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| 2 | <!DOCTYPE html
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| 3 | PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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| 4 | "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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| 5 |
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| 6 | <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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| 7 | <head>
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| 8 | <meta name="AUTHOR" content="[email protected] (Benjamin Kosnik)" />
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| 9 | <meta name="KEYWORDS" content="c++, libstdc++, gdb, g++, debug" />
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| 10 | <meta name="DESCRIPTION" content="Debugging C++ binaries" />
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| 11 | <meta name="GENERATOR" content="vi and ten fingers" />
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| 12 | <title>Debugging schemes and strategies</title>
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| 13 | <link rel="StyleSheet" href="lib3styles.css" />
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| 14 | </head>
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| 15 | <body>
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| 16 |
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| 17 | <h1 class="centered"><a name="top">Debugging schemes and strategies</a></h1>
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| 18 |
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| 19 | <p class="fineprint"><em>
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| 20 | The latest version of this document is always available at
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| 21 | <a href="http://gcc.gnu.org/onlinedocs/libstdc++/debug.html">
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| 22 | http://gcc.gnu.org/onlinedocs/libstdc++/debug.html</a>.
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| 23 | </em></p>
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| 24 |
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| 25 | <p><em>
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| 26 | To the <a href="http://gcc.gnu.org/libstdc++/">libstdc++-v3 homepage</a>.
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| 27 | </em></p>
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| 28 |
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| 29 | <!-- ####################################################### -->
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| 30 | <hr />
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| 31 | <p>There are numerous things that can be done to improve the ease with
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| 32 | which C++ binaries are debugged when using the GNU C++
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| 33 | tool chain. Here are some things to keep in mind when debugging C++
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| 34 | code with GNU tools.
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| 35 | </p>
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| 36 |
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| 37 | <h3 class="left"><a name="gplusplus">Compiler flags determine debug info</a></h3>
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| 38 | <p>The default optimizations and debug flags for a libstdc++ build are
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| 39 | <code>-g -O2</code>. However, both debug and optimization flags can
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| 40 | be varied to change debugging characteristics. For instance,
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| 41 | turning off all optimization via the <code>-g -O0</code> flag will
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| 42 | disable inlining, so that stepping through all functions, including
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| 43 | inlined constructors and destructors, is possible. Or, the debug
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| 44 | format that the compiler and debugger use to communicate
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| 45 | information about source constructs can be changed via <code>
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| 46 | -gdwarf-2 </code> or <code> -gstabs </code> flags: some debugging
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| 47 | formats permit more expressive type and scope information to be
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| 48 | shown in gdb.
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| 49 | The default debug information for a particular platform can be
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| 50 | identified via the value set by the PREFERRED_DEBUGGING_TYPE macro
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| 51 | in the gcc sources.
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| 52 | </p>
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| 53 |
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| 54 | <p>Many other options are available: please see
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| 55 | <a href="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging%20Options">"Options for Debugging Your Program"</a>
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| 56 | in Using the GNU Compiler Collection (GCC) for a complete list.
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| 57 | </p>
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| 58 |
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| 59 |
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| 60 | <h3 class="left"><a name="lib">Using special flags to make a debug binary</a></h3>
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| 61 | <p>There are two ways to build libstdc++ with debug flags. The first
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| 62 | is to run make from the toplevel in a freshly-configured tree with
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| 63 | specialized debug <code>CXXFLAGS</code>, as in
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| 64 | </p>
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| 65 | <pre>
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| 66 | make CXXFLAGS='-g3 -O0' all
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| 67 | </pre>
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| 68 |
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| 69 | <p>This quick and dirty approach is often sufficient for quick
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| 70 | debugging tasks, but the lack of state can be confusing in the long
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| 71 | term.
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| 72 | </p>
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| 73 | <p>A second approach is to use the configuration flags
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| 74 | </p>
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| 75 | <pre>
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| 76 | --enable-debug
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| 77 | </pre>
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| 78 | <p>and perhaps</p>
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| 79 | <pre>
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| 80 | --enable-debug-flags='...'
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| 81 | </pre>
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| 82 | <p>to create a separate debug build. Both the normal build and the
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| 83 | debug build will persist, without having to specify
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| 84 | <code>CXXFLAGS</code>, and the debug library will be installed in a
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| 85 | separate directory tree, in <code>(prefix)/lib/debug</code>. For
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| 86 | more information, look at the <a href="configopts.html">configuration
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| 87 | options</a> document.
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| 88 | </p>
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| 89 |
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| 90 |
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| 91 | <h3 class="left"><a name="mem">Tips for memory leak hunting</a></h3>
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| 92 |
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| 93 | <p>There are various third party memory tracing and debug utilities
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| 94 | that can be used to provide detailed memory allocation information
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| 95 | about C++ code. An exhaustive list of tools is not going to be
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| 96 | attempted, but includes <code>mtrace</code>, <code>valgrind</code>,
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| 97 | <code>mudflap</code>, and <code>purify</code>. Also highly
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| 98 | recommended are <code>libcwd</code> and some other one that I
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| 99 | forget right now.
