toolchain/gcc/bugs.html

   1 <html>
   2
   3 <head>
   4 <title>GCC Bugs</title>
   5 </head>
   6
   7 <body>
   8 <h1>GCC Bugs</h1>
   9
  10 <p>The latest version of this document is always available at
  11 <a href="http://gcc.gnu.org/bugs.html">http://gcc.gnu.org/bugs.html</a>.</p>
  12
  13 <hr />
  14
  15 <h2>Table of Contents</h2>
  16 <ul>
  17 <li><a href="#report">Reporting Bugs</a>
  18   <ul>
  19   <li><a href="#need">What we need</a></li>
  20   <li><a href="#dontwant">What we DON'T want</a></li>
  21   <li><a href="#where">Where to post it</a></li>
  22   <li><a href="#detailed">Detailed bug reporting instructions</a></li>
  23   <li><a href="#gnat">Detailed bug reporting instructions for GNAT</a></li>
  24   <li><a href="#pch">Detailed bug reporting instructions when using a precompiled header</a></li>
  25   </ul>
  26 </li>
  27 <li><a href="#known">Frequently Reported Bugs in GCC</a>
  28   <ul>
  29   <li><a href="#cxx">C++</a>
  30     <ul>
  31     <li><a href="#missing">Missing features</a></li>
  32     <li><a href="#fixed34">Bugs fixed in the 3.4 series</a></li>
  33     </ul>
  34   </li>
  35   <li><a href="#fortran">Fortran</a></li>
  36   </ul>
  37 </li>
  38 <li><a href="#nonbugs">Non-bugs</a>
  39   <ul>
  40   <li><a href="#nonbugs_general">General</a></li>
  41   <li><a href="#nonbugs_c">C</a></li>
  42   <li><a href="#nonbugs_cxx">C++</a>
  43     <ul>
  44     <li><a href="#upgrading">Common problems when upgrading the compiler</a></li>
  45     </ul>
  46   </li>
  47   </ul>
  48 </li>
  49 </ul>
  50
  51 <hr />
  52
  53 <h1><a name="report">Reporting Bugs</a></h1>
  54
  55 <p>The main purpose of a bug report is to enable us to fix the bug. The
  56 most important prerequisite for this is that the report must be complete and
  57 self-contained, which we explain in detail below.</p>
  58
  59 <p>Before you report a bug, please check the
  60 <a href="#known">list of well-known bugs</a> and, <strong>if possible
  61 in any way, try a current development snapshot</strong>.
  62 If you want to report a bug with versions of GCC before 3.1 we strongly
  63 recommend upgrading to the current release first.</p>
  64
  65 <p>Before reporting that GCC compiles your code incorrectly, please
  66 compile it with <code>gcc -Wall</code> and see whether this shows
  67 anything wrong with your code that could be the cause instead of a bug
  68 in GCC.</p>
  69
  70 <h2>Summarized bug reporting instructions</h2>
  71
  72 <p>After this summary, you'll find detailed bug reporting
  73 instructions, that explain how to obtain some of the information
  74 requested in this summary.</p>
  75
  76 <h3><a name="need">What we need</a></h3>
  77
  78 <p>Please include in your bug report all of the following items, the first
  79 three of which can be obtained from the output of <code>gcc -v</code>:</p>
  80
  81 <ul>
  82   <li>the exact version of GCC;</li>
  83   <li>the system type;</li>
  84   <li>the options given when GCC was configured/built;</li>
  85   <li>the complete command line that triggers the bug;</li>
  86   <li>the compiler output (error messages, warnings, etc.); and</li>
  87   <li>the <em>preprocessed</em> file (<code>*.i*</code>) that triggers the
  88   bug, generated by adding <code>-save-temps</code> to the complete
  89   compilation command, or, in the case of a bug report for the GNAT front end,
  90   a complete set of source files (see below).</li>
  91 </ul>
  92
  93 <h3><a name="dontwant">What we do <strong>not</strong> want</a></h3>
  94
  95 <ul>
  96   <li>A source file that <code>#include</code>s header files that are left
  97   out of the bug report (see above)</li>
  98
  99   <li>That source file and a collection of header files.</li>
 100
 101   <li>An attached archive (tar, zip, shar, whatever) containing all
 102   (or some :-) of the above.</li>
 103
 104   <li>A code snippet that won't cause the compiler to produce the
 105   exact output mentioned in the bug report (e.g., a snippet with just
 106   a few lines around the one that <b>apparently</b> triggers the bug,
 107   with some pieces replaced with ellipses or comments for extra
 108   obfuscation :-)</li>
 109
 110   <li>The location (URL) of the package that failed to build (we won't
 111   download it, anyway, since you've already given us what we need to
 112   duplicate the bug, haven't you? :-)</li>
 113
 114   <li>An error that occurs only some of the times a certain file is
 115   compiled, such that retrying a sufficient number of times results in
 116   a successful compilation; this is a symptom of a hardware problem,
 117   not of a compiler bug (sorry)</li>
 118
 119   <li>E-mail messages that complement previous, incomplete bug
 120   reports. Post a new, self-contained, full bug report instead, if
 121   possible as a follow-up to the original bug report</li>
 122
 123   <li>Assembly files (<code>*.s</code>) produced by the compiler, or any
 124   binary files, such as object files, executables, core files, or
 125   precompiled header files</li>
 126
 127   <li>Duplicate bug reports, or reports of bugs already fixed in the
 128   development tree, especially those that have already been reported
 129   as fixed last week :-)</li>
 130
 131   <li>Bugs in the assembler, the linker or the C library.  These are
 132   separate projects, with separate mailing lists and different bug
 133   reporting procedures</li>
 134
 135   <li>Bugs in releases or snapshots of GCC not issued by the GNU
 136   Project.  Report them to whoever provided you with the release</li>
 137
 138   <li>Questions about the correctness or the expected behavior of
 139   certain constructs that are not GCC extensions.  Ask them in forums
 140   dedicated to the discussion of the programming language</li>
 141 </ul>
 142
 143 <h3><a name="where">Where to post it</a></h3>
 144
 145 <p>Please submit your bug report directly to the
 146 <a href="http://gcc.gnu.org/bugzilla/">GCC bug database</a>.
