1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
|
@c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
@c 1999, 2000, 2001, 2003, 2004, 2005, 2006, 2007, 2008, 2009
@c Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.
@node Trouble
@chapter Known Causes of Trouble with GCC
@cindex bugs, known
@cindex installation trouble
@cindex known causes of trouble
This section describes known problems that affect users of GCC@. Most
of these are not GCC bugs per se---if they were, we would fix them.
But the result for a user may be like the result of a bug.
Some of these problems are due to bugs in other software, some are
missing features that are too much work to add, and some are places
where people's opinions differ as to what is best.
@menu
* Actual Bugs:: Bugs we will fix later.
* Cross-Compiler Problems:: Common problems of cross compiling with GCC.
* Interoperation:: Problems using GCC with other compilers,
and with certain linkers, assemblers and debuggers.
* Incompatibilities:: GCC is incompatible with traditional C.
* Fixed Headers:: GCC uses corrected versions of system header files.
This is necessary, but doesn't always work smoothly.
* Standard Libraries:: GCC uses the system C library, which might not be
compliant with the ISO C standard.
* Disappointments:: Regrettable things we can't change, but not quite bugs.
* C++ Misunderstandings:: Common misunderstandings with GNU C++.
* Non-bugs:: Things we think are right, but some others disagree.
* Warnings and Errors:: Which problems in your code get warnings,
and which get errors.
@end menu
@node Actual Bugs
@section Actual Bugs We Haven't Fixed Yet
@itemize @bullet
@item
The @code{fixincludes} script interacts badly with automounters; if the
directory of system header files is automounted, it tends to be
unmounted while @code{fixincludes} is running. This would seem to be a
bug in the automounter. We don't know any good way to work around it.
@end itemize
@node Cross-Compiler Problems
@section Cross-Compiler Problems
You may run into problems with cross compilation on certain machines,
for several reasons.
@itemize @bullet
@item
At present, the program @file{mips-tfile} which adds debug
support to object files on MIPS systems does not work in a cross
compile environment.
@end itemize
@node Interoperation
@section Interoperation
This section lists various difficulties encountered in using GCC
together with other compilers or with the assemblers, linkers,
libraries and debuggers on certain systems.
@itemize @bullet
@item
On many platforms, GCC supports a different ABI for C++ than do other
compilers, so the object files compiled by GCC cannot be used with object
files generated by another C++ compiler.
An area where the difference is most apparent is name mangling. The use
of different name mangling is intentional, to protect you from more subtle
problems.
Compilers differ as to many internal details of C++ implementation,
including: how class instances are laid out, how multiple inheritance is
implemented, and how virtual function calls are handled. If the name
encoding were made the same, your programs would link against libraries
provided from other compilers---but the programs would then crash when
run. Incompatible libraries are then detected at link time, rather than
at run time.
@item
On some BSD systems, including some versions of Ultrix, use of profiling
causes static variable destructors (currently used only in C++) not to
be run.
@item
On some SGI systems, when you use @option{-lgl_s} as an option,
it gets translated magically to @samp{-lgl_s -lX11_s -lc_s}.
Naturally, this does not happen when you use GCC@.
You must specify all three options explicitly.
@item
On a SPARC, GCC aligns all values of type @code{double} on an 8-byte
boundary, and it expects every @code{double} to be so aligned. The Sun
compiler usually gives @code{double} values 8-byte alignment, with one
exception: function arguments of type @code{double} may not be aligned.
As a result, if a function compiled with Sun CC takes the address of an
argument of type @code{double} and passes this pointer of type
@code{double *} to a function compiled with GCC, dereferencing the
pointer may cause a fatal signal.
One way to solve this problem is to compile your entire program with GCC@.