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| 100 | </p>
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| 101 |
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| 102 | <p>Regardless of the memory debugging tool being used, there is one
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| 103 | thing of great importance to keep in mind when debugging C++ code
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| 104 | that uses <code>new</code> and <code>delete</code>:
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| 105 | there are different kinds of allocation schemes that can be used by
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| 106 | <code> std::allocator </code>. For implementation details, see this
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| 107 | <a href="ext/howto.html#3"> document</a> and look specifically for
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| 108 | <code>GLIBCPP_FORCE_NEW</code>.
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| 109 | </p>
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| 110 |
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| 111 | <p>In a nutshell, the default allocator used by <code>
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| 112 | std::allocator</code> is a high-performance pool allocator, and can
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| 113 | give the mistaken impression that memory is being leaked, when in
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| 114 | reality the memory is still being used by the library and is reclaimed
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| 115 | after program termination.
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| 116 | </p>
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| 117 |
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| 118 | <p>For valgrind, there are some specific items to keep in mind. First
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| 119 | of all, use a version of valgrind that will work with current GNU
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| 120 | C++ tools: the first that can do this is valgrind 1.0.4, but later
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| 121 | versions should work at least as well. Second of all, use a
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| 122 | completely unoptimized build to avoid confusing valgrind. Third,
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| 123 | use GLIBCPP_FORCE_NEW to keep extraneous pool allocation noise from
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| 124 | cluttering debug information.
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| 125 | </p>
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| 126 |
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| 127 | <p>Fourth, it may be necessary to force deallocation in other
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| 128 | libraries as well, namely the "C" library. On linux, this can be
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| 129 | accomplished with the appropriate use of the
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| 130 | <code>__cxa_atexit</code> or <code>atexit</code> functions.
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| 131 | </p>
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| 132 |
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| 133 | <pre>
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| 134 | #include <cstdlib>
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| 135 |
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| 136 | extern "C" void __libc_freeres(void);
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| 137 |
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| 138 | void do_something() { }
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| 139 |
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| 140 | int main()
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| 141 | {
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| 142 | atexit(__libc_freeres);
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| 143 | do_something();
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| 144 | return 0;
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| 145 | }
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| 146 | </pre>
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| 147 |
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| 148 |
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| 149 | <p>or, using <code>__cxa_atexit</code>:</p>
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| 150 |
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| 151 | <pre>
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| 152 | extern "C" void __libc_freeres(void);
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| 153 | extern "C" int __cxa_atexit(void (*func) (void *), void *arg, void *d);
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| 154 |
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| 155 | void do_something() { }
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| 156 |
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| 157 | int main()
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| 158 | {
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| 159 | extern void* __dso_handle __attribute__ ((__weak__));
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| 160 | __cxa_atexit((void (*) (void *)) __libc_freeres, NULL,
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| 161 | &__dso_handle ? __dso_handle : NULL);
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| 162 | do_test();
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| 163 | return 0;
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| 164 | }
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| 165 | </pre>
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| 166 |
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| 167 | <p>Suggested valgrind flags, given the suggestions above about setting
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| 168 | up the runtime environment, library, and test file, might be:
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| 169 | </p>
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| 170 | <pre>
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| 171 | valgrind -v --num-callers=20 --leak-check=yes --leak-resolution=high --show-reachable=yes a.out
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| 172 | </pre>
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| 173 |
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| 174 |
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| 175 | <h3 class="left"><a name="gdb">Some gdb strategies</a></h3>
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| 176 | <p>Many options are available for gdb itself: please see <a
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| 177 | href="http://sources.redhat.com/gdb/current/onlinedocs/gdb_13.html#SEC109">
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| 178 | "GDB features for C++" </a> in the gdb documentation. Also
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| 179 | recommended: the other parts of this manual.
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| 180 | </p>
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| 181 |
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| 182 | <p>These settings can either be switched on in at the gdb command
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| 183 | line, or put into a .gdbint file to establish default debugging
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| 184 | characteristics, like so:
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| 185 | </p>
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| 186 |
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| 187 | <pre>
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| 188 | set print pretty on
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| 189 | set print object on
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| 190 | set print static-members on
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| 191 | set print vtbl on
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| 192 | set print demangle on
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| 193 | set demangle-style gnu-v3
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| 194 | </pre>
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| 195 |
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| 196 |
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| 197 | <h3 class="left"><a name="verbterm">Tracking uncaught exceptions</a></h3>
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| 198 | <p>The <a href="19_diagnostics/howto.html#4">verbose termination handler</a>
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| 199 | gives information about uncaught exceptions which are killing the
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| 200 | program. It is described in the linked-to page.
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| 201 | </p>
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| 202 |
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| 203 |
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| 204 | <p>Return <a href="#top">to the top of the page</a> or
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| 205 | <a href="http://gcc.gnu.org/libstdc++/">to the libstdc++ homepage</a>.
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| 206 | </p>
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| 207 |
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| 208 |
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| 209 | <!-- ####################################################### -->
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| 210 |
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| 211 | <hr />
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| 212 | <p class="fineprint"><em>
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| 213 | See <a href="17_intro/license.html">license.html</a> for copying conditions.
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| 214 | Comments and suggestions are welcome, and may be sent to
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| 215 | <a href="mailto:[email protected]">the libstdc++ mailing list</a>.
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| 216 | </em></p>
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| 217 |
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| 218 |
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| 219 | </body>
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| 220 | </html>
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