1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!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-NS Stylesheets V1.78.1" /><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="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> 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 <cstdlib> 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 &__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. 230 Standard Contents 231 </td></tr></table></div></body></html>