1<sect1 id="manual.intro.using.debug" xreflabel="Debugging Support"> 2<?dbhtml filename="debug.html"?> 3 4<sect1info> 5 <keywordset> 6 <keyword> 7 C++ 8 </keyword> 9 <keyword> 10 debug 11 </keyword> 12 </keywordset> 13</sect1info> 14 15<title>Debugging Support</title> 16 17<para> 18 There are numerous things that can be done to improve the ease with 19 which C++ binaries are debugged when using the GNU tool chain. Here 20 are some of them. 21</para> 22 23<sect2 id="debug.compiler"> 24<title>Using <command>g++</command></title> 25 <para> 26 Compiler flags determine how debug information is transmitted 27 between compilation and debug or analysis tools. 28 </para> 29 30 <para> 31 The default optimizations and debug flags for a libstdc++ build 32 are <code>-g -O2</code>. However, both debug and optimization 33 flags can be varied to change debugging characteristics. For 34 instance, turning off all optimization via the <code>-g -O0 35 -fno-inline</code> flags will disable inlining and optimizations, 36 and add debugging information, so that stepping through all functions, 37 (including inlined constructors and destructors) is possible. In 38 addition, <code>-fno-eliminate-unused-debug-types</code> can be 39 used when additional debug information, such as nested class info, 40 is desired. 41</para> 42 43<para> 44 Or, the debug format that the compiler and debugger use to 45 communicate information about source constructs can be changed via 46 <code>-gdwarf-2</code> or <code>-gstabs</code> flags: some debugging 47 formats permit more expressive type and scope information to be 48 shown in GDB. Expressiveness can be enhanced by flags like 49 <code>-g3</code>. The default debug information for a particular 50 platform can be identified via the value set by the 51 PREFERRED_DEBUGGING_TYPE macro in the gcc sources. 52</para> 53 54<para> 55 Many other options are available: please see <ulink 56 url="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging%20Options">"Options 57 for Debugging Your Program"</ulink> in Using the GNU Compiler 58 Collection (GCC) for a complete list. 59</para> 60</sect2> 61 62<sect2 id="debug.req"> 63<title>Debug Versions of Library Binary Files</title> 64 65<para> 66 If you would like debug symbols in libstdc++, there are two ways to 67 build libstdc++ with debug flags. The first is to run make from the 68 toplevel in a freshly-configured tree with 69</para> 70<programlisting> 71 --enable-libstdcxx-debug 72</programlisting> 73<para>and perhaps</para> 74<programlisting> 75 --enable-libstdcxx-debug-flags='...' 76</programlisting> 77<para> 78 to create a separate debug build. Both the normal build and the 79 debug build will persist, without having to specify 80 <code>CXXFLAGS</code>, and the debug library will be installed in a 81 separate directory tree, in <code>(prefix)/lib/debug</code>. For 82 more information, look at the <link 83 linkend="manual.intro.setup.configure">configuration</link> section. 84</para> 85 86<para> 87 A second approach is to use the configuration flags 88</para> 89<programlisting> 90 make CXXFLAGS='-g3 -fno-inline -O0' all 91</programlisting> 92 93<para> 94 This quick and dirty approach is often sufficient for quick 95 debugging tasks, when you cannot or don't want to recompile your 96 application to use the <link linkend="manual.ext.debug_mode">debug mode</link>.</para> 97</sect2> 98 99<sect2 id="debug.memory"> 100<title>Memory Leak Hunting</title> 101 102<para> 103 There are various third party memory tracing and debug utilities 104 that can be used to provide detailed memory allocation information 105 about C++ code. An exhaustive list of tools is not going to be 106 attempted, but includes <code>mtrace</code>, <code>valgrind</code>, 107 <code>mudflap</code>, and the non-free commercial product 108 <code>purify</code>. In addition, <code>libcwd</code> has a 109 replacement for the global new and delete operators that can track 110 memory allocation and deallocation and provide useful memory 111 statistics. 112</para> 113 114<para> 115 Regardless of the memory debugging tool being used, there is one 116 thing of great importance to keep in mind when debugging C++ code 117 that uses <code>new</code> and <code>delete</code>: there are 118 different kinds of allocation schemes that can be used by <code> 119 std::allocator </code>. For implementation details, see the <link 120 linkend="manual.ext.allocator.mt">mt allocator</link> documentation and 121 look specifically for <code>GLIBCXX_FORCE_NEW</code>. 122</para> 123 124<para> 125 In a nutshell, the default allocator used by <code> 126 std::allocator</code> is a high-performance pool allocator, and can 127 give the mistaken impression that in a suspect executable, memory is 128 being leaked, when in reality the memory "leak" is a pool being used 129 by the library's allocator and is reclaimed after program 130 termination. 131</para> 132 133<para> 134 For valgrind, there are some specific items to keep in mind. First 135 of all, use a version of valgrind that will work with current GNU 136 C++ tools: the first that can do this is valgrind 1.0.4, but later 137 versions should work at least as well. Second of all, use a 138 completely unoptimized build to avoid confusing valgrind. Third, use 139 GLIBCXX_FORCE_NEW to keep extraneous pool allocation noise from 140 cluttering debug information. 