 147 Alternatively, you can use the <code>gccbug</code> script that mails your bug
 148 report to the bug database.
 149 <br />
 150 Only if all this is absolutely impossible, mail all information to
 151 <a href="mailto:gcc-bugs@gcc.gnu.org">gcc-bugs@gcc.gnu.org</a>.</p>
 152
 153 <h2><a name="detailed">Detailed bug reporting instructions</a></h2>
 154
 155 <p>Please refer to the <a href="#gnat">next section</a> when reporting
 156 bugs in GNAT, the Ada compiler, or to the <a href="#pch">one after
 157 that</a> when reporting bugs that appear when using a precompiled header.</p>
 158
 159 <p>In general, all the information we need can be obtained by
 160 collecting the command line below, as well as its output and the
 161 preprocessed file it generates.</p>
 162
 163 <blockquote><p><code>gcc -v -save-temps <i>all-your-options
 164 source-file</i></code></p></blockquote>
 165
 166 <p>Typically the preprocessed file (extension <code>.i</code> for C or
 167 <code>.ii</code> for C++, and <code>.f</code> if the preprocessor is used on
 168 Fortran files) will be large, so please compress the
 169 resulting file with one of the popular compression programs such as
 170 bzip2, gzip, zip or compress (in
 171 decreasing order of preference).  Use maximum compression
 172 (<code>-9</code>) if available.  Please include the compressed
 173 preprocessor output in your bug report, even if the source code is
 174 freely available elsewhere; it makes the job of our volunteer testers
 175 much easier.</p>
 176
 177 <p>The <b>only</b> excuses to not send us the preprocessed sources are
 178 (i) if you've found a bug in the preprocessor, (ii) if you've reduced
 179 the testcase to a small file that doesn't include any other file or
 180 (iii) if the bug appears only when using precompiled headers.  If you
 181 can't post the preprocessed sources because they're proprietary code,
 182 then try to create a small file that triggers the same problem.</p>
 183
 184 <p>Since we're supposed to be able to re-create the assembly output
 185 (extension <code>.s</code>), you usually should not include
 186 it in the bug report, although you may want to post parts of it to
 187 point out assembly code you consider to be wrong.</p>
 188
 189 <p>Whether to use MIME attachments or <code>uuencode</code> is up to
 190 you.  In any case, make sure the compiler command line, version and
 191 error output are in plain text, so that we don't have to decode the
 192 bug report in order to tell who should take care of it.  A meaningful
 193 subject indicating language and platform also helps.</p>
 194
 195 <p>Please avoid posting an archive (.tar, .shar or .zip); we generally
 196 need just a single file to reproduce the bug (the .i/.ii/.f preprocessed
 197 file), and, by storing it in an archive, you're just making our
 198 volunteers' jobs harder.  Only when your bug report requires multiple
 199 source files to be reproduced should you use an archive.  This is, for example,
 200 the case if you are using <code>INCLUDE</code> directives in Fortran code,
 201 which are not processed by the preprocessor, but the compiler.  In that case,
 202 we need the main file and all <code>INCLUDE</code>d files.  In any case,
 203 make sure the compiler version, error message, etc, are included in
 204 the body of your bug report as plain text, even if needlessly
 205 duplicated as part of an archive.</p>
 206
 207 <p>If you fail to supply enough information for a bug report to be
 208 reproduced, someone will probably ask you to post additional
 209 information (or just ignore your bug report, if they're in a bad day,
 210 so try to get it right on the first posting :-).  In this case, please
 211 post the additional information to the bug reporting mailing list, not
 212 just to the person who requested it, unless explicitly told so.  If
 213 possible, please include in this follow-up all the information you had
 214 supplied in the incomplete bug report (including the preprocessor
 215 output), so that the new bug report is self-contained.</p>
 216
 217 <h2><a name="gnat">Detailed bug reporting instructions for GNAT</a></h2>
 218
 219 <p>See the <a href="#detailed">previous section</a> for bug reporting
 220 instructions for GCC language implementations other than Ada.</p>
 221
 222 <p>Bug reports have to contain at least the following information in
 223 order to be useful:</p>
 224
 225 <ul>
 226 <li>the exact version of GCC, as shown by "<code>gcc -v</code>";</li>
 227 <li>the system type;</li>
