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ort</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="using_excep
tions.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><t
d 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="t
itlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.intro.us
ing.debug"></a>Debugging Support</h2></div></div></div><p> | |
| 4 There are numerous things that can be done to improve the ease with | |
| 5 which C++ binaries are debugged when using the GNU tool chain. Here | |
| 6 are some of them. | |
| 7 </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> | |
| 8 Compiler flags determine how debug information is transmitted | |
| 9 between compilation and debug or analysis tools. | |
| 10 </p><p> | |
| 11 The default optimizations and debug flags for a libstdc++ build | |
| 12 are <code class="code">-g -O2</code>. However, both debug and optimization | |
| 13 flags can be varied to change debugging characteristics. For | |
| 14 instance, turning off all optimization via the <code class="code">-g -O0 | |
| 15 -fno-inline</code> flags will disable inlining and optimizations, | |
| 16 and add debugging information, so that stepping through all functions, | |
| 17 (including inlined constructors and destructors) is possible. In | |
| 18 addition, <code class="code">-fno-eliminate-unused-debug-types</code> can be | |
| 19 used when additional debug information, such as nested class info, | |
| 20 is desired. | |
| 21 </p><p> | |
| 22 Or, the debug format that the compiler and debugger use to | |
| 23 communicate information about source constructs can be changed via | |
| 24 <code class="code">-gdwarf-2</code> or <code class="code">-gstabs</code> flags
: some debugging | |
| 25 formats permit more expressive type and scope information to be | |
| 26 shown in gdb. Expressiveness can be enhanced by flags like | |
| 27 <code class="code">-g3</code>. The default debug information for a particular | |
| 28 platform can be identified via the value set by the | |
| 29 PREFERRED_DEBUGGING_TYPE macro in the gcc sources. | |
| 30 </p><p> | |
| 31 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 | |
| 32 for Debugging Your Program"</a> in Using the GNU Compiler | |
| 33 Collection (GCC) for a complete list. | |
| 34 </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 F
iles</h3></div></div></div><p> | |
| 35 If you would like debug symbols in libstdc++, there are two ways to | |
| 36 build libstdc++ with debug flags. The first is to run make from the | |
| 37 toplevel in a freshly-configured tree 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 to create a separate debug build. Both the normal build and the | |
| 44 debug build will persist, without having to specify | |
| 45 <code class="code">CXXFLAGS</code>, and the debug library will be installed in
a | |
| 46 separate directory tree, in <code class="code">(prefix)/lib/debug</code>. For | |
| 47 more information, look at the <a class="link" href="configure.html" title="Con
figure">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 3
0. Debug Mode">debug mode</a>.</p></div><div class="sect2" lang="en" xml:lang="e
n"><div class="titlepage"><div><div><h3 class="title"><a id="debug.memory"></a>M
emory 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">v
algrind</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" hr
ef="ext_allocators.html#manual.ext.allocator.mt" title="mt_allocator">mt allocat
or</a> documentation and | |
| 71 look specifically for <code class="code">GLIBCXX_FORCE_NEW</code>. | |
| 72 </p><p> | |
| 73 In a nutshell, the default allocator used by <code class="code"> | |
| 74 std::allocator</code> 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="progra
mlisting"> | |
| 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-r
eachable=yes a.out | |
| 124 </pre></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><d
iv><div><h3 class="title"><a id="debug.gdb"></a>Using <span class="command"><str
ong>gdb</strong></span></h3></div></div></div><p> | |
| 125 </p><p> | |
| 126 Many options are available for gdb itself: please see <a class="ulink" href="h
ttp://sources.redhat.com/gdb/current/onlinedocs/gdb_13.html#SEC125" target="_top
"> | |
| 127 "GDB features for C++" </a> in the gdb documentation. Also | |
| 128 recommended: the other parts of this manual. | |
| 129 </p><p> | |
| 130 These settings can either be switched on in at the gdb command line, | |
| 131 or put into a .gdbint file to establish default debugging | |
| 132 characteristics, like so: | |
| 133 </p><pre class="programlisting"> | |
| 134 set print pretty on | |
| 135 set print object on | |
| 136 set print static-members on | |
| 137 set print vtbl on | |
| 138 set print demangle on | |
| 139 set demangle-style gnu-v3 | |
| 140 </pre></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><d
iv><div><h3 class="title"><a id="debug.exceptions"></a>Tracking uncaught excepti
ons</h3></div></div></div><p> | |
| 141 The <a class="link" href="verbose_termination.html" title="Verbose Terminate H
andler">verbose | |
| 142 termination handler</a> gives information about uncaught | |
| 143 exceptions which are killing the program. It is described in the | |
| 144 linked-to page. | |
| 145 </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 Mo
de">Debug Mode</a> | |
| 146 has compile and run-time checks for many containers. | |
| 147 </p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><d
iv><div><h3 class="title"><a id="debug.compile_time_checks"></a>Compile Time Che
cking</h3></div></div></div><p> The <a class="link" href="ext_compile_checks.htm
l" title="Chapter 29. Compile Time Checks">Compile-Time | |
| 148 Checks</a> Extension has compile-time checks for many algorithms. | |
| 149 </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navi
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| 150 Support | |
| 151 | |
| 152 </td></tr></table></div></body></html> | |
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