Another solution is to modify the function that is compiled with
Sun CC to copy the argument into a local variable; local variables
are always properly aligned. A third solution is to modify the function
that uses the pointer to dereference it via the following function
@code{access_double} instead of directly with @samp{*}:
@smallexample
inline double
access_double (double *unaligned_ptr)
@{
union d2i @{ double d; int i[2]; @};
union d2i *p = (union d2i *) unaligned_ptr;
union d2i u;
u.i[0] = p->i[0];
u.i[1] = p->i[1];
return u.d;
@}
@end smallexample
@noindent
Storing into the pointer can be done likewise with the same union.
@item
On Solaris, the @code{malloc} function in the @file{libmalloc.a} library
may allocate memory that is only 4 byte aligned. Since GCC on the
SPARC assumes that doubles are 8 byte aligned, this may result in a
fatal signal if doubles are stored in memory allocated by the
@file{libmalloc.a} library.
The solution is to not use the @file{libmalloc.a} library. Use instead
@code{malloc} and related functions from @file{libc.a}; they do not have
this problem.
@item
On the HP PA machine, ADB sometimes fails to work on functions compiled
with GCC@. Specifically, it fails to work on functions that use
@code{alloca} or variable-size arrays. This is because GCC doesn't
generate HP-UX unwind descriptors for such functions. It may even be
impossible to generate them.
@item
Debugging (@option{-g}) is not supported on the HP PA machine, unless you use
the preliminary GNU tools.
@item
Taking the address of a label may generate errors from the HP-UX
PA assembler. GAS for the PA does not have this problem.
@item
Using floating point parameters for indirect calls to static functions
will not work when using the HP assembler. There simply is no way for GCC
to specify what registers hold arguments for static functions when using
the HP assembler. GAS for the PA does not have this problem.
@item
In extremely rare cases involving some very large functions you may
receive errors from the HP linker complaining about an out of bounds
unconditional branch offset. This used to occur more often in previous
versions of GCC, but is now exceptionally rare. If you should run
into it, you can work around by making your function smaller.
@item
GCC compiled code sometimes emits warnings from the HP-UX assembler of
the form:
@smallexample
(warning) Use of GR3 when
frame >= 8192 may cause conflict.
@end smallexample
These warnings are harmless and can be safely ignored.
@item
In extremely rare cases involving some very large functions you may
receive errors from the AIX Assembler complaining about a displacement
that is too large. If you should run into it, you can work around by
making your function smaller.
@item
The @file{libstdc++.a} library in GCC relies on the SVR4 dynamic
linker semantics which merges global symbols between libraries and
applications, especially necessary for C++ streams functionality.
This is not the default behavior of AIX shared libraries and dynamic
linking. @file{libstdc++.a} is built on AIX with ``runtime-linking''
enabled so that symbol merging can occur. To utilize this feature,
the application linked with @file{libstdc++.a} must include the
@option{-Wl,-brtl} flag on the link line. G++ cannot impose this
because this option may interfere with the semantics of the user
program and users may not always use @samp{g++} to link his or her
application. Applications are not required to use the
@option{-Wl,-brtl} flag on the link line---the rest of the
@file{libstdc++.a} library which is not dependent on the symbol
merging semantics will continue to function correctly.
@item
An application can interpose its own definition of functions for
functions invoked by @file{libstdc++.a} with ``runtime-linking''
enabled on AIX@. To accomplish this the application must be linked
with ``runtime-linking'' option and the functions explicitly must be
exported by the application (@option{-Wl,-brtl,-bE:exportfile}).
@item
AIX on the RS/6000 provides support (NLS) for environments outside of
the United States. Compilers and assemblers use NLS to support
locale-specific representations of various objects including
floating-point numbers (@samp{.} vs @samp{,} for separating decimal
fractions). There have been problems reported where the library linked
with GCC does not produce the same floating-point formats that the
assembler accepts. If you have this problem, set the @env{LANG}
environment variable to @samp{C} or @samp{En_US}.
@item
@opindex fdollars-in-identifiers
Even if you specify @option{-fdollars-in-identifiers},
you cannot successfully use @samp{$} in identifiers on the RS/6000 due
to a restriction in the IBM assembler. GAS supports these
identifiers.
