libstdc++-v3/docs/html/debug.html

   1 <?xml version="1.0" encoding="ISO-8859-1"?>
   2 <!DOCTYPE html
   3           PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
   4           "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
   5
   6 <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
   7 <head>
   8    <meta name="AUTHOR" content="bkoz@gcc.gnu.org (Benjamin Kosnik)" />
   9    <meta name="KEYWORDS" content="c++, libstdc++, gdb, g++, debug" />
  10    <meta name="DESCRIPTION" content="Debugging C++ binaries" />
  11    <meta name="GENERATOR" content="vi and ten fingers" />
  12    <title>Debugging schemes and strategies</title>
  13 <link rel="StyleSheet" href="lib3styles.css" type="text/css" />
  14 <link rel="Copyright" href="17_intro/license.html" type="text/html" />
  15 </head>
  16 <body>
  17
  18 <h1 class="centered"><a name="top">Debugging schemes and strategies</a></h1>
  19
  20 <p class="fineprint"><em>
  21    The latest version of this document is always available at
  22    <a href="http://gcc.gnu.org/onlinedocs/libstdc++/debug.html">
  23    http://gcc.gnu.org/onlinedocs/libstdc++/debug.html</a>.
  24 </em></p>
  25
  26 <p><em>
  27    To the <a href="http://gcc.gnu.org/libstdc++/">libstdc++-v3 homepage</a>.
  28 </em></p>
  29
  30 <!-- ####################################################### -->
  31 <hr />
  32 <p>There are numerous things that can be done to improve the ease with
  33    which C++ binaries are debugged when using the GNU
  34    tool chain. Here are some of them.
  35 </p>
  36
  37 <h3 class="left"><a name="gplusplus">Compiler flags determine debug info</a></h3>
  38 <p>The default optimizations and debug flags for a libstdc++ build are
  39    <code>-g -O2</code>. However, both debug and optimization flags can
  40    be varied to change debugging characteristics. For instance,
  41    turning off all optimization via the <code>-g -O0</code> flag will
  42    disable inlining, so that stepping through all functions, including
  43    inlined constructors and destructors, is possible. In addition,
  44    <code>-fno-eliminate-unused-debug-types<code> can be used when
  45    additional debug information, such as nested class info, is desired.
  46 </p>
  47
  48 <p>Or, the debug format that the compiler and debugger use to communicate
  49    information about source constructs can be changed via <code>
  50    -gdwarf-2 </code> or <code> -gstabs </code> flags: some debugging
  51    formats permit more expressive type and scope information to be
  52    shown in gdb.  The default debug information for a particular
  53    platform can be identified via the value set by the
  54    PREFERRED_DEBUGGING_TYPE macro in the gcc sources.
  55 </p>
  56
  57 <p>Many other options are available: please see
  58 <a href="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging%20Options">"Options for Debugging Your Program"</a>
  59    in Using the GNU Compiler Collection (GCC) for a complete list.
  60 </p>
  61
  62 <h3 class="left"><a name="lib">Using special flags to make a debug binary</a></h3>
  63 <p>If you would like debug symbols in libstdc++, there are two ways to
  64   build libstdc++ with debug flags. The first is to run make from the
  65   toplevel in a freshly-configured tree with
  66 <pre>
  67      --enable-libstdcxx-debug
  68 </pre>
  69 <p>and perhaps</p>
  70 <pre>
  71      --enable-libstdcxx-debug-flags='...'
  72 </pre>
  73 <p>to create a separate debug build. Both the normal build and the
  74    debug build will persist, without having to specify
  75    <code>CXXFLAGS</code>, and the debug library will be installed in a
  76    separate directory tree, in <code>(prefix)/lib/debug</code>. For
  77    more information, look at the <a href="configopts.html">configuration
  78    options</a> document.
