libstdc++-v3/docs/html/faq/index.html

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  19
  20 <h1 class="centered">libstdc++ Frequently Asked Questions</h1>
  21
  22 <p class="fineprint"><em>
  23    The latest version of this document is always available at
  24    <a href="http://gcc.gnu.org/onlinedocs/libstdc++/faq/">
  25    http://gcc.gnu.org/onlinedocs/libstdc++/faq/</a>.  The main documentation
  26    page is at
  27    <a href="http://gcc.gnu.org/onlinedocs/libstdc++/documentation.html">
  28    http://gcc.gnu.org/onlinedocs/libstdc++/documentation.html</a>.
  29 </em></p>
  30
  31 <p><em>
  32    To the <a href="http://gcc.gnu.org/libstdc++/">libstdc++-v3 homepage</a>.
  33 </em></p>
  34
  35 <!-- ####################################################### -->
  36 <hr />
  37 <h1>Questions</h1>
  38 <ol>
  39    <li><a href="#1_0">General Information</a>
  40    <!-- I suspect these will mostly be links to/into existing documents. -->
  41    <ol>
  42       <li><a href="#1_1">What is libstdc++-v3?</a> </li>
  43       <li><a href="#1_2">Why should I use libstdc++?</a> </li>
  44       <li><a href="#1_3">Who's in charge of it?</a> </li>
  45       <li><a href="#1_4">How do I get libstdc++?</a> </li>
  46       <li><a href="#1_5">When is libstdc++ going to be finished?</a> </li>
  47       <li><a href="#1_6">How do I contribute to the effort?</a> </li>
  48       <li><a href="#1_7">What happened to libg++?  I need that!</a> </li>
  49       <li><a href="#1_8">What if I have more questions?</a> </li>
  50       <li><a href="#1_9">What are the license terms for libstdc++-v3?</a> </li>
  51    </ol>
  52    </li>
  53
  54    <li><a href="#2_0">Installation</a>
  55       <ol>
  56          <li><a href="#2_1">How do I install libstdc++-v3?</a> </li>
  57          <li><a href="#2_2">[removed]</a> </li>
  58          <li><a href="#2_3">What is this CVS thing that you keep
  59                             mentioning?</a> </li>
  60          <li><a href="#2_4">How do I know if it works?</a> </li>
  61          <li><a href="#2_5">This library is HUGE!  And what's libsupc++?</a> </li>
  62       </ol>
  63    </li>
  64
  65    <li><a href="#3_0">Platform-Specific Issues</a>
  66       <ol>
  67          <li><a href="#3_1">Can libstdc++-v3 be used with &lt;my
  68                             favorite compiler&gt;?</a> </li>
  69          <li><a href="#3_2">[removed]</a> </li>
  70          <li><a href="#3_3">[removed]</a> </li>
  71          <li><a href="#3_4">I can't use 'long long' on Solaris</a> </li>
  72          <li><a href="#3_5"><code>_XOPEN_SOURCE</code> /
  73                             <code>_GNU_SOURCE</code> / etc is always defined</a>
  74          </li>
  75          <li><a href="#3_6">OS X ctype.h is broken!  How can I hack it?</a></li>
  76          <li><a href="#3_7">Threading is broken on i386</a></li>
  77          <li><a href="#3_8">Recent GNU/Linux glibc required?</a></li>
  78          <li><a href="#3_9">Can't use wchar_t/wstring on FreeBSD</a></li>
  79          <li><a href="#3_10">MIPS atomic operations</a></li>
  80       </ol>
  81    </li>
  82
  83    <li><a href="#4_0">Known Bugs and Non-Bugs</a>
  84       <ol>
  85          <li><a href="#4_1">What works already?</a> </li>
  86          <li><a href="#4_2">Bugs in gcc/g++ (not libstdc++-v3)</a> </li>
  87          <li><a href="#4_3">Bugs in the C++ language/lib specification</a> </li>
  88          <li><a href="#4_4">Things in libstdc++ that only look like bugs</a><ul>
  89            <li><a href="#4_4_iostreamclear">reopening a stream fails</a> </li>
  90            <li><a href="#4_4_Weff">-Weffc++ complains too much</a> </li>
  91            <li><a href="#4_4_rel_ops">&quot;ambiguous overloads&quot;
  92                                after including an old-style header</a> </li>
  93            <li><a href="#4_4_interface">The g++-3 headers are
  94                                <strong>not ours</strong></a> </li>
  95            <li><a href="#4_4_glibc">compilation errors from streambuf.h</a> </li>
  96            <li><a href="#4_4_checks">errors about <em>*Concept</em> and
  97                                <em>constraints</em> in the STL...</a> </li>
  98            <li><a href="#4_4_dlsym">program crashes when using library code
  99                                in a dynamically-loaded library</a> </li>
 100            <li><a href="#4_4_leak">"memory leaks" in containers</a> </li>
 101          </ul>
 102          </li>
 103          <li><a href="#4_5">Aw, that's easy to fix!</a> </li>
 104       </ol>
 105    </li>
 106
 107    <li><a href="#5_0">Miscellaneous</a>
 108       <ol>
 109          <li><a href="#5_1">string::iterator is not char*;
 110                             vector&lt;T&gt;::iterator is not T*</a> </li>
 111          <li><a href="#5_2">What's next after libstdc++-v3?</a> </li>
 112          <li><a href="#5_3">What about the STL from SGI?</a> </li>
 113          <li><a href="#5_4">Extensions and Backward Compatibility</a> </li>
 114          <li><a href="#5_5">[removed]</a> </li>
 115          <li><a href="#5_6">Is libstdc++-v3 thread-safe?</a> </li>
 116          <li><a href="#5_7">How do I get a copy of the ISO C++ Standard?</a> </li>
 117          <li><a href="#5_8">What's an ABI and why is it so messy?</a> </li>
 118       </ol>
 119    </li>
 120
 121 </ol>
 122
 123 <hr />
 124
 125 <!-- ####################################################### -->
 126
 127 <h1><a name="1_0">1.0 General Information</a></h1>
 128 <!-- I suspect these will mostly be links to/into existing documents. -->
 129    <h2><a name="1_1">1.1 What is libstdc++-v3?</a></h2>
 130       <p>The GNU Standard C++ Library v3 is an
 131          ongoing project to implement the ISO 14882 Standard C++ library
 132          as described in chapters 17 through 27 and annex D.  As the
 133          library reaches stable plateaus, it is captured in a snapshot
 134          and released.  The latest release is
 135          <a href="http://gcc.gnu.org/libstdc++/index.html#download">the
 136          fourteenth snapshot</a> but newer versions have been included
 137          in recent GCC releases.  For those who want to see exactly how
 138          far the project has come, or just want the latest
 139          bleeding-edge code, the up-to-date source is available over
 140          anonymous CVS, and can even be browsed over the Web (see
 141          <a href="#1_4">1.4</a> below).
