buildtools.git

<section xmlns="http://docbook.org/ns/docbook" version="5.0" 
	 xml:id="manual.intro.using.debug" xreflabel="Debugging Support">
<?dbhtml filename="debug.html"?>

<info><title>Debugging Support</title>
  <keywordset>
    <keyword>
      C++
    </keyword>
    <keyword>
      debug
    </keyword>
  </keywordset>
</info>



<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>

<section xml:id="debug.compiler"><info><title>Using <command>g++</command></title></info>

  <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 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging%20Options">"Options
  for Debugging Your Program"</link> in Using the GNU Compiler
  Collection (GCC) for a complete list.
</para>
</section>

<section xml:id="debug.req"><info><title>Debug Versions of Library Binary Files</title></info>


<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>
</section>

<section xml:id="debug.memory"><info><title>Memory Leak Hunting</title></info>


<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>

</section>

<section xml:id="debug.races"><info><title>Data Race Hunting</title></info>
<para>
  All synchronization primitives used in the library internals need to be
  understood by race detectors so that they do not produce false reports.
</para>

<para>
  Two annotation macros are used to explain low-level synchronization 
  to race detectors:
  <code>_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE()</code> and
  <code> _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER()</code>.
  By default, these macros are defined empty -- anyone who wants
  to use a race detector needs to redefine them to call an
  appropriate API.
  Since these macros are empty by default when the library is built,
  redefining them will only affect inline functions and template
  instantiations which are compiled in user code. This allows annotation
  of templates such as <code>shared_ptr</code>, but not code which is
  only instantiated in the library.
  In order to annotate <code>basic_string</code> reference counting it
  is necessary to disable extern templates (by defining 
  <code>_GLIBCXX_EXTERN_TEMPLATE=-1</code>) or to rebuild the 
  <code>.so</code> file.
  Annotating the remaining atomic operations (at the time of writing these
  are in <code>ios_base::Init::~Init</code>, <code>locale::_Impl</code> and
  <code>locale::facet</code>) requires rebuilding the <code>.so</code> file.
</para>

<para>
  The approach described above is known to work with the following race
  detection tools:
  <link xmlns:xlink="http://www.w3.org/1999/xlink" 
  xlink:href="http://valgrind.org/docs/manual/drd-manual.html">
  DRD</link>,
  <link xmlns:xlink="http://www.w3.org/1999/xlink" 
  xlink:href="http://valgrind.org/docs/manual/hg-manual.html"> 
  Helgrind</link>, and
  <link xmlns:xlink="http://www.w3.org/1999/xlink" 
  xlink:href="http://code.google.com/p/data-race-test"> 
  ThreadSanitizer</link>.
</para>

<para>
  With DRD, Helgrind and ThreadSanitizer you will need to define
  the macros like this:
<programlisting>
  #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(A) ANNOTATE_HAPPENS_BEFORE(A)
  #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(A)  ANNOTATE_HAPPENS_AFTER(A)
</programlisting>
  Refer to the documentation of each particular tool for details.
</para>

</section>

<section xml:id="debug.gdb"><info><title>Using <command>gdb</command></title></info>

  <para>
  </para>

<para>
  Many options are available for GDB itself: please see <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://sources.redhat.com/gdb/current/onlinedocs/gdb/">
  "GDB features for C++" </link> 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:
  <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://sourceware.org/gdb/wiki/STLSupport"> "GDB Support
  for STL" </link> 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
  <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://sourceware.org/gdb/"> "GDB: The GNU Project
  Debugger" </link>.
</para>

</section>

<section xml:id="debug.exceptions"><info><title>Tracking uncaught exceptions</title></info>

<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>
</section>

<section xml:id="debug.debug_mode"><info><title>Debug Mode</title></info>

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

<section xml:id="debug.compile_time_checks"><info><title>Compile Time Checking</title></info>

  <para> The <link linkend="manual.ext.compile_checks">Compile-Time
  Checks</link> Extension has compile-time checks for many algorithms.
  </para>
</section>

<section xml:id="debug.profile_mode" xreflabel="debug.profile_mode"><info><title>Profile-based Performance Analysis</title></info>

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

</section>