<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 include 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="https://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.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>

<para>
  There are also lightweight assertions for checking function preconditions,
  such as checking for out-of-bounds indices when accessing a
  <classname>std::vector</classname>. These can be enabled without using
  the full Debug Mode, by using <option>-D_GLIBCXX_ASSERTIONS</option>
  (see <xref linkend="manual.intro.using.macros"/>).
</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 kill the program.
</para>
</section>

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

<para>
  On many targets GCC supports AddressSanitizer, a fast memory error detector,
  which is enabled by the <option>-fsanitize=address</option> option.
</para>

<para>
  The <classname>std::vector</classname> implementation has additional
  instrumentation to work with AddressSanitizer, but this has to be enabled
  explicitly by using <option>-D_GLIBCXX_SANITIZE_VECTOR</option>
  (see <xref linkend="manual.intro.using.macros"/>).
</para>

<para>
  There are also 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> (no longer supported since GCC 4.9.0), ElectricFence,
  and the non-free commercial product <code>purify</code>.
  In addition, <code>libcwd</code>, jemalloc and TCMalloc have replacements
  for the global <code>new</code> and <code>delete</code> operators
  that can track memory allocation and deallocation and provide useful
  memory statistics.
</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 better. Second, using an unoptimized build
  might avoid confusing valgrind.
</para>

<para>
  Third, it may be necessary to force deallocation in other libraries
  as well, namely the "C" library. On GNU/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 xml:id="debug.memory.mtalloc">
<info><title>Non-memory leaks in Pool and MT allocators</title></info>

<para>
  There are different kinds of allocation schemes that can be used by
  <code>std::allocator</code>. Prior to GCC 3.4.0 the default was to use
  a pooling allocator, <classname>pool_allocator</classname>,
  which is still available as the optional
  <classname>__pool_alloc</classname> extension.
  Another optional extension, <classname>__mt_alloc</classname>,
  is a high-performance pool allocator.
</para>

<para>
  In a suspect executable these pooling allocators can give
  the mistaken impression that 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>
  If you're using memory debugging tools on a program that uses
  one of these pooling allocators, you can set the environment variable
  <literal>GLIBCXX_FORCE_NEW</literal> to keep extraneous pool allocation
  noise from cluttering debug information.
  For more details, see the
  <link linkend="manual.ext.allocator.mt">mt allocator</link>
  documentation and look specifically for <code>GLIBCXX_FORCE_NEW</code>.
</para>

</section>

</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.  Code which is only instantiated in
  the library needs to be recompiled with the annotation macros defined.
  That can be done by rebuilding the entire
  <filename class="libraryfile">libstdc++.so</filename> file but a simpler
  alternative exists for ELF platforms such as GNU/Linux, because ELF
  symbol interposition allows symbols defined in the shared library to be
  overridden by symbols with the same name that appear earlier in the
  runtime search path. This means you only need to recompile the functions
  that are affected by the annotation macros, which can be done by
  recompiling individual files.
  Annotating <code>std::string</code> and <code>std::wstring</code>
  reference counting can be done by disabling extern templates (by defining
  <code>_GLIBCXX_EXTERN_TEMPLATE=-1</code>) or by rebuilding the
  <filename>src/string-inst.cc</filename> file.
  Annotating the remaining atomic operations (at the time of writing these
  are in <code>ios_base::Init::~Init</code>, <code>locale::_Impl</code>,
  <code>locale::facet</code> and <code>thread::_M_start_thread</code>)
  requires rebuilding the relevant source files.
</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="https://valgrind.org/docs/manual/drd-manual.html">
  DRD</link>,
  <link xmlns:xlink="http://www.w3.org/1999/xlink"
  xlink:href="https://valgrind.org/docs/manual/hg-manual.html">
  Helgrind</link>, and
  <link xmlns:xlink="http://www.w3.org/1999/xlink"
  xlink:href="https://github.com/google/sanitizers">
  ThreadSanitizer</link> (this refers to ThreadSanitizer v1, not the
  new "tsan" feature built-in to GCC itself).
</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="https://sourceware.org/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 <filename>.gdbinit</filename> 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 containers and other
  classes are distributed with GCC from version 4.5.0 and should be installed
  alongside the libstdc++ shared library files and found automatically by
  GDB.
</para>

<para>
  Depending where libstdc++ is installed, GDB might refuse to auto-load
  the python printers and print a warning instead.
  If this happens the python printers can be enabled by following the
  instructions GDB gives for setting your <code>auto-load safe-path</code>
  in your <filename>.gdbinit</filename> configuration file.
</para>

<para>
  Once loaded, standard library classes that the printers support
  should print in a more human-readable format.  To print the classes
  in the old style, use the <userinput>/r</userinput> (raw) switch in the
  print command (i.e., <userinput>print /r foo</userinput>).  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="https://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="https://sourceware.org/gdb/"> "GDB: The GNU Project
  Debugger" </link>.
</para>

</section>

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

<para>
  As described above, libstdc++ is built with debug symbols enabled by default,
  but because it's also built with optimizations the code can be hard to
  follow when stepping into the library in a debugger.
</para>

<para>
  If you would like to debug <filename>libstdc++.so</filename> itself,
  there are two ways to build an unoptimized libstdc++ with debug flags.
  The first is to create a separate debug build by running make from the
  top-level of a tree freshly-configured with
</para>
<programlisting>
     --enable-libstdcxx-debug
</programlisting>
<para>and perhaps</para>
<programlisting>
     --enable-libstdcxx-debug-flags='...'
</programlisting>
<para>
  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>

</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.
  These checks were designed for C++98 and have not been updated to work
  with C++11 and later standards. They might be removed at a future date.
  </para>
</section>

</section>