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
  57 and self-contained.</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 try a current development snapshot</strong>.
  62 If you want to report a bug with versions of GCC before 3.4 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>Assembly files (<code>*.s</code>) produced by the compiler, or any
 120   binary files, such as object files, executables, core files, or
 121   precompiled header files</li>
 122
 123   <li>Duplicate bug reports, or reports of bugs already fixed in the
 124   development tree, especially those that have already been reported
 125   as fixed last week :-)</li>
 126
 127   <li>Bugs in the assembler, the linker or the C library.  These are
 128   separate projects, with separate mailing lists and different bug
 129   reporting procedures</li>
 130
 131   <li>Bugs in releases or snapshots of GCC not issued by the GNU
 132   Project.  Report them to whoever provided you with the release</li>
 133
 134   <li>Questions about the correctness or the expected behavior of
 135   certain constructs that are not GCC extensions.  Ask them in forums
 136   dedicated to the discussion of the programming language</li>
 137 </ul>
 138
 139 <h3><a name="where">Where to post it</a></h3>
 140
 141 <p>Please submit your bug report directly to the
 142 <a href="http://gcc.gnu.org/bugzilla/">GCC bug database</a>.
 143 Alternatively, you can use the <code>gccbug</code> script that mails your bug
 144 report to the bug database.
 145 <br />
 146 Only if all this is absolutely impossible, mail all information to
 147 <a href="mailto:gcc-bugs@gcc.gnu.org">gcc-bugs@gcc.gnu.org</a>.</p>
 148
 149 <h2><a name="detailed">Detailed bug reporting instructions</a></h2>
 150
 151 <p>Please refer to the <a href="#gnat">next section</a> when reporting
 152 bugs in GNAT, the Ada compiler, or to the <a href="#pch">one after
 153 that</a> when reporting bugs that appear when using a precompiled header.</p>
 154
 155 <p>In general, all the information we need can be obtained by
 156 collecting the command line below, as well as its output and the
 157 preprocessed file it generates.</p>
 158
 159 <blockquote><p><code>gcc -v -save-temps <i>all-your-options
 160 source-file</i></code></p></blockquote>
 161
 162 <p>The <b>only</b> excuses to not send us the preprocessed sources are
 163 (i) if you've found a bug in the preprocessor, (ii) if you've reduced
 164 the testcase to a small file that doesn't include any other file or
 165 (iii) if the bug appears only when using precompiled headers.  If you
 166 can't post the preprocessed sources because they're proprietary code,
 167 then try to create a small file that triggers the same problem.</p>
 168
 169 <p>Since we're supposed to be able to re-create the assembly output
 170 (extension <code>.s</code>), you usually should not include
 171 it in the bug report, although you may want to post parts of it to
 172 point out assembly code you consider to be wrong.</p>
 173
 174 <p>Please avoid posting an archive (.tar, .shar or .zip); we generally
 175 need just a single file to reproduce the bug (the .i/.ii/.f preprocessed
 176 file), and, by storing it in an archive, you're just making our
 177 volunteers' jobs harder.  Only when your bug report requires multiple
 178 source files to be reproduced should you use an archive.  This is, for example,
 179 the case if you are using <code>INCLUDE</code> directives in Fortran code,
 180 which are not processed by the preprocessor, but the compiler.  In that case,
 181 we need the main file and all <code>INCLUDE</code>d files.  In any case,
 182 make sure the compiler version, error message, etc, are included in
 183 the body of your bug report as plain text, even if needlessly
 184 duplicated as part of an archive.</p>
 185
 186 <h2><a name="gnat">Detailed bug reporting instructions for GNAT</a></h2>
 187
 188 <p>See the <a href="#detailed">previous section</a> for bug reporting
 189 instructions for GCC language implementations other than Ada.</p>
 190
 191 <p>Bug reports have to contain at least the following information in
 192 order to be useful:</p>
 193
 194 <ul>
 195 <li>the exact version of GCC, as shown by "<code>gcc -v</code>";</li>
 196 <li>the system type;</li>
 197 <li>the options when GCC was configured/built;</li>
 198 <li>the exact command line passed to the <code>gcc</code> program
