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  15     "><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 bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Debugging Support</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="using_exceptions.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td width="20%" 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" style="clear: both"><a name="manual.intro.using.debug"></a>Debugging Support</h2></div></div></div><p>
  16   There are numerous things that can be done to improve the ease with
  17   which C++ binaries are debugged when using the GNU tool chain. Here
  18   are some of them.
  19 </p><div class="section" title="Using g++"><div class="titlepage"><div><div><h3 class="title"><a name="debug.compiler"></a>Using <span class="command"><strong>g++</strong></span></h3></div></div></div><p>
  20     Compiler flags determine how debug information is transmitted
  21     between compilation and debug or analysis tools.
  22   </p><p>
  23     The default optimizations and debug flags for a libstdc++ build
  24     are <code class="code">-g -O2</code>. However, both debug and optimization
  25     flags can be varied to change debugging characteristics. For
  26     instance, turning off all optimization via the <code class="code">-g -O0
  27     -fno-inline</code> flags will disable inlining and optimizations,
  28     and add debugging information, so that stepping through all functions,
  29     (including inlined constructors and destructors) is possible. In
  30     addition, <code class="code">-fno-eliminate-unused-debug-types</code> can be
  31     used when additional debug information, such as nested class info,
  32     is desired.
  33 </p><p>
  34   Or, the debug format that the compiler and debugger use to
  35   communicate information about source constructs can be changed via
  36   <code class="code">-gdwarf-2</code> or <code class="code">-gstabs</code> flags: some debugging
  37   formats permit more expressive type and scope information to be
  38   shown in GDB. Expressiveness can be enhanced by flags like
  39   <code class="code">-g3</code>. The default debug information for a particular
  40   platform can be identified via the value set by the
  41   PREFERRED_DEBUGGING_TYPE macro in the gcc sources.
  42 </p><p>
  43   Many other options are available: please see <a class="link" href="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging%20Options" target="_top">"Options
  44   for Debugging Your Program"</a> in Using the GNU Compiler
  45   Collection (GCC) for a complete list.
  46 </p></div><div class="section" title="Debug Versions of Library Binary Files"><div class="titlepage"><div><div><h3 class="title"><a name="debug.req"></a>Debug Versions of Library Binary Files</h3></div></div></div><p>
  47   If you would like debug symbols in libstdc++, there are two ways to
  48   build libstdc++ with debug flags. The first is to create a separate
  49   debug build by running make from the top-level of a tree
  50   freshly-configured with
  51 </p><pre class="programlisting">
  52      --enable-libstdcxx-debug
  53 </pre><p>and perhaps</p><pre class="programlisting">
  54      --enable-libstdcxx-debug-flags='...'
  55 </pre><p>
  56   Both the normal build and the debug build will persist, without
  57   having to specify <code class="code">CXXFLAGS</code>, and the debug library will
  58   be installed in a separate directory tree, in <code class="code">(prefix)/lib/debug</code>.
  59   For more information, look at the
  60   <a class="link" href="configure.html" title="Configure">configuration</a> section.
  61 </p><p>
  62   A second approach is to use the configuration flags
  63 </p><pre class="programlisting">
  64      make CXXFLAGS='-g3 -fno-inline -O0' all
  65 </pre><p>
  66   This quick and dirty approach is often sufficient for quick
  67   debugging tasks, when you cannot or don't want to recompile your
  68   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 name="debug.memory"></a>Memory Leak Hunting</h3></div></div></div><p>
  69   There are various third party memory tracing and debug utilities
  70   that can be used to provide detailed memory allocation information
  71   about C++ code. An exhaustive list of tools is not going to be
  72   attempted, but includes <code class="code">mtrace</code>, <code class="code">valgrind</code>,
  73   <code class="code">mudflap</code>, and the non-free commercial product
  74   <code class="code">purify</code>. In addition, <code class="code">libcwd</code> has a
  75   replacement for the global new and delete operators that can track
  76   memory allocation and deallocation and provide useful memory
  77   statistics.
  78 </p><p>
  79   Regardless of the memory debugging tool being used, there is one
  80   thing of great importance to keep in mind when debugging C++ code
  81   that uses <code class="code">new</code> and <code class="code">delete</code>: there are
  82   different kinds of allocation schemes that can be used by <code class="code">
  83   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
  84   look specifically for <code class="code">GLIBCXX_FORCE_NEW</code>.
  85 </p><p>
  86   In a nutshell, the optional <code class="classname">mt_allocator</code>
  87   is a high-performance pool allocator, and can
  88   give the mistaken impression that in a suspect executable, memory is
  89   being leaked, when in reality the memory "leak" is a pool being used
  90   by the library's allocator and is reclaimed after program
  91   termination.