 228 <li>the options when GCC was configured/built;</li>
 229 <li>the exact command line passed to the <code>gcc</code> program
 230 triggering the bug
 231 (not just the flags passed to <code>gnatmake</code>, but
 232 <code>gnatmake</code> prints the parameters it passed to <code>gcc</code>)</li>
 233 <li>a collection of source files for reproducing the bug,
 234 preferably a minimal set (see below);</li>
 235 <li>a description of the expected behavior;</li>
 236 <li>a description of actual behavior.</li>
 237 </ul>
 238
 239 <p>If your code depends on additional source files (usually package
 240 specifications), submit the source code for these compilation units in
 241 a single file that is acceptable input to <code>gnatchop</code>,
 242 i.e. contains no non-Ada text.  If the compilation terminated
 243 normally, you can usually obtain a list of dependencies using the
 244 "<code>gnatls -d <i>main_unit</i></code>" command, where
 245 <code><i>main_unit</i></code> is the file name of the main compilation
 246 unit (which is also passed to <code>gcc</code>).</p>
 247
 248 <p>If you report a bug which causes the compiler to print a bug box,
 249 include that bug box in your report, and do not forget to send all the
 250 source files listed after the bug box along with your report.</p>
 251
 252 <p>If you use <code>gnatprep</code>, be sure to send in preprocessed
 253 sources (unless you have to report a bug in <code>gnatprep</code>).</p>
 254
 255 <p>When you have checked that your report meets these criteria, please
 256 submit it according to our <a href="#where">generic instructions</a>.
 257 (If you use a mailing list for reporting, please include an
 258 "<code>[Ada]</code>" tag in the subject.)</p>
 259
 260 <h2><a name="pch">Detailed bug reporting instructions when using a
 261 precompiled header</a></h2>
 262
 263 <p>If you're encountering a bug when using a precompiled header, the
 264 first thing to do is to delete the precompiled header, and try running
 265 the same GCC command again.  If the bug happens again, the bug doesn't
 266 really involve precompiled headers, please report it without using
 267 them by following the instructions <a href="#detailed">above</a>.</p>
 268
 269 <p>If you've found a bug while <i>building</i> a precompiled header
 270 (for instance, the compiler crashes), follow the usual instructions
 271 <a href="#detailed">above</a>.</p>
 272
 273 <p>If you've found a real precompiled header bug, what we'll need to
 274 reproduce it is the sources to build the precompiled header (as a
 275 single <code>.i</code> file), the source file that uses the
 276 precompiled header, any other headers that source file includes, and
 277 the command lines that you used to build the precompiled header and to
 278 use it.</p>
 279
 280 <p>Please <strong>don't</strong> send us the actual precompiled
 281 header.  It is likely to be very large and we can't use it to
 282 reproduce the problem.</p>
 283
 284 <hr />
 285
 286 <h1><a name="known">Frequently Reported Bugs in GCC</a></h1>
 287
 288 <p>This is a list of bugs in GCC that are reported very often, but not
 289 yet fixed. While it is certainly better to fix bugs instead of documenting
 290 them, this document might save people the effort of writing a bug report
 291 when the bug is already well-known.</p>
 292
 293 <p>There are many reasons why a reported bug doesn't get fixed.
 294 It might be difficult to fix, or fixing it might break compatibility.
 295 Often, reports get a low priority when there is a simple work-around.
 296 In particular, bugs caused by invalid code have a simple work-around:
 297 <em>fix the code</em>.</p>
 298
 299 <hr />
 300
 301 <h2><a name="cxx">C++</a></h2>
 302
 303 <h3><a name="missing">Missing features</a></h3>
 304
 305 <dl>
 306
 307 <dt>The <code>export</code> keyword is not implemented.</dt>
 308 <dd><p>Most C++ compilers (G++ included) do not yet implement
 309 <code>export</code>, which is necessary for separate compilation of
 310 template declarations and definitions. Without <code>export</code>, a
 311 template definition must be in scope to be used. The obvious
 312 workaround is simply to place all definitions in the header
 313 itself. Alternatively, the compilation unit containing template
 314 definitions may be included from the header.</p></dd>
 315
 316 </dl>
 317
 318 <h3><a name="fixed34">Bugs fixed in the 3.4 series</a></h3>
 319
 320 <p>The following bugs are present up to (and including) GCC 3.3.x.