 199 triggering the bug
 200 (not just the flags passed to <code>gnatmake</code>, but
 201 <code>gnatmake</code> prints the parameters it passed to <code>gcc</code>)</li>
 202 <li>a collection of source files for reproducing the bug,
 203 preferably a minimal set (see below);</li>
 204 <li>a description of the expected behavior;</li>
 205 <li>a description of actual behavior.</li>
 206 </ul>
 207
 208 <p>If your code depends on additional source files (usually package
 209 specifications), submit the source code for these compilation units in
 210 a single file that is acceptable input to <code>gnatchop</code>,
 211 i.e. contains no non-Ada text.  If the compilation terminated
 212 normally, you can usually obtain a list of dependencies using the
 213 "<code>gnatls -d <i>main_unit</i></code>" command, where
 214 <code><i>main_unit</i></code> is the file name of the main compilation
 215 unit (which is also passed to <code>gcc</code>).</p>
 216
 217 <p>If you report a bug which causes the compiler to print a bug box,
 218 include that bug box in your report, and do not forget to send all the
 219 source files listed after the bug box along with your report.</p>
 220
 221 <p>If you use <code>gnatprep</code>, be sure to send in preprocessed
 222 sources (unless you have to report a bug in <code>gnatprep</code>).</p>
 223
 224 <p>When you have checked that your report meets these criteria, please
 225 submit it according to our <a href="#where">generic instructions</a>.
 226 (If you use a mailing list for reporting, please include an
 227 "<code>[Ada]</code>" tag in the subject.)</p>
 228
 229 <h2><a name="pch">Detailed bug reporting instructions when using a
 230 precompiled header</a></h2>
 231
 232 <p>If you're encountering a bug when using a precompiled header, the
 233 first thing to do is to delete the precompiled header, and try running
 234 the same GCC command again.  If the bug happens again, the bug doesn't
 235 really involve precompiled headers, please report it without using
 236 them by following the instructions <a href="#detailed">above</a>.</p>
 237
 238 <p>If you've found a bug while <i>building</i> a precompiled header
 239 (for instance, the compiler crashes), follow the usual instructions
 240 <a href="#detailed">above</a>.</p>
 241
 242 <p>If you've found a real precompiled header bug, what we'll need to
 243 reproduce it is the sources to build the precompiled header (as a
 244 single <code>.i</code> file), the source file that uses the
 245 precompiled header, any other headers that source file includes, and
 246 the command lines that you used to build the precompiled header and to
 247 use it.</p>
 248
 249 <p>Please <strong>don't</strong> send us the actual precompiled
 250 header.  It is likely to be very large and we can't use it to
 251 reproduce the problem.</p>
 252
 253 <hr />
 254
 255 <h1><a name="known">Frequently Reported Bugs in GCC</a></h1>
 256
 257 <p>This is a list of bugs in GCC that are reported very often, but not
 258 yet fixed. While it is certainly better to fix bugs instead of documenting
 259 them, this document might save people the effort of writing a bug report
 260 when the bug is already well-known.</p>
 261
 262 <p>There are many reasons why a reported bug doesn't get fixed.
 263 It might be difficult to fix, or fixing it might break compatibility.
 264 Often, reports get a low priority when there is a simple work-around.
 265 In particular, bugs caused by invalid code have a simple work-around:
 266 <em>fix the code</em>.</p>
 267
 268 <hr />
 269
 270 <h2><a name="cxx">C++</a></h2>
 271
 272 <h3><a name="missing">Missing features</a></h3>
 273
 274 <dl>
 275
 276 <dt>The <code>export</code> keyword is not implemented.</dt>
 277 <dd><p>Most C++ compilers (G++ included) do not yet implement
 278 <code>export</code>, which is necessary for separate compilation of
 279 template declarations and definitions. Without <code>export</code>, a
 280 template definition must be in scope to be used. The obvious
 281 workaround is simply to place all definitions in the header
 282 itself. Alternatively, the compilation unit containing template
 283 definitions may be included from the header.</p></dd>
 284
 285 </dl>
 286
 287 <h3><a name="fixed34">Bugs fixed in the 3.4 series</a></h3>
 288
 289 <p>The following bugs are present up to (and including) GCC 3.3.x.
