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<html>
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<head>
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<title>GCC Bugs</title>
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</head>
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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
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908
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