@end itemize
@node Incompatibilities
@section Incompatibilities of GCC
@cindex incompatibilities of GCC
@opindex traditional
There are several noteworthy incompatibilities between GNU C and K&R
(non-ISO) versions of C@.
@itemize @bullet
@cindex string constants
@cindex read-only strings
@cindex shared strings
@item
GCC normally makes string constants read-only. If several
identical-looking string constants are used, GCC stores only one
copy of the string.
@cindex @code{mktemp}, and constant strings
One consequence is that you cannot call @code{mktemp} with a string
constant argument. The function @code{mktemp} always alters the
string its argument points to.
@cindex @code{sscanf}, and constant strings
@cindex @code{fscanf}, and constant strings
@cindex @code{scanf}, and constant strings
Another consequence is that @code{sscanf} does not work on some very
old systems when passed a string constant as its format control string
or input. This is because @code{sscanf} incorrectly tries to write
into the string constant. Likewise @code{fscanf} and @code{scanf}.
The solution to these problems is to change the program to use
@code{char}-array variables with initialization strings for these
purposes instead of string constants.
@item
@code{-2147483648} is positive.
This is because 2147483648 cannot fit in the type @code{int}, so
(following the ISO C rules) its data type is @code{unsigned long int}.
Negating this value yields 2147483648 again.
@item
GCC does not substitute macro arguments when they appear inside of
string constants. For example, the following macro in GCC
@smallexample
#define foo(a) "a"
@end smallexample
@noindent
will produce output @code{"a"} regardless of what the argument @var{a} is.
@cindex @code{setjmp} incompatibilities
@cindex @code{longjmp} incompatibilities
@item
When you use @code{setjmp} and @code{longjmp}, the only automatic
variables guaranteed to remain valid are those declared
@code{volatile}. This is a consequence of automatic register
allocation. Consider this function:
@smallexample
jmp_buf j;
foo ()
@{
int a, b;
a = fun1 ();
if (setjmp (j))
return a;
a = fun2 ();
/* @r{@code{longjmp (j)} may occur in @code{fun3}.} */
return a + fun3 ();
@}
@end smallexample
Here @code{a} may or may not be restored to its first value when the
@code{longjmp} occurs. If @code{a} is allocated in a register, then
its first value is restored; otherwise, it keeps the last value stored
in it.
@opindex W
If you use the @option{-W} option with the @option{-O} option, you will
get a warning when GCC thinks such a problem might be possible.
@item
Programs that use preprocessing directives in the middle of macro
arguments do not work with GCC@. For example, a program like this
will not work:
@smallexample
@group
foobar (
#define luser
hack)
@end group
@end smallexample
ISO C does not permit such a construct.
@item
K&R compilers allow comments to cross over an inclusion boundary
(i.e.@: started in an include file and ended in the including file).
@cindex external declaration scope
@cindex scope of external declarations
@cindex declaration scope
@item
Declarations of external variables and functions within a block apply
only to the block containing the declaration. In other words, they
have the same scope as any other declaration in the same place.
In some other C compilers, a @code{extern} declaration affects all the
rest of the file even if it happens within a block.
@item
In traditional C, you can combine @code{long}, etc., with a typedef name,
as shown here:
@smallexample
typedef int foo;
typedef long foo bar;
@end smallexample
In ISO C, this is not allowed: @code{long} and other type modifiers
require an explicit @code{int}.
@cindex typedef names as function parameters
@item
PCC allows typedef names to be used as function parameters.
@item
Traditional C allows the following erroneous pair of declarations to
appear together in a given scope:
@smallexample
typedef int foo;
typedef foo foo;
@end smallexample
@item
GCC treats all characters of identifiers as significant. According to
K&R-1 (2.2), ``No more than the first eight characters are significant,
although more may be used.''. Also according to K&R-1 (2.2), ``An
identifier is a sequence of letters and digits; the first character must
be a letter. The underscore _ counts as a letter.'', but GCC also
allows dollar signs in identifiers.