 290 They have been fixed in 3.4.0.</p>
 291
 292 <dl>
 293
 294 <dt>Two-stage name-lookup.</dt>
 295
 296 <dd><p>GCC did not implement two-stage name-lookup (also see
 297 <a href="#new34">below</a>).</p></dd>
 298
 299 <dt>Covariant return types.</dt>
 300
 301 <dd><p>GCC did not implement non-trivial covariant returns.</p></dd>
 302
 303 <dt>Parse errors for "simple" code.</dt>
 304
 305 <dd><p>GCC gave parse errors for seemingly simple code, such as</p>
 306
 307 <blockquote><pre>
 308 struct A
 309 {
 310   A();
 311   A(int);
 312 };
 313
 314 struct B
 315 {
 316   B(A);
 317   B(A,A);
 318   void foo();
 319 };
 320
 321 A bar()
 322 {
 323   B b(A(),A(1));  // Variable b, initialized with two temporaries
 324   B(A(2)).foo();  // B temporary, initialized with A temporary
 325   return (A());   // return A temporary
 326 }
 327 </pre></blockquote>
 328
 329 <p>Although being valid code, each of the three lines with a comment was
 330 rejected by GCC.  The work-arounds for older compiler versions proposed
 331 below do not change the semantics of the programs at all.</p>
 332
 333 <p>The problem in the first case was that GCC started to parse the
 334 declaration of <code>b</code> as a function called <code>b</code> returning
 335 <code>B</code>, taking a function returning <code>A</code> as an argument.
 336 When it encountered the <code>1</code>, it was too late.  To show the
 337 compiler that this should be really an expression, a comma operator with
 338 a dummy argument could be used:</p>
 339
 340 <blockquote><pre>
 341 B b((0,A()),A(1));
 342 </pre></blockquote>
 343
 344 <p>The work-around for simpler cases like the second one was to add
 345 additional parentheses around the expressions that were mistaken as
 346 declarations:</p>
 347
 348 <blockquote><pre>
 349 (B(A(2))).foo();
 350 </pre></blockquote>
 351
 352 <p>In the third case, however, additional parentheses were causing
 353 the problems: The compiler interpreted <code>A()</code> as a function
 354 (taking no arguments, returning <code>A</code>), and <code>(A())</code>
 355 as a cast lacking an expression to be casted, hence the parse error.
 356 The work-around was to omit the parentheses:</p>
 357
 358 <blockquote><pre>
 359 return A();
 360 </pre></blockquote>
 361
 362 <p>This problem occurred in a number of variants; in <code>throw</code>
 363 statements, people also frequently put the object in parentheses.</p></dd>
 364
 365 </dl>
 366
 367 <hr />
 368
 369 <h2><a name="fortran">Fortran</a></h2>
 370
 371 <p>G77 bugs are documented in the G77 manual rather than
 372 explicitly listed here.  Please see
 373 <a href="http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/Trouble.html">Known Causes of
 374 Trouble with GNU Fortran</a> in the G77 manual.</p>
 375
 376 <hr />
 377
 378 <h1><a name="nonbugs">Non-bugs</a></h1>
 379
 380 <p>The following are not actually bugs, but are reported often
 381 enough to warrant a mention here.</p>
 382
 383 <p>It is not always a bug in the compiler, if code which "worked" in a
 384 previous version, is now rejected.  Earlier versions of GCC sometimes were
 385 less picky about standard conformance and accepted invalid source code.
 386 In addition, programming languages themselves change, rendering code
 387 invalid that used to be conforming (this holds especially for C++).