 142       </p>
 143       <p>The older libstdc++-v2 project is no longer maintained; the code
 144          has been completely replaced and rewritten.
 145          <a href="#4_4_interface">If you are using V2</a>, then you need to
 146          report bugs to your system vendor, not to the V3 list.
 147       </p>
 148       <p>A more formal description of the V3 goals can be found in the
 149          official <a href="../17_intro/DESIGN">design document</a>.
 150       </p>
 151
 152 <hr />
 153    <h2><a name="1_2">1.2 Why should I use libstdc++?</a></h2>
 154       <p>The completion of the ISO C++ standardization gave the
 155          C++ community a powerful set of reuseable tools in the form
 156          of the C++ Standard Library.  However, all existing C++
 157          implementations are (as the Draft Standard used to say)
 158          &quot;incomplet and incorrekt,&quot; and many suffer from
 159          limitations of the compilers that use them.
 160       </p>
 161       <p>The GNU C/C++/FORTRAN/&lt;pick-a-language&gt; compiler
 162          (<code>gcc</code>, <code>g++</code>, etc) is widely considered to be
 163          one of the leading compilers in the world.  Its development
 164          has recently been taken over by the
 165          <a href="http://gcc.gnu.org/">GCC team</a>.  All of
 166          the rapid development and near-legendary
 167          <a href="http://gcc.gnu.org/gcc-3.0/buildstat.html">portability</a>
 168          that are the hallmarks of an open-source project are being
 169          applied to libstdc++.
 170       </p>
 171       <p>That means that all of the Standard classes and functions
 172          (such as <code>string</code>, <code>vector&lt;&gt;</code>, iostreams,
 173          and algorithms) will be freely available and fully compliant.
 174          Programmers will no longer need to &quot;roll their own&quot;
 175          nor be worried about platform-specific incompatibilities.
 176       </p>
 177
 178 <hr />
 179    <h2><a name="1_3">1.3 Who's in charge of it?</a></h2>
 180       <p>The libstdc++ project is contributed to by several developers
 181          all over the world, in the same way as GCC or Linux.
 182          Benjamin Kosnik, Gabriel Dos Reis, Phil Edwards, Ulrich Drepper,
 183          Loren James Rittle, and Paolo Carlini are the lead maintainers of
 184          the CVS archive.
 185       </p>
 186       <p>Development and discussion is held on the libstdc++ mailing
 187          list.  Subscribing to the list, or searching the list
 188          archives, is open to everyone.  You can read instructions for
 189          doing so on the <a href="http://gcc.gnu.org/libstdc++/">homepage</a>.
 190          If you have questions, ideas, code, or are just curious, sign up!
 191       </p>
 192
 193 <hr />
 194    <h2><a name="1_4">1.4 How do I get libstdc++?</a></h2>
 195       <p>The <a href="http://gcc.gnu.org/libstdc++/">homepage</a>
 196          has instructions for retrieving the latest CVS sources, and for
 197          browsing the CVS sources over the web.
 198       </p>
 199       <p>Stable versions of libstdc++-v3 are included with releases of
 200          <a href="http://gcc.gnu.org/releases.html">the GCC compilers</a>.
 201       </p>
 202       <p>The subset commonly known as the Standard Template Library
 203          (chapters 23 through 25, mostly) is adapted from the final release
 204          of the SGI STL.
 205       </p>
 206
 207 <hr />
 208    <h2><a name="1_5">1.5 When is libstdc++ going to be finished?</a></h2>
 209 <!--      <p>Nathan Myers gave the best of all possible answers in <a
 210          href="http://www.deja.com/getdoc.xp?AN=469581698&fmt=text">a
 211          Usenet article</a>.</p>
 212 which is no longer available, thanks deja...-->
 213       <p>Nathan Myers gave the best of all possible answers, responding to a
 214          Usenet article asking this question:  <em>Sooner, if you help.</em>
 215       </p>
 216
 217 <hr />
 218    <h2><a name="1_6">1.6 How do I contribute to the effort?</a></h2>
 219       <p>Here is <a href="../17_intro/contribute.html">a
 220          page devoted to this topic</a>.  Subscribing to the mailing
 221          list (see above, or the homepage) is a very good idea if you
 222          have something to contribute, or if you have spare time and
 223          want to help.  Contributions don't have to be in the form of
 224          source code; anybody who is willing to help write
 225          documentation, for example, or has found a bug in code that
 226          we all thought was working, is more than welcome!
 227       </p>
 228
 229 <hr />
 230    <h2><a name="1_7">1.7 What happened to libg++?  I need that!</a></h2>
 231       <p>The most recent libg++ README states that libg++ is no longer
 232          being actively maintained.  It should not be used for new
 233          projects, and is only being kicked along to support older code.
 234       </p>
 235       <p>The libg++ was designed and created when there was no Standard
 236          to provide guidance.  Classes like linked lists are now provided
 237          for by <code>list&lt;T&gt;</code> and do not need to be created by
 238          <code>genclass</code>.  (For that matter, templates exist now and
 239          are well-supported, whereas genclass (mostly) predates them.)
 240       </p>
 241       <p>There are other classes in libg++ that are not specified in the
 242          ISO Standard (e.g., statistical analysis).  While there are a
 243          lot of really useful things that are used by a lot of people
 244          (e.g., statistics :-), the Standards Committee couldn't include
 245          everything, and so a lot of those &quot;obvious&quot; classes
 246          didn't get included.
 247       </p>
 248       <p>Since libstdc++ is an implementation of the Standard Library, we
 249          have no plans at this time to include non-Standard utilities
 250          in the implementation, however handy they are.  (The extensions
 251          provided in the SGI STL aren't maintained by us and don't get
 252          a lot of our attention, because they don't require a lot of our
 253          time.)  It is entirely plausable that the &quot;useful stuff&quot;
 254          from libg++ might be extracted into an updated utilities library,
 255          but nobody has stated such a project yet.
 256       </p>
 257       <p>(The <a href="http://www.boost.org/">Boost</a> site houses free
 258          C++ libraries that do varying things, and happened to be started
 259          by members of the Standards Committee.  Certain &quot;useful
 260          stuff&quot; classes will probably migrate there.)
 261       </p>
 262       <p>For the bold and/or desperate, the
 263          <a href="http://gcc.gnu.org/extensions.html">GCC extensions page</a>
 264          describes where to find the last libg++ source.
