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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Debugging Support</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.0" /><meta name="keywords" content="&#10;      C++&#10;    , &#10;      debug&#10;    " /><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      library&#10;    " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="using.html" title="Chapter 3. Using" /><link rel="prev" href="using_exceptions.html" title="Exceptions" /><link rel="next" href="support.html" title="Part II.  Support" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Debugging Support</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="using_exceptions.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td width="20%" align="right"> <a accesskey="n" href="support.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.intro.using.debug"></a>Debugging Support</h2></div></div></div><p>
  There are numerous things that can be done to improve the ease with
  which C++ binaries are debugged when using the GNU tool chain. Here
  are some of them.
</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="debug.compiler"></a>Using <span class="command"><strong>g++</strong></span></h3></div></div></div><p> 
    Compiler flags determine how debug information is transmitted
    between compilation and debug or analysis tools.
  </p><p>
    The default optimizations and debug flags for a libstdc++ build
    are <code class="code">-g -O2</code>. However, both debug and optimization
    flags can be varied to change debugging characteristics. For
    instance, turning off all optimization via the <code class="code">-g -O0
    -fno-inline</code> flags will disable inlining and optimizations,
    and add debugging information, so that stepping through all functions,
    (including inlined constructors and destructors) is possible. In
    addition, <code class="code">-fno-eliminate-unused-debug-types</code> can be
    used when additional debug information, such as nested class info,
    is desired.
</p><p>
  Or, the debug format that the compiler and debugger use to
  communicate information about source constructs can be changed via
  <code class="code">-gdwarf-2</code> or <code class="code">-gstabs</code> flags: some debugging
  formats permit more expressive type and scope information to be
  shown in gdb. Expressiveness can be enhanced by flags like
  <code class="code">-g3</code>. The default debug information for a particular
  platform can be identified via the value set by the
  PREFERRED_DEBUGGING_TYPE macro in the gcc sources.
</p><p>
  Many other options are available: please see <a class="ulink" href="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging%20Options" target="_top">"Options
  for Debugging Your Program"</a> in Using the GNU Compiler
  Collection (GCC) for a complete list.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="debug.req"></a>Debug Versions of Library Binary Files</h3></div></div></div><p>
  If you would like debug symbols in libstdc++, there are two ways to
  build libstdc++ with debug flags. The first is to run make from the
  toplevel in a freshly-configured tree with
</p><pre class="programlisting">
     --enable-libstdcxx-debug
</pre><p>and perhaps</p><pre class="programlisting">
     --enable-libstdcxx-debug-flags='...'
</pre><p>
  to create a separate debug build. Both the normal build and the
  debug build will persist, without having to specify
  <code class="code">CXXFLAGS</code>, and the debug library will be installed in a
  separate directory tree, in <code class="code">(prefix)/lib/debug</code>. For
  more information, look at the <a class="link" href="configure.html" title="Configure">configuration</a> section.
</p><p>
  A second approach is to use the configuration flags 
</p><pre class="programlisting">
     make CXXFLAGS='-g3 -fno-inline -O0' all
</pre><p>
  This quick and dirty approach is often sufficient for quick
  debugging tasks, when you cannot or don't want to recompile your
  application to use the <a class="link" href="debug_mode.html" title="Chapter 30. Debug Mode">debug mode</a>.</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="debug.memory"></a>Memory Leak Hunting</h3></div></div></div><p>
  There are various third party memory tracing and debug utilities
  that can be used to provide detailed memory allocation information
  about C++ code. An exhaustive list of tools is not going to be
  attempted, but includes <code class="code">mtrace</code>, <code class="code">valgrind</code>,
  <code class="code">mudflap</code>, and the non-free commercial product
  <code class="code">purify</code>. In addition, <code class="code">libcwd</code> has a
  replacement for the global new and delete operators that can track
  memory allocation and deallocation and provide useful memory
  statistics.
</p><p>
  Regardless of the memory debugging tool being used, there is one
  thing of great importance to keep in mind when debugging C++ code
  that uses <code class="code">new</code> and <code class="code">delete</code>: there are
  different kinds of allocation schemes that can be used by <code class="code">
  std::allocator </code>. For implementation details, see the <a class="link" href="ext_allocators.html#manual.ext.allocator.mt" title="mt_allocator">mt allocator</a> documentation and
  look specifically for <code class="code">GLIBCXX_FORCE_NEW</code>.
</p><p>
  In a nutshell, the default allocator used by <code class="code">
  std::allocator</code> is a high-performance pool allocator, and can
  give the mistaken impression that in a suspect executable, memory is
  being leaked, when in reality the memory "leak" is a pool being used
  by the library's allocator and is reclaimed after program
  termination.
</p><p>
  For valgrind, there are some specific items to keep in mind. First
  of all, use a version of valgrind that will work with current GNU
  C++ tools: the first that can do this is valgrind 1.0.4, but later
  versions should work at least as well. Second of all, use a
  completely unoptimized build to avoid confusing valgrind. Third, use
  GLIBCXX_FORCE_NEW to keep extraneous pool allocation noise from
  cluttering debug information.
</p><p>
  Fourth, it may be necessary to force deallocation in other libraries
  as well, namely the "C" library. On linux, this can be accomplished
  with the appropriate use of the <code class="code">__cxa_atexit</code> or
  <code class="code">atexit</code> functions.
</p><pre class="programlisting">
   #include &lt;cstdlib&gt;