 388 In either case, you should update your code to match recent language
 389 standards.</p>
 390
 391 <hr />
 392
 393 <h2><a name="nonbugs_general">General</a></h2>
 394
 395 <dl>
 396 <dt>Problems with floating point numbers - the
 397 <a href="http://gcc.gnu.org/PR323">most often reported non-bug</a>.</dt>
 398 <dd><p>In a number of cases, GCC appears to perform floating point
 399 computations incorrectly. For example, the C++ program</p>
 400 <blockquote><pre>
 401 #include &lt;iostream&gt;
 402
 403 int main()
 404 {
 405   double a = 0.5;
 406   double b = 0.01;
 407   std::cout &lt;&lt; (int)(a / b) &lt;&lt; std::endl;
 408   return 0;
 409 }
 410 </pre></blockquote>
 411 <p>might print 50 on some systems and optimization levels, and 49 on
 412 others.</p>
 413
 414 <p>This is the result of <em>rounding</em>: The computer cannot
 415 represent all real numbers exactly, so it has to use
 416 approximations. When computing with approximation, the computer needs
 417 to round to the nearest representable number.</p>
 418
 419 <p>This is not a bug in the compiler, but an inherent limitation of
 420 the floating point types. Please study
 421 <a href="http://www.validlab.com/goldberg/paper.ps">this paper</a>
 422 for more information.</p></dd>
 423 </dl>
 424
 425 <hr />
 426
 427 <h2><a name="nonbugs_c">C</a></h2>
 428
 429 <dl>
 430 <dt>Increment/decrement operator (<code>++</code>/<code>--</code>) not
 431 working as expected - a <a href="http://gcc.gnu.org/PR11751">problem with
 432 many variations</a>.</dt>
 433
 434 <dd><p>The following expressions have unpredictable results:</p>
 435 <blockquote><pre>
 436 x[i]=++i
 437 foo(i,++i)
 438 i*(++i)                 /* special case with foo=="operator*" */
 439 std::cout &lt;&lt; i &lt;&lt; ++i   /* foo(foo(std::cout,i),++i)          */
 440 </pre></blockquote>
 441 <p>since the <code>i</code> without increment can be evaluated before or
 442 after <code>++i</code>.</p>
 443
 444 <p>The C and C++ standards have the notion of "sequence points". Everything
 445 that happens between two sequence points happens in an unspecified order,
 446 but it has to happen after the first and before the second sequence point.
 447 The end of a statement and a function call are examples for sequence points,
 448 whereas assignments and the comma between function arguments are not.</p>
 449
 450 <p>Modifying a value twice between two sequence points as shown in the
 451 following examples is even worse:</p>
 452 <blockquote><pre>
 453 i=++i
 454 foo(++i,++i)
 455 (++i)*(++i)               /* special case with foo=="operator*" */
 456 std::cout &lt;&lt; ++i &lt;&lt; ++i   /* foo(foo(std::cout,++i),++i)        */
 457 </pre></blockquote>
 458 <p>This leads to undefined behavior (i.e. the compiler can do
 459 anything).</p></dd>
 460
 461
 462 <dt>Casting does not work as expected when optimization is turned on.</dt>
 463
 464 <dd><p>This is often caused by a violation of aliasing rules, which are part
 465 of the ISO C standard.  These rules say that a program is invalid if you try
 466 to access a variable through a pointer of an incompatible type.  This is
 467 happening in the following example where a short is accessed through a
 468 pointer to integer (the code assumes 16-bit <code>short</code>s and 32-bit
 469 <code>int</code>s):</p>
 470 <blockquote><pre>
 471 #include &lt;stdio.h&gt;
 472
 473 int main()
 474 {
 475   short a[2];
 476
 477   a[0]=0x1111;
 478   a[1]=0x1111;
 479
 480   *(int *)a = 0x22222222; /* violation of aliasing rules */
 481
 482   printf("%x %x\n", a[0], a[1]);
 483   return 0;
 484 }
 485 </pre></blockquote>
 486 <p>The aliasing rules were designed to allow compilers more aggressive
 487 optimization.  Basically, a compiler can assume that all changes to variables
 488 happen through pointers or references to variables of a type compatible to
 489 the accessed variable.  Dereferencing a pointer that violates the aliasing
 490 rules results in undefined behavior.</p>
 491
 492 <p>In the case above, the compiler may assume that no access through an
 493 integer pointer can change the array <code>a</code>, consisting of shorts.
 494 Thus, <code>printf</code> may be called with the original values of
 495 <code>a[0]</code> and <code>a[1]</code>.  What really happens is up to
 496 the compiler and may change with architecture and optimization level.</p>
 497
 498 <p>Recent versions of GCC turn on the option <code>-fstrict-aliasing</code>
 499 (which allows alias-based optimizations) by default with <code>-O2</code>.