  92 </p><p>
  93   For valgrind, there are some specific items to keep in mind. First
  94   of all, use a version of valgrind that will work with current GNU
  95   C++ tools: the first that can do this is valgrind 1.0.4, but later
  96   versions should work at least as well. Second of all, use a
  97   completely unoptimized build to avoid confusing valgrind. Third, use
  98   GLIBCXX_FORCE_NEW to keep extraneous pool allocation noise from
  99   cluttering debug information.
 100 </p><p>
 101   Fourth, it may be necessary to force deallocation in other libraries
 102   as well, namely the "C" library. On linux, this can be accomplished
 103   with the appropriate use of the <code class="code">__cxa_atexit</code> or
 104   <code class="code">atexit</code> functions.
 105 </p><pre class="programlisting">
 106    #include &lt;cstdlib&gt;
 107
 108    extern "C" void __libc_freeres(void);
 109
 110    void do_something() { }
 111
 112    int main()
 113    {
 114      atexit(__libc_freeres);
 115      do_something();
 116      return 0;
 117    }
 118 </pre><p>or, using <code class="code">__cxa_atexit</code>:</p><pre class="programlisting">
 119    extern "C" void __libc_freeres(void);
 120    extern "C" int __cxa_atexit(void (*func) (void *), void *arg, void *d);
 121
 122    void do_something() { }
 123
 124    int main()
 125    {
 126       extern void* __dso_handle __attribute__ ((__weak__));
 127       __cxa_atexit((void (*) (void *)) __libc_freeres, NULL,
 128                    &amp;__dso_handle ? __dso_handle : NULL);
 129       do_test();
 130       return 0;
 131    }
 132 </pre><p>
 133   Suggested valgrind flags, given the suggestions above about setting
 134   up the runtime environment, library, and test file, might be:
 135 </p><pre class="programlisting">
 136    valgrind -v --num-callers=20 --leak-check=yes --leak-resolution=high --show-reachable=yes a.out
 137 </pre></div><div class="section" title="Data Race Hunting"><div class="titlepage"><div><div><h3 class="title"><a name="debug.races"></a>Data Race Hunting</h3></div></div></div><p>
 138   All synchronization primitives used in the library internals need to be
 139   understood by race detectors so that they do not produce false reports.
 140 </p><p>
 141   Two annotation macros are used to explain low-level synchronization
 142   to race detectors:
 143   <code class="code">_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE()</code> and
 144   <code class="code"> _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER()</code>.
 145   By default, these macros are defined empty -- anyone who wants
 146   to use a race detector needs to redefine them to call an
 147   appropriate API.
 148   Since these macros are empty by default when the library is built,
 149   redefining them will only affect inline functions and template
 150   instantiations which are compiled in user code. This allows annotation
 151   of templates such as <code class="code">shared_ptr</code>, but not code which is
 152   only instantiated in the library.  Code which is only instantiated in
 153   the library needs to be recompiled with the annotation macros defined.
 154   That can be done by rebuilding the entire
 155   <code class="filename">libstdc++.so</code> file but a simpler
 156   alternative exists for ELF platforms such as GNU/Linux, because ELF
 157   symbol interposition allows symbols defined in the shared library to be
 158   overridden by symbols with the same name that appear earlier in the
 159   runtime search path. This means you only need to recompile the functions
 160   that are affected by the annotation macros, which can be done by
 161   recompiling individual files.
 162   Annotating <code class="code">std::string</code> and <code class="code">std::wstring</code>
 163   reference counting can be done by disabling extern templates (by defining
 164   <code class="code">_GLIBCXX_EXTERN_TEMPLATE=-1</code>) or by rebuilding the
 165   <code class="filename">src/string-inst.cc</code> file.
 166   Annotating the remaining atomic operations (at the time of writing these
 167   are in <code class="code">ios_base::Init::~Init</code>, <code class="code">locale::_Impl</code>,
 168   <code class="code">locale::facet</code> and <code class="code">thread::_M_start_thread</code>)
 169   requires rebuilding the relevant source files.