   extern "C" void __libc_freeres(void);

   void do_something() { }

   int main()
   {
     atexit(__libc_freeres);
     do_something();
     return 0;
   }
</pre><p>or, using <code class="code">__cxa_atexit</code>:</p><pre class="programlisting">
   extern "C" void __libc_freeres(void);
   extern "C" int __cxa_atexit(void (*func) (void *), void *arg, void *d);

   void do_something() { }

   int main()
   {
      extern void* __dso_handle __attribute__ ((__weak__));
      __cxa_atexit((void (*) (void *)) __libc_freeres, NULL, 
                   &amp;__dso_handle ? __dso_handle : NULL);
      do_test();
      return 0;
   }
</pre><p>
  Suggested valgrind flags, given the suggestions above about setting
  up the runtime environment, library, and test file, might be:
</p><pre class="programlisting"> 
   valgrind -v --num-callers=20 --leak-check=yes --leak-resolution=high --show-reachable=yes a.out
</pre></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="debug.gdb"></a>Using <span class="command"><strong>gdb</strong></span></h3></div></div></div><p> 
  </p><p>
  Many options are available for gdb itself: please see <a class="ulink" href="http://sources.redhat.com/gdb/current/onlinedocs/gdb_13.html#SEC125" target="_top">
  "GDB features for C++" </a> in the gdb documentation. Also
  recommended: the other parts of this manual.
</p><p>
  These settings can either be switched on in at the gdb command line,
  or put into a .gdbint file to establish default debugging
  characteristics, like so:
</p><pre class="programlisting">
   set print pretty on
   set print object on
   set print static-members on
   set print vtbl on
   set print demangle on
   set demangle-style gnu-v3
</pre></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="debug.exceptions"></a>Tracking uncaught exceptions</h3></div></div></div><p>
  The <a class="link" href="verbose_termination.html" title="Verbose Terminate Handler">verbose
  termination handler</a> gives information about uncaught
  exceptions which are killing the program.  It is described in the
  linked-to page.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="debug.debug_mode"></a>Debug Mode</h3></div></div></div><p> The <a class="link" href="debug_mode.html" title="Chapter 30. Debug Mode">Debug Mode</a>
  has compile and run-time checks for many containers.
  </p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="debug.compile_time_checks"></a>Compile Time Checking</h3></div></div></div><p> The <a class="link" href="ext_compile_checks.html" title="Chapter 29. Compile Time Checks">Compile-Time
  Checks</a> Extension has compile-time checks for many algorithms.
  </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="using_exceptions.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="using.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="support.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Exceptions </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Part II. 
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