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  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

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  18. <title>Interrupts (Debugging with GDB)</title>

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  63. <a name="Interrupts"></a>

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  66. Next: <a href="Notification-Packets.html#Notification-Packets" accesskey="n" rel="next">Notification Packets</a>, Previous: <a href="Host-I_002fO-Packets.html#Host-I_002fO-Packets" accesskey="p" rel="prev">Host I/O Packets</a>, Up: <a href="Remote-Protocol.html#Remote-Protocol" accesskey="u" rel="up">Remote Protocol</a> &nbsp; [<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>

  67. </div>

  68. <hr>

  69. <a name="Interrupts-1"></a>

  70. <h3 class="section">E.8 Interrupts</h3>

  71. <a name="index-interrupts-_0028remote-protocol_0029"></a>

  72. <a name="interrupting-remote-targets"></a>

  73. <p>In all-stop mode, when a program on the remote target is running,

  74. <small>GDB</small> may attempt to interrupt it by sending a &lsquo;<samp>Ctrl-C</samp>&rsquo;,

  75. <code>BREAK</code> or a <code>BREAK</code> followed by <code>g</code>, control of which

  76. is specified via <small>GDB</small>&rsquo;s &lsquo;<samp>interrupt-sequence</samp>&rsquo;.

  77. </p>

  78. <p>The precise meaning of <code>BREAK</code> is defined by the transport

  79. mechanism and may, in fact, be undefined.  <small>GDB</small> does not

  80. currently define a <code>BREAK</code> mechanism for any of the network

  81. interfaces except for TCP, in which case <small>GDB</small> sends the

  82. <code>telnet</code> BREAK sequence.

  83. </p>

  84. <p>&lsquo;<samp>Ctrl-C</samp>&rsquo;, on the other hand, is defined and implemented for all

  85. transport mechanisms.  It is represented by sending the single byte

  86. <code>0x03</code> without any of the usual packet overhead described in

  87. the Overview section (see <a href="Overview.html#Overview">Overview</a>).  When a <code>0x03</code> byte is

  88. transmitted as part of a packet, it is considered to be packet data

  89. and does <em>not</em> represent an interrupt.  E.g., an &lsquo;<samp>X</samp>&rsquo; packet

  90. (see <a href="Packets.html#X-packet">X packet</a>), used for binary downloads, may include an unescaped

  91. <code>0x03</code> as part of its packet.

  92. </p>

  93. <p><code>BREAK</code> followed by <code>g</code> is also known as Magic SysRq g.

  94. When Linux kernel receives this sequence from serial port,

  95. it stops execution and connects to gdb.

  96. </p>

  97. <p>In non-stop mode, because packet resumptions are asynchronous

  98. (see <a href="Packets.html#vCont-packet">vCont packet</a>), <small>GDB</small> is always free to send a remote

  99. command to the remote stub, even when the target is running.  For that

  100. reason, <small>GDB</small> instead sends a regular packet (see <a href="Packets.html#vCtrlC-packet">vCtrlC packet</a>) with the usual packet framing instead of the single byte

  101. <code>0x03</code>.

  102. </p>

  103. <p>Stubs are not required to recognize these interrupt mechanisms and the

  104. precise meaning associated with receipt of the interrupt is

  105. implementation defined.  If the target supports debugging of multiple

  106. threads and/or processes, it should attempt to interrupt all 

  107. currently-executing threads and processes.

  108. If the stub is successful at interrupting the

  109. running program, it should send one of the stop

  110. reply packets (see <a href="Stop-Reply-Packets.html#Stop-Reply-Packets">Stop Reply Packets</a>) to <small>GDB</small> as a result

  111. of successfully stopping the program in all-stop mode, and a stop reply

  112. for each stopped thread in non-stop mode.

  113. Interrupts received while the

  114. program is stopped are queued and the program will be interrupted when

  115. it is resumed next time.

  116. </p>

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