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<sect1 id="manual.intro.using.debug" xreflabel="Debugging Support">
<?dbhtml filename="debug.html"?>

<sect1info>
  <keywordset>
    <keyword>
      C++
    </keyword>
    <keyword>
      debug
    </keyword>
  </keywordset>
</sect1info>

<title>Debugging Support</title>

<para>
  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.
</para>

<sect2 id="debug.compiler">
<title>Using <command>g++</command></title>
  <para>
    Compiler flags determine how debug information is transmitted
    between compilation and debug or analysis tools.
  </para>

  <para>
    The default optimizations and debug flags for a libstdc++ build
    are <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>-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>-fno-eliminate-unused-debug-types</code> can be
    used when additional debug information, such as nested class info,
    is desired.
</para>

<para>
  Or, the debug format that the compiler and debugger use to
  communicate information about source constructs can be changed via
  <code>-gdwarf-2</code> or <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>-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.
</para>

<para>
  Many other options are available: please see <ulink
  url="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging%20Options">"Options
  for Debugging Your Program"</ulink> in Using the GNU Compiler
  Collection (GCC) for a complete list.
</para>
</sect2>

<sect2 id="debug.req">
<title>Debug Versions of Library Binary Files</title>

<para>
  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
</para>
<programlisting>
     --enable-libstdcxx-debug
</programlisting>
<para>and perhaps</para>
<programlisting>
     --enable-libstdcxx-debug-flags='...'
</programlisting>
<para>
  to create a separate debug build. Both the normal build and the
  debug build will persist, without having to specify
  <code>CXXFLAGS</code>, and the debug library will be installed in a
  separate directory tree, in <code>(prefix)/lib/debug</code>. For
  more information, look at the <link
  linkend="manual.intro.setup.configure">configuration</link> section.
</para>

<para>
  A second approach is to use the configuration flags
</para>
<programlisting>
     make CXXFLAGS='-g3 -fno-inline -O0' all
</programlisting>

<para>
  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 <link linkend="manual.ext.debug_mode">debug mode</link>.</para>
</sect2>

<sect2 id="debug.memory">
<title>Memory Leak Hunting</title>

<para>
  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>mtrace</code>, <code>valgrind</code>,
  <code>mudflap</code>, and the non-free commercial product
  <code>purify</code>. In addition, <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.
</para>

<para>
  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>new</code> and <code>delete</code>: there are
  different kinds of allocation schemes that can be used by <code>
  std::allocator </code>. For implementation details, see the <link
  linkend="manual.ext.allocator.mt">mt allocator</link> documentation and
  look specifically for <code>GLIBCXX_FORCE_NEW</code>.
</para>

<para>
  In a nutshell, the default allocator used by <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.
</para>

<para>
  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.
</para>

<para>
  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>__cxa_atexit</code> or
  <code>atexit</code> functions.
</para>

<programlisting>
   #include &lt;cstdlib&gt;

   extern "C" void __libc_freeres(void);

   void do_something() { }

   int main()
   {
     atexit(__libc_freeres);
     do_something();
     return 0;
   }
</programlisting>


<para>or, using <code>__cxa_atexit</code>:</para>

<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;
   }
</programlisting>

<para>
  Suggested valgrind flags, given the suggestions above about setting
  up the runtime environment, library, and test file, might be:
</para>
<programlisting>
   valgrind -v --num-callers=20 --leak-check=yes --leak-resolution=high --show-reachable=yes a.out
</programlisting>

</sect2>

<sect2 id="debug.gdb">
<title>Using <command>gdb</command></title>
  <para>
  </para>

<para>
  Many options are available for gdb itself: please see <ulink
  url="http://sources.redhat.com/gdb/current/onlinedocs/gdb_13.html#SEC125">
  "GDB features for C++" </ulink> in the gdb documentation. Also
  recommended: the other parts of this manual.
</para>

<para>
  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:
</para>

<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
</programlisting>

<para>
  Starting with version 7.0, GDB includes support for writing
  pretty-printers in Python.  Pretty printers for STL classes are
  distributed with GCC from version 4.5.0.  The most recent version of
  these printers are always found in libstdc++ svn repository.
  To enable these printers, check-out the latest printers to a local
  directory:
</para>

<programlisting>
  svn co svn://gcc.gnu.org/svn/gcc/trunk/libstdc++-v3/python
</programlisting>

<para>
  Next, add the following section to your ~/.gdbinit  The path must
  match the location where the Python module above was checked-out.
  So if checked out to: /home/maude/gdb_printers/, the path would be as
  written in the example below.
</para>

<programlisting>
  python
  import sys
  sys.path.insert(0, '/home/maude/gdb_printers/python')
  from libstdcxx.v6.printers import register_libstdcxx_printers
  register_libstdcxx_printers (None)
  end
</programlisting>

<para>
  The path should be the only element that needs to be adjusted in the
  example.  Once loaded, STL classes that the printers support
  should print in a more human-readable format.  To print the classes
  in the old style, use the /r (raw) switch in the print command
  (i.e., print /r foo).  This will print the classes as if the Python
  pretty-printers were not loaded.
</para>

<para>
  For additional information on STL support and GDB please visit:
  <ulink url="http://sourceware.org/gdb/wiki/STLSupport"> "GDB Support
  for STL" </ulink> in the GDB wiki.  Additionally, in-depth
  documentation and discussion of the pretty printing feature can be
  found in "Pretty Printing" node in the GDB manual.  You can find
  on-line versions of the GDB user manual in GDB's homepage, at
  <ulink url="http://sourceware.org/gdb/"> "GDB: The GNU Project
  Debugger" </ulink>.
</para>

</sect2>

<sect2 id="debug.exceptions">
<title>Tracking uncaught exceptions</title>
<para>
  The <link linkend="support.termination.verbose">verbose
  termination handler</link> gives information about uncaught
  exceptions which are killing the program.  It is described in the
  linked-to page.
</para>
</sect2>

<sect2 id="debug.debug_mode">
<title>Debug Mode</title>
  <para> The <link linkend="manual.ext.debug_mode">Debug Mode</link>
  has compile and run-time checks for many containers.
  </para>
</sect2>

<sect2 id="debug.compile_time_checks">
<title>Compile Time Checking</title>
  <para> The <link linkend="manual.ext.compile_checks">Compile-Time
  Checks</link> Extension has compile-time checks for many algorithms.
  </para>
</sect2>

<sect2 id="debug.profile_mode" xreflabel="debug.profile_mode">
<title>Profile-based Performance Analysis</title>
  <para> The <link linkend="manual.ext.profile_mode">Profile-based
  Performance Analysis</link> Extension has performance checks for many
  algorithms.
  </para>
</sect2>

</sect1>