 321 They have been fixed in 3.4.0.</p>
 322
 323 <dl>
 324
 325 <dt>Two-stage name-lookup.</dt>
 326
 327 <dd><p>GCC did not implement two-stage name-lookup (also see
 328 <a href="#new34">below</a>).</p></dd>
 329
 330 <dt>Covariant return types.</dt>
 331
 332 <dd><p>GCC did not implement non-trivial covariant returns.</p></dd>
 333
 334 <dt>Parse errors for "simple" code.</dt>
 335
 336 <dd><p>GCC gave parse errors for seemingly simple code, such as</p>
 337
 338 <blockquote><pre>
 339 struct A
 340 {
 341   A();
 342   A(int);
 343 };
 344
 345 struct B
 346 {
 347   B(A);
 348   B(A,A);
 349   void foo();
 350 };
 351
 352 A bar()
 353 {
 354   B b(A(),A(1));  // Variable b, initialized with two temporaries
 355   B(A(2)).foo();  // B temporary, initialized with A temporary
 356   return (A());   // return A temporary
 357 }
 358 </pre></blockquote>
 359
 360 <p>Although being valid code, each of the three lines with a comment was
 361 rejected by GCC.  The work-arounds for older compiler versions proposed
 362 below do not change the semantics of the programs at all.</p>
 363
 364 <p>The problem in the first case was that GCC started to parse the
 365 declaration of <code>b</code> as a function called <code>b</code> returning
 366 <code>B</code>, taking a function returning <code>A</code> as an argument.
 367 When it encountered the <code>1</code>, it was too late.  To show the
 368 compiler that this should be really an expression, a comma operator with
 369 a dummy argument could be used:</p>
 370
 371 <blockquote><pre>
 372 B b((0,A()),A(1));
 373 </pre></blockquote>
 374
 375 <p>The work-around for simpler cases like the second one was to add
 376 additional parentheses around the expressions that were mistaken as
 377 declarations:</p>
 378
 379 <blockquote><pre>
 380 (B(A(2))).foo();
 381 </pre></blockquote>
 382
 383 <p>In the third case, however, additional parentheses were causing
 384 the problems: The compiler interpreted <code>A()</code> as a function
 385 (taking no arguments, returning <code>A</code>), and <code>(A())</code>
 386 as a cast lacking an expression to be casted, hence the parse error.
 387 The work-around was to omit the parentheses:</p>
 388
 389 <blockquote><pre>
 390 return A();
 391 </pre></blockquote>
 392
 393 <p>This problem occured in a number of variants; in <code>throw</code>
 394 statements, people also frequently put the object in parentheses.</p></dd>
 395
 396 </dl>
 397
 398 <hr />
 399
 400 <h2><a name="fortran">Fortran</a></h2>
 401
 402 <p>Fortran bugs are documented in the G77 manual rather than
 403 explicitly listed here.  Please see
 404 <a href="http://gcc.gnu.org/onlinedocs/g77/Trouble.html">Known Causes of
 405 Trouble with GNU Fortran</a> in the G77 manual.</p>
 406
 407 <hr />
 408
 409 <h1><a name="nonbugs">Non-bugs</a></h1>
 410
 411 <p>The following are not actually bugs, but are reported often
 412 enough to warrant a mention here.</p>
 413
 414 <p>It is not always a bug in the compiler, if code which "worked" in a
 415 previous version, is now rejected.  Earlier versions of GCC sometimes were
 416 less picky about standard conformance and accepted invalid source code.
 417 In addition, programming languages themselves change, rendering code
 418 invalid that used to be conforming (this holds especially for C++).
 419 In either case, you should update your code to match recent language
 420 standards.</p>
 421
 422 <hr />
 423
 424 <h2><a name="nonbugs_general">General</a></h2>
 425
 426 <dl>
 427 <dt>Problems with floating point numbers - the
 428 <a href="http://gcc.gnu.org/PR323">most often reported non-bug</a>.</dt>
 429 <dd><p>In a number of cases, GCC appears to perform floating point
 430 computations incorrectly. For example, the C++ program</p>
 431 <blockquote><pre>
 432 #include &lt;iostream&gt;
 433
 434 int main()
 435 {
 436   double a = 0.5;
 437   double b = 0.01;
 438   std::cout &lt;&lt; (int)(a / b) &lt;&lt; std::endl;
 439   return 0;
 440 }
 441 </pre></blockquote>
 442 <p>might print 50 on some systems and optimization levels, and 49 on
 443 others.</p>
 444
 445 <p>This is the result of <em>rounding</em>: The computer cannot
 446 represent all real numbers exactly, so it has to use
 447 approximations. When computing with approximation, the computer needs
 448 to round to the nearest representable number.</p>
 449
 450 <p>This is not a bug in the compiler, but an inherent limitation of
 451 the floating point types. Please study
 452 <a href="http://www.validlab.com/goldberg/paper.ps">this paper</a>
 453 for more information.</p></dd>
 454 </dl>
 455
 456 <hr />
 457
 458 <h2><a name="nonbugs_c">C</a></h2>
 459
 460 <dl>
 461 <dt>Increment/decrement operator (<code>++</code>/<code>--</code>) not
 462 working as expected - a <a href="http://gcc.gnu.org/PR11751">problem with
 463 many variations</a>.</dt>
 464
 465 <dd><p>The following expressions have unpredictable results:</p>
 466 <blockquote><pre>
 467 x[i]=++i
 468 foo(i,++i)
 469 i*(++i)                 /* special case with foo=="operator*" */
 470 std::cout &lt;&lt; i &lt;&lt; ++i   /* foo(foo(std::cout,i),++i)          */
 471 </pre></blockquote>
 472 <p>since the <code>i</code> without increment can be evaluated before or
 473 after <code>++i</code>.</p>
 474
 475 <p>The C and C++ standards have the notion of "sequence points". Everything
 476 that happens between two sequence points happens in an unspecified order,
 477 but it has to happen after the first and before the second sequence point.
