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<head>
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<meta name="AUTHOR" content="bkoz@gcc.gnu.org (Benjamin Kosnik)" />
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<meta name="KEYWORDS" content="c++, libstdc++, gdb, g++, debug" />
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<meta name="DESCRIPTION" content="Debugging C++ binaries" />
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<meta name="GENERATOR" content="vi and ten fingers" />
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<title>Debugging schemes and strategies</title>
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<link rel="StyleSheet" href="lib3styles.css" type="text/css" />
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<link rel="Copyright" href="17_intro/license.html" type="text/html" />
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</head>
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<body>
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<h1 class="centered"><a name="top">Debugging schemes and strategies</a></h1>
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<p class="fineprint"><em>
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The latest version of this document is always available at
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<a href="http://gcc.gnu.org/onlinedocs/libstdc++/debug.html">
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http://gcc.gnu.org/onlinedocs/libstdc++/debug.html</a>.
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</em></p>
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<p><em>
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To the <a href="http://gcc.gnu.org/libstdc++/">libstdc++-v3 homepage</a>.
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</em></p>
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<!-- ####################################################### -->
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<hr />
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<p>There are numerous things that can be done to improve the ease with
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which C++ binaries are debugged when using the GNU
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tool chain. Here are some of them.
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</p>
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<h3 class="left"><a name="gplusplus">Compiler flags determine debug info</a></h3>
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<p>The default optimizations and debug flags for a libstdc++ build are
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<code>-g -O2</code>. However, both debug and optimization flags can
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be varied to change debugging characteristics. For instance,
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turning off all optimization via the <code>-g -O0</code> flag will
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disable inlining, so that stepping through all functions, including
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inlined constructors and destructors, is possible. In addition,
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<code>-fno-eliminate-unused-debug-types</code> can be used when
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additional debug information, such as nested class info, is desired.
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</p>
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<p>Or, the debug format that the compiler and debugger use to communicate
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information about source constructs can be changed via <code>
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-gdwarf-2 </code> or <code> -gstabs </code> flags: some debugging
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formats permit more expressive type and scope information to be
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shown in gdb. The default debug information for a particular
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platform can be identified via the value set by the
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PREFERRED_DEBUGGING_TYPE macro in the gcc sources.
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</p>
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<p>Many other options are available: please see
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<a href="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging%20Options">"Options for Debugging Your Program"</a>
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in Using the GNU Compiler Collection (GCC) for a complete list.
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</p>
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<h3 class="left"><a name="lib">Using special flags to make a debug binary</a></h3>
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<p>If you would like debug symbols in libstdc++, there are two ways to
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build libstdc++ with debug flags. The first is to run make from the
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toplevel in a freshly-configured tree with
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</p>
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<pre>
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--enable-libstdcxx-debug
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</pre>
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<p>and perhaps</p>
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<pre>
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--enable-libstdcxx-debug-flags='...'
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</pre>
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<p>to create a separate debug build. Both the normal build and the
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debug build will persist, without having to specify
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<code>CXXFLAGS</code>, and the debug library will be installed in a
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separate directory tree, in <code>(prefix)/lib/debug</code>. For
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more information, look at the <a href="configopts.html">configuration
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options</a> document.