 265       </p>
 266
 267 <hr />
 268    <h2><a name="1_8">1.8 What if I have more questions?</a></h2>
 269       <p>If you have read the README and RELEASE-NOTES files, and your
 270          question remains unanswered, then just ask the mailing list.
 271          At present, you do not need to be subscribed to the list to
 272          send a message to it.  More information is available on the
 273          homepage (including how to browse the list archives); to send
 274          to the list, use <a href="mailto:libstdc++@gcc.gnu.org">
 275          <code>libstdc++@gcc.gnu.org</code></a>.
 276       </p>
 277       <p>If you have a question that you think should be included here,
 278          or if you have a question <em>about</em> a question/answer here,
 279          contact <a href="mailto:pme@gcc.gnu.org">Phil Edwards</a>
 280          or <a href="mailto:gdr@gcc.gnu.org">Gabriel Dos Reis</a>.
 281       </p>
 282
 283 <hr />
 284    <h2><a name="1_9">1.9 What are the license terms for libstdc++-v3?</a></h2>
 285       <p>See <a href="../17_intro/license.html">our license description</a>
 286          for these and related questions.
 287       </p>
 288
 289 <hr />
 290 <h1><a name="2_0">2.0 Installation</a></h1>
 291    <h2><a name="2_1">2.1 How do I install libstdc++-v3?</a></h2>
 292       <p>Complete instructions are not given here (this is a FAQ, not
 293          an installation document), but the tools required are few:
 294       </p>
 295          <ul>
 296             <li> A 3.x release of GCC.  Note that building GCC is much
 297                  easier and more automated than building the GCC 2.[78]
 298                  series was.  If you are using GCC 2.95, you can still
 299                  build earlier snapshots of libstdc++.
 300             </li>
 301             <li> GNU Make is recommended, but should not be required.
 302             </li>
 303             <li> The GNU Autotools are needed if you are messing with
 304                  the configury or makefiles.
 305             </li>
 306          </ul>
 307       <p>The file <a href="../documentation.html">documentation.html</a>
 308          provides a good overview of the steps necessary to build, install,
 309          and use the library.  Instructions for configuring the library
 310          with new flags such as --enable-threads are there also, as well as
 311          patches and instructions for working with GCC 2.95.
 312       </p>
 313       <p>The top-level install.html and
 314          <a href="../17_intro/RELEASE-NOTES">RELEASE-NOTES</a> files contain
 315          the exact build and installation instructions.  You may wish to
 316          browse those files over CVSweb ahead of time to get a feel for
 317          what's required.  RELEASE-NOTES is located in the
 318          &quot;.../docs/17_intro/&quot; directory of the distribution.
 319       </p>
 320
 321 <hr />
 322    <h2><a name="2_2">2.2 [removed]</a></h2>
 323       <p>This question has become moot and has been removed.  The stub
 324          is here to preserve numbering (and hence links/bookmarks).
 325       </p>
 326
 327 <hr />
 328    <h2><a name="2_3">2.3 What is this CVS thing that you
 329                          keep mentioning?</a></h2>
 330       <p>The <em>Concurrent Versions System</em> is one of several revision
 331          control packages.  It was selected for GNU projects because it's
 332          free (speech), free (beer), and very high quality.  The <a
 333          href="http://www.gnu.org/software/cvs/cvs.html">CVS entry in
 334          the GNU software catalogue</a> has a better description as
 335          well as a
 336          <a href="http://www.cvshome.org/">link to the makers of CVS</a>.
 337       </p>
 338       <p>The &quot;anonymous client checkout&quot; feature of CVS is
 339          similar to anonymous FTP in that it allows anyone to retrieve
 340          the latest libstdc++ sources.
 341       </p>
 342       <p>After the first of April, American users will have a
 343          &quot;/pharmacy&quot; command-line option...
 344          <!-- wonder how long that'll live -->
 345       </p>
 346
 347 <hr />
 348    <h2><a name="2_4">2.4 How do I know if it works?</a></h2>
 349       <p>libstdc++-v3 comes with its own testsuite.  You do not need
 350          to actually install the library (&quot;<code>make
 351          install</code>&quot;) to run the testsuite, but you do need
 352          DejaGNU, as described
 353          <a href="http://gcc.gnu.org/install/test.html">here</a>.
 354       </p>
 355       <p>To run the testsuite on the library after building it, use
 356          &quot;make check&quot; while in your build directory.  To run
 357          the testsuite on the library after building and installing it,
 358          use &quot;make check-install&quot; instead.
 359       </p>
 360       <p>If you find bugs in the testsuite programs themselves, or if you
 361          think of a new test program that should be added to the suite,
 362          <strong>please</strong> write up your idea and send it to the list!
 363       </p>
 364
 365 <hr />
 366    <h2><a name="2_5">2.5 This library is HUGE!  And what's libsupc++?</a></h2>
 367       <p>Usually the size of libraries on disk isn't noticeable.  When a
 368          link editor (or simply &quot;linker&quot;) pulls things from a
 369          static archive library, only the necessary object files are copied
 370          into your executable, not the entire library.  Unfortunately, even
 371          if you only need a single function or variable from an object file,
 372          the entire object file is extracted.  (There's nothing unique to C++
 373          or libstdc++-v3 about this; it's just common behavior, given here
 374          for background reasons.)
 375       </p>
 376       <p>Some of the object files which make up libstdc++.a are rather large.
 377          If you create a statically-linked executable with
 378          <code> -static</code>, those large object files are suddenly part
 379          of your executable.  Historically the best way around this was to
 380          only place a very few functions (often only a single one) in each
 381          source/object file; then extracting a single function is the same
 382          as extracting a single .o file.  For libstdc++-v3 this is only
 383          possible to a certain extent; the object files in question contain
 384          template classes and template functions, pre-instantiated, and
 385          splitting those up causes severe maintenance headaches.
 386       </p>
 387       <p>It's not a bug, and it's not really a problem.  Nevertheless, some
 388          people don't like it, so here are two pseudo-solutions:
 389       </p>
 390       <p>If the only functions from libstdc++.a which you need are language
 391          support functions (those listed in
 392          <a href="../18_support/howto.html">clause 18</a> of the standard,
 393          e.g., <code>new</code> and <code>delete</code>), then try linking
 394          against <code>libsupc++.a</code> (usually specifying
 395          <code>-lsupc++</code> when calling g++ for the final link step will
 396          do it).  This library contains only those support routines, one per
 397          object file.  But if you are using anything from the rest of the
 398          library, such as IOStreams or vectors, then you'll still need
 399          pieces from <code>libstdc++.a</code>.
