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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Debugging Support</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="C++, debug" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><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="std_contents.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 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="std_contents.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.intro.using.debug"></a>Debugging Support</h2></div></div></div><p>
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There are numerous things that can be done to improve the ease with
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which C++ binaries are debugged when using the GNU tool chain. Here
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are some of them.
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</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>
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Compiler flags determine how debug information is transmitted
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between compilation and debug or analysis tools.
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</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" target="_top">"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"><div class="titlepage"><div><div><h3 class="title"><a id="debug.req"></a>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 create a separate
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debug build by running make from the top-level of a tree
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freshly-configured 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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Both the normal build and the debug build will persist, without
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having to specify <code class="code">CXXFLAGS</code>, and the debug library will
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be installed in a separate directory tree, in <code class="code">(prefix)/lib/debug</code>.
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For more information, look at the
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<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"><div class="titlepage"><div><div><h3 class="title"><a id="debug.memory"></a>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
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look specifically for <code class="code">GLIBCXX_FORCE_NEW</code>.
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</p><p>
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In a nutshell, the optional <code class="classname">mt_allocator</code>
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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
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by the library's allocator and is reclaimed after program
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termination.
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</p><p>
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For valgrind, there are some specific items to keep in mind. First
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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.
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</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
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with the appropriate use of the <code class="code">__cxa_atexit</code> or
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<code class="code">atexit</code> functions.
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</p><pre class="programlisting">
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#include <cstdlib>
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extern "C" void __libc_freeres(void);
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void do_something() { }
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int main()
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{
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atexit(__libc_freeres);
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do_something();
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return 0;
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}
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</pre><p>or, using <code class="code">__cxa_atexit</code>:</p><pre class="programlisting">
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extern "C" void __libc_freeres(void);
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extern "C" int __cxa_atexit(void (*func) (void *), void *arg, void *d);
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void do_something() { }
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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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&__dso_handle ? __dso_handle : NULL);
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do_test();
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return 0;
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}
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</pre><p>
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Suggested valgrind flags, given the suggestions above about setting
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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"><div class="titlepage"><div><div><h3 class="title"><a id="debug.races"></a>Data Race Hunting</h3></div></div></div><p>
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All synchronization primitives used in the library internals need to be
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understood by race detectors so that they do not produce false reports.
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</p><p>
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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>.
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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
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appropriate API.
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Since these macros are empty by default when the library is built,
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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
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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
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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.
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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.
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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>,
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<code class="code">locale::facet</code> and <code class="code">thread::_M_start_thread</code>)
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requires rebuilding the relevant source files.
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</p><p>
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The approach described above is known to work with the following race
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detection tools:
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<a class="link" href="http://valgrind.org/docs/manual/drd-manual.html" target="_top">
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DRD</a>,
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<a class="link" href="http://valgrind.org/docs/manual/hg-manual.html" target="_top">
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Helgrind</a>, and
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<a class="link" href="https://github.com/google/sanitizers" target="_top">
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ThreadSanitizer</a> (this refers to ThreadSanitizer v1, not the
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new "tsan" feature built-in to GCC itself).
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</p><p>
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With DRD, Helgrind and ThreadSanitizer you will need to define
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the macros like this:
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</p><pre class="programlisting">
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#define _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(A) ANNOTATE_HAPPENS_BEFORE(A)
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#define _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(A) ANNOTATE_HAPPENS_AFTER(A)
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</pre><p>
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Refer to the documentation of each particular tool for details.
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</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>
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</p><p>
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Many options are available for GDB itself: please see <a class="link" href="http://sourceware.org/gdb/current/onlinedocs/gdb/" target="_top">
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"GDB features for C++" </a> in the GDB documentation. Also
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recommended: the other parts of this manual.
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</p><p>
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These settings can either be switched on in at the GDB command line,
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or put into a <code class="filename">.gdbinit</code> file to establish default
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debugging characteristics, like so:
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</p><pre class="programlisting">
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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
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set demangle-style gnu-v3
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</pre><p>
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Starting with version 7.0, GDB includes support for writing
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pretty-printers in Python. Pretty printers for containers and other
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classes are distributed with GCC from version 4.5.0 and should be installed
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alongside the libstdc++ shared library files and found automatically by
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GDB.
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</p><p>
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Depending where libstdc++ is installed, GDB might refuse to auto-load
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the python printers and print a warning instead.
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If this happens the python printers can be enabled by following the
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instructions GDB gives for setting your <code class="code">auto-load safe-path</code>
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in your <code class="filename">.gdbinit</code> configuration file.
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</p><p>
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Once loaded, standard library classes that the printers support
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should print in a more human-readable format. To print the classes
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in the old style, use the <strong class="userinput"><code>/r</code></strong> (raw) switch in the
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print command (i.e., <strong class="userinput"><code>print /r foo</code></strong>). This will
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print the classes as if the Python pretty-printers were not loaded.
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</p><p>
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For additional information on STL support and GDB please visit:
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<a class="link" href="http://sourceware.org/gdb/wiki/STLSupport" target="_top"> "GDB Support
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for STL" </a> in the GDB wiki. Additionally, in-depth
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documentation and discussion of the pretty printing feature can be
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found in "Pretty Printing" node in the GDB manual. You can find
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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/" target="_top"> "GDB: The GNU Project
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Debugger" </a>.
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</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>
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The <a class="link" href="termination.html#support.termination.verbose" title="Verbose Terminate Handler">verbose
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termination handler</a> gives information about uncaught
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exceptions which kill the program.
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</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>
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has compile and run-time checks for many containers.
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</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
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Checks</a> extension has compile-time checks for many algorithms.
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</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
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Performance Analysis</a> extension has performance checks for many
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algorithms.
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</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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Standard Contents
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</td></tr></table></div></body></html> |