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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Debugging Support</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content="&#10;      C++&#10;    , &#10;      debug&#10;    "/><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      library&#10;    "/><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      runtime&#10;    , &#10;      library&#10;    "/><link rel="home" href="../index.html" title="The GNU C++ Library"/><link rel="up" href="using.html" title="Chapter 3. Using"/><link rel="prev" href="using_exceptions.html" title="Exceptions"/><link rel="next" href="bk01pt02.html" title="Part II.  Standard Contents"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Debugging Support</th></tr><tr><td align="left"><a accesskey="p" href="using_exceptions.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td align="right"> <a accesskey="n" href="bk01pt02.html">Next</a></td></tr></table><hr/></div><div class="section" title="Debugging Support"><div class="titlepage"><div><div><h2 class="title"><a id="manual.intro.using.debug"/>Debugging Support</h2></div></div></div><p>
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  There are numerous things that can be done to improve the ease with
5 
  which C++ binaries are debugged when using the GNU tool chain. Here
6 
  are some of them.
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</p><div class="section" title="Using g++"><div class="titlepage"><div><div><h3 class="title"><a id="debug.compiler"/>Using <span class="command"><strong>g++</strong></span></h3></div></div></div><p>
8 
    Compiler flags determine how debug information is transmitted
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    between compilation and debug or analysis tools.
10 
  </p><p>
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    The default optimizations and debug flags for a libstdc++ build
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    are <code class="code">-g -O2</code>. However, both debug and optimization
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    flags can be varied to change debugging characteristics. For
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    instance, turning off all optimization via the <code class="code">-g -O0
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    -fno-inline</code> flags will disable inlining and optimizations,
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    and add debugging information, so that stepping through all functions,
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    (including inlined constructors and destructors) is possible. In
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    addition, <code class="code">-fno-eliminate-unused-debug-types</code> can be
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    used when additional debug information, such as nested class info,
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    is desired.
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</p><p>
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  Or, the debug format that the compiler and debugger use to
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  communicate information about source constructs can be changed via
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  <code class="code">-gdwarf-2</code> or <code class="code">-gstabs</code> flags: some debugging
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  formats permit more expressive type and scope information to be
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  shown in GDB. Expressiveness can be enhanced by flags like
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  <code class="code">-g3</code>. The default debug information for a particular
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  platform can be identified via the value set by the
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  PREFERRED_DEBUGGING_TYPE macro in the gcc sources.
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</p><p>
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  Many other options are available: please see <a class="link" href="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging%20Options">"Options
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  for Debugging Your Program"</a> in Using the GNU Compiler
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  Collection (GCC) for a complete list.
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</p></div><div class="section" title="Debug Versions of Library Binary Files"><div class="titlepage"><div><div><h3 class="title"><a id="debug.req"/>Debug Versions of Library Binary Files</h3></div></div></div><p>
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  If you would like debug symbols in libstdc++, there are two ways to
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  build libstdc++ with debug flags. The first is to run make from the
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  toplevel in a freshly-configured tree with
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</p><pre class="programlisting">
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     --enable-libstdcxx-debug
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</pre><p>and perhaps</p><pre class="programlisting">
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     --enable-libstdcxx-debug-flags='...'
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</pre><p>
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  to create a separate debug build. Both the normal build and the
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  debug build will persist, without having to specify
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  <code class="code">CXXFLAGS</code>, and the debug library will be installed in a
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  separate directory tree, in <code class="code">(prefix)/lib/debug</code>. For
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  more information, look at the <a class="link" href="configure.html" title="Configure">configuration</a> section.
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</p><p>
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  A second approach is to use the configuration flags
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</p><pre class="programlisting">
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     make CXXFLAGS='-g3 -fno-inline -O0' all
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</pre><p>
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  This quick and dirty approach is often sufficient for quick
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  debugging tasks, when you cannot or don't want to recompile your
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  application to use the <a class="link" href="debug_mode.html" title="Chapter 17. Debug Mode">debug mode</a>.</p></div><div class="section" title="Memory Leak Hunting"><div class="titlepage"><div><div><h3 class="title"><a id="debug.memory"/>Memory Leak Hunting</h3></div></div></div><p>
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  There are various third party memory tracing and debug utilities
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  that can be used to provide detailed memory allocation information
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  about C++ code. An exhaustive list of tools is not going to be
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  attempted, but includes <code class="code">mtrace</code>, <code class="code">valgrind</code>,
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  <code class="code">mudflap</code>, and the non-free commercial product
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  <code class="code">purify</code>. In addition, <code class="code">libcwd</code> has a
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  replacement for the global new and delete operators that can track
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  memory allocation and deallocation and provide useful memory
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  statistics.
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</p><p>
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  Regardless of the memory debugging tool being used, there is one
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  thing of great importance to keep in mind when debugging C++ code
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  that uses <code class="code">new</code> and <code class="code">delete</code>: there are
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  different kinds of allocation schemes that can be used by <code class="code">
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  std::allocator </code>. For implementation details, see the <a class="link" href="mt_allocator.html" title="Chapter 20. The mt_allocator">mt allocator</a> documentation and
71 
  look specifically for <code class="code">GLIBCXX_FORCE_NEW</code>.
72 
</p><p>
73 
  In a nutshell, the default allocator used by <code class="code">
74 
  std::allocator</code> is a high-performance pool allocator, and can
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  give the mistaken impression that in a suspect executable, memory is
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  being leaked, when in reality the memory "leak" is a pool being used
77 
  by the library's allocator and is reclaimed after program
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  termination.
79 
</p><p>
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  For valgrind, there are some specific items to keep in mind. First
81 
  of all, use a version of valgrind that will work with current GNU
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  C++ tools: the first that can do this is valgrind 1.0.4, but later
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  versions should work at least as well. Second of all, use a
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  completely unoptimized build to avoid confusing valgrind. Third, use
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  GLIBCXX_FORCE_NEW to keep extraneous pool allocation noise from
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  cluttering debug information.
87 
</p><p>
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  Fourth, it may be necessary to force deallocation in other libraries
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  as well, namely the "C" library. On linux, this can be accomplished
90 
  with the appropriate use of the <code class="code">__cxa_atexit</code> or
91 
  <code class="code">atexit</code> functions.
92 
</p><pre class="programlisting">
93 
   #include &lt;cstdlib&gt;
94 
 