@cindex whitespace
@item
PCC allows whitespace in the middle of compound assignment operators
such as @samp{+=}. GCC, following the ISO standard, does not
allow this.
@cindex apostrophes
@cindex '
@item
GCC complains about unterminated character constants inside of
preprocessing conditionals that fail. Some programs have English
comments enclosed in conditionals that are guaranteed to fail; if these
comments contain apostrophes, GCC will probably report an error. For
example, this code would produce an error:
@smallexample
#if 0
You can't expect this to work.
#endif
@end smallexample
The best solution to such a problem is to put the text into an actual
C comment delimited by @samp{/*@dots{}*/}.
@item
Many user programs contain the declaration @samp{long time ();}. In the
past, the system header files on many systems did not actually declare
@code{time}, so it did not matter what type your program declared it to
return. But in systems with ISO C headers, @code{time} is declared to
return @code{time_t}, and if that is not the same as @code{long}, then
@samp{long time ();} is erroneous.
The solution is to change your program to use appropriate system headers
(@code{<time.h>} on systems with ISO C headers) and not to declare
@code{time} if the system header files declare it, or failing that to
use @code{time_t} as the return type of @code{time}.
@cindex @code{float} as function value type
@item
When compiling functions that return @code{float}, PCC converts it to
a double. GCC actually returns a @code{float}. If you are concerned
with PCC compatibility, you should declare your functions to return
@code{double}; you might as well say what you mean.
@cindex structures
@cindex unions
@item
When compiling functions that return structures or unions, GCC
output code normally uses a method different from that used on most
versions of Unix. As a result, code compiled with GCC cannot call
a structure-returning function compiled with PCC, and vice versa.
The method used by GCC is as follows: a structure or union which is
1, 2, 4 or 8 bytes long is returned like a scalar. A structure or union
with any other size is stored into an address supplied by the caller
(usually in a special, fixed register, but on some machines it is passed
on the stack). The target hook @code{TARGET_STRUCT_VALUE_RTX}
tells GCC where to pass this address.
By contrast, PCC on most target machines returns structures and unions
of any size by copying the data into an area of static storage, and then
returning the address of that storage as if it were a pointer value.
The caller must copy the data from that memory area to the place where
the value is wanted. GCC does not use this method because it is
slower and nonreentrant.
On some newer machines, PCC uses a reentrant convention for all
structure and union returning. GCC on most of these machines uses a
compatible convention when returning structures and unions in memory,
but still returns small structures and unions in registers.
@opindex fpcc-struct-return
You can tell GCC to use a compatible convention for all structure and
union returning with the option @option{-fpcc-struct-return}.
@cindex preprocessing tokens
@cindex preprocessing numbers
@item
GCC complains about program fragments such as @samp{0x74ae-0x4000}
which appear to be two hexadecimal constants separated by the minus
operator. Actually, this string is a single @dfn{preprocessing token}.
Each such token must correspond to one token in C@. Since this does not,
GCC prints an error message. Although it may appear obvious that what
is meant is an operator and two values, the ISO C standard specifically
requires that this be treated as erroneous.
A @dfn{preprocessing token} is a @dfn{preprocessing number} if it
begins with a digit and is followed by letters, underscores, digits,
periods and @samp{e+}, @samp{e-}, @samp{E+}, @samp{E-}, @samp{p+},
@samp{p-}, @samp{P+}, or @samp{P-} character sequences. (In strict C89
mode, the sequences @samp{p+}, @samp{p-}, @samp{P+} and @samp{P-} cannot
appear in preprocessing numbers.)
To make the above program fragment valid, place whitespace in front of
the minus sign. This whitespace will end the preprocessing number.
@end itemize
@node Fixed Headers
@section Fixed Header Files
GCC needs to install corrected versions of some system header files.
This is because most target systems have some header files that won't
work with GCC unless they are changed. Some have bugs, some are
incompatible with ISO C, and some depend on special features of other
compilers.
