<?xml version="1.0" encoding="UTF-8" standalone="no"?><!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 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 withwhich C++ binaries are debugged when using the GNU tool chain. Hereare 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 transmittedbetween compilation and debug or analysis tools.</p><p>The default optimizations and debug flags for a libstdc++ buildare <code class="code">-g -O2</code>. However, both debug and optimizationflags can be varied to change debugging characteristics. Forinstance, 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. Inaddition, <code class="code">-fno-eliminate-unused-debug-types</code> can beused when additional debug information, such as nested class info,is desired.</p><p>Or, the debug format that the compiler and debugger use tocommunicate information about source constructs can be changed via<code class="code">-gdwarf-2</code> or <code class="code">-gstabs</code> flags: some debuggingformats permit more expressive type and scope information to beshown in gdb. Expressiveness can be enhanced by flags like<code class="code">-g3</code>. The default debug information for a particularplatform can be identified via the value set by thePREFERRED_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">"Optionsfor Debugging Your Program"</a> in Using the GNU CompilerCollection (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 tobuild libstdc++ with debug flags. The first is to run make from thetoplevel 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 thedebug build will persist, without having to specify<code class="code">CXXFLAGS</code>, and the debug library will be installed in aseparate directory tree, in <code class="code">(prefix)/lib/debug</code>. Formore 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 quickdebugging tasks, when you cannot or don't want to recompile yourapplication 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 utilitiesthat can be used to provide detailed memory allocation informationabout C++ code. An exhaustive list of tools is not going to beattempted, 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 areplacement for the global new and delete operators that can trackmemory allocation and deallocation and provide useful memorystatistics.</p><p>Regardless of the memory debugging tool being used, there is onething of great importance to keep in mind when debugging C++ codethat uses <code class="code">new</code> and <code class="code">delete</code>: there aredifferent 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 andlook 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 cangive the mistaken impression that in a suspect executable, memory isbeing leaked, when in reality the memory "leak" is a pool being usedby the library's allocator and is reclaimed after programtermination.</p><p>For valgrind, there are some specific items to keep in mind. Firstof all, use a version of valgrind that will work with current GNUC++ tools: the first that can do this is valgrind 1.0.4, but laterversions should work at least as well. Second of all, use acompletely unoptimized build to avoid confusing valgrind. Third, useGLIBCXX_FORCE_NEW to keep extraneous pool allocation noise fromcluttering debug information.</p><p>Fourth, it may be necessary to force deallocation in other librariesas well, namely the "C" library. On linux, this can be accomplishedwith 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 settingup 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. Alsorecommended: 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 debuggingcharacteristics, like so:</p><pre class="programlisting">set print pretty onset print object onset print static-members onset print vtbl onset print demangle onset 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">verbosetermination handler</a> gives information about uncaughtexceptions which are killing the program. It is described in thelinked-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-TimeChecks</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</td></tr></table></div></body></html>