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</p>
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<p>A second approach is to use the configuration flags
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</p>
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<pre>
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make CXXFLAGS='-g3 -O0' all
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</pre>
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<p>This quick and dirty approach is often sufficient for quick
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debugging tasks, when you cannot or don't want to recompile your
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application to use the <a href="#safe">debug mode</a>.</p>
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<h3 class="left"><a name="safe">The libstdc++ debug mode</a></h3>
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<p>By default, libstdc++ is built with efficiency in mind, and
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therefore performs little or no error checking that is not required
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by the C++ standard. This means that programs that incorrectly use
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the C++ standard library will exhibit behavior that is not portable
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and may not even be predictable, because they tread into
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implementation-specific or undefined behavior. To detect some of
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these errors before they can become problematic, libstdc++ offers a
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debug mode that provides additional checking of library facilities,
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and will report errors in the use of libstdc++ as soon as they can
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be detected by emitting a description of the problem to standard
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error and aborting the program. This debug mode is available with
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GCC 3.4.0 and later versions. </p>
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<p>The libstdc++ debug mode performs checking for many areas of the C++
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standard, but the focus is on checking interactions among standard
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iterators, containers, and algorithms, including:</p>
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<ul>
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<li><em>Safe iterators</em>: Iterators keep track of the
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container whose elements they reference, so errors such as
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incrementing a past-the-end iterator or dereferencing an iterator
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that points to a container that has been destructed are diagnosed
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immediately.</li>
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<li><em>Algorithm preconditions</em>: Algorithms attempt to
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validate their input parameters to detect errors as early as
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possible. For instance, the <code>set_intersection</code>
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algorithm requires that its iterator
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parameters <code>first1</code> and <code>last1</code> form a valid
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iterator range, and that the sequence
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[<code>first1</code>, <code>last1</code>) is sorted according to
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the same predicate that was passed
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to <code>set_intersection</code>; the libstdc++ debug mode will
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detect an error if the sequence is not sorted or was sorted by a
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different predicate.</li>
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</ul>
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<h4 class="left">Using the libstdc++ debug mode</h4>
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<p>To use the libstdc++ debug mode, compile your application with the
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compiler flag <code>-D_GLIBCXX_DEBUG</code>. Note that this flag
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changes the sizes and behavior of standard class templates such
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as <code>std::vector</code>, and therefore you can only link code
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compiled with debug mode and code compiled without debug mode if no
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instantiation of a container is passed between the two translation
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units.</p>
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<p>For information about the design of the libstdc++ debug mode,
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please see the <a href="debug_mode.html">libstdc++ debug mode design
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document</a>.</p>
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<h4 class="left">Using the debugging containers without debug
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mode</h4>
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<p>When it is not feasible to recompile your entire application, or
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only specific containers need checking, debugging containers are
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available as GNU extensions. These debugging containers are
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functionally equivalent to the standard drop-in containers used in
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debug mode, but they are available in a separate namespace as GNU
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extensions and may be used in programs compiled with either release
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mode or with debug mode. The
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following table provides the names and headers of the debugging
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containers:
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</p>
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<table title="Debugging containers" border="1">
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<tr>
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<th>Container</th>
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<th>Header</th>
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<th>Debug container</th>
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<th>Debug header</th>
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</tr>
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<tr>
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<td>std::bitset</td>
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<td><bitset></td>
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<td>__gnu_debug::bitset</td>
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<td><debug/bitset></td>
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</tr>
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<tr>
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<td>std::deque</td>
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<td><deque></td>
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<td>__gnu_debug::deque</td>
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<td><debug/deque></td>
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</tr>
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<tr>
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<td>std::list</td>
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<td><list></td>
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<td>__gnu_debug::list</td>
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<td><debug/list></td>
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</tr>
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<tr>
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<td>std::map</td>
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<td><map></td>
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<td>__gnu_debug::map</td>
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<td><debug/map></td>
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</tr>
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<tr>
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<td>std::multimap</td>
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<td><map></td>
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<td>__gnu_debug::multimap</td>
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<td><debug/map></td>
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</tr>
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<tr>
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<td>std::multiset</td>
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<td><set></td>
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<td>__gnu_debug::multiset</td>
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<td><debug/set></td>
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</tr>
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<tr>
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<td>std::set</td>
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<td><set></td>
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<td>__gnu_debug::set</td>
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<td><debug/set></td>
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</tr>
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<tr>
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<td>std::string</td>