Installing GCC automatically creates and installs the fixed header
files, by running a program called @code{fixincludes}. Normally, you
don't need to pay attention to this. But there are cases where it
doesn't do the right thing automatically.
@itemize @bullet
@item
If you update the system's header files, such as by installing a new
system version, the fixed header files of GCC are not automatically
updated. They can be updated using the @command{mkheaders} script
installed in
@file{@var{libexecdir}/gcc/@var{target}/@var{version}/install-tools/}.
@item
On some systems, header file directories contain
machine-specific symbolic links in certain places. This makes it
possible to share most of the header files among hosts running the
same version of the system on different machine models.
The programs that fix the header files do not understand this special
way of using symbolic links; therefore, the directory of fixed header
files is good only for the machine model used to build it.
It is possible to make separate sets of fixed header files for the
different machine models, and arrange a structure of symbolic links so
as to use the proper set, but you'll have to do this by hand.
@end itemize
@node Standard Libraries
@section Standard Libraries
@opindex Wall
GCC by itself attempts to be a conforming freestanding implementation.
@xref{Standards,,Language Standards Supported by GCC}, for details of
what this means. Beyond the library facilities required of such an
implementation, the rest of the C library is supplied by the vendor of
the operating system. If that C library doesn't conform to the C
standards, then your programs might get warnings (especially when using
@option{-Wall}) that you don't expect.
For example, the @code{sprintf} function on SunOS 4.1.3 returns
@code{char *} while the C standard says that @code{sprintf} returns an
@code{int}. The @code{fixincludes} program could make the prototype for
this function match the Standard, but that would be wrong, since the
function will still return @code{char *}.
If you need a Standard compliant library, then you need to find one, as
GCC does not provide one. The GNU C library (called @code{glibc})
provides ISO C, POSIX, BSD, SystemV and X/Open compatibility for
GNU/Linux and HURD-based GNU systems; no recent version of it supports
other systems, though some very old versions did. Version 2.2 of the
GNU C library includes nearly complete C99 support. You could also ask
your operating system vendor if newer libraries are available.
@node Disappointments
@section Disappointments and Misunderstandings
These problems are perhaps regrettable, but we don't know any practical
way around them.
@itemize @bullet
@item
Certain local variables aren't recognized by debuggers when you compile
with optimization.
This occurs because sometimes GCC optimizes the variable out of
existence. There is no way to tell the debugger how to compute the
value such a variable ``would have had'', and it is not clear that would
be desirable anyway. So GCC simply does not mention the eliminated
variable when it writes debugging information.
You have to expect a certain amount of disagreement between the
executable and your source code, when you use optimization.
@cindex conflicting types
@cindex scope of declaration
@item
Users often think it is a bug when GCC reports an error for code
like this:
@smallexample
int foo (struct mumble *);
struct mumble @{ @dots{} @};
int foo (struct mumble *x)
@{ @dots{} @}
@end smallexample
This code really is erroneous, because the scope of @code{struct
mumble} in the prototype is limited to the argument list containing it.
It does not refer to the @code{struct mumble} defined with file scope
immediately below---they are two unrelated types with similar names in
different scopes.
But in the definition of @code{foo}, the file-scope type is used
because that is available to be inherited. Thus, the definition and
the prototype do not match, and you get an error.
This behavior may seem silly, but it's what the ISO standard specifies.
It is easy enough for you to make your code work by moving the
definition of @code{struct mumble} above the prototype. It's not worth
being incompatible with ISO C just to avoid an error for the example
shown above.
@item
Accesses to bit-fields even in volatile objects works by accessing larger
objects, such as a byte or a word. You cannot rely on what size of
object is accessed in order to read or write the bit-field; it may even
vary for a given bit-field according to the precise usage.
If you care about controlling the amount of memory that is accessed, use
volatile but do not use bit-fields.