 478 The end of a statement and a function call are examples for sequence points,
 479 whereas assignments and the comma between function arguments are not.</p>
 480
 481 <p>Modifying a value twice between two sequence points as shown in the
 482 following examples is even worse:</p>
 483 <blockquote><pre>
 484 i=++i
 485 foo(++i,++i)
 486 (++i)*(++i)               /* special case with foo=="operator*" */
 487 std::cout &lt;&lt; ++i &lt;&lt; ++i   /* foo(foo(std::cout,++i),++i)        */
 488 </pre></blockquote>
 489 <p>This leads to undefined behavior (i.e. the compiler can do
 490 anything).</p></dd>
 491
 492
 493 <dt>Casting does not work as expected when optimization is turned on.</dt>
 494
 495 <dd><p>This is often caused by a violation of aliasing rules, which are part
 496 of the ISO C standard.  These rules say that a program is invalid if you try
 497 to access a variable through a pointer of an incompatible type.  This is
 498 happening in the following example where a short is accessed through a
 499 pointer to integer (the code assumes 16-bit <code>short</code>s and 32-bit
 500 <code>int</code>s):</p>
 501 <blockquote><pre>
 502 #include &lt;stdio.h&gt;
 503
 504 int main()
 505 {
 506   short a[2];
 507
 508   a[0]=0x1111;
 509   a[1]=0x1111;
 510
 511   *(int *)a = 0x22222222; /* violation of aliasing rules */
 512
 513   printf("%x %x\n", a[0], a[1]);
 514   return 0;
 515 }
 516 </pre></blockquote>
 517 <p>The aliasing rules were designed to allow compilers more aggressive
 518 optimization.  Basically, a compiler can assume that all changes to variables
 519 happen through pointers or references to variables of a type compatible to
 520 the accessed variable.  Dereferencing a pointer that violates the aliasing
 521 rules results in undefined behavior.</p>
 522
 523 <p>In the case above, the compiler may assume that no access through an
 524 integer pointer can change the array <code>a</code>, consisting of shorts.
 525 Thus, <code>printf</code> may be called with the original values of
 526 <code>a[0]</code> and <code>a[1]</code>.  What really happens is up to
 527 the compiler and may change with architecture and optimization level.</p>
 528
 529 <p>Recent versions of GCC turn on the option <code>-fstrict-aliasing</code>
 530 (which allows alias-based optimizations) by default with <code>-O2</code>.
 531 And some architectures then really print "1111 1111" as result.  Without
 532 optimization the executable will generate the "expected" output
 533 "2222 2222".</p>
 534
 535 <p>To disable optimizations based on alias-analysis for faulty legacy code,
 536 the option <code>-fno-strict-aliasing</code> can be used as a work-around.</p>
 537
 538 <p>The option <code>-Wstrict-aliasing</code> (which is included in
 539 <code>-Wall</code>) warns about some - but not all - cases of violation
 540 of aliasing rules when <code>-fstrict-aliasing</code> is active.</p>
 541
 542 <p>To fix the code above, you can use a <code>union</code> instead of a
 543 cast (note that this is a GCC extension which might not work with other
 544 compilers):</p>
 545 <blockquote><pre>
 546 #include &lt;stdio.h&gt;
 547
 548 int main()
 549 {
 550   union
 551   {
 552     short a[2];
 553     int i;
 554   } u;
 555
 556   u.a[0]=0x1111;
 557   u.a[1]=0x1111;
 558
 559   u.i = 0x22222222;
 560
 561   printf("%x %x\n", u.a[0], u.a[1]);
 562   return 0;
 563 }
 564 </pre></blockquote>
 565 <p>Now the result will always be "2222 2222".</p>
 566
 567 <p>For some more insight into the subject, please have a look at
 568 <a href="http://mail-index.NetBSD.org/tech-kern/2003/08/11/0001.html">this
 569 article</a>.</p></dd>
 570
 571
 572 <dt>Cannot use preprocessor directive in macro arguments.</dt>
 573 <dd><p>Let me guess... you used an older version of GCC to compile code
 574 that looks something like this:</p>
 575 <blockquote><pre>
 576   memcpy(dest, src,
 577 #ifdef PLATFORM1
 578          12
 579 #else
 580          24
 581 #endif
 582         );
 583 </pre></blockquote>
 584 <p>and you got a whole pile of error messages:</p>
 585 <blockquote><pre>
 586 test.c:11: warning: preprocessing directive not recognized within macro arg
 587 test.c:11: warning: preprocessing directive not recognized within macro arg
 588 test.c:11: warning: preprocessing directive not recognized within macro arg
 589 test.c: In function `foo':
 590 test.c:6: undefined or invalid # directive
 591 test.c:8: undefined or invalid # directive
 592 test.c:9: parse error before `24'
 593 test.c:10: undefined or invalid # directive
 594 </pre></blockquote>
 595
 596 <p>This is because your C library's <code>&lt;string.h&gt;</code> happens
 597 to define <code>memcpy</code> as a macro - which is perfectly legitimate.