  79 </p>
  80
  81 <p>A second approach is to use the configuration flags
  82 </p>
  83 <pre>
  84      make CXXFLAGS='-g3 -O0' all
  85 </pre>
  86
  87 <p>This quick and dirty approach is often sufficient for quick
  88   debugging tasks, when you cannot or don't want to recompile your
  89   application to use the <a href="#safe">debug mode</a>.</p>
  90
  91 <h3 class="left"><a name="safe">The libstdc++ debug mode</a></h3>
  92 <p>By default, libstdc++ is built with efficiency in mind, and
  93   therefore performs little or no error checking that is not required
  94   by the C++ standard. This means that programs that incorrectly use
  95   the C++ standard library will exhibit behavior that is not portable
  96   and may not even be predictable, because they tread into
  97   implementation-specific or undefined behavior. To detect some of
  98   these errors before they can become problematic, libstdc++ offers a
  99   debug mode that provides additional checking of library facilities,
 100   and will report errors in the use of libstdc++ as soon as they can
 101   be detected by emitting a description of the problem to standard
 102   error and aborting the program. </p>
 103
 104 <p>The libstdc++ debug mode performs checking for many areas of the C++
 105   standard, but the focus is on checking interactions among standard
 106   iterators, containers, and algorithms, including:</p>
 107
 108   <ul>
 109     <li><em>Safe iterators</em>: Iterators keep track of the
 110     container whose elements they reference, so errors such as
 111     incrementing a past-the-end iterator or dereferencing an iterator
 112     that points to a container that has been destructed are diagnosed
 113     immediately.</li>
 114
 115     <li><em>Algorithm preconditions</em>: Algorithms attempt to
 116     validate their input parameters to detect errors as early as
 117     possible. For instance, the <code>set_intersection</code>
 118     algorithm requires that its iterator
 119     parameters <code>first1</code> and <code>last1</code> form a valid
 120     iterator range, and that the sequence
 121     [<code>first1</code>, <code>last1</code>) is sorted according to
 122     the same predicate that was passed
 123     to <code>set_intersection</code>; the libstdc++ debug mode will
 124     detect an error if the sequence is not sorted or was sorted by a
 125     different predicate.</li>
 126   </ul>
 127
 128 <h4 class="left">Using the libstdc++ debug mode</h4>
 129 <p>To use the libstdc++ debug mode, compile your application with the
 130   compiler flag <code>-D_GLIBCXX_DEBUG</code>. Note that this flag
 131   changes the sizes and behavior of standard class templates such
 132   as <code>std::vector</code>, and therefore you can only link code
 133   compiled with debug mode and code compiled without debug mode if no
 134   instantiation of a container is passed between the two translation
 135   units.</p>
 136
 137 <p>For information about the design of the libstdc++ debug mode,
 138   please see the <a href="debug_mode.html">libstdc++ debug mode design
 139   document</a>.</p>
 140
 141 <h4 class="left">Using the debugging containers without debug
 142   mode</h4>
 143 <p>When it is not feasible to recompile your entire application, or
 144   only specific containers need checking, debugging containers are
 145   available as GNU extensions. These debugging containers are
 146   functionally equivalent to the standard drop-in containers used in
 147   debug mode, but they are available in a separate namespace as GNU
 148   extensions and may be used in programs compiled with either release
 149   mode or with debug mode. The
 150   following table provides the names and headers of the debugging
 151   containers:
 152
 153 <table title="Debugging containers" border="1">
 154   <tr>
 155     <th>Container</th>
 156     <th>Header</th>
 157     <th>Debug container</th>
 158     <th>Debug header</th>
 159   </tr>
 160   <tr>
 161     <td>std::bitset</td>
 162     <td>&lt;bitset&gt;</td>
 163     <td>__gnu_debug::bitset</td>
 164     <td>&lt;debug/bitset&gt;</td>
 165   </tr>
 166   <tr>
 167     <td>std::deque</td>
 168     <td>&lt;deque&gt;</td>
 169     <td>__gnu_debug::deque</td>
 170     <td>&lt;debug/deque&gt;</td>
 171   </tr>
 172   <tr>
 173     <td>std::list</td>
 174     <td>&lt;list&gt;</td>
 175     <td>__gnu_debug::list</td>
 176     <td>&lt;debug/list&gt;</td>
 177   </tr>
 178   <tr>
 179     <td>std::map</td>
 180     <td>&lt;map&gt;</td>
 181     <td>__gnu_debug::map</td>
 182     <td>&lt;debug/map&gt;</td>
 183   </tr>
 184   <tr>
 185     <td>std::multimap</td>
 186     <td>&lt;map&gt;</td>
 187     <td>__gnu_debug::multimap</td>
 188     <td>&lt;debug/map&gt;</td>
 189   </tr>
 190   <tr>
 191     <td>std::multiset</td>
 192     <td>&lt;set&gt;</td>
 193     <td>__gnu_debug::multiset</td>
 194     <td>&lt;debug/set&gt;</td>
 195   </tr>
 196   <tr>
 197     <td>std::set</td>
 198     <td>&lt;set&gt;</td>
 199     <td>__gnu_debug::set</td>
 200     <td>&lt;debug/set&gt;</td>
 201   </tr>
 202   <tr>
 203     <td>std::string</td>
