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<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>
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</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>
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<div id="footer-text">
Last updated
2020-02-22 05:12:29 UTC
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