@item
GCC comes with shell scripts to fix certain known problems in system
header files. They install corrected copies of various header files in
a special directory where only GCC will normally look for them. The
scripts adapt to various systems by searching all the system header
files for the problem cases that we know about.
If new system header files are installed, nothing automatically arranges
to update the corrected header files. They can be updated using the
@command{mkheaders} script installed in
@file{@var{libexecdir}/gcc/@var{target}/@var{version}/install-tools/}.
@item
@cindex floating point precision
On 68000 and x86 systems, for instance, you can get paradoxical results
if you test the precise values of floating point numbers. For example,
you can find that a floating point value which is not a NaN is not equal
to itself. This results from the fact that the floating point registers
hold a few more bits of precision than fit in a @code{double} in memory.
Compiled code moves values between memory and floating point registers
at its convenience, and moving them into memory truncates them.
@opindex ffloat-store
You can partially avoid this problem by using the @option{-ffloat-store}
option (@pxref{Optimize Options}).
@item
On AIX and other platforms without weak symbol support, templates
need to be instantiated explicitly and symbols for static members
of templates will not be generated.
@item
On AIX, GCC scans object files and library archives for static
constructors and destructors when linking an application before the
linker prunes unreferenced symbols. This is necessary to prevent the
AIX linker from mistakenly assuming that static constructor or
destructor are unused and removing them before the scanning can occur.
All static constructors and destructors found will be referenced even
though the modules in which they occur may not be used by the program.
This may lead to both increased executable size and unexpected symbol
references.
@end itemize
@node C++ Misunderstandings
@section Common Misunderstandings with GNU C++
@cindex misunderstandings in C++
@cindex surprises in C++
@cindex C++ misunderstandings
C++ is a complex language and an evolving one, and its standard
definition (the ISO C++ standard) was only recently completed. As a
result, your C++ compiler may occasionally surprise you, even when its
behavior is correct. This section discusses some areas that frequently
give rise to questions of this sort.
@menu
* Static Definitions:: Static member declarations are not definitions
* Name lookup:: Name lookup, templates, and accessing members of base classes
* Temporaries:: Temporaries may vanish before you expect
* Copy Assignment:: Copy Assignment operators copy virtual bases twice
@end menu
@node Static Definitions
@subsection Declare @emph{and} Define Static Members
@cindex C++ static data, declaring and defining
@cindex static data in C++, declaring and defining
@cindex declaring static data in C++
@cindex defining static data in C++
When a class has static data members, it is not enough to @emph{declare}
the static member; you must also @emph{define} it. For example:
@smallexample
class Foo
@{
@dots{}
void method();
static int bar;
@};
@end smallexample
This declaration only establishes that the class @code{Foo} has an
@code{int} named @code{Foo::bar}, and a member function named
@code{Foo::method}. But you still need to define @emph{both}
@code{method} and @code{bar} elsewhere. According to the ISO
standard, you must supply an initializer in one (and only one) source
file, such as:
@smallexample
int Foo::bar = 0;
@end smallexample
Other C++ compilers may not correctly implement the standard behavior.
As a result, when you switch to @command{g++} from one of these compilers,
you may discover that a program that appeared to work correctly in fact
does not conform to the standard: @command{g++} reports as undefined
symbols any static data members that lack definitions.
@node Name lookup
@subsection Name lookup, templates, and accessing members of base classes
@cindex base class members
@cindex two-stage name lookup
@cindex dependent name lookup
The C++ standard prescribes that all names that are not dependent on
template parameters are bound to their present definitions when parsing
a template function or class.@footnote{The C++ standard just uses the
term ``dependent'' for names that depend on the type or value of
template parameters. This shorter term will also be used in the rest of
this section.} Only names that are dependent are looked up at the point
of instantiation. For example, consider
@smallexample
void foo(double);
struct A @{
template <typename T>
void f () @{
foo (1); // @r{1}
int i = N; // @r{2}
T t;
t.bar(); // @r{3}
foo (t); // @r{4}
@}
static const int N;
@};
@end smallexample
Here, the names @code{foo} and @code{N} appear in a context that does
not depend on the type of @code{T}. The compiler will thus require that
they are defined in the context of use in the template, not only before
the point of instantiation, and will here use @code{::foo(double)} and
@code{A::N}, respectively. In particular, it will convert the integer
value to a @code{double} when passing it to @code{::foo(double)}.
