libstdc++-v3/doc/html/manual/debug.html

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   3   There are numerous things that can be done to improve the ease with
   4   which C++ binaries are debugged when using the GNU tool chain. Here
   5   are some of them.
   6 </p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="debug.compiler"></a>Using <span class="command"><strong>g++</strong></span></h3></div></div></div><p>
   7     Compiler flags determine how debug information is transmitted
   8     between compilation and debug or analysis tools.
   9   </p><p>
  10     The default optimizations and debug flags for a libstdc++ build
  11     are <code class="code">-g -O2</code>. However, both debug and optimization
  12     flags can be varied to change debugging characteristics. For
  13     instance, turning off all optimization via the <code class="code">-g -O0
  14     -fno-inline</code> flags will disable inlining and optimizations,
  15     and add debugging information, so that stepping through all functions,
  16     (including inlined constructors and destructors) is possible. In
  17     addition, <code class="code">-fno-eliminate-unused-debug-types</code> can be
  18     used when additional debug information, such as nested class info,
  19     is desired.
  20 </p><p>
  21   Or, the debug format that the compiler and debugger use to
  22   communicate information about source constructs can be changed via
  23   <code class="code">-gdwarf-2</code> or <code class="code">-gstabs</code> flags: some debugging
  24   formats permit more expressive type and scope information to be
  25   shown in GDB. Expressiveness can be enhanced by flags like
  26   <code class="code">-g3</code>. The default debug information for a particular
  27   platform can be identified via the value set by the
  28   PREFERRED_DEBUGGING_TYPE macro in the GCC sources.
  29 </p><p>
  30   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
  31   for Debugging Your Program"</a> in Using the GNU Compiler
  32   Collection (GCC) for a complete list.
  33 </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="debug.req"></a>Debug Versions of Library Binary Files</h3></div></div></div><p>
  34   If you would like debug symbols in libstdc++, there are two ways to
  35   build libstdc++ with debug flags. The first is to create a separate
  36   debug build by running make from the top-level of a tree
  37   freshly-configured with
  38 </p><pre class="programlisting">
  39      --enable-libstdcxx-debug
  40 </pre><p>and perhaps</p><pre class="programlisting">
  41      --enable-libstdcxx-debug-flags='...'
  42 </pre><p>
  43   Both the normal build and the debug build will persist, without
  44   having to specify <code class="code">CXXFLAGS</code>, and the debug library will
  45   be installed in a separate directory tree, in <code class="code">(prefix)/lib/debug</code>.
  46   For more information, look at the
  47   <a class="link" href="configure.html" title="Configure">configuration</a> section.
  48 </p><p>
  49   A second approach is to use the configuration flags
  50 </p><pre class="programlisting">
  51      make CXXFLAGS='-g3 -fno-inline -O0' all
  52 </pre><p>
  53   This quick and dirty approach is often sufficient for quick
  54   debugging tasks, when you cannot or don't want to recompile your
  55   application to use the <a class="link" href="debug_mode.html" title="Chapter 17. Debug Mode">debug mode</a>.</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="debug.memory"></a>Memory Leak Hunting</h3></div></div></div><p>
  56   There are various third party memory tracing and debug utilities
  57   that can be used to provide detailed memory allocation information
  58   about C++ code. An exhaustive list of tools is not going to be
  59   attempted, but includes <code class="code">mtrace</code>, <code class="code">valgrind</code>,
  60   <code class="code">mudflap</code>, and the non-free commercial product
  61   <code class="code">purify</code>. In addition, <code class="code">libcwd</code> has a
  62   replacement for the global new and delete operators that can track
  63   memory allocation and deallocation and provide useful memory
  64   statistics.
  65 </p><p>
  66   Regardless of the memory debugging tool being used, there is one
  67   thing of great importance to keep in mind when debugging C++ code
  68   that uses <code class="code">new</code> and <code class="code">delete</code>: there are
  69   different kinds of allocation schemes that can be used by <code class="code">
  70   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 optional <code class="classname">mt_allocator</code>
  74   is a high-performance pool allocator, and can
  75   give the mistaken impression that in a suspect executable, memory is
  76   being leaked, when in reality the memory "leak" is a pool being used
  77   by the library's allocator and is reclaimed after program
  78   termination.
  79 </p><p>
  80   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
  82   C++ tools: the first that can do this is valgrind 1.0.4, but later
  83   versions should work at least as well. Second of all, use a
  84   completely unoptimized build to avoid confusing valgrind. Third, use
  85   GLIBCXX_FORCE_NEW to keep extraneous pool allocation noise from
  86   cluttering debug information.
  87 </p><p>
  88   Fourth, it may be necessary to force deallocation in other libraries
  89   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()
 100    {
 101      atexit(__libc_freeres);
 102      do_something();
 103      return 0;
 104    }
 105 </pre><p>or, using <code class="code">__cxa_atexit</code>:</p><pre class="programlisting">
 106    extern "C" void __libc_freeres(void);
 107    extern "C" int __cxa_atexit(void (*func) (void *), void *arg, void *d);
 108
 109    void do_something() { }
 110
 111    int main()
 112    {
 113       extern void* __dso_handle __attribute__ ((__weak__));
 114       __cxa_atexit((void (*) (void *)) __libc_freeres, NULL,
 115                    &amp;__dso_handle ? __dso_handle : NULL);
 116       do_test();
 117       return 0;
 118    }
 119 </pre><p>
 120   Suggested valgrind flags, given the suggestions above about setting
 121   up the runtime environment, library, and test file, might be:
 122 </p><pre class="programlisting">
 123    valgrind -v --num-callers=20 --leak-check=yes --leak-resolution=high --show-reachable=yes a.out
 124 </pre></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="debug.races"></a>Data Race Hunting</h3></div></div></div><p>
 125   All synchronization primitives used in the library internals need to be
 126   understood by race detectors so that they do not produce false reports.