 400       </p>
 401       <p>The second method is one we hope to incorporate into the library
 402          build process.  Some platforms can place each function and variable
 403          into its own section in a .o file.  The GNU linker can then perform
 404          garbage collection on unused sections; this reduces the situation
 405          to only copying needed functions into the executable, as before,
 406          but all happens automatically.
 407       </p>
 408       <p>Unfortunately the garbage collection in GNU ld is buggy; sections
 409          (corresponding to functions and variables) which <em>are</em> used
 410          are mistakenly removed, leading to horrible crashes when your
 411          executable starts up.  For the time being, this feature is not used
 412          when building the library.
 413       </p>
 414
 415 <hr />
 416 <h1><a name="3_0">3.0 Platform-Specific Issues</a></h1>
 417    <h2><a name="3_1">3.1 Can libstdc++-v3 be used with &lt;my
 418                          favorite compiler&gt;?</a></h2>
 419       <p>Probably not.  Yet.</p>
 420       <p>Because GCC advances so rapidly, development and testing of
 421          libstdc++ is being done almost entirely under that compiler.
 422          If you are curious about whether other, lesser compilers
 423          (*grin*) support libstdc++, you are more than welcome to try.
 424          Configuring and building the library (see above) will still
 425          require certain tools, however.  Also keep in mind that
 426          <em>building</em> libstdc++ does not imply that your compiler
 427          will be able to <em>use</em> all of the features found in the
 428          C++ Standard Library.
 429       </p>
 430       <p>Since the goal of ISO Standardization is for all C++
 431          implementations to be able to share code, the final libstdc++
 432          should, in theory, be usable under any ISO-compliant
 433          compiler.  It will still be targeted and optimized for
 434          GCC/g++, however.
 435       </p>
 436
 437 <hr />
 438    <h2><a name="3_2">3.2 [removed]</a></h2>
 439       <p>This question has become moot and has been removed.  The stub
 440          is here to preserve numbering (and hence links/bookmarks).
 441       </p>
 442
 443 <hr />
 444    <h2><a name="3_3">3.3 [removed]</a></h2>
 445       <p>This question has become moot and has been removed.  The stub
 446          is here to preserve numbering (and hence links/bookmarks).
 447       </p>
 448
 449 <hr />
 450    <h2><a name="3_4">3.4 I can't use 'long long' on Solaris</a></h2>
 451       <p>By default we try to support the C99 <code>long long</code> type.
 452          This requires that certain functions from your C library be present.
 453       </p>
 454       <p>Up through release 3.0.2 the tests performed were too general, and
 455          this feature was disabled when it did not need to be.  The most
 456          commonly reported platform affected was Solaris.
 457       </p>
 458       <p>This has been fixed for 3.0.3 and onwards.
 459       </p>
 460
 461 <hr />
 462    <h2><a name="3_5">3.5 <code>_XOPEN_SOURCE</code> / <code>_GNU_SOURCE</code>
 463                          / etc is always defined</a></h2>
 464       <p>On Solaris, g++ (but not gcc) always defines the preprocessor
 465          macro <code>_XOPEN_SOURCE</code>.  On GNU/Linux, the same happens
 466          with <code>_GNU_SOURCE</code>.  (This is not an exhaustive list;
 467          other macros and other platforms are also affected.)
 468       </p>
 469       <p>These macros are typically used in C library headers, guarding new
 470          versions of functions from their older versions.  The C++ standard
 471          library includes the C standard library, but it requires the C90
 472          version, which for backwards-compatability reasons is often not the
 473          default for many vendors.
 474       </p>
 475       <p>More to the point, the C++ standard requires behavior which is only
 476          available on certain platforms after certain symbols are defined.
 477          Usually the issue involves I/O-related typedefs.  In order to
 478          ensure correctness, the compiler simply predefines those symbols.
 479       </p>
 480       <p>Note that it's not enough to #define them only when the library is
 481          being built (during installation).  Since we don't have an 'export'
 482          keyword, much of the library exists as headers, which means that
 483          the symbols must also be defined as your programs are parsed and
 484          compiled.
 485       </p>
 486       <p>To see which symbols are defined, look for CPLUSPLUS_CPP_SPEC in
 487          the gcc config headers for your target (and try changing them to
 488          see what happens when building complicated code).  You can also run
 489          <code>&quot;g++ -E -dM - &lt; /dev/null&quot;</code> to display
 490          a list of predefined macros for any particular installation.
 491       </p>
 492       <p>This has been discussed on the mailing lists
 493          <a href="http://gcc.gnu.org/cgi-bin/htsearch?method=and&amp;format=builtin-long&amp;sort=score&amp;words=_XOPEN_SOURCE+Solaris">quite a bit</a>.
 494       </p>
 495       <p>This method is something of a wart.  We'd like to find a cleaner
 496          solution, but nobody yet has contributed the time.
 497       </p>
 498
 499 <hr />
 500    <h2><a name="3_6">3.6 OS X ctype.h is broken!  How can I hack it?</a></h2>
 501       <p>This is a long-standing bug in the OS X support.  Fortunately,
 502          the patch is quite simple, and well-known.
 503          <a href="http://gcc.gnu.org/ml/gcc/2002-03/msg00817.html"> Here's a
 504          link to the solution.</a>
 505       </p>
 506
 507 <hr />
 508    <h2><a name="3_7">3.7 Threading is broken on i386</a></h2>
 509       <p>Support for atomic integer operations is/was broken on i386
 510          platforms.  The assembly code accidentally used opcodes that are
 511          only available on the i486 and later.  So if you configured GCC
 512          to target, for example, i386-linux, but actually used the programs
 513          on an i686, then you would encounter no problems.  Only when
 514          actually running the code on a i386 will the problem appear.
 515       </p>
 516       <p>This is fixed in 3.2.2.
 517       </p>
 518
 519 <hr />
 520    <h2><a name="3_8">3.8 Recent GNU/Linux glibc required?</a></h2>
 521       <p>When running on GNU/Linux, libstdc++ 3.2.1 (shared library version
 522          5.0.1) and later uses localization and formatting code from the system
 523          C library (glibc) version 2.2.5.  That version of glibc is over a
 524          year old and contains necessary bugfixes.  Many GNU/Linux distros make
 525          glibc version 2.3.x available now.
 526       </p>
 527       <p>The guideline is simple:  the more recent the C++ library, the
 528          more recent the C library.  (This is also documented in the main
 529          GCC installation instructions.)
 530       </p>
 531
 532 <hr />
 533    <h2><a name="3_9">3.9 Can't use wchar_t/wstring on FreeBSD</a></h2>
 534       <p>At the moment there are a few problems in FreeBSD's support for
 535          wide character functions, and as a result the libstdc++ configury
 536          decides that wchar_t support should be disabled.  Once the underlying
 537          problems are fixed in FreeBSD (soon), the library support will
 538          automatically enable itself.