 500 And some architectures then really print "1111 1111" as result.  Without
 501 optimization the executable will generate the "expected" output
 502 "2222 2222".</p>
 503
 504 <p>To disable optimizations based on alias-analysis for faulty legacy code,
 505 the option <code>-fno-strict-aliasing</code> can be used as a work-around.</p>
 506
 507 <p>The option <code>-Wstrict-aliasing</code> (which is included in
 508 <code>-Wall</code>) warns about some - but not all - cases of violation
 509 of aliasing rules when <code>-fstrict-aliasing</code> is active.</p>
 510
 511 <p>To fix the code above, you can use a <code>union</code> instead of a
 512 cast (note that this is a GCC extension which might not work with other
 513 compilers):</p>
 514 <blockquote><pre>
 515 #include &lt;stdio.h&gt;
 516
 517 int main()
 518 {
 519   union
 520   {
 521     short a[2];
 522     int i;
 523   } u;
 524
 525   u.a[0]=0x1111;
 526   u.a[1]=0x1111;
 527
 528   u.i = 0x22222222;
 529
 530   printf("%x %x\n", u.a[0], u.a[1]);
 531   return 0;
 532 }
 533 </pre></blockquote>
 534 <p>Now the result will always be "2222 2222".</p>
 535
 536 <p>For some more insight into the subject, please have a look at
 537 <a href="http://mail-index.NetBSD.org/tech-kern/2003/08/11/0001.html">this
 538 article</a>.</p></dd>
 539
 540
 541 <dt>Cannot use preprocessor directive in macro arguments.</dt>
 542 <dd><p>Let me guess... you used an older version of GCC to compile code
 543 that looks something like this:</p>
 544 <blockquote><pre>
 545   memcpy(dest, src,
 546 #ifdef PLATFORM1
 547          12
 548 #else
 549          24
 550 #endif
 551         );
 552 </pre></blockquote>
 553 <p>and you got a whole pile of error messages:</p>
 554 <blockquote><pre>
 555 test.c:11: warning: preprocessing directive not recognized within macro arg
 556 test.c:11: warning: preprocessing directive not recognized within macro arg
 557 test.c:11: warning: preprocessing directive not recognized within macro arg
 558 test.c: In function `foo':
 559 test.c:6: undefined or invalid # directive
 560 test.c:8: undefined or invalid # directive
 561 test.c:9: parse error before `24'
 562 test.c:10: undefined or invalid # directive
 563 </pre></blockquote>
 564
 565 <p>This is because your C library's <code>&lt;string.h&gt;</code> happens
 566 to define <code>memcpy</code> as a macro - which is perfectly legitimate.
 567 In recent versions of glibc, for example, <code>printf</code> is among those
 568 functions which are implemented as macros.</p>
 569
 570 <p>Versions of GCC prior to 3.3 did not allow you to put <code>#ifdef</code>
 571 (or any other preprocessor directive) inside the arguments of a macro.  The
 572 code therefore would not compile.</p>
 573
 574 <p>As of GCC 3.3 this kind of construct is always accepted and the
 575 preprocessor will probably do what you expect, but see the manual for
 576 detailed semantics.</p>
 577
 578 <p>However, this kind of code is not portable.  It is "undefined behavior"
 579 according to the C standard; that means different compilers may do
 580 different things with it.  It is always possible to rewrite code which
 581 uses conditionals inside macros so that it doesn't.  You could write
 582 the above example</p>
 583 <blockquote><pre>
 584 #ifdef PLATFORM1
 585    memcpy(dest, src, 12);
 586 #else
 587    memcpy(dest, src, 24);
 588 #endif
 589 </pre></blockquote>
 590 <p>This is a bit more typing, but I personally think it's better style
 591 in addition to being more portable.</p></dd>
 592
 593
 594 <dt>Cannot initialize a static variable with <code>stdin</code>.</dt>
 595 <dd><p>This has nothing to do with GCC, but people ask us about it a
 596 lot.  Code like this:</p>
 597
 598 <blockquote><pre>
 599 #include &lt;stdio.h&gt;
 600
 601 FILE *yyin = stdin;
 602 </pre></blockquote>
 603
 604 <p>will not compile with GNU libc, because <code>stdin</code> is not a
 605 constant.  This was done deliberately, to make it easier to maintain
 606 binary compatibility when the type <code>FILE</code> needs to be changed.