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<td><string></td>
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<td>__gnu_debug::string</td>
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<td><debug/string></td>
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</tr>
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<tr>
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<td>std::wstring</td>
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<td><string></td>
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<td>__gnu_debug::wstring</td>
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<td><debug/string></td>
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</tr>
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<tr>
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<td>std::basic_string</td>
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<td><string></td>
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<td>__gnu_debug::basic_string</td>
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<td><debug/string></td>
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</tr>
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<tr>
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<td>std::vector</td>
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<td><vector></td>
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<td>__gnu_debug::vector</td>
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<td><debug/vector></td>
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</tr>
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<tr>
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<td>__gnu_cxx::hash_map</td>
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<td><ext/hash_map></td>
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<td>__gnu_debug::hash_map</td>
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<td><debug/hash_map></td>
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</tr>
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<tr>
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<td>__gnu_cxx::hash_multimap</td>
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<td><ext/hash_map></td>
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<td>__gnu_debug::hash_multimap</td>
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<td><debug/hash_map></td>
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</tr>
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<tr>
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<td>__gnu_cxx::hash_set</td>
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<td><ext/hash_set></td>
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<td>__gnu_debug::hash_set</td>
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<td><debug/hash_set></td>
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</tr>
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<tr>
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<td>__gnu_cxx::hash_multiset</td>
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<td><ext/hash_set></td>
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<td>__gnu_debug::hash_multiset</td>
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<td><debug/hash_set></td>
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</tr>
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</table>
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<h4 class="left">Debug mode semantics</h4>
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<p>A program that uses the C++ standard library correctly
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will maintain the same semantics under debug mode as it had with
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the normal (release) library. All functional and exception-handling
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guarantees made by the normal library also hold for the debug mode
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library, with one exception: performance guarantees made by the
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normal library may not hold in the debug mode library. For
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instance, erasing an element in a <code>std::list</code> is a
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constant-time operation in normal library, but in debug mode it is
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linear in the number of iterators that reference that particular
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list. So while your (correct) program won't change its results, it
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is likely to execute more slowly.</p>
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<p>libstdc++ includes many extensions to the C++ standard library. In
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some cases the extensions are obvious, such as the hashed
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associative containers, whereas other extensions give predictable
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results to behavior that would otherwise be undefined, such as
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throwing an exception when a <code>std::basic_string</code> is
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constructed from a NULL character pointer. This latter category also
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includes implementation-defined and unspecified semantics, such as
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the growth rate of a vector. Use of these extensions is not
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considered incorrect, so code that relies on them will not be
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rejected by debug mode. However, use of these extensions may affect
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the portability of code to other implementations of the C++ standard
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library, and is therefore somewhat hazardous. For this reason, the
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libstdc++ debug mode offers a "pedantic" mode (similar to
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GCC's <code>-pedantic</code> compiler flag) that attempts to emulate
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the semantics guaranteed by the C++ standard. For
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instance, constructing a <code>std::basic_string</code> with a NULL
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character pointer would result in an exception under normal mode or
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non-pedantic debug mode (this is a libstdc++ extension), whereas
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under pedantic debug mode libstdc++ would signal an error. To enable
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the pedantic debug mode, compile your program with
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both <code>-D_GLIBCXX_DEBUG</code>
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and <code>-D_GLIBCXX_DEBUG_PEDANTIC</code> .
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(N.B. In GCC 3.4.x and 4.0.0, due to a bug,
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<code>-D_GLIBXX_DEBUG_PEDANTIC</code> was also needed. The problem has
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been fixed in GCC 4.0.1 and later versions.) </p>
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|
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<p>The following library components provide extra debugging
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capabilities in debug mode:</p>
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<ul>
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<li><code>std::basic_string</code> (no safe iterators)</li>
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<li><code>std::bitset</code></li>
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<li><code>std::deque</code></li>
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<li><code>__gnu_cxx::hash_map</code></li>
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<li><code>__gnu_cxx::hash_multimap</code></li>
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<li><code>__gnu_cxx::hash_multiset</code></li>
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<li><code>__gnu_cxx::hash_set</code></li>
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<li><code>std::list</code></li>
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<li><code>std::map</code></li>
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<li><code>std::multimap</code></li>
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<li><code>std::multiset</code></li>
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<li><code>std::set</code></li>
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<li><code>std::vector</code></li>
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</ul>
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| 312 |
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<h3 class="left"><a name="mem">Tips for memory leak hunting</a></h3>
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|
|
| 315 |
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<p>There are various third party memory tracing and debug utilities
|
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|
that can be used to provide detailed memory allocation information
|
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|
|
about C++ code. An exhaustive list of tools is not going to be
|
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|
|
attempted, but includes <code>mtrace</code>, <code>valgrind</code>,
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<code>mudflap</code>, and the non-free commercial product
|
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|
|
<code>purify</code>. In addition, <code>libcwd</code> has a
|
| 321 |
|
|
replacement for the global new and delete operators that can track
|
| 322 |
|
|
memory allocation and deallocation and provide useful memory
|
| 323 |
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|
statistics.