 598 In recent versions of glibc, for example, <code>printf</code> is among those
 599 functions which are implemented as macros.</p>
 600
 601 <p>Versions of GCC prior to 3.3 did not allow you to put <code>#ifdef</code>
 602 (or any other preprocessor directive) inside the arguments of a macro.  The
 603 code therefore would not compile.</p>
 604
 605 <p>As of GCC 3.3 this kind of construct is always accepted and the
 606 preprocessor will probably do what you expect, but see the manual for
 607 detailed semantics.</p>
 608
 609 <p>However, this kind of code is not portable.  It is "undefined behavior"
 610 according to the C standard; that means different compilers may do
 611 different things with it.  It is always possible to rewrite code which
 612 uses conditionals inside macros so that it doesn't.  You could write
 613 the above example</p>
 614 <blockquote><pre>
 615 #ifdef PLATFORM1
 616    memcpy(dest, src, 12);
 617 #else
 618    memcpy(dest, src, 24);
 619 #endif
 620 </pre></blockquote>
 621 <p>This is a bit more typing, but I personally think it's better style
 622 in addition to being more portable.</p></dd>
 623
 624
 625 <dt>Cannot initialize a static variable with <code>stdin</code>.</dt>
 626 <dd><p>This has nothing to do with GCC, but people ask us about it a
 627 lot.  Code like this:</p>
 628
 629 <blockquote><pre>
 630 #include &lt;stdio.h&gt;
 631
 632 FILE *yyin = stdin;
 633 </pre></blockquote>
 634
 635 <p>will not compile with GNU libc, because <code>stdin</code> is not a
 636 constant.  This was done deliberately, to make it easier to maintain
 637 binary compatibility when the type <code>FILE</code> needs to be changed.
 638 It is surprising for people used to traditional Unix C libraries, but it
 639 is permitted by the C standard.</p>
 640
 641 <p>This construct commonly occurs in code generated by old versions of
 642 lex or yacc.  We suggest you try regenerating the parser with a
 643 current version of flex or bison, respectively.  In your own code, the
 644 appropriate fix is to move the initialization to the beginning of
 645 main.</p>
 646
 647 <p>There is a common misconception that the GCC developers are
 648 responsible for GNU libc.  These are in fact two entirely separate
 649 projects; please check the
 650 <a href="http://www.gnu.org/software/libc/">GNU libc web pages</a>
 651 for details.
 652 </p></dd>
 653 </dl>
 654
 655 <hr />
 656
 657 <h2><a name="nonbugs_cxx">C++</a></h2>
 658
 659 <dl>
 660 <dt>Nested classes can access private members and types of the containing
 661 class.</dt>
 662
 663 <dd><p>Defect report 45 clarifies that nested classes are members of the
 664 class they are nested in, and so are granted access to private members of
 665 that class.</p></dd>
 666
 667 <dt>G++ emits two copies of constructors and destructors.</dt>
 668
 669 <dd><p>In general there are <em>three</em> types of constructors (and
 670 destructors).</p>
 671 <ol>
 672 <li>The complete object constructor/destructor.</li>
 673 <li>The base object constructor/destructor.</li>
 674 <li>The allocating constructor/deallocating destructor.</li>
 675 </ol>
 676 <p>The first two are different, when virtual base classes are involved.
 677 </p></dd>
 678
 679 <dt>Global destructors are not run in the correct order.</dt>
 680
 681 <dd><p>Global destructors should be run in the reverse order of their
 682 constructors <em>completing</em>. In most cases this is the same as
 683 the reverse order of constructors <em>starting</em>, but sometimes it
 684 is different, and that is important. You need to compile and link your
 685 programs with <code>--use-cxa-atexit</code>. We have not turned this
 686 switch on by default, as it requires a <code>cxa</code> aware runtime
 687 library (<code>libc</code>, <code>glibc</code>, or equivalent).</p></dd>
 688
 689 <dt>Classes in exception specifiers must be complete types.</dt>
 690
 691 <dd><p>[15.4]/1 tells you that you cannot have an incomplete type, or
 692 pointer to incomplete (other than <code><i>cv</i> void *</code>) in
 693 an exception specification.</p></dd>
 694
 695 <dt>Exceptions don't work in multithreaded applications.</dt>
 696
 697 <dd><p>You need to rebuild g++ and libstdc++ with
 698 <code>--enable-threads</code>.  Remember, C++ exceptions are not like
 699 hardware interrupts. You cannot throw an exception in one thread and
 700 catch it in another. You cannot throw an exception from a signal
 701 handler and catch it in the main thread.</p></dd>
 702
 703 <dt>Templates, scoping, and digraphs.</dt>
 704
 705 <dd><p>If you have a class in the global namespace, say named <code>X</code>,
 706 and want to give it as a template argument to some other class, say
 707 <code>std::vector</code>, then <code>std::vector&lt;::X&gt;</code>
 708 fails with a parser error.</p>
 709
 710 <p>The reason is that the standard mandates that the sequence
 711 <code>&lt;:</code> is treated as if it were the token <code>[</code>.
