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97 lines
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<title>Interrupts - Debugging with GDB</title>
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<a name="Interrupts"></a>
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Next: <a rel="next" accesskey="n" href="Notification-Packets.html#Notification-Packets">Notification Packets</a>,
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Previous: <a rel="previous" accesskey="p" href="Host-I_002fO-Packets.html#Host-I_002fO-Packets">Host I/O Packets</a>,
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<h3 class="section">E.8 Interrupts</h3>
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<p><a name="index-interrupts-_0028remote-protocol_0029-3589"></a><a name="interrupting-remote-targets"></a>In all-stop mode, when a program on the remote target is running,
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<span class="sc">gdb</span> may attempt to interrupt it by sending a ‘<samp><span class="samp">Ctrl-C</span></samp>’,
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<code>BREAK</code> or a <code>BREAK</code> followed by <code>g</code>, control of which
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is specified via <span class="sc">gdb</span>'s ‘<samp><span class="samp">interrupt-sequence</span></samp>’.
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<p>The precise meaning of <code>BREAK</code> is defined by the transport
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mechanism and may, in fact, be undefined. <span class="sc">gdb</span> does not
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currently define a <code>BREAK</code> mechanism for any of the network
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interfaces except for TCP, in which case <span class="sc">gdb</span> sends the
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<code>telnet</code> BREAK sequence.
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<p>‘<samp><span class="samp">Ctrl-C</span></samp>’, on the other hand, is defined and implemented for all
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transport mechanisms. It is represented by sending the single byte
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<code>0x03</code> without any of the usual packet overhead described in
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the Overview section (see <a href="Overview.html#Overview">Overview</a>). When a <code>0x03</code> byte is
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transmitted as part of a packet, it is considered to be packet data
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and does <em>not</em> represent an interrupt. E.g., an ‘<samp><span class="samp">X</span></samp>’ packet
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(see <a href="X-packet.html#X-packet">X packet</a>), used for binary downloads, may include an unescaped
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<code>0x03</code> as part of its packet.
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<p><code>BREAK</code> followed by <code>g</code> is also known as Magic SysRq g.
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When Linux kernel receives this sequence from serial port,
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it stops execution and connects to gdb.
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<p>In non-stop mode, because packet resumptions are asynchronous
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(see <a href="vCont-packet.html#vCont-packet">vCont packet</a>), <span class="sc">gdb</span> is always free to send a remote
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command to the remote stub, even when the target is running. For that
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reason, <span class="sc">gdb</span> instead sends a regular packet (see <a href="vCtrlC-packet.html#vCtrlC-packet">vCtrlC packet</a>) with the usual packet framing instead of the single byte
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<code>0x03</code>.
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<p>Stubs are not required to recognize these interrupt mechanisms and the
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precise meaning associated with receipt of the interrupt is
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implementation defined. If the target supports debugging of multiple
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threads and/or processes, it should attempt to interrupt all
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currently-executing threads and processes.
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If the stub is successful at interrupting the
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running program, it should send one of the stop
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reply packets (see <a href="Stop-Reply-Packets.html#Stop-Reply-Packets">Stop Reply Packets</a>) to <span class="sc">gdb</span> as a result
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of successfully stopping the program in all-stop mode, and a stop reply
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for each stopped thread in non-stop mode.
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Interrupts received while the
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program is stopped are queued and the program will be interrupted when
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it is resumed next time.
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