 607 It is surprising for people used to traditional Unix C libraries, but it
 608 is permitted by the C standard.</p>
 609
 610 <p>This construct commonly occurs in code generated by old versions of
 611 lex or yacc.  We suggest you try regenerating the parser with a
 612 current version of flex or bison, respectively.  In your own code, the
 613 appropriate fix is to move the initialization to the beginning of
 614 main.</p>
 615
 616 <p>There is a common misconception that the GCC developers are
 617 responsible for GNU libc.  These are in fact two entirely separate
 618 projects; please check the
 619 <a href="http://www.gnu.org/software/libc/">GNU libc web pages</a>
 620 for details.
 621 </p></dd>
 622 </dl>
 623
 624 <hr />
 625
 626 <h2><a name="nonbugs_cxx">C++</a></h2>
 627
 628 <dl>
 629 <dt>Nested classes can access private members and types of the containing
 630 class.</dt>
 631
 632 <dd><p>Defect report 45 clarifies that nested classes are members of the
 633 class they are nested in, and so are granted access to private members of
 634 that class.</p></dd>
 635
 636 <dt>G++ emits two copies of constructors and destructors.</dt>
 637
 638 <dd><p>In general there are <em>three</em> types of constructors (and
 639 destructors).</p>
 640 <ol>
 641 <li>The complete object constructor/destructor.</li>
 642 <li>The base object constructor/destructor.</li>
 643 <li>The allocating constructor/deallocating destructor.</li>
 644 </ol>
 645 <p>The first two are different, when virtual base classes are involved.
 646 </p></dd>
 647
 648 <dt>Global destructors are not run in the correct order.</dt>
 649
 650 <dd><p>Global destructors should be run in the reverse order of their
 651 constructors <em>completing</em>. In most cases this is the same as
 652 the reverse order of constructors <em>starting</em>, but sometimes it
 653 is different, and that is important. You need to compile and link your
 654 programs with <code>--use-cxa-atexit</code>. We have not turned this
 655 switch on by default, as it requires a <code>cxa</code> aware runtime
 656 library (<code>libc</code>, <code>glibc</code>, or equivalent).</p></dd>
 657
 658 <dt>Classes in exception specifiers must be complete types.</dt>
 659
 660 <dd><p>[15.4]/1 tells you that you cannot have an incomplete type, or
 661 pointer to incomplete (other than <code><i>cv</i> void *</code>) in
 662 an exception specification.</p></dd>
 663
 664 <dt>Exceptions don't work in multithreaded applications.</dt>
 665
 666 <dd><p>You need to rebuild g++ and libstdc++ with
 667 <code>--enable-threads</code>.  Remember, C++ exceptions are not like
 668 hardware interrupts. You cannot throw an exception in one thread and
 669 catch it in another. You cannot throw an exception from a signal
 670 handler and catch it in the main thread.</p></dd>
 671
 672 <dt>Templates, scoping, and digraphs.</dt>
 673
 674 <dd><p>If you have a class in the global namespace, say named <code>X</code>,
 675 and want to give it as a template argument to some other class, say
 676 <code>std::vector</code>, then <code>std::vector&lt;::X&gt;</code>
 677 fails with a parser error.</p>
 678
 679 <p>The reason is that the standard mandates that the sequence
 680 <code>&lt;:</code> is treated as if it were the token <code>[</code>.