 712 (There are several such combinations of characters - they are called
 713 <em>digraphs</em>.) Depending on the version, the compiler then reports
 714 a parse error before the character <code>:</code> (the colon before
 715 <code>X</code>) or a missing closing bracket <code>]</code>.</p>
 716
 717 <p>The simplest way to avoid this is to write <code>std::vector&lt;
 718 ::X&gt;</code>, i.e. place a space between the opening angle bracket
 719 and the scope operator.</p></dd>
 720
 721
 722 <dt><a name="cxx_rvalbind">Copy constructor access check while
 723 initializing a reference.</a></dt>
 724
 725 <dd><p>Consider this code:</p>
 726
 727 <blockquote><pre>
 728 class A
 729 {
 730 public:
 731   A();
 732
 733 private:
 734   A(const A&amp;);   // private copy ctor
 735 };
 736
 737 A makeA(void);
 738 void foo(const A&amp;);
 739
 740 void bar(void)
 741 {
 742   foo(A());       // error, copy ctor is not accessible
 743   foo(makeA());   // error, copy ctor is not accessible
 744
 745   A a1;
 746   foo(a1);        // OK, a1 is a lvalue
 747 }</pre></blockquote>
 748
 749 <p>Starting with GCC 3.4.0, binding an rvalue to a const reference requires
 750 an accessible copy constructor. This might be surprising at first sight,
 751 especially since most popular compilers do not correctly implement this
 752 rule.</p>
 753
 754 <p>The C++ Standard says that a temporary object should be created in
 755 this context and its contents filled with a copy of the object we are
 756 trying to bind to the reference; it also says that the temporary copy
 757 can be elided, but the semantic constraints (eg. accessibility) of the
 758 copy constructor still have to be checked.</p>
 759
 760 <p>For further information, you can consult the following paragraphs of
 761 the C++ standard: [dcl.init.ref]/5, bullet 2, sub-bullet 1, and
 762 [class.temporary]/2.</p></dd>
 763 </dl>
 764
 765 <h3><a name="upgrading">Common problems when upgrading the compiler</a></h3>
 766
 767 <h4>ABI changes</h4>
 768
 769 <p>The C++ application binary interface (ABI) consists of two
 770 components: the first defines how the elements of classes are laid
 771 out, how functions are called, how function names are mangled, etc;
 772 the second part deals with the internals of the objects in libstdc++.
 773 Although we strive for a non-changing ABI, so far we have had to
 774 modify it with each major release.  If you change your compiler to a
 775 different major release <em>you must recompile all libraries that
 776 contain C++ code</em>.  If you fail to do so you risk getting linker
 777 errors or malfunctioning programs.  Some of our Java support libraries
 778 also contain C++ code, so you might want to recompile all libraries to
 779 be safe.  It should not be necessary to recompile if you have changed
 780 to a bug-fix release of the same version of the compiler; bug-fix
 781 releases are careful to avoid ABI changes. See also the
 782 <a href="onlinedocs/gcc/Compatibility.html">compatibility section</a>
 783 of the GCC manual.</p>
 784
 785 <p>Remark: A major release is designated by a change to the first or second
 786 component of the two- or three-part version number.  A minor (bug-fix)
 787 release is designated by a change to the third component only.  Thus GCC
 788 3.2 and 3.3 are major releases, while 3.3.1 and 3.3.2 are bug-fix releases
 789 for GCC 3.3.  With the 3.4 series we are introducing a new naming scheme;
 790 the first release of this series is 3.4.0 instead of just 3.4.</p>
 791
 792 <h4>Standard conformance</h4>
 793
 794 <p>With each release, we try to make G++ conform closer to the ISO C++ standard
 795 (available at
 796 <a href="http://www.ncits.org/cplusplus.htm">http://www.ncits.org/cplusplus.htm</a>).
 797 We have also implemented some of the core and library defect reports
 798 (available at
 799 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html">http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html</a>
 800 &amp;
 801 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html">http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html</a>
 802 respectively).</p>
 803
 804 <p>Non-conforming legacy code that worked with older versions of GCC may be
 805 rejected by more recent compilers.  There is no command-line switch to ensure
 806 compatibility in general, because trying to parse standard-conforming and
 807 old-style code at the same time would render the C++ frontend unmaintainable.