 170 </p><p>
 171   The approach described above is known to work with the following race
 172   detection tools:
 173   <a class="link" href="http://valgrind.org/docs/manual/drd-manual.html" target="_top">
 174   DRD</a>,
 175   <a class="link" href="http://valgrind.org/docs/manual/hg-manual.html" target="_top">
 176   Helgrind</a>, and
 177   <a class="link" href="http://code.google.com/p/data-race-test" target="_top">
 178   ThreadSanitizer</a>.
 179 </p><p>
 180   With DRD, Helgrind and ThreadSanitizer you will need to define
 181   the macros like this:
 182 </p><pre class="programlisting">
 183   #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(A) ANNOTATE_HAPPENS_BEFORE(A)
 184   #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(A)  ANNOTATE_HAPPENS_AFTER(A)
 185 </pre><p>
 186   Refer to the documentation of each particular tool for details.
 187 </p></div><div class="section" title="Using gdb"><div class="titlepage"><div><div><h3 class="title"><a name="debug.gdb"></a>Using <span class="command"><strong>gdb</strong></span></h3></div></div></div><p>
 188   </p><p>
 189   Many options are available for GDB itself: please see <a class="link" href="http://sources.redhat.com/gdb/current/onlinedocs/gdb/" target="_top">
 190   "GDB features for C++" </a> in the GDB documentation. Also
 191   recommended: the other parts of this manual.
 192 </p><p>
 193   These settings can either be switched on in at the GDB command line,
 194   or put into a .gdbint file to establish default debugging
 195   characteristics, like so:
 196 </p><pre class="programlisting">
 197    set print pretty on
 198    set print object on
 199    set print static-members on
 200    set print vtbl on
 201    set print demangle on
 202    set demangle-style gnu-v3
 203 </pre><p>
 204   Starting with version 7.0, GDB includes support for writing
 205   pretty-printers in Python.  Pretty printers for STL classes are
 206   distributed with GCC from version 4.5.0.  The most recent version of
 207   these printers are always found in libstdc++ svn repository.
 208   To enable these printers, check-out the latest printers to a local
 209   directory:
 210 </p><pre class="programlisting">
 211   svn co svn://gcc.gnu.org/svn/gcc/trunk/libstdc++-v3/python
 212 </pre><p>
 213   Next, add the following section to your ~/.gdbinit  The path must
 214   match the location where the Python module above was checked-out.
 215   So if checked out to: /home/maude/gdb_printers/, the path would be as
 216   written in the example below.
 217 </p><pre class="programlisting">
 218   python
 219   import sys
 220   sys.path.insert(0, '/home/maude/gdb_printers/python')
 221   from libstdcxx.v6.printers import register_libstdcxx_printers
 222   register_libstdcxx_printers (None)
 223   end
 224 </pre><p>
 225   The path should be the only element that needs to be adjusted in the
 226   example.  Once loaded, STL classes that the printers support
 227   should print in a more human-readable format.  To print the classes
 228   in the old style, use the /r (raw) switch in the print command
 229   (i.e., print /r foo).  This will print the classes as if the Python
 230   pretty-printers were not loaded.
 231 </p><p>
 232   For additional information on STL support and GDB please visit:
 233   <a class="link" href="http://sourceware.org/gdb/wiki/STLSupport" target="_top"> "GDB Support
 234   for STL" </a> in the GDB wiki.  Additionally, in-depth
 235   documentation and discussion of the pretty printing feature can be
 236   found in "Pretty Printing" node in the GDB manual.  You can find
 237   on-line versions of the GDB user manual in GDB's homepage, at
 238   <a class="link" href="http://sourceware.org/gdb/" target="_top"> "GDB: The GNU Project
 239   Debugger" </a>.
 240 </p></div><div class="section" title="Tracking uncaught exceptions"><div class="titlepage"><div><div><h3 class="title"><a name="debug.exceptions"></a>Tracking uncaught exceptions</h3></div></div></div><p>
 241   The <a class="link" href="termination.html#support.termination.verbose" title="Verbose Terminate Handler">verbose
 242   termination handler</a> gives information about uncaught
 243   exceptions which are killing the program.  It is described in the
 244   linked-to page.
 245 </p></div><div class="section" title="Debug Mode"><div class="titlepage"><div><div><h3 class="title"><a name="debug.debug_mode"></a>Debug Mode</h3></div></div></div><p> The <a class="link" href="debug_mode.html" title="Chapter 17. Debug Mode">Debug Mode</a>
 246   has compile and run-time checks for many containers.
 247   </p></div><div class="section" title="Compile Time Checking"><div class="titlepage"><div><div><h3 class="title"><a name="debug.compile_time_checks"></a>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
 248   Checks</a> Extension has compile-time checks for many algorithms.
 249   </p></div><div class="section" title="Profile-based Performance Analysis"><div class="titlepage"><div><div><h3 class="title"><a name="debug.profile_mode"></a>Profile-based Performance Analysis</h3></div></div></div><p> The <a class="link" href="profile_mode.html" title="Chapter 19. Profile Mode">Profile-based
 250   Performance Analysis</a> Extension has performance checks for many
 251   algorithms.
 252   </p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="using_exceptions.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="using.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="bk01pt02.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Exceptions </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Part II.
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