 681 (There are several such combinations of characters - they are called
 682 <em>digraphs</em>.) Depending on the version, the compiler then reports
 683 a parse error before the character <code>:</code> (the colon before
 684 <code>X</code>) or a missing closing bracket <code>]</code>.</p>
 685
 686 <p>The simplest way to avoid this is to write <code>std::vector&lt;
 687 ::X&gt;</code>, i.e. place a space between the opening angle bracket
 688 and the scope operator.</p></dd>
 689
 690
 691 <dt><a name="cxx_rvalbind">Copy constructor access check while
 692 initializing a reference.</a></dt>
 693
 694 <dd><p>Consider this code:</p>
 695
 696 <blockquote><pre>
 697 class A
 698 {
 699 public:
 700   A();
 701
 702 private:
 703   A(const A&amp;);   // private copy ctor
 704 };
 705
 706 A makeA(void);
 707 void foo(const A&amp;);
 708
 709 void bar(void)
 710 {
 711   foo(A());       // error, copy ctor is not accessible
 712   foo(makeA());   // error, copy ctor is not accessible
 713
 714   A a1;
 715   foo(a1);        // OK, a1 is a lvalue
 716 }</pre></blockquote>
 717
 718 <p>Starting with GCC 3.4.0, binding an rvalue to a const reference requires
 719 an accessible copy constructor. This might be surprising at first sight,
 720 especially since most popular compilers do not correctly implement this
 721 rule.</p>
 722
 723 <p>The C++ Standard says that a temporary object should be created in
 724 this context and its contents filled with a copy of the object we are
 725 trying to bind to the reference; it also says that the temporary copy
 726 can be elided, but the semantic constraints (eg. accessibility) of the
 727 copy constructor still have to be checked.</p>
 728
 729 <p>For further information, you can consult the following paragraphs of
 730 the C++ standard: [dcl.init.ref]/5, bullet 2, sub-bullet 1, and
 731 [class.temporary]/2.</p></dd>
 732 </dl>
 733
 734 <h3><a name="upgrading">Common problems when upgrading the compiler</a></h3>
 735
 736 <h4>ABI changes</h4>
 737
 738 <p>The C++ application binary interface (ABI) consists of two
 739 components: the first defines how the elements of classes are laid
 740 out, how functions are called, how function names are mangled, etc;
 741 the second part deals with the internals of the objects in libstdc++.
 742 Although we strive for a non-changing ABI, so far we have had to
 743 modify it with each major release.  If you change your compiler to a
 744 different major release <em>you must recompile all libraries that
 745 contain C++ code</em>.  If you fail to do so you risk getting linker
 746 errors or malfunctioning programs.  Some of our Java support libraries
 747 also contain C++ code, so you might want to recompile all libraries to
 748 be safe.  It should not be necessary to recompile if you have changed
 749 to a bug-fix release of the same version of the compiler; bug-fix
 750 releases are careful to avoid ABI changes. See also the
 751 <a href="http://gcc.gnu.org/onlinedocs/gcc/Compatibility.html">compatibility
 752 section</a> of the GCC manual.</p>
 753
 754 <p>Remark: A major release is designated by a change to the first or second
 755 component of the two- or three-part version number.  A minor (bug-fix)
 756 release is designated by a change to the third component only.  Thus GCC
 757 3.2 and 3.3 are major releases, while 3.3.1 and 3.3.2 are bug-fix releases
 758 for GCC 3.3.  With the 3.4 series we are introducing a new naming scheme;
 759 the first release of this series is 3.4.0 instead of just 3.4.</p>
 760
 761 <h4>Standard conformance</h4>
 762
 763 <p>With each release, we try to make G++ conform closer to the ISO C++ standard
 764 (available at
 765 <a href="http://www.ncits.org/cplusplus.htm">http://www.ncits.org/cplusplus.htm</a>).
 766 We have also implemented some of the core and library defect reports
 767 (available at
 768 <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>
 769 &amp;
 770 <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>
 771 respectively).</p>
 772
 773 <p>Non-conforming legacy code that worked with older versions of GCC may be
 774 rejected by more recent compilers.  There is no command-line switch to ensure
 775 compatibility in general, because trying to parse standard-conforming and
 776 old-style code at the same time would render the C++ frontend unmaintainable.
 777 However, some non-conforming constructs are allowed when the command-line
 778 option <code>-fpermissive</code> is used.</p>
 779
 780 <p>Two milestones in standard conformance are GCC 3.0 (including a major
 781 overhaul of the standard library) and the 3.4.0 version (with its new C++
 782 parser).</p>
 783
 784 <h4>New in GCC 3.0</h4>
 785
 786 <ul>
 787
 788 <li>The standard library is much more conformant, and uses the
 789 <code>std::</code> namespace (which is now a real namespace, not an
 790 alias for <code>::</code>).</li>
 791
 792 <li>The standard header files for the c library don't end with
 793 <code>.h</code>, but begin with <code>c</code> (i.e.
 794 <code>&lt;cstdlib&gt;</code> rather than <code>&lt;stdlib.h&gt;</code>).