 808 However, some non-conforming constructs are allowed when the command-line
 809 option <code>-fpermissive</code> is used.</p>
 810
 811 <p>Two milestones in standard conformance are GCC 3.0 (including a major
 812 overhaul of the standard library) and the 3.4.0 version (with its new C++
 813 parser).</p>
 814
 815 <h4>New in GCC 3.0</h4>
 816
 817 <ul>
 818
 819 <li>The standard library is much more conformant, and uses the
 820 <code>std::</code> namespace (which is now a real namespace, not an
 821 alias for <code>::</code>).</li>
 822
 823 <li>The standard header files for the c library don't end with
 824 <code>.h</code>, but begin with <code>c</code> (i.e.
 825 <code>&lt;cstdlib&gt;</code> rather than <code>&lt;stdlib.h&gt;</code>).
 826 The <code>.h</code> names are still available, but are deprecated.</li>
 827
 828 <li><code>&lt;strstream&gt;</code> is deprecated, use
 829 <code>&lt;sstream&gt;</code> instead.</li>
 830
 831 <li><code>streambuf::seekoff</code> &amp;
 832 <code>streambuf::seekpos</code> are private, instead use
 833 <code>streambuf::pubseekoff</code> &amp;
 834 <code>streambuf::pubseekpos</code> respectively.</li>
 835
 836 <li>If <code>std::operator &lt;&lt; (std::ostream &amp;, long long)</code>
 837 doesn't exist, you need to recompile libstdc++ with
 838 <code>--enable-long-long</code>.</li>
 839
 840 </ul>
 841
 842 <p>If you get lots of errors about things like <code>cout</code> not being
 843 found, you've most likely forgotten to tell the compiler to look in the
 844 <code>std::</code> namespace.  There are several ways to do this:</p>
 845
 846 <ul>
 847
 848 <li>Say <code>std::cout</code> at the call.  This is the most explicit
 849 way of saying what you mean.</li>
 850
 851 <li>Say <code>using std::cout;</code> somewhere before the call.  You
 852 will need to do this for each function or type you wish to use from the
 853 standard library.</li>
 854
 855 <li>Say <code>using namespace std;</code> somewhere before the call.
 856 This is the quick-but-dirty fix. This brings the <em>whole</em> of the
 857 <code>std::</code> namespace into scope.  <em>Never</em> do this in a
 858 header file, as every user of your header file will be affected by this
 859 decision.</li>
 860
 861 </ul>
 862
 863 <h4><a name="new34">New in GCC 3.4.0</a></h4>
 864
 865 <p>The new parser brings a lot of improvements, especially concerning
 866 name-lookup.</p>
 867
 868 <ul>
 869
 870 <li>The "implicit typename" extension got removed (it was already deprecated
 871 since GCC 3.1), so that the following code is now rejected, see [14.6]:
 872 <blockquote><pre>
 873 template &lt;typename&gt; struct A
 874 {
 875     typedef int X;
 876 };
 877
 878 template &lt;typename T&gt; struct B
 879 {
 880     A&lt;T&gt;::X          x;  // error
 881     typename A&lt;T&gt;::X y;  // OK
 882 };
 883
 884 B&lt;void&gt; b;
 885 </pre></blockquote></li>
 886
 887 <li>For similar reasons, the following code now requires the
 888 <code>template</code> keyword, see [14.2]:
 889 <blockquote><pre>
 890 template &lt;typename&gt; struct A
 891 {
 892     template &lt;int&gt; struct X {};
 893 };
 894
 895 template &lt;typename T&gt; struct B
 896 {
 897     typename A&lt;T&gt;::X&lt;0&gt;          x;  // error
 898     typename A&lt;T&gt;::template X&lt;0&gt; y;  // OK
 899 };
 900
 901 B&lt;void&gt; b;
 902 </pre></blockquote></li>
 903
 904 <li>We now have two-stage name-lookup, so that the following code is
 905 rejected, see [14.6]/9:
 906 <blockquote><pre>
 907 template &lt;typename T&gt; int foo()
 908 {
 909     return i;  // error
 910 }
 911 </pre></blockquote></li>
 912
 913 <li>This also affects members of base classes, see [14.6.2]:
 914 <blockquote><pre>
 915 template &lt;typename&gt; struct A
 916 {
 917     int i, j;
 918 };
 919
 920 template &lt;typename T&gt; struct B : A&lt;T&gt;
 921 {
 922     int foo1() { return i; }       // error
 923     int foo2() { return this-&gt;i; } // OK
 924     int foo3() { return B&lt;T&gt;::i; } // OK
 925     int foo4() { return A&lt;T&gt;::i; } // OK
 926
 927     using A&lt;T&gt;::j;
 928     int foo5() { return j; }       // OK
 929 };
 930 </pre></blockquote></li>
 931
 932 </ul>
 933
 934 <p>In addition to the problems listed above, the manual contains a section on
 935 <a href="http://gcc.gnu.org/onlinedocs/gcc/C---Misunderstandings.html">Common
 936 Misunderstandings with GNU C++</a>.</p>
 937
 938 </body>
 939 </html>