buildtools.git

<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" xreflabel="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</code>
    flag will disable inlining, 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.  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" xreflabel="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 <ulink
  url="configopts.html">configuration options</ulink> document.
</para>

<para>
  A second approach is to use the configuration flags 
</para>
<programlisting>
     make CXXFLAGS='-g3 -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 <ulink url="#safe">debug mode</ulink>.</para>
</sect2>
 
<sect2 id="debug.memory" xreflabel="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 <ulink
  url="ext/mt_allocator.html">mt allocator</ulink> 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" xreflabel="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#SEC109">
  "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>
</sect2>

<sect2 id="debug.exceptions" xreflabel="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" xreflabel="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" xreflabel="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>

</sect1>