141</para> 142 143<para> 144 Fourth, it may be necessary to force deallocation in other libraries 145 as well, namely the "C" library. On linux, this can be accomplished 146 with the appropriate use of the <code>__cxa_atexit</code> or 147 <code>atexit</code> functions. 148</para> 149 150<programlisting> 151 #include <cstdlib> 152 153 extern "C" void __libc_freeres(void); 154 155 void do_something() { } 156 157 int main() 158 { 159 atexit(__libc_freeres); 160 do_something(); 161 return 0; 162 } 163</programlisting> 164 165 166<para>or, using <code>__cxa_atexit</code>:</para> 167 168<programlisting> 169 extern "C" void __libc_freeres(void); 170 extern "C" int __cxa_atexit(void (*func) (void *), void *arg, void *d); 171 172 void do_something() { } 173 174 int main() 175 { 176 extern void* __dso_handle __attribute__ ((__weak__)); 177 __cxa_atexit((void (*) (void *)) __libc_freeres, NULL, 178 &__dso_handle ? __dso_handle : NULL); 179 do_test(); 180 return 0; 181 } 182</programlisting> 183 184<para> 185 Suggested valgrind flags, given the suggestions above about setting 186 up the runtime environment, library, and test file, might be: 187</para> 188<programlisting> 189 valgrind -v --num-callers=20 --leak-check=yes --leak-resolution=high --show-reachable=yes a.out 190</programlisting> 191 192</sect2> 193 194<sect2 id="debug.gdb"> 195<title>Using <command>gdb</command></title> 196 <para> 197 </para> 198 199<para> 200 Many options are available for GDB itself: please see <ulink 201 url="http://sources.redhat.com/gdb/current/onlinedocs/gdb/"> 202 "GDB features for C++" </ulink> in the GDB documentation. Also 203 recommended: the other parts of this manual. 204</para> 205 206<para> 207 These settings can either be switched on in at the GDB command line, 208 or put into a .gdbint file to establish default debugging 209 characteristics, like so: 210</para> 211 212<programlisting> 213 set print pretty on 214 set print object on 215 set print static-members on 216 set print vtbl on 217 set print demangle on 218 set demangle-style gnu-v3 219</programlisting> 220 221<para> 222 Starting with version 7.0, GDB includes support for writing 223 pretty-printers in Python. Pretty printers for STL classes are 224 distributed with GCC from version 4.5.0. The most recent version of 225 these printers are always found in libstdc++ svn repository. 226 To enable these printers, check-out the latest printers to a local 227 directory: 228</para> 229 230<programlisting> 231 svn co svn://gcc.gnu.org/svn/gcc/trunk/libstdc++-v3/python 232</programlisting> 233 234<para> 235 Next, add the following section to your ~/.gdbinit The path must 236 match the location where the Python module above was checked-out. 237 So if checked out to: /home/maude/gdb_printers/, the path would be as 238 written in the example below. 239</para> 240 241<programlisting> 242 python 243 import sys 244 sys.path.insert(0, '/home/maude/gdb_printers/python') 245 from libstdcxx.v6.printers import register_libstdcxx_printers 246 register_libstdcxx_printers (None) 247 end 248</programlisting> 249 250<para> 251 The path should be the only element that needs to be adjusted in the 252 example. Once loaded, STL classes that the printers support 253 should print in a more human-readable format. To print the classes 254 in the old style, use the /r (raw) switch in the print command 255 (i.e., print /r foo). This will print the classes as if the Python 256 pretty-printers were not loaded. 257</para> 258 259<para> 260 For additional information on STL support and GDB please visit: 261 <ulink url="http://sourceware.org/gdb/wiki/STLSupport"> "GDB Support 262 for STL" </ulink> in the GDB wiki. Additionally, in-depth 263 documentation and discussion of the pretty printing feature can be 264 found in "Pretty Printing" node in the GDB manual. You can find 265 on-line versions of the GDB user manual in GDB's homepage, at 266 <ulink url="http://sourceware.org/gdb/"> "GDB: The GNU Project 267 Debugger" </ulink>. 268</para> 269 270</sect2> 271 272<sect2 id="debug.exceptions"> 273<title>Tracking uncaught exceptions</title> 274<para> 275 The <link linkend="support.termination.verbose">verbose 276 termination handler</link> gives information about uncaught 277 exceptions which are killing the program. It is described in the 278 linked-to page. 279</para> 280</sect2> 281 282<sect2 id="debug.debug_mode"> 283<title>Debug Mode</title> 284 <para> The <link linkend="manual.ext.debug_mode">Debug Mode</link> 285 has compile and run-time checks for many containers. 286 </para> 287</sect2> 288 289<sect2 id="debug.compile_time_checks"> 290<title>Compile Time Checking</title> 291 <para> The <link linkend="manual.ext.compile_checks">Compile-Time 292 Checks</link> Extension has compile-time checks for many algorithms. 293 </para> 294</sect2> 295 296<sect2 id="debug.profile_mode" xreflabel="debug.profile_mode"> 297<title>Profile-based Performance Analysis</title> 298 <para> The <link linkend="manual.ext.profile_mode">Profile-based 299 Performance Analysis</link> Extension has performance checks for many 300 algorithms. 301 </para> 302</sect2> 303 304</sect1> 305