Conversely, @code{bar} and the call to @code{foo} in the fourth marked
line are used in contexts that do depend on the type of @code{T}, so
they are only looked up at the point of instantiation, and you can
provide declarations for them after declaring the template, but before
instantiating it. In particular, if you instantiate @code{A::f<int>},
the last line will call an overloaded @code{::foo(int)} if one was
provided, even if after the declaration of @code{struct A}.
This distinction between lookup of dependent and non-dependent names is
called two-stage (or dependent) name lookup. G++ implements it
since version 3.4.
Two-stage name lookup sometimes leads to situations with behavior
different from non-template codes. The most common is probably this:
@smallexample
template <typename T> struct Base @{
int i;
@};
template <typename T> struct Derived : public Base<T> @{
int get_i() @{ return i; @}
@};
@end smallexample
In @code{get_i()}, @code{i} is not used in a dependent context, so the
compiler will look for a name declared at the enclosing namespace scope
(which is the global scope here). It will not look into the base class,
since that is dependent and you may declare specializations of
@code{Base} even after declaring @code{Derived}, so the compiler can't
really know what @code{i} would refer to. If there is no global
variable @code{i}, then you will get an error message.
In order to make it clear that you want the member of the base class,
you need to defer lookup until instantiation time, at which the base
class is known. For this, you need to access @code{i} in a dependent
context, by either using @code{this->i} (remember that @code{this} is of
type @code{Derived<T>*}, so is obviously dependent), or using
@code{Base<T>::i}. Alternatively, @code{Base<T>::i} might be brought
into scope by a @code{using}-declaration.
Another, similar example involves calling member functions of a base
class:
@smallexample
template <typename T> struct Base @{
int f();
@};
template <typename T> struct Derived : Base<T> @{
int g() @{ return f(); @};
@};
@end smallexample
Again, the call to @code{f()} is not dependent on template arguments
(there are no arguments that depend on the type @code{T}, and it is also
not otherwise specified that the call should be in a dependent context).
Thus a global declaration of such a function must be available, since
the one in the base class is not visible until instantiation time. The
compiler will consequently produce the following error message:
@smallexample
x.cc: In member function `int Derived<T>::g()':
x.cc:6: error: there are no arguments to `f' that depend on a template
parameter, so a declaration of `f' must be available
x.cc:6: error: (if you use `-fpermissive', G++ will accept your code, but
allowing the use of an undeclared name is deprecated)
@end smallexample
To make the code valid either use @code{this->f()}, or
@code{Base<T>::f()}. Using the @option{-fpermissive} flag will also let
the compiler accept the code, by marking all function calls for which no
declaration is visible at the time of definition of the template for
later lookup at instantiation time, as if it were a dependent call.
We do not recommend using @option{-fpermissive} to work around invalid
code, and it will also only catch cases where functions in base classes
are called, not where variables in base classes are used (as in the
example above).
Note that some compilers (including G++ versions prior to 3.4) get these
examples wrong and accept above code without an error. Those compilers
do not implement two-stage name lookup correctly.
@node Temporaries
@subsection Temporaries May Vanish Before You Expect
@cindex temporaries, lifetime of
@cindex portions of temporary objects, pointers to
It is dangerous to use pointers or references to @emph{portions} of a
temporary object. The compiler may very well delete the object before
you expect it to, leaving a pointer to garbage. The most common place
where this problem crops up is in classes like string classes,
especially ones that define a conversion function to type @code{char *}
or @code{const char *}---which is one reason why the standard
@code{string} class requires you to call the @code{c_str} member
function. However, any class that returns a pointer to some internal
structure is potentially subject to this problem.