 539       </p>
 540       <p>You can fix the problems yourself, and learn more about the situation,
 541          by reading
 542          <a href="http://gcc.gnu.org/ml/libstdc++/2003-02/subjects.html#00286">
 543          this short thread</a> (&quot;_GLIBCPP_USE_WCHAR_T undefined in
 544          FreeBSD's c++config.h?&quot;).
 545       </p>
 546
 547 <hr />
 548    <h2><a name="3_10">3.10 MIPS atomic operations</a></h2>
 549       <p>The atomic locking routines for MIPS targets requires MIPS II
 550          and later.  A patch went in just after the 3.3.0 release to
 551          make mips* use the generic implementation instead.  You can also
 552          configure for mipsel-elf as a workaround.
 553       </p>
 554       <p>mips*-*-linux* continues to use the MIPS II routines, and more
 555          work in this area is expected.
 556       </p>
 557
 558 <hr />
 559 <h1><a name="4_0">4.0 Known Bugs and Non-Bugs</a></h1>
 560    <em>Note that this section can get rapdily outdated -- such is the
 561        nature of an open-source project.  For the latest information, join
 562        the mailing list or look through recent archives.   The RELEASE-
 563        NOTES and BUGS files are generally kept up-to-date.</em>
 564
 565    <p>For 3.0.1, the most common &quot;bug&quot; is an apparently missing
 566       &quot;<code>../</code>&quot; in include/Makefile, resulting in files
 567       like gthr.h and gthr-single.h not being found.  Please read
 568       <a href="http://gcc.gnu.org/install/configure.html">the configuration
 569       instructions for GCC</a>,
 570       specifically the part about configuring in a separate build directory,
 571       and how strongly recommended it is.  Building in the source directory
 572       is fragile, is rarely tested, and tends to break, as in this case.
 573       This was fixed for 3.0.2.
 574    </p>
 575
 576    <p>For 3.1, the most common &quot;bug&quot; is a parse error when using
 577       <code>&lt;fstream&gt;</code>, ending with a message,
 578       &quot;<code>bits/basic_file.h:52: parse error before `{'
 579       token</code>.&quot;  Please read
 580       <a href="http://gcc.gnu.org/install/">the installation instructions for
 581       GCC</a>, specifically the part about not installing newer versions on
 582       top of older versions.  If you install 3.1 over a 3.0.x release, then
 583       the wrong basic_file.h header will be found (its location changed
 584       between releases).
 585    </p>
 586
 587    <p><strong>Please do not report these as bugs.  We know about them.</strong>
 588       Reporting this -- or any other problem that's already been fixed --
 589       hinders the development of GCC, because we have to take time to
 590       respond to your report.  Thank you.
 591    </p>
 592
 593    <h2><a name="4_1">4.1 What works already?</a></h2>
 594       <p>Short answer:  Pretty much everything <em>works</em> except for some
 595          corner cases.  Also, localization is incomplete.  For whether it works
 596          well, or as you expect it to work, see 5.2.
 597       </p>
 598       <p>Long answer:  See the docs/html/17_intro/CHECKLIST file, which is
 599          badly outdated...
 600       </p>
 601       <p>What follows is a verbatim clip from the &quot;Status&quot; section
 602          of the RELEASE-NOTES for the latest snapshot.  For a list of
 603          fixed bugs, see that file.
 604       </p>
 605
 606 <!-- Yeah, I meant that "verbatim clip" thing literally... :-)  -->
 607
 608 <pre>
 609 New:
 610 </pre>
 611
 612
 613 <hr />
 614    <h2><a name="4_2">4.2 Bugs in gcc/g++ (not libstdc++-v3)</a></h2>
 615       <p>This is by no means meant to be complete nor exhaustive, but
 616          mentions some problems that users may encounter when building
 617          or using libstdc++.  If you are experiencing one of these
 618          problems, you can find more information on the libstdc++ and
 619          the GCC mailing lists.
 620       </p>
 621       <p>Before reporting a bug, examine the
 622          <a href="http://gcc.gnu.org/bugs.html">bugs database</a> with the
 623          category set to &quot;libstdc++&quot;.  The BUGS file in the source
 624          tree also tracks known serious problems.
 625       </p>
 626       <ul>
 627          <li>Debugging is problematic, due to bugs in line-number generation
 628              (mostly fixed in the compiler) and gdb lagging behind the
 629              compiler (lack of personnel).  We recommend configuring the
 630              compiler using <code>--with-dwarf2</code> if the DWARF2
 631              debugging format is not already the default on your platform.
 632              Also,
 633 <a href="http://gcc.gnu.org/ml/libstdc++/2002-02/msg00034.html">changing your
 634              GDB settings</a> can have a profound effect on your C++ debugging
 635              experiences.  :-)</li>
 636       </ul>
 637
 638 <hr />
 639    <h2><a name="4_3">4.3 Bugs in the C++ language/lib specification</a></h2>
 640       <p>Yes, unfortunately, there are some.  In a
 641          <a href="http://gcc.gnu.org/ml/libstdc++/1998/msg00006.html">message
 642          to the list</a>, Nathan Myers announced that he has started a list of
 643          problems in the ISO C++ Standard itself, especially with
 644          regard to the chapters that concern the library.  The list
 645          itself is
 646          <a href="http://www.cantrip.org/draft-bugs.txt">posted on his
 647          website</a>.  Developers who are having problems interpreting
 648          the Standard may wish to consult his notes.
 649       </p>
 650       <p>For those people who are not part of the ISO Library Group
 651          (i.e., nearly all of us needing to read this page in the first
 652          place :-), a public list of the library defects is occasionally
 653          published <a href="http://anubis.dkuug.dk/jtc1/sc22/wg21/">here</a>.
 654          Some of these have resulted in <a href="#5_2">code changes</a>.
 655       </p>
 656
 657 <hr />
 658    <h2><a name="4_4">4.4 Things in libstdc++ that only look like bugs</a></h2>
 659       <p>There are things which are not bugs in the compiler (4.2) nor
 660          the language specification (4.3), but aren't really bugs in
 661          libstdc++, either.  Really!  Please do not report these as bugs.
 662       </p>
 663       <p><a name="4_4_Weff"><strong>-Weffc++</strong></a>
 664          The biggest of these is the quadzillions of warnings about the
 665          library headers emitted when <code>-Weffc++</code> is used.  Making
 666          libstdc++ &quot;-Weffc++-clean&quot; is not a goal of the project,
 667          for a few reasons.  Mainly, that option tries to enforce
 668          object-oriented programming, while the Standard Library isn't
 669          necessarily trying to be OO.