 204     <td>&lt;string&gt;</td>
 205     <td>__gnu_debug::string</td>
 206     <td>&lt;debug/string&gt;</td>
 207   </tr>
 208   <tr>
 209     <td>std::wstring</td>
 210     <td>&lt;string&gt;</td>
 211     <td>__gnu_debug::wstring</td>
 212     <td>&lt;debug/string&gt;</td>
 213   </tr>
 214   <tr>
 215     <td>std::basic_string</td>
 216     <td>&lt;string&gt;</td>
 217     <td>__gnu_debug::basic_string</td>
 218     <td>&lt;debug/string&gt;</td>
 219   </tr>
 220   <tr>
 221     <td>std::vector</td>
 222     <td>&lt;vector&gt;</td>
 223     <td>__gnu_debug::vector</td>
 224     <td>&lt;debug/vector&gt;</td>
 225   </tr>
 226   <tr>
 227     <td>__gnu_cxx::hash_map</td>
 228     <td>&lt;ext/hash_map&gt;</td>
 229     <td>__gnu_debug::hash_map</td>
 230     <td>&lt;debug/hash_map&gt;</td>
 231   </tr>
 232   <tr>
 233     <td>__gnu_cxx::hash_multimap</td>
 234     <td>&lt;ext/hash_map&gt;</td>
 235     <td>__gnu_debug::hash_multimap</td>
 236     <td>&lt;debug/hash_map&gt;</td>
 237   </tr>
 238   <tr>
 239     <td>__gnu_cxx::hash_set</td>
 240     <td>&lt;ext/hash_set&gt;</td>
 241     <td>__gnu_debug::hash_set</td>
 242     <td>&lt;debug/hash_set&gt;</td>
 243   </tr>
 244   <tr>
 245     <td>__gnu_cxx::hash_multiset</td>
 246     <td>&lt;ext/hash_set&gt;</td>
 247     <td>__gnu_debug::hash_multiset</td>
 248     <td>&lt;debug/hash_set&gt;</td>
 249   </tr>
 250 </table>
 251
 252 <h4 class="left">Debug mode semantics</h4>
 253 <p>A program that uses the C++ standard library correctly
 254   will maintain the same semantics under debug mode as it had with
 255   the normal (release) library. All functional and exception-handling
 256   guarantees made by the normal library also hold for the debug mode
 257   library, with one exception: performance guarantees made by the
 258   normal library may not hold in the debug mode library. For
 259   instance, erasing an element in a <code>std::list</code> is a
 260   constant-time operation in normal library, but in debug mode it is
 261   linear in the number of iterators that reference that particular
 262   list. So while your (correct) program won't change its results, it
 263   is likely to execute more slowly.</p>
 264
 265 <p>libstdc++ includes many extensions to the C++ standard library. In
 266   some cases the extensions are obvious, such as the hashed
 267   associative containers, whereas other extensions give predictable
 268   results to behavior that would otherwise be undefined, such as
 269   throwing an exception when a <code>std::basic_string</code> is
 270   constructed from a NULL character pointer. This latter category also
 271   includes implementation-defined and unspecified semantics, such as
 272   the growth rate of a vector. Use of these extensions is not
 273   considered incorrect, so code that relies on them will not be
 274   rejected by debug mode. However, use of these extensions may affect
 275   the portability of code to other implementations of the C++ standard
 276   library, and is therefore somewhat hazardous. For this reason, the
 277   libstdc++ debug mode offers a "pedantic" mode (similar to
 278   GCC's <code>-pedantic</code> compiler flag) that attempts to emulate
 279   the semantics guaranteed by the C++ standard. For
 280   instance, constructing a <code>std::basic_string</code> with a NULL
 281   character pointer would result in an exception under normal mode or
 282   non-pedantic debug mode (this is a libstdc++ extension), whereas
 283   under pedantic debug mode libstdc++ would signal an error. To enable
 284   the pedantic debug mode, compile your program with
 285   both <code>-D_GLIBCXX_DEBUG</code>
 286   and <code>-D_GLIBCXX_DEBUG_PEDANTIC</code> .</p>
 287
 288 <p>The following library components provide extra debugging
 289   capabilities in debug mode:</p>
 290 <ul>
 291   <li><code>std::basic_string</code> (no safe iterators)</li>
 292   <li><code>std::bitset</code></li>
 293   <li><code>std::deque</code></li>
 294   <li><code>__gnu_cxx::hash_map</code></li>
 295   <li><code>__gnu_cxx::hash_multimap</code></li>
 296   <li><code>__gnu_cxx::hash_multiset</code></li>
 297   <li><code>__gnu_cxx::hash_set</code></li>
 298   <li><code>std::list</code></li>
 299   <li><code>std::map</code></li>
 300   <li><code>std::multimap</code></li>
 301   <li><code>std::multiset</code></li>
 302   <li><code>std::set</code></li>
 303   <li><code>std::vector</code></li>
 304 </ul>
 305
 306
 307 <h3 class="left"><a name="mem">Tips for memory leak hunting</a></h3>
 308
 309 <p>There are various third party memory tracing and debug utilities
 310    that can be used to provide detailed memory allocation information
 311    about C++ code. An exhaustive list of tools is not going to be
 312    attempted, but includes <code>mtrace</code>, <code>valgrind</code>,
 313    <code>mudflap</code>, and the non-free commercial product
 314    <code>purify</code>. In addition, <code>libcwd</code> has a
 315    replacement for the global new and delete operators that can track
 316    memory allocation and deallocation and provide useful memory
 317    statistics.