95 
   extern "C" void __libc_freeres(void);
96 
 
97 
   void do_something() { }
98 
 
99 
   int main()
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   {
101 
     atexit(__libc_freeres);
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     do_something();
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     return 0;
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   }
105 
</pre><p>or, using <code class="code">__cxa_atexit</code>:</p><pre class="programlisting">
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   extern "C" void __libc_freeres(void);
107 
   extern "C" int __cxa_atexit(void (*func) (void *), void *arg, void *d);
108 
 
109 
   void do_something() { }
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111 
   int main()
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   {
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      extern void* __dso_handle __attribute__ ((__weak__));
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      __cxa_atexit((void (*) (void *)) __libc_freeres, NULL,
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                   &amp;__dso_handle ? __dso_handle : NULL);
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      do_test();
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      return 0;
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   }
119 
</pre><p>
120 
  Suggested valgrind flags, given the suggestions above about setting
121 
  up the runtime environment, library, and test file, might be:
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</p><pre class="programlisting">
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   valgrind -v --num-callers=20 --leak-check=yes --leak-resolution=high --show-reachable=yes a.out
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</pre></div><div class="section" title="Data Race Hunting"><div class="titlepage"><div><div><h3 class="title"><a id="debug.races"/>Data Race Hunting</h3></div></div></div><p>
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  All synchronization primitives used in the library internals need to be
126 
  understood by race detectors so that they do not produce false reports.
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</p><p>
128 
  Two annotation macros are used to explain low-level synchronization
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  to race detectors:
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  <code class="code">_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE()</code> and
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  <code class="code"> _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER()</code>.
132 
  By default, these macros are defined empty -- anyone who wants
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  to use a race detector needs to redefine them to call an
134 
  appropriate API.
135 
  Since these macros are empty by default when the library is built,
136 
  redefining them will only affect inline functions and template
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  instantiations which are compiled in user code. This allows annotation
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  of templates such as <code class="code">shared_ptr</code>, but not code which is
139 
  only instantiated in the library.  Code which is only instantiated in
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  the library needs to be recompiled with the annotation macros defined.
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  That can be done by rebuilding the entire
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  <code class="filename">libstdc++.so</code> file but a simpler
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  alternative exists for ELF platforms such as GNU/Linux, because ELF
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  symbol interposition allows symbols defined in the shared library to be
145 
  overridden by symbols with the same name that appear earlier in the
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  runtime search path. This means you only need to recompile the functions
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  that are affected by the annotation macros, which can be done by
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  recompiling individual files.
149 
  Annotating <code class="code">std::string</code> and <code class="code">std::wstring</code>
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  reference counting can be done by disabling extern templates (by defining
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  <code class="code">_GLIBCXX_EXTERN_TEMPLATE=-1</code>) or by rebuilding the
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  <code class="filename">src/string-inst.cc</code> file.
153 
  Annotating the remaining atomic operations (at the time of writing these
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  are in <code class="code">ios_base::Init::~Init</code>, <code class="code">locale::_Impl</code>,
155 
  <code class="code">locale::facet</code> and <code class="code">thread::_M_start_thread</code>)
156 
  requires rebuilding the relevant source files.
157 
</p><p>
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  The approach described above is known to work with the following race
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  detection tools:
160 
  <a class="link" href="http://valgrind.org/docs/manual/drd-manual.html">
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  DRD</a>,
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  <a class="link" href="http://valgrind.org/docs/manual/hg-manual.html">
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  Helgrind</a>, and
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  <a class="link" href="http://code.google.com/p/data-race-test">
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  ThreadSanitizer</a>.
166 
</p><p>
167 
  With DRD, Helgrind and ThreadSanitizer you will need to define