 670       </p>
 671       <p><a name="4_4_iostreamclear"><strong>reopening a stream fails</strong>
 672          </a> Did I just say that -Weffc++ was our biggest false-bug report?
 673          I lied.  (It used to be.)  Today it seems to be reports that after
 674          executing a sequence like
 675       </p>
 676       <pre>
 677     #include &lt;fstream&gt;
 678     ...
 679     std::fstream  fs(&quot;a_file&quot;);
 680     // .
 681     // . do things with fs...
 682     // .
 683     fs.close();
 684     fs.open(&quot;a_new_file&quot;);</pre>
 685       <p>all operations on the re-opened <code>fs</code> will fail, or at
 686          least act very strangely.  Yes, they often will, especially if
 687          <code>fs</code> reached the EOF state on the previous file.  The
 688          reason is that the state flags are <strong>not</strong> cleared
 689          on a successful call to open().  The standard unfortunately did
 690          not specify behavior in this case, and to everybody's great sorrow,
 691          the <a href="../ext/howto.html#5">proposed LWG resolution in
 692          DR #22</a> is to leave the flags unchanged.  You must insert a call
 693          to <code>fs.clear()</code> between the calls to close() and open(),
 694          and then everything will work like we all expect it to work.
 695       </p>
 696       <p><a name="4_4_rel_ops"><strong>rel_ops</strong></a>
 697          Another is the <code>rel_ops</code> namespace and the template
 698          comparison operator functions contained therein.  If they become
 699          visible in the same namespace as other comparison functions
 700          (e.g., '<code>using</code>' them and the &lt;iterator&gt; header),
 701          then you will suddenly be faced with huge numbers of ambiguity
 702          errors.  This was discussed on the -v3 list; Nathan Myers
 703          <a href="http://gcc.gnu.org/ml/libstdc++/2001-01/msg00247.html">sums
 704          things up here</a>.  The collisions with vector/string iterator
 705          types have been fixed for 3.1.  <!-- more links to email here -->
 706       </p>
 707       <h3><a name="4_4_interface">The g++-3 headers are <em>not ours</em></a></h3>
 708       <p>If you have found an extremely broken header file which is
 709          causing problems for you, look carefully before submitting a
 710          &quot;high&quot; priority bug report (which you probably shouldn't
 711          do anyhow; see the last paragraph of the page describing
 712          <a href="http://gcc.gnu.org/gnatswrite.html">the GCC bug database</a>).
 713       </p>
 714       <p>If the headers are in <code>${prefix}/include/g++-3</code>, or if
 715          the installed library's name looks like <code>libstdc++-2.10.a</code>
 716          or <code>libstdc++-libc6-2.10.so</code>, then you are using the old
 717          libstdc++-v2 library, which is nonstandard and unmaintained.  Do not
 718          report problems with -v2 to the -v3 mailing list.
 719       </p>
 720       <p>For GCC versions 3.0 and 3.1 the libstdc++-v3 header files are
 721          installed in <code>${prefix}/include/g++-v3</code> (see the 'v'?).
 722          Starting with version 3.2 the headers are installed in
 723          <code>${prefix}/include/c++/${version}</code> as this prevents
 724          headers from previous versions being found by mistake.
 725       </p>
 726       <p><a name="4_4_glibc"><strong>glibc</strong></a>
 727          If you're on a GNU/Linux system and have just upgraded to
 728          glibc 2.2, but are still using gcc 2.95.2, then you should have
 729          read the glibc FAQ, specifically 2.34:
 730       </p>
 731       <pre>
 732 2.34.   When compiling C++ programs, I get a compilation error in streambuf.h.
 733
 734 {BH} You are using g++ 2.95.2? After upgrading to glibc 2.2, you need to
 735 apply a patch to the include files in /usr/include/g++, because the fpos_t
 736 type has changed in glibc 2.2.  The patch is at
 737 http://clisp.cons.org/~haible/gccinclude-glibc-2.2-compat.diff
 738    </pre>
 739       <p>Note that 2.95.x shipped with the
 740          <a href="#4_4_interface">old v2 library</a> which is no longer
 741          maintained.  Also note that gcc 2.95.3 fixes this problem, but
 742          requires a separate patch for libstdc++-v3.
 743       </p>
 744       <p><a name="4_4_checks"><strong>concept checks</strong></a>
 745          If you see compilation errors containing messages about
 746          <code> <em>foo</em>Concept </code>and a<code> constraints </code>
 747          member function, then most likely you have violated one of the
 748          requirements for types used during instantiation of template
 749          containers and functions.  For example, EqualityComparableConcept
 750          appears if your types must be comparable with == and you have not
 751          provided this capability (a typo, or wrong visibility, or you
 752          just plain forgot, etc).
 753       </p>
 754       <p>More information, including how to optionally enable/disable the
 755          checks, is available
 756          <a href="../19_diagnostics/howto.html#3">here</a>.
 757       </p>
 758       <p><a name="4_4_dlsym"><strong>dlopen/dlsym</strong></a>
 759          If you are using the C++ library across dynamically-loaded
 760          objects, make certain that you are passing the correct options
 761          when compiling and linking:
 762       </p>
 763       <pre>
 764     // compile your library components
 765     g++ -fPIC -c a.cc
 766     g++ -fPIC -c b.cc
 767     ...
 768     g++ -fPIC -c z.cc
 769
 770     // create your library
 771     g++ -fPIC -shared -rdynamic -o libfoo.so a.o b.o ... z.o
 772
 773     // link the executable
 774     g++ -fPIC -rdynamic -o foo ... -L. -lfoo -ldl</pre>
 775       <p><a name="4_4_leak"><strong>"memory leaks" in containers</strong></a>
 776          A few people have reported that the standard containers appear
 777          to leak memory when tested with memory checkers such as
 778          <a href="http://developer.kde.org/~sewardj/">valgrind</a>.
 779          The library's default allocators keep free memory in a pool
 780          for later reuse, rather than returning it to the OS.  Although
 781          this memory is always reachable by the library and is never
 782          lost, memory debugging tools can report it as a leak.  If you
 783          want to test the library for memory leaks please read
 784          <a href="../debug.html#mem">Tips for memory leak hunting</a>
 785          first.
 786       </p>
 787
 788 <hr />
 789    <h2><a name="4_5">4.5 Aw, that's easy to fix!</a></h2>
 790       <p>If you have found a bug in the library and you think you have
 791          a working fix, then send it in!  The main GCC site has a page
 792          on <a href="http://gcc.gnu.org/contribute.html">submitting
 793          patches</a> that covers the procedure, but for libstdc++ you
 794          should also send the patch to our mailing list in addition to
 795          the GCC patches mailing list.  The libstdc++
 796          <a href="../17_intro/contribute.html">contributors' page</a>
 797          also talks about how to submit patches.