For example, a program may use a function @code{strfunc} that returns
@code{string} objects, and another function @code{charfunc} that
operates on pointers to @code{char}:
@smallexample
string strfunc ();
void charfunc (const char *);
void
f ()
@{
const char *p = strfunc().c_str();
@dots{}
charfunc (p);
@dots{}
charfunc (p);
@}
@end smallexample
@noindent
In this situation, it may seem reasonable to save a pointer to the C
string returned by the @code{c_str} member function and use that rather
than call @code{c_str} repeatedly. However, the temporary string
created by the call to @code{strfunc} is destroyed after @code{p} is
initialized, at which point @code{p} is left pointing to freed memory.
Code like this may run successfully under some other compilers,
particularly obsolete cfront-based compilers that delete temporaries
along with normal local variables. However, the GNU C++ behavior is
standard-conforming, so if your program depends on late destruction of
temporaries it is not portable.
The safe way to write such code is to give the temporary a name, which
forces it to remain until the end of the scope of the name. For
example:
@smallexample
const string& tmp = strfunc ();
charfunc (tmp.c_str ());
@end smallexample
@node Copy Assignment
@subsection Implicit Copy-Assignment for Virtual Bases
When a base class is virtual, only one subobject of the base class
belongs to each full object. Also, the constructors and destructors are
invoked only once, and called from the most-derived class. However, such
objects behave unspecified when being assigned. For example:
@smallexample
struct Base@{
char *name;
Base(char *n) : name(strdup(n))@{@}
Base& operator= (const Base& other)@{
free (name);
name = strdup (other.name);
@}
@};
struct A:virtual Base@{
int val;
A():Base("A")@{@}
@};
struct B:virtual Base@{
int bval;
B():Base("B")@{@}
@};
struct Derived:public A, public B@{
Derived():Base("Derived")@{@}
@};
void func(Derived &d1, Derived &d2)
@{
d1 = d2;
@}
@end smallexample
The C++ standard specifies that @samp{Base::Base} is only called once
when constructing or copy-constructing a Derived object. It is
unspecified whether @samp{Base::operator=} is called more than once when
the implicit copy-assignment for Derived objects is invoked (as it is
inside @samp{func} in the example).
G++ implements the ``intuitive'' algorithm for copy-assignment: assign all
direct bases, then assign all members. In that algorithm, the virtual
base subobject can be encountered more than once. In the example, copying
proceeds in the following order: @samp{val}, @samp{name} (via
@code{strdup}), @samp{bval}, and @samp{name} again.
If application code relies on copy-assignment, a user-defined
copy-assignment operator removes any uncertainties. With such an
operator, the application can define whether and how the virtual base
subobject is assigned.
@node Non-bugs
@section Certain Changes We Don't Want to Make
This section lists changes that people frequently request, but which
we do not make because we think GCC is better without them.
@itemize @bullet
@item
Checking the number and type of arguments to a function which has an
old-fashioned definition and no prototype.
Such a feature would work only occasionally---only for calls that appear
in the same file as the called function, following the definition. The
only way to check all calls reliably is to add a prototype for the
function. But adding a prototype eliminates the motivation for this
feature. So the feature is not worthwhile.
@item
Warning about using an expression whose type is signed as a shift count.
Shift count operands are probably signed more often than unsigned.
Warning about this would cause far more annoyance than good.
@item
Warning about assigning a signed value to an unsigned variable.
Such assignments must be very common; warning about them would cause
more annoyance than good.
@item
Warning when a non-void function value is ignored.
C contains many standard functions that return a value that most
programs choose to ignore. One obvious example is @code{printf}.
Warning about this practice only leads the defensive programmer to
clutter programs with dozens of casts to @code{void}. Such casts are
required so frequently that they become visual noise. Writing those
casts becomes so automatic that they no longer convey useful
information about the intentions of the programmer. For functions
where the return value should never be ignored, use the
@code{warn_unused_result} function attribute (@pxref{Function
Attributes}