168 
  the macros like this:
169 
</p><pre class="programlisting">
170 
  #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(A) ANNOTATE_HAPPENS_BEFORE(A)
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  #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(A)  ANNOTATE_HAPPENS_AFTER(A)
172 
</pre><p>
173 
  Refer to the documentation of each particular tool for details.
174 
</p></div><div class="section" title="Using gdb"><div class="titlepage"><div><div><h3 class="title"><a id="debug.gdb"/>Using <span class="command"><strong>gdb</strong></span></h3></div></div></div><p>
175 
  </p><p>
176 
  Many options are available for GDB itself: please see <a class="link" href="http://sources.redhat.com/gdb/current/onlinedocs/gdb/">
177 
  "GDB features for C++" </a> in the GDB documentation. Also
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  recommended: the other parts of this manual.
179 
</p><p>
180 
  These settings can either be switched on in at the GDB command line,
181 
  or put into a .gdbint file to establish default debugging
182 
  characteristics, like so:
183 
</p><pre class="programlisting">
184 
   set print pretty on
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   set print object on
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   set print static-members on
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   set print vtbl on
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   set print demangle on
189 
   set demangle-style gnu-v3
190 
</pre><p>
191 
  Starting with version 7.0, GDB includes support for writing
192 
  pretty-printers in Python.  Pretty printers for STL classes are
193 
  distributed with GCC from version 4.5.0.  The most recent version of
194 
  these printers are always found in libstdc++ svn repository.
195 
  To enable these printers, check-out the latest printers to a local
196 
  directory:
197 
</p><pre class="programlisting">
198 
  svn co svn://gcc.gnu.org/svn/gcc/trunk/libstdc++-v3/python
199 
</pre><p>
200 
  Next, add the following section to your ~/.gdbinit  The path must
201 
  match the location where the Python module above was checked-out.
202 
  So if checked out to: /home/maude/gdb_printers/, the path would be as
203 
  written in the example below.
204 
</p><pre class="programlisting">
205 
  python
206 
  import sys
207 
  sys.path.insert(0, '/home/maude/gdb_printers/python')
208 
  from libstdcxx.v6.printers import register_libstdcxx_printers
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  register_libstdcxx_printers (None)
210 
  end
211 
</pre><p>
212 
  The path should be the only element that needs to be adjusted in the
213 
  example.  Once loaded, STL classes that the printers support
214 
  should print in a more human-readable format.  To print the classes
215 
  in the old style, use the /r (raw) switch in the print command
216 
  (i.e., print /r foo).  This will print the classes as if the Python
217 
  pretty-printers were not loaded.
218 
</p><p>
219 
  For additional information on STL support and GDB please visit:
220 
  <a class="link" href="http://sourceware.org/gdb/wiki/STLSupport"> "GDB Support
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  for STL" </a> in the GDB wiki.  Additionally, in-depth
222 
  documentation and discussion of the pretty printing feature can be
223 
  found in "Pretty Printing" node in the GDB manual.  You can find
224 
  on-line versions of the GDB user manual in GDB's homepage, at
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  <a class="link" href="http://sourceware.org/gdb/"> "GDB: The GNU Project
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  Debugger" </a>.
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</p></div><div class="section" title="Tracking uncaught exceptions"><div class="titlepage"><div><div><h3 class="title"><a id="debug.exceptions"/>Tracking uncaught exceptions</h3></div></div></div><p>
228 
  The <a class="link" href="termination.html#support.termination.verbose" title="Verbose Terminate Handler">verbose
229 
  termination handler</a> gives information about uncaught
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  exceptions which are killing the program.  It is described in the
231 
  linked-to page.
232 
</p></div><div class="section" title="Debug Mode"><div class="titlepage"><div><div><h3 class="title"><a id="debug.debug_mode"/>Debug Mode</h3></div></div></div><p> The <a class="link" href="debug_mode.html" title="Chapter 17. Debug Mode">Debug Mode</a>
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  has compile and run-time checks for many containers.
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  </p></div><div class="section" title="Compile Time Checking"><div class="titlepage"><div><div><h3 class="title"><a id="debug.compile_time_checks"/>Compile Time Checking</h3></div></div></div><p> The <a class="link" href="ext_compile_checks.html" title="Chapter 16. Compile Time Checks">Compile-Time
235 
  Checks</a> Extension has compile-time checks for many algorithms.
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  </p></div><div class="section" title="Profile-based Performance Analysis"><div class="titlepage"><div><div><h3 class="title"><a id="debug.profile_mode"/>Profile-based Performance Analysis</h3></div></div></div><p> The <a class="link" href="profile_mode.html" title="Chapter 19. Profile Mode">Profile-based
237 
  Performance Analysis</a> Extension has performance checks for many
238 
  algorithms.
239 
  </p></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="using_exceptions.html">Prev</a> </td><td align="center"><a accesskey="u" href="using.html">Up</a></td><td align="right"> <a accesskey="n" href="bk01pt02.html">Next</a></td></tr><tr><td align="left" valign="top">Exceptions </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Part II. 
240 
    Standard Contents
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  </td></tr></table></div></body></html>

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