 798       </p>
 799       <p>In addition to the description, the patch, and the ChangeLog
 800          entry, it is a Good Thing if you can additionally create a small
 801          test program to test for the presence of the bug that your
 802          patch fixes.  Bugs have a way of being reintroduced; if an old
 803          bug creeps back in, it will be caught immediately by the
 804          <a href="#2_4">testsuite</a> -- but only if such a test exists.
 805       </p>
 806
 807 <hr />
 808 <h1><a name="5_0">5.0 Miscellaneous</a></h1>
 809    <h2><a name="5_1">5.1 string::iterator is not char*;
 810                      vector&lt;T&gt;::iterator is not T*</a></h2>
 811       <p>If you have code that depends on container&lt;T&gt; iterators
 812          being implemented as pointer-to-T, your code is broken.
 813       </p>
 814       <p>While there are arguments for iterators to be implemented in
 815          that manner, A) they aren't very good ones in the long term,
 816          and B) they were never guaranteed by the Standard anyway.  The
 817          type-safety achieved by making iterators a real class rather
 818          than a typedef for <code>T*</code> outweighs nearly all opposing
 819          arguments.
 820       </p>
 821       <p>Code which does assume that a vector iterator <code> i </code>
 822          is a pointer can often be fixed by changing <code> i </code> in
 823          certain expressions to <code> &amp;*i </code>.  Future revisions
 824          of the Standard are expected to bless this usage for
 825          vector&lt;&gt; (but not for basic_string&lt;&gt;).
 826       </p>
 827
 828 <hr />
 829    <h2><a name="5_2">5.2 What's next after libstdc++-v3?</a></h2>
 830       <p>Hopefully, not much.  The goal of libstdc++-v3 is to produce
 831          a fully-compliant, fully-portable Standard Library.  After that,
 832          we're mostly done:  there won't <em>be</em> any more compliance
 833          work to do.  However:
 834       </p>
 835       <ol>
 836       <li><p>The ISO Committee will meet periodically to review Defect Reports
 837          in the C++ Standard.  Undoubtedly some of these will result in
 838          changes to the Standard, which will be reflected in patches to
 839          libstdc++.  Some of that is already happening, see 4.2.  Some of
 840          those changes are being predicted by the library maintainers, and
 841          we add code to the library based on what the current proposed
 842          resolution specifies.  Those additions are listed in
 843          <a href="../ext/howto.html#5">the extensions page</a>.
 844       </p></li>
 845       <li><p>Performance tuning.  Lots of performance tuning.  This too is
 846          already underway for post-3.0 releases, starting with memory
 847          expansion in container classes and buffer usage in synchronized
 848          stream objects.
 849       </p></li>
 850       <li><p>An ABI for libstdc++ is being developed, so that
 851          multiple binary-incompatible copies of the library can be replaced
 852          with a single backwards-compatible library, like libgcc_s.so is.
 853       </p></li>
 854       <li><p>The current libstdc++ contains extensions to the Library which
 855          must be explicitly requested by client code (for example, the
 856          hash tables from SGI).  Other extensions may be added to
 857          libstdc++-v3 if they seem to be &quot;standard&quot; enough.
 858          (For example, the &quot;long long&quot; type from C99.)
 859          Bugfixes and rewrites (to improve or fix thread safety, for
 860          instance) will of course be a continuing task.
 861       </p></li>
 862       </ol>
 863       <p><a href="http://gcc.gnu.org/ml/libstdc++/1999/msg00080.html">This
 864          question</a> about the next libstdc++ prompted some brief but
 865          interesting
 866          <a href="http://gcc.gnu.org/ml/libstdc++/1999/msg00084.html">speculation</a>.
 867       </p>
 868
 869 <hr />
 870    <h2><a name="5_3">5.3 What about the STL from SGI?</a></h2>
 871       <p>The <a href="http://www.sgi.com/Technology/STL/">STL from SGI</a>,
 872          version 3.3, was the most recent merge of the STL codebase.  The
 873          code in libstdc++ contains many fixes and changes, and it is
 874          very likely that the SGI code is no longer under active
 875          development.  We expect that no future merges will take place.
 876       </p>
 877       <p>In particular, <code>string</code> is not from SGI and makes no
 878          use of their &quot;rope&quot; class (which is included as an
 879          optional extension), nor is <code>valarray</code> and some others.
 880          Classes like <code>vector&lt;&gt;</code> are, however we have
 881          made significant changes to them since then.
 882       </p>
 883       <p>The FAQ for SGI's STL (one jump off of their main page) is
 884          recommended reading.
 885       </p>
 886
 887 <hr />
 888    <h2><a name="5_4">5.4 Extensions and Backward Compatibility</a></h2>
 889       <p>Headers in the <code>ext</code> and <code>backward</code>
 890          subdirectories should be referred to by their relative paths:
 891          <!-- Careful, the leading spaces in PRE show up directly. -->
 892       </p>
 893       <pre>
 894       #include &lt;ext/hash_map&gt; </pre>
 895       <p>rather than using <code>-I</code> or other options.  This is more
 896          portable and forward-compatible.  (The situation is the same as
 897          that of other headers whose directories are not searched directly,
 898          e.g., <code>&lt;sys/stat.h&gt;</code>, <code>&lt;X11/Xlib.h&gt;</code>.
 899       </p>
 900
 901       <p>The extensions are no longer in the global or <code>std</code>
 902          namespaces, instead they are declared in the <code>__gnu_cxx</code>
 903          namespace. For maximum portability, consider defining a namespace
 904          alias to use to talk about extensions, e.g.:
 905       </p>
 906       <pre>
 907       #ifdef __GNUC__
 908       #if __GNUC__ &lt; 3
 909         #include &lt;hash_map.h&gt;
 910         namespace Sgi { using ::hash_map; }; // inherit globals
 911       #else
 912         #include &lt;ext/hash_map&gt;
 913         #if __GNUC_MINOR__ == 0
 914           namespace Sgi = std;               // GCC 3.0
 915         #else
 916           namespace Sgi = ::__gnu_cxx;       // GCC 3.1 and later
 917         #endif
 918       #endif
 919       #else      // ...  there are other compilers, right?
 920         namespace Sgi = std;
 921       #endif
 922
 923       Sgi::hash_map&lt;int,int&gt; my_map; </pre>
 924       <p>This is a bit cleaner than defining typedefs for all the
 925          instantiations you might need.