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| 324 |
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</p>
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| 325 |
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|
|
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<p>Regardless of the memory debugging tool being used, there is one
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thing of great importance to keep in mind when debugging C++ code
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|
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that uses <code>new</code> and <code>delete</code>:
|
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|
|
there are different kinds of allocation schemes that can be used by
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<code> std::allocator </code>. For implementation details, see this
|
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|
|
<a href="ext/howto.html#3"> document</a> and look specifically for
|
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|
|
<code>GLIBCXX_FORCE_NEW</code>.
|
| 333 |
|
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</p>
|
| 334 |
|
|
|
| 335 |
|
|
<p>In a nutshell, the default allocator used by <code>
|
| 336 |
|
|
std::allocator</code> is a high-performance pool allocator, and can
|
| 337 |
|
|
give the mistaken impression that in a suspect executable, memory
|
| 338 |
|
|
is being leaked, when in reality the memory "leak" is a pool being
|
| 339 |
|
|
used by the library's allocator and is reclaimed after program
|
| 340 |
|
|
termination.
|
| 341 |
|
|
</p>
|
| 342 |
|
|
|
| 343 |
|
|
<p>For valgrind, there are some specific items to keep in mind. First
|
| 344 |
|
|
of all, use a version of valgrind that will work with current GNU
|
| 345 |
|
|
C++ tools: the first that can do this is valgrind 1.0.4, but later
|
| 346 |
|
|
versions should work at least as well. Second of all, use a
|
| 347 |
|
|
completely unoptimized build to avoid confusing valgrind. Third,
|
| 348 |
|
|
use GLIBCXX_FORCE_NEW to keep extraneous pool allocation noise from
|
| 349 |
|
|
cluttering debug information.
|
| 350 |
|
|
</p>
|
| 351 |
|
|
|
| 352 |
|
|
<p>Fourth, it may be necessary to force deallocation in other
|
| 353 |
|
|
libraries as well, namely the "C" library. On linux, this can be
|
| 354 |
|
|
accomplished with the appropriate use of the
|
| 355 |
|
|
<code>__cxa_atexit</code> or <code>atexit</code> functions.