 795 The <code>.h</code> names are still available, but are deprecated.</li>
 796
 797 <li><code>&lt;strstream&gt;</code> is deprecated, use
 798 <code>&lt;sstream&gt;</code> instead.</li>
 799
 800 <li><code>streambuf::seekoff</code> &amp;
 801 <code>streambuf::seekpos</code> are private, instead use
 802 <code>streambuf::pubseekoff</code> &amp;
 803 <code>streambuf::pubseekpos</code> respectively.</li>
 804
 805 <li>If <code>std::operator &lt;&lt; (std::ostream &amp;, long long)</code>
 806 doesn't exist, you need to recompile libstdc++ with
 807 <code>--enable-long-long</code>.</li>
 808
 809 </ul>
 810
 811 <p>If you get lots of errors about things like <code>cout</code> not being
 812 found, you've most likely forgotten to tell the compiler to look in the
 813 <code>std::</code> namespace.  There are several ways to do this:</p>
 814
 815 <ul>
 816
 817 <li>Say <code>std::cout</code> at the call.  This is the most explicit
 818 way of saying what you mean.</li>
 819
 820 <li>Say <code>using std::cout;</code> somewhere before the call.  You
 821 will need to do this for each function or type you wish to use from the
 822 standard library.</li>
 823
 824 <li>Say <code>using namespace std;</code> somewhere before the call.
 825 This is the quick-but-dirty fix. This brings the <em>whole</em> of the
 826 <code>std::</code> namespace into scope.  <em>Never</em> do this in a
 827 header file, as every user of your header file will be affected by this
 828 decision.</li>
 829
 830 </ul>
 831
 832 <h4><a name="new34">New in GCC 3.4.0</a></h4>
 833
 834 <p>The new parser brings a lot of improvements, especially concerning
 835 name-lookup.</p>
 836
 837 <ul>
 838
 839 <li>The "implicit typename" extension got removed (it was already deprecated
 840 since GCC 3.1), so that the following code is now rejected, see [14.6]:
 841 <blockquote><pre>
 842 template &lt;typename&gt; struct A
 843 {
 844     typedef int X;
 845 };
 846
 847 template &lt;typename T&gt; struct B
 848 {
 849     A&lt;T&gt;::X          x;  // error
 850     typename A&lt;T&gt;::X y;  // OK
 851 };
 852
 853 B&lt;void&gt; b;
 854 </pre></blockquote></li>
 855
 856 <li>For similar reasons, the following code now requires the
 857 <code>template</code> keyword, see [14.2]:
 858 <blockquote><pre>
 859 template &lt;typename&gt; struct A
 860 {
 861     template &lt;int&gt; struct X {};
 862 };
 863
 864 template &lt;typename T&gt; struct B
 865 {
 866     typename A&lt;T&gt;::X&lt;0&gt;          x;  // error
 867     typename A&lt;T&gt;::template X&lt;0&gt; y;  // OK
 868 };
 869
 870 B&lt;void&gt; b;
 871 </pre></blockquote></li>
 872
 873 <li>We now have two-stage name-lookup, so that the following code is
 874 rejected, see [14.6]/9:
 875 <blockquote><pre>
 876 template &lt;typename T&gt; int foo()
 877 {
 878     return i;  // error
 879 }
 880 </pre></blockquote></li>
 881
 882 <li>This also affects members of base classes, see [14.6.2]:
 883 <blockquote><pre>
 884 template &lt;typename&gt; struct A
 885 {
 886     int i, j;
 887 };
 888
 889 template &lt;typename T&gt; struct B : A&lt;T&gt;
 890 {
 891     int foo1() { return i; }       // error
 892     int foo2() { return this-&gt;i; } // OK
 893     int foo3() { return B&lt;T&gt;::i; } // OK
 894     int foo4() { return A&lt;T&gt;::i; } // OK
 895
 896     using A&lt;T&gt;::j;
 897     int foo5() { return j; }       // OK
 898 };
 899 </pre></blockquote></li>
 900
 901 </ul>
 902
 903 <p>In addition to the problems listed above, the manual contains a section on
 904 <a href="http://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Misunderstandings.html">
 905 Common Misunderstandings with GNU C++</a>.</p>
 906
 907 </body>
 908 </html>