 926       </p>
 927       <p>Extensions to the library have
 928          <a href="../ext/howto.html">their own page</a>.
 929       </p>
 930
 931 <hr />
 932    <h2><a name="5_5">5.5 [removed]</a></h2>
 933       <p>This question has become moot and has been removed.  The stub
 934          is here to preserve numbering (and hence links/bookmarks).
 935       </p>
 936
 937 <hr />
 938    <h2><a name="5_6">5.6 Is libstdc++-v3 thread-safe?</a></h2>
 939       <p>libstdc++-v3 strives to be thread-safe when all of the following
 940          conditions are met:
 941       </p>
 942       <ul>
 943        <li>The system's libc is itself thread-safe,</li>
 944        <li><code>gcc -v</code> reports a thread model other than 'single',</li>
 945        <li>[pre-3.3 only] a non-generic implementation of atomicity.h
 946            exists for the architecture in question.</li>
 947       </ul>
 948       <p>The user-code must guard against concurrent method calls which may
 949          access any particular library object's state.  Typically, the
 950          application programmer may infer what object locks must be held
 951          based on the objects referenced in a method call.  Without getting
 952          into great detail, here is an example which requires user-level
 953          locks:
 954       </p>
 955       <pre>
 956      library_class_a shared_object_a;
 957
 958      thread_main () {
 959        library_class_b *object_b = new library_class_b;
 960        shared_object_a.add_b (object_b);   // must hold lock for shared_object_a
 961        shared_object_a.mutate ();          // must hold lock for shared_object_a
 962      }
 963
 964      // Multiple copies of thread_main() are started in independent threads.</pre>
 965       <p>Under the assumption that object_a and object_b are never exposed to
 966          another thread, here is an example that should not require any
 967          user-level locks:
 968       </p>
 969       <pre>
 970      thread_main () {
 971        library_class_a object_a;
 972        library_class_b *object_b = new library_class_b;
 973        object_a.add_b (object_b);
 974        object_a.mutate ();
 975      } </pre>
 976       <p>All library objects are safe to use in a multithreaded program as
 977          long as each thread carefully locks out access by any other
 978          thread while it uses any object visible to another thread, i.e.,
 979          treat library objects like any other shared resource.  In general,
 980          this requirement includes both read and write access to objects;
 981          unless otherwise documented as safe, do not assume that two threads
 982          may access a shared standard library object at the same time.
 983       </p>
 984       <p>See chapters <a href="../17_intro/howto.html#3">17</a> (library
 985          introduction), <a href="../23_containers/howto.html#3">23</a>
 986          (containers), and <a href="../27_io/howto.html#9">27</a> (I/O) for
 987          more information.
 988       </p>
 989
 990 <hr />
 991    <h2><a name="5_7">5.7 How do I get a copy of the ISO C++ Standard?</a></h2>
 992       <p>Copies of the full ISO 14882 standard are available on line via the
 993          ISO mirror site for committee members.  Non-members, or those who
 994          have not paid for the privilege of sitting on the committee and
 995          sustained their two-meeting commitment for voting rights, may get a
 996          copy of the standard from their respective national standards
 997          organization.  In the USA, this national standards organization is
 998          ANSI and their website is right <a href="http://www.ansi.org">here</a>.
 999          (And if you've already registered with them, clicking this link will
1000          take you to directly to the place where you can
1001 <a href="http://webstore.ansi.org/ansidocstore/product.asp?sku=ISO%2FIEC+14882%2D1998">buy
1002          the standard on-line</a>.
1003       </p>
1004       <p>Who is your country's member body?  Visit the
1005          <a href="http://www.iso.ch/">ISO homepage</a> and find out!
1006       </p>
1007
1008 <hr />
1009    <h2><a name="5_8">5.8 What's an ABI and why is it so messy?</a></h2>
1010       <p>&quot;ABI&quot; stands for &quot;Application Binary Interface.&quot;
1011          Conventionally, it refers to a great mass of details about how
1012          arguments are arranged on the call stack and/or in registers, and
1013          how various types are arranged and padded in structs.  A single CPU
1014          design may suffer multiple ABIs designed by different development
1015          tool vendors who made different choices, or even by the same vendor
1016          for different target applications or compiler versions.  In ideal
1017          circumstances the CPU designer presents one ABI and all the OSes and
1018          compilers use it.  In practice every ABI omits details that compiler
1019          implementers (consciously or accidentally) must choose for themselves.
1020       </p>
1021       <p>That ABI definition suffices for compilers to generate code so a
1022          program can interact safely with an OS and its lowest-level libraries.
1023          Users usually want an ABI to encompass more detail, allowing libraries
1024          built with different compilers (or different releases of the same
1025          compiler!) to be linked together.  For C++, this includes many more
1026          details than for C, and CPU designers (for good reasons elaborated
1027          below) have not stepped up to publish C++ ABIs.  The details include
1028          virtual function implementation, struct inheritance layout, name
1029          mangling, and exception handling.  Such an ABI has been defined for
1030          GNU C++, and is immediately useful for embedded work relying only on
1031          a &quot;free-standing implementation&quot; that doesn't include (much
1032          of) the standard library.  It is a good basis for the work to come.
1033       </p>
1034       <p>A useful C++ ABI must also incorporate many details of the standard
1035          library implementation.  For a C ABI, the layouts of a few structs
1036          (such as FILE, stat, jmpbuf, and the like) and a few macros suffice.
1037          For C++, the details include the complete set of names of functions
1038          and types used, the offsets of class members and virtual functions,
1039          and the actual definitions of all inlines.  C++ exposes many more
1040          library details to the caller than C does.  It makes defining
1041          a complete ABI a much bigger undertaking, and requires not just
1042          documenting library implementation details, but carefully designing
1043          those details so that future bug fixes and optimizations don't
1044          force breaking the ABI.
1045       </p>
1046       <p>There are ways to help isolate library implementation details from the
1047          ABI, but they trade off against speed.  Library details used in
1048          inner loops (e.g., getchar) must be exposed and frozen for all
1049          time, but many others may reasonably be kept hidden from user code,
1050          so they may later be changed.  Deciding which, and implementing
1051          the decisions, must happen before you can reasonably document a
1052          candidate C++ ABI that encompasses the standard library.
1053       </p>
1054
1055 <!-- ####################################################### -->
1056
1057 <hr />
1058 <p class="fineprint"><em>
1059 See <a href="../17_intro/license.html">license.html</a> for copying conditions.
1060 Comments and suggestions are welcome, and may be sent to
1061 <a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
1062 </em></p>
1063
1064
1065 </body>
1066 </html>
1067