|
<dt>‘<samp>O <var>XX</var>…</samp>’</dt>
|
<dd><p>‘<samp><var>XX</var>…</samp>’ is hex encoding of <small>ASCII</small> data, to be
|
written as the program’s console output. This can happen at any time
|
while the program is running and the debugger should continue to wait
|
for ‘<samp>W</samp>’, ‘<samp>T</samp>’, etc. This reply is not permitted in non-stop mode.
|
</p>
|
</dd>
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<dt>‘<samp>F <var>call-id</var>,<var>parameter</var>…</samp>’</dt>
|
<dd><p><var>call-id</var> is the identifier which says which host system call should
|
be called. This is just the name of the function. Translation into the
|
correct system call is only applicable as it’s defined in <small>GDB</small>.
|
See <a href="File_002dI_002fO-Remote-Protocol-Extension.html#File_002dI_002fO-Remote-Protocol-Extension">File-I/O Remote Protocol Extension</a>, for a list of implemented
|
system calls.
|
</p>
|
<p>‘<samp><var>parameter</var>…</samp>’ is a list of parameters as defined for
|
this very system call.
|
</p>
|
<p>The target replies with this packet when it expects <small>GDB</small> to
|
call a host system call on behalf of the target. <small>GDB</small> replies
|
with an appropriate ‘<samp>F</samp>’ packet and keeps up waiting for the next
|
reply packet from the target. The latest ‘<samp>C</samp>’, ‘<samp>c</samp>’, ‘<samp>S</samp>’
|
or ‘<samp>s</samp>’ action is expected to be continued. See <a href="File_002dI_002fO-Remote-Protocol-Extension.html#File_002dI_002fO-Remote-Protocol-Extension">File-I/O Remote Protocol Extension</a>, for more details.
|
</p>
|
</dd>
|
</dl>
|
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<hr>
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<div class="header">
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<p>
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Next: <a href="General-Query-Packets.html#General-Query-Packets" accesskey="n" rel="next">General Query Packets</a>, Previous: <a href="Packets.html#Packets" accesskey="p" rel="previous">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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