 127 </p><p>
 128   Two annotation macros are used to explain low-level synchronization
 129   to race detectors:
 130   <code class="code">_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE()</code> and
 131   <code class="code"> _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER()</code>.
 132   By default, these macros are defined empty -- anyone who wants
 133   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
 137   instantiations which are compiled in user code. This allows annotation
 138   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
 140   the library needs to be recompiled with the annotation macros defined.
 141   That can be done by rebuilding the entire
 142   <code class="filename">libstdc++.so</code> file but a simpler
 143   alternative exists for ELF platforms such as GNU/Linux, because ELF
 144   symbol interposition allows symbols defined in the shared library to be
 145   overridden by symbols with the same name that appear earlier in the
 146   runtime search path. This means you only need to recompile the functions
 147   that are affected by the annotation macros, which can be done by
 148   recompiling individual files.
 149   Annotating <code class="code">std::string</code> and <code class="code">std::wstring</code>
 150   reference counting can be done by disabling extern templates (by defining
 151   <code class="code">_GLIBCXX_EXTERN_TEMPLATE=-1</code>) or by rebuilding the
 152   <code class="filename">src/string-inst.cc</code> file.
 153   Annotating the remaining atomic operations (at the time of writing these
 154   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>
 158   The approach described above is known to work with the following race
 159   detection tools:
 160   <a class="link" href="http://valgrind.org/docs/manual/drd-manual.html" target="_top">
 161   DRD</a>,
 162   <a class="link" href="http://valgrind.org/docs/manual/hg-manual.html" target="_top">
 163   Helgrind</a>, and
 164   <a class="link" href="http://code.google.com/p/data-race-test/" target="_top">
 165   ThreadSanitizer</a> (this refers to ThreadSanitizer v1, not the
 166   new "tsan" feature built-in to GCC itself).
 167 </p><p>
 168   With DRD, Helgrind and ThreadSanitizer you will need to define
 169   the macros like this:
 170 </p><pre class="programlisting">
 171   #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(A) ANNOTATE_HAPPENS_BEFORE(A)
 172   #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(A)  ANNOTATE_HAPPENS_AFTER(A)
 173 </pre><p>
 174   Refer to the documentation of each particular tool for details.
 175 </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="debug.gdb"></a>Using <span class="command"><strong>gdb</strong></span></h3></div></div></div><p>
 176   </p><p>
 177   Many options are available for GDB itself: please see <a class="link" href="http://sourceware.org/gdb/current/onlinedocs/gdb/" target="_top">
 178   "GDB features for C++" </a> in the GDB documentation. Also
 179   recommended: the other parts of this manual.
 180 </p><p>
 181   These settings can either be switched on in at the GDB command line,
 182   or put into a <code class="filename">.gdbinit</code> file to establish default
 183   debugging characteristics, like so:
 184 </p><pre class="programlisting">
 185    set print pretty on
 186    set print object on
 187    set print static-members on
 188    set print vtbl on
 189    set print demangle on
 190    set demangle-style gnu-v3
 191 </pre><p>
 192   Starting with version 7.0, GDB includes support for writing
 193   pretty-printers in Python.  Pretty printers for containers and other
 194   classes are distributed with GCC from version 4.5.0 and should be installed
 195   alongside the libstdc++ shared library files and found automatically by
 196   GDB.
 197 </p><p>
 198   Depending where libstdc++ is installed, GDB might refuse to auto-load
 199   the python printers and print a warning instead.
 200   If this happens the python printers can be enabled by following the
 201   instructions GDB gives for setting your <code class="code">auto-load safe-path</code>
 202   in your <code class="filename">.gdbinit</code> configuration file.
 203 </p><p>
 204   Once loaded, standard library classes that the printers support
 205   should print in a more human-readable format.  To print the classes
 206   in the old style, use the <strong class="userinput"><code>/r</code></strong> (raw) switch in the
 207   print command (i.e., <strong class="userinput"><code>print /r foo</code></strong>).  This will
 208   print the classes as if the Python pretty-printers were not loaded.
 209 </p><p>
 210   For additional information on STL support and GDB please visit:
 211   <a class="link" href="http://sourceware.org/gdb/wiki/STLSupport" target="_top"> "GDB Support
 212   for STL" </a> in the GDB wiki.  Additionally, in-depth
 213   documentation and discussion of the pretty printing feature can be
 214   found in "Pretty Printing" node in the GDB manual.  You can find
 215   on-line versions of the GDB user manual in GDB's homepage, at
 216   <a class="link" href="http://sourceware.org/gdb/" target="_top"> "GDB: The GNU Project
 217   Debugger" </a>.
 218 </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="debug.exceptions"></a>Tracking uncaught exceptions</h3></div></div></div><p>
 219   The <a class="link" href="termination.html#support.termination.verbose" title="Verbose Terminate Handler">verbose
 220   termination handler</a> gives information about uncaught
 221   exceptions which kill the program.
 222 </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="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>
 223   has compile and run-time checks for many containers.
 224   </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="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
 225   Checks</a> extension has compile-time checks for many algorithms.
 226   </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="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
 227   Performance Analysis</a> extension has performance checks for many
 228   algorithms.
 229   </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="std_contents.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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