 318 </p>
 319
 320 <p>Regardless of the memory debugging tool being used, there is one
 321    thing of great importance to keep in mind when debugging C++ code
 322    that uses <code>new</code> and <code>delete</code>:
 323    there are different kinds of allocation schemes that can be used by
 324    <code> std::allocator </code>. For implementation details, see this
 325    <a href="ext/howto.html#3"> document</a> and look specifically for
 326    <code>GLIBCXX_FORCE_NEW</code>.
 327 </p>
 328
 329 <p>In a nutshell, the default allocator used by <code>
 330    std::allocator</code> is a high-performance pool allocator, and can
 331    give the mistaken impression that in a suspect executable, memory
 332    is being leaked, when in reality the memory "leak" is a pool being
 333    used by the library's allocator and is reclaimed after program
 334    termination.
 335 </p>
 336
 337 <p>For valgrind, there are some specific items to keep in mind. First
 338    of all, use a version of valgrind that will work with current GNU
 339    C++ tools: the first that can do this is valgrind 1.0.4, but later
 340    versions should work at least as well. Second of all, use a
 341    completely unoptimized build to avoid confusing valgrind. Third,
 342    use GLIBCXX_FORCE_NEW to keep extraneous pool allocation noise from
 343    cluttering debug information.
 344 </p>
 345
 346 <p>Fourth, it may be necessary to force deallocation in other
 347    libraries as well, namely the "C" library. On linux, this can be
 348    accomplished with the appropriate use of the
 349    <code>__cxa_atexit</code> or <code>atexit</code> functions.
 350 </p>
 351
 352 <pre>
 353    #include &lt;cstdlib&gt;
 354
 355    extern "C" void __libc_freeres(void);
 356
 357    void do_something() { }
 358
 359    int main()
 360    {
 361      atexit(__libc_freeres);
 362      do_something();
 363      return 0;
 364    }
 365 </pre>
 366
 367
 368 <p>or, using <code>__cxa_atexit</code>:</p>
 369
 370 <pre>
 371    extern "C" void __libc_freeres(void);
 372    extern "C" int __cxa_atexit(void (*func) (void *), void *arg, void *d);
 373
 374    void do_something() { }
 375
 376    int main()
 377    {
 378       extern void* __dso_handle __attribute__ ((__weak__));
 379       __cxa_atexit((void (*) (void *)) __libc_freeres, NULL,
 380                    &amp;__dso_handle ? __dso_handle : NULL);
 381       do_test();
 382       return 0;
 383    }
 384 </pre>
 385
 386 <p>Suggested valgrind flags, given the suggestions above about setting
 387    up the runtime environment, library, and test file, might be:
 388 </p>
 389 <pre>
 390    valgrind -v --num-callers=20 --leak-check=yes --leak-resolution=high --show-reachable=yes a.out
 391 </pre>
 392
 393
 394 <h3 class="left"><a name="gdb">Some gdb strategies</a></h3>
 395 <p>Many options are available for gdb itself: please see <a
 396    href="http://sources.redhat.com/gdb/current/onlinedocs/gdb_13.html#SEC109">
 397    "GDB features for C++" </a> in the gdb documentation. Also
 398    recommended: the other parts of this manual.
 399 </p>
 400
 401 <p>These settings can either be switched on in at the gdb command
 402    line, or put into a .gdbint file to establish default debugging
 403    characteristics, like so:
 404 </p>
 405
 406 <pre>
 407    set print pretty on
 408    set print object on
 409    set print static-members on
 410    set print vtbl on
 411    set print demangle on
 412    set demangle-style gnu-v3
 413 </pre>
 414
 415
 416 <h3 class="left"><a name="verbterm">Tracking uncaught exceptions</a></h3>
 417 <p>The <a href="19_diagnostics/howto.html#4">verbose termination handler</a>
 418    gives information about uncaught exceptions which are killing the
 419    program.  It is described in the linked-to page.
 420 </p>
 421
 422
 423 <p>Return <a href="#top">to the top of the page</a> or
 424    <a href="http://gcc.gnu.org/libstdc++/">to the libstdc++ homepage</a>.
 425 </p>
 426
 427
 428 <!-- ####################################################### -->
 429
 430 <hr />
 431 <p class="fineprint"><em>
 432 See <a href="17_intro/license.html">license.html</a> for copying conditions.
 433 Comments and suggestions are welcome, and may be sent to
 434 <a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
 435 </em></p>
 436
 437
 438 </body>
 439 </html>