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<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"> <head> <meta http-equiv="Content-Type" content="application/xhtml+xml; charset=UTF-8" /> <meta name="generator" content="AsciiDoc 9.0.0rc1" /> <title>How to use gdb</title> <link rel="stylesheet" href="../xsf.css" type="text/css" /> <script type="text/javascript" src="../asciidoc-xhtml11.js"></script> <script type="text/javascript"> /*<![CDATA[*/ asciidoc.install(2); /*]]>*/ </script> </head> <body class="article"> <div id="header"> <h1><a href="../index.html">XSF</a> / How to use gdb</h1> <span id="author">Cyril Brulebois</span><br /> <span id="email"><code><<a href="mailto:kibi@debian.org">kibi@debian.org</a>></code></span><br /> <div id="toc"> <div id="toctitle">Table of Contents</div> <noscript><p><b>JavaScript must be enabled in your browser to display the table of contents.</b></p></noscript> </div> </div> <div id="content"> <div class="sect1"> <h2 id="_foreword">Foreword</h2> <div class="sectionbody"> <div class="paragraph"><p>One should note that X is responsible for VT switching, meaning switching between an X session and console terminals. In other words, <code>Ctrl+Alt+Fn</code> is handled by X. If X is stopped, for example because it’s running under <code>gdb</code>, one can no longer switch to another VT. That’s why we’re recommending using a second machine to debug X. Nevertheless, here are some instructions to attempt debugging X with a single machine.</p></div> <div class="sect2"> <h3 id="_one_machine_approach">One-machine approach</h3> <div class="paragraph"><p>This is a <em>post-mortem</em> approach. The idea is to run X with the <code>-core</code> option. Once it dies, a core file (<code>/etc/X11/core</code>) is generated, and can be loaded from <code>gdb</code>.</p></div> <div class="paragraph"><p>Follow these steps:</p></div> <div class="olist arabic"><ol class="arabic"> <li> <p> Getting a core file. </p> </li> <li> <p> Loading a core file. </p> </li> <li> <p> Displaying/saving a backtrace. </p> </li> </ol></div> </div> <div class="sect2"> <h3 id="_two_machine_approach">Two-machine approach</h3> <div class="paragraph"><p>You pay the “need a second machine” price, but that buys you interactivity. Just log into the first machine from the second one, using <code>ssh</code>.</p></div> <div class="paragraph"><p>Follow these steps:</p></div> <div class="olist arabic"><ol class="arabic"> <li> <p> Attaching/Starting X from gdb. </p> </li> <li> <p> Displaying/saving a backtrace. </p> </li> </ol></div> </div> <div class="sect2"> <h3 id="_needed_packages">Needed packages</h3> <div class="paragraph"><p>Obviously, <code>gdb</code> is needed; <code>xserver-xorg-core-dbg</code> contains the debugging symbols for the server itself. Other needed packages can be determined by looking at the backtrace. <strong>FIXME: More info about that.</strong></p></div> <div style="page-break-after:always"></div> </div> </div> </div> <div class="sect1"> <h2 id="_actual_gdb_work">Actual gdb work</h2> <div class="sectionbody"> <div class="sect2"> <h3 id="_getting_a_core_file">Getting a core file</h3> <div class="ulist"><ul> <li> <p> Using <code>gdm3</code> 3.4.1 and above: uncomment <code>Enable = true</code> in the <code>[debug]</code> section of the <code>/etc/gdm3/daemon.conf</code> file. </p> </li> <li> <p> Using an older <code>gdm3</code> package: The idea is to tweak the daemon’s <code>LocalXserverCommand</code> setting, adding the <code>-core</code> option. As of <code>gdm3 2.30</code>, the defaults can be found in <code>/usr/share/gdm/gdm.schemas</code>. Sample <code>/etc/gdm3/daemon.conf</code> excerpt: </p> </li> </ul></div> <div class="listingblock"> <div class="content"> <pre><code>[daemon] LocalXserverCommand=/usr/bin/Xorg -br -verbose -audit 0 -novtswitch -core</code></pre> </div></div> <div class="ulist"><ul> <li> <p> Using <code>kdm</code>: One should look for the <code>ServerArgsLocal</code> variable in the <code>/etc/kde4/kdm/kdmrc</code> file, and add <code>-core</code> there. Example: </p> </li> </ul></div> <div class="listingblock"> <div class="content"> <pre><code>ServerArgsLocal=-br -nolisten tcp -core</code></pre> </div></div> <div class="ulist"><ul> <li> <p> Using <code>xdm</code>: It’s sufficient to add <code>-core</code> to the command configured through <code>/etc/X11/xdm/Xservers</code>, for example: </p> </li> </ul></div> <div class="listingblock"> <div class="content"> <pre><code>:0 local /usr/bin/X :0 vt7 -nolisten tcp -core</code></pre> </div></div> </div> <div class="sect2"> <h3 id="_loading_a_core_file">Loading a core file</h3> <div class="paragraph"><p>That’s trivial, one just needs to pass both the core file and the path to the binary:</p></div> <div class="listingblock"> <div class="content"> <pre><code># gdb -c /etc/X11/core /usr/bin/Xorg</code></pre> </div></div> <div class="paragraph"><p>Now <code>gdb</code> is ready to display backtraces.</p></div> </div> <div class="sect2"> <h3 id="_attaching_x_from_gdb">Attaching X from gdb</h3> <div class="paragraph"><p>The way of starting X doesn’t really matter, as <code>gdb</code> makes it possible to attach a running process. If there’s a single X instance running, that will do the job:</p></div> <div class="listingblock"> <div class="content"> <pre><code># gdb attach $(pidof X) [---GDB starts---] (gdb) handle SIGPIPE nostop (gdb) cont</code></pre> </div></div> <div class="paragraph"><p>If there are several instances, one can use <code>ps aux</code> to determine the PID of the appropriate instance (2nd column → <code>$pid</code>), and then attach it:</p></div> <div class="listingblock"> <div class="content"> <pre><code># gdb attach $pid [---GDB starts---] (gdb) handle SIGPIPE nostop (gdb) cont</code></pre> </div></div> </div> <div class="sect2"> <h3 id="_starting_x_from_gdb">Starting X from gdb</h3> <div class="paragraph"><p>In case X crashes at start-up, one can start X from <code>gdb</code> in the following way. In this example, the only parameter is the display, but more parameters can be added.</p></div> <div class="listingblock"> <div class="content"> <pre><code># gdb --args Xorg :0 [---GDB starts---] (gdb) handle SIGPIPE nostop (gdb) run</code></pre> </div></div> </div> <div class="sect2"> <h3 id="_what_is_sigpipe">What is SIGPIPE?</h3> <div class="paragraph"><p><code>SIGPIPE</code> is a signal that can reach the X server quite easily, especially when performing a VT switch, or refreshing large parts of the screen. That’s why we ask <code>gdb</code> not to stop when such a signal is caught, thanks to the <code>handle SIGPIPE nostop</code> command.</p></div> </div> <div class="sect2"> <h3 id="_how_to_display_a_backtrace">How to display a backtrace?</h3> <div class="paragraph"><p>Once X is crashed, for example because it received a <code>SIGSEGV</code> (segmentation fault, usually because of a NULL pointer dereference), or a <code>SIGBUS</code>, one gets back to the <code>gdb</code> prompt. One can then request a backtrace (<code>bt</code>) or a full backtrace (<code>bt full</code>). The latter is what developers are usually interested in, because variable values are also available.</p></div> <div class="listingblock"> <div class="content"> <pre><code>(gdb) bt (gdb) bt full</code></pre> </div></div> </div> <div class="sect2"> <h3 id="_how_to_save_a_backtrace">How to save a backtrace?</h3> <div class="paragraph"><p>To save a recording of the gdb session to a file (<code>gdb.txt</code> by default):</p></div> <div class="listingblock"> <div class="content"> <pre><code>(gdb) set logging on</code></pre> </div></div> <div class="paragraph"><p>To save in a different file, use this instead:</p></div> <div class="listingblock"> <div class="content"> <pre><code>(gdb) set logging file my-file.txt (gdb) set logging on</code></pre> </div></div> <div class="paragraph"><p>Once logging is enabled, you can request a (full) backtrace using the previous commands.</p></div> </div> </div> </div> </div> <div id="footnotes"><hr /></div> <div id="footer"> <div id="footer-text"> Last updated 2018-12-14 12:34:22 UTC </div> </div> </body> </html>