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<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"><head><title>Debugging Support</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><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="bk01pt02.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 align="left"><a accesskey="p" href="using_exceptions.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td 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"><a id="manual.intro.using.debug"/>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="section" title="Using g++"><div class="titlepage"><div><div><h3 class="title"><a id="debug.compiler"/>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="link" href="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging%20Options">"Options for Debugging Your Program"</a> in Using the GNU Compiler Collection (GCC) for a complete list. </p></div><div class="section" title="Debug Versions of Library Binary Files"><div class="titlepage"><div><div><h3 class="title"><a id="debug.req"/>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 17. Debug Mode">debug mode</a>.</p></div><div class="section" title="Memory Leak Hunting"><div class="titlepage"><div><div><h3 class="title"><a id="debug.memory"/>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="mt_allocator.html" title="Chapter 20. The 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="section" title="Data Race Hunting"><div class="titlepage"><div><div><h3 class="title"><a id="debug.races"/>Data Race Hunting</h3></div></div></div><p> All synchronization primitives used in the library internals need to be understood by race detectors so that they do not produce false reports. </p><p> Two annotation macros are used to explain low-level synchronization to race detectors: <code class="code">_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE()</code> and <code class="code"> _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER()</code>. By default, these macros are defined empty -- anyone who wants to use a race detector needs to redefine them to call an appropriate API. Since these macros are empty by default when the library is built, redefining them will only affect inline functions and template instantiations which are compiled in user code. This allows annotation of templates such as <code class="code">shared_ptr</code>, but not code which is only instantiated in the library. Code which is only instantiated in the library needs to be recompiled with the annotation macros defined. That can be done by rebuilding the entire <code class="filename">libstdc++.so</code> file but a simpler alternative exists for ELF platforms such as GNU/Linux, because ELF symbol interposition allows symbols defined in the shared library to be overridden by symbols with the same name that appear earlier in the runtime search path. This means you only need to recompile the functions that are affected by the annotation macros, which can be done by recompiling individual files. Annotating <code class="code">std::string</code> and <code class="code">std::wstring</code> reference counting can be done by disabling extern templates (by defining <code class="code">_GLIBCXX_EXTERN_TEMPLATE=-1</code>) or by rebuilding the <code class="filename">src/string-inst.cc</code> file. Annotating the remaining atomic operations (at the time of writing these are in <code class="code">ios_base::Init::~Init</code>, <code class="code">locale::_Impl</code>, <code class="code">locale::facet</code> and <code class="code">thread::_M_start_thread</code>) requires rebuilding the relevant source files. </p><p> The approach described above is known to work with the following race detection tools: <a class="link" href="http://valgrind.org/docs/manual/drd-manual.html"> DRD</a>, <a class="link" href="http://valgrind.org/docs/manual/hg-manual.html"> Helgrind</a>, and <a class="link" href="http://code.google.com/p/data-race-test"> ThreadSanitizer</a>. </p><p> With DRD, Helgrind and ThreadSanitizer you will need to define the macros like this: </p><pre class="programlisting"> #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(A) ANNOTATE_HAPPENS_BEFORE(A) #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(A) ANNOTATE_HAPPENS_AFTER(A) </pre><p> Refer to the documentation of each particular tool for details. </p></div><div class="section" title="Using gdb"><div class="titlepage"><div><div><h3 class="title"><a id="debug.gdb"/>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="link" href="http://sources.redhat.com/gdb/current/onlinedocs/gdb/"> "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><p> Starting with version 7.0, GDB includes support for writing pretty-printers in Python. Pretty printers for STL classes are distributed with GCC from version 4.5.0. The most recent version of these printers are always found in libstdc++ svn repository. To enable these printers, check-out the latest printers to a local directory: </p><pre class="programlisting"> svn co svn://gcc.gnu.org/svn/gcc/trunk/libstdc++-v3/python </pre><p> Next, add the following section to your ~/.gdbinit The path must match the location where the Python module above was checked-out. So if checked out to: /home/maude/gdb_printers/, the path would be as written in the example below. </p><pre class="programlisting"> python import sys sys.path.insert(0, '/home/maude/gdb_printers/python') from libstdcxx.v6.printers import register_libstdcxx_printers register_libstdcxx_printers (None) end </pre><p> The path should be the only element that needs to be adjusted in the example. Once loaded, STL classes that the printers support should print in a more human-readable format. To print the classes in the old style, use the /r (raw) switch in the print command (i.e., print /r foo). This will print the classes as if the Python pretty-printers were not loaded. </p><p> For additional information on STL support and GDB please visit: <a class="link" href="http://sourceware.org/gdb/wiki/STLSupport"> "GDB Support for STL" </a> in the GDB wiki. Additionally, in-depth documentation and discussion of the pretty printing feature can be found in "Pretty Printing" node in the GDB manual. You can find on-line versions of the GDB user manual in GDB's homepage, at <a class="link" href="http://sourceware.org/gdb/"> "GDB: The GNU Project Debugger" </a>. </p></div><div class="section" title="Tracking uncaught exceptions"><div class="titlepage"><div><div><h3 class="title"><a id="debug.exceptions"/>Tracking uncaught exceptions</h3></div></div></div><p> The <a class="link" href="termination.html#support.termination.verbose" 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="section" title="Debug Mode"><div class="titlepage"><div><div><h3 class="title"><a id="debug.debug_mode"/>Debug Mode</h3></div></div></div><p> The <a class="link" href="debug_mode.html" title="Chapter 17. Debug Mode">Debug Mode</a> has compile and run-time checks for many containers. </p></div><div class="section" title="Compile Time Checking"><div class="titlepage"><div><div><h3 class="title"><a id="debug.compile_time_checks"/>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 Checks</a> Extension has compile-time checks for many algorithms. </p></div><div class="section" title="Profile-based Performance Analysis"><div class="titlepage"><div><div><h3 class="title"><a id="debug.profile_mode"/>Profile-based Performance Analysis</h3></div></div></div><p> The <a class="link" href="profile_mode.html" title="Chapter 19. Profile Mode">Profile-based Performance Analysis</a> Extension has performance checks for many algorithms. </p></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="using_exceptions.html">Prev</a> </td><td align="center"><a accesskey="u" href="using.html">Up</a></td><td align="right"> <a accesskey="n" href="bk01pt02.html">Next</a></td></tr><tr><td align="left" valign="top">Exceptions </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Part II. Standard Contents </td></tr></table></div></body></html>
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