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| <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 V1.74.0" /><meta name="keywords" content=" C++ , debug " /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="using.html" title="Chapter 3. Using" /><link rel="prev" href="using_exceptions.html" title="Exceptions" /><link rel="next" href="support.html" title="Part II. Support" /></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="support.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><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> |
| 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. |
| </p><div class="sect2" lang="en" xml:lang="en"><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> |
| Compiler flags determine how debug information is transmitted |
| between compilation and debug or analysis tools. |
| </p><p> |
| The default optimizations and debug flags for a libstdc++ build |
| are <code class="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 class="code">-g -O0 |
| -fno-inline</code> flags will disable inlining and optimizations, |
| and add debugging information, so that stepping through all functions, |
| (including inlined constructors and destructors) is possible. In |
| addition, <code class="code">-fno-eliminate-unused-debug-types</code> can be |
| used when additional debug information, such as nested class info, |
| is desired. |
| </p><p> |
| Or, the debug format that the compiler and debugger use to |
| communicate information about source constructs can be changed via |
| <code class="code">-gdwarf-2</code> or <code class="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 class="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. |
| </p><p> |
| Many other options are available: please see <a class="ulink" href="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging%20Options" target="_top">"Options |
| for Debugging Your Program"</a> in Using the GNU Compiler |
| Collection (GCC) for a complete list. |
| </p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="debug.req"></a>Debug Versions of Library Binary Files</h3></div></div></div><p> |
| If you would like debug symbols in libstdc++, there are two ways to |
| build libstdc++ with debug flags. The first is to run make from the |
| toplevel in a freshly-configured tree with |
| </p><pre class="programlisting"> |
| --enable-libstdcxx-debug |
| </pre><p>and perhaps</p><pre class="programlisting"> |
| --enable-libstdcxx-debug-flags='...' |
| </pre><p> |
| to create a separate debug build. Both the normal build and the |
| debug build will persist, without having to specify |
| <code class="code">CXXFLAGS</code>, and the debug library will be installed in a |
| separate directory tree, in <code class="code">(prefix)/lib/debug</code>. For |
| more information, look at the <a class="link" href="configure.html" title="Configure">configuration</a> section. |
| </p><p> |
| A second approach is to use the configuration flags |
| </p><pre class="programlisting"> |
| make CXXFLAGS='-g3 -fno-inline -O0' all |
| </pre><p> |
| This quick and dirty approach is often sufficient for quick |
| debugging tasks, when you cannot or don't want to recompile your |
| application to use the <a class="link" href="debug_mode.html" title="Chapter 30. Debug Mode">debug mode</a>.</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="debug.memory"></a>Memory Leak Hunting</h3></div></div></div><p> |
| There are 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 class="code">mtrace</code>, <code class="code">valgrind</code>, |
| <code class="code">mudflap</code>, and the non-free commercial product |
| <code class="code">purify</code>. In addition, <code class="code">libcwd</code> has a |
| replacement for the global new and delete operators that can track |
| memory allocation and deallocation and provide useful memory |
| statistics. |
| </p><p> |
| Regardless of the memory debugging tool being used, there is one |
| thing of great importance to keep in mind when debugging C++ code |
| that uses <code class="code">new</code> and <code class="code">delete</code>: there are |
| different kinds of allocation schemes that can be used by <code class="code"> |
| std::allocator </code>. For implementation details, see the <a class="link" href="ext_allocators.html#manual.ext.allocator.mt" title="mt_allocator">mt allocator</a> documentation and |
| look specifically for <code class="code">GLIBCXX_FORCE_NEW</code>. |
| </p><p> |
| In a nutshell, the default allocator used by <code class="code"> |
| std::allocator</code> is a high-performance pool allocator, and can |
| give the mistaken impression that in a suspect executable, 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. |
| </p><p> |
| 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 at least as well. Second of all, use a |
| completely unoptimized build to avoid confusing valgrind. Third, use |
| GLIBCXX_FORCE_NEW to keep extraneous pool allocation noise from |
| cluttering debug information. |
| </p><p> |
| Fourth, it may be necessary to force deallocation in other libraries |
| as well, namely the "C" library. On linux, this can be accomplished |
| with the appropriate use of the <code class="code">__cxa_atexit</code> or |
| <code class="code">atexit</code> functions. |
| </p><pre class="programlisting"> |
| #include <cstdlib> |
| |
| extern "C" void __libc_freeres(void); |
| |
| void do_something() { } |
| |
| int main() |
| { |
| atexit(__libc_freeres); |
| do_something(); |
| return 0; |
| } |
| </pre><p>or, using <code class="code">__cxa_atexit</code>:</p><pre class="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, |
| &__dso_handle ? __dso_handle : NULL); |
| do_test(); |
| return 0; |
| } |
| </pre><p> |
| Suggested valgrind flags, given the suggestions above about setting |
| up the runtime environment, library, and test file, might be: |
| </p><pre class="programlisting"> |
| valgrind -v --num-callers=20 --leak-check=yes --leak-resolution=high --show-reachable=yes a.out |
| </pre></div><div class="sect2" lang="en" xml:lang="en"><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> |
| </p><p> |
| Many options are available for gdb itself: please see <a class="ulink" href="http://sources.redhat.com/gdb/current/onlinedocs/gdb_13.html#SEC125" target="_top"> |
| "GDB features for C++" </a> in the gdb documentation. Also |
| recommended: the other parts of this manual. |
| </p><p> |
| These settings can either be switched on in at the gdb command line, |
| or put into a .gdbint file to establish default debugging |
| characteristics, like so: |
| </p><pre class="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 |
| </pre></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="debug.exceptions"></a>Tracking uncaught exceptions</h3></div></div></div><p> |
| The <a class="link" href="verbose_termination.html" title="Verbose Terminate Handler">verbose |
| termination handler</a> gives information about uncaught |
| exceptions which are killing the program. It is described in the |
| linked-to page. |
| </p></div><div class="sect2" lang="en" xml:lang="en"><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 30. Debug Mode">Debug Mode</a> |
| has compile and run-time checks for many containers. |
| </p></div><div class="sect2" lang="en" xml:lang="en"><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 29. Compile Time Checks">Compile-Time |
| Checks</a> Extension has compile-time checks for many algorithms. |
| </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="support.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Exceptions </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Part II. |
| Support |
| |
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