|
| 356 |
|
|
</p>
|
| 357 |
|
|
|
| 358 |
|
|
<pre>
|
| 359 |
|
|
#include <cstdlib>
|
| 360 |
|
|
|
| 361 |
|
|
extern "C" void __libc_freeres(void);
|
| 362 |
|
|
|
| 363 |
|
|
void do_something() { }
|
| 364 |
|
|
|
| 365 |
|
|
int main()
|
| 366 |
|
|
{
|
| 367 |
|
|
atexit(__libc_freeres);
|
| 368 |
|
|
do_something();
|
| 369 |
|
|
return 0;
|
| 370 |
|
|
}
|
| 371 |
|
|
</pre>
|
| 372 |
|
|
|
| 373 |
|
|
|
| 374 |
|
|
<p>or, using <code>__cxa_atexit</code>:</p>
|
| 375 |
|
|
|
| 376 |
|
|
<pre>
|
| 377 |
|
|
extern "C" void __libc_freeres(void);
|
| 378 |
|
|
extern "C" int __cxa_atexit(void (*func) (void *), void *arg, void *d);
|
| 379 |
|
|
|
| 380 |
|
|
void do_something() { }
|
| 381 |
|
|
|
| 382 |
|
|
int main()
|
| 383 |
|
|
{
|
| 384 |
|
|
extern void* __dso_handle __attribute__ ((__weak__));
|
| 385 |
|
|
__cxa_atexit((void (*) (void *)) __libc_freeres, NULL,
|
| 386 |
|
|
&__dso_handle ? __dso_handle : NULL);
|
| 387 |
|
|
do_test();
|
| 388 |
|
|
return 0;
|
| 389 |
|
|
}
|
| 390 |
|
|
</pre>
|
| 391 |
|
|
|
| 392 |
|
|
<p>Suggested valgrind flags, given the suggestions above about setting
|
| 393 |
|
|
up the runtime environment, library, and test file, might be:
|
| 394 |
|
|
</p>
|
| 395 |
|
|
<pre>
|
| 396 |
|
|
valgrind -v --num-callers=20 --leak-check=yes --leak-resolution=high --show-reachable=yes a.out
|
| 397 |
|
|
</pre>
|
| 398 |
|
|
|
| 399 |
|
|
|
| 400 |
|
|
<h3 class="left"><a name="gdb">Some gdb strategies</a></h3>
|
| 401 |
|
|
<p>Many options are available for gdb itself: please see <a
|
| 402 |
|
|
href="http://sources.redhat.com/gdb/current/onlinedocs/gdb_13.html#SEC109">
|
| 403 |
|
|
"GDB features for C++" </a> in the gdb documentation. Also
|
| 404 |
|
|
recommended: the other parts of this manual.
|
| 405 |
|
|
</p>
|
| 406 |
|
|
|
| 407 |
|
|
<p>These settings can either be switched on in at the gdb command
|
| 408 |
|
|
line, or put into a .gdbint file to establish default debugging
|
| 409 |
|
|
characteristics, like so:
|
| 410 |
|
|
</p>
|
| 411 |
|
|
|
| 412 |
|
|
<pre>
|
| 413 |
|
|
set print pretty on
|
| 414 |
|
|
set print object on
|
| 415 |
|
|
set print static-members on
|
| 416 |
|
|
set print vtbl on
|
| 417 |
|
|
set print demangle on
|
| 418 |
|
|
set demangle-style gnu-v3
|
| 419 |
|
|
</pre>
|
| 420 |
|
|
|
| 421 |
|
|
|
| 422 |
|
|
<h3 class="left"><a name="verbterm">Tracking uncaught exceptions</a></h3>
|
| 423 |
|
|
<p>The <a href="19_diagnostics/howto.html#4">verbose termination handler</a>
|
| 424 |
|
|
gives information about uncaught exceptions which are killing the
|
| 425 |
|
|
program. It is described in the linked-to page.
|
| 426 |
|
|
</p>
|
| 427 |
|
|
|
| 428 |
|
|
|
| 429 |
|
|
<p>Return <a href="#top">to the top of the page</a> or
|
| 430 |
|
|
<a href="http://gcc.gnu.org/libstdc++/">to the libstdc++ homepage</a>.
|
| 431 |
|
|
</p>
|
| 432 |
|
|
|
| 433 |
|
|
|
| 434 |
|
|
<!-- ####################################################### -->
|
| 435 |
|
|
|
| 436 |
|
|
<hr />
|
| 437 |
|
|
<p class="fineprint"><em>
|
| 438 |
|
|
See <a href="17_intro/license.html">license.html</a> for copying conditions.
|
| 439 |
|
|
Comments and suggestions are welcome, and may be sent to
|
| 440 |
|
|
<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
|
| 441 |
|
|
</em></p>
|
| 442 |
|
|
|
| 443 |
|
|
|
| 444 |
|
|
</body>
|
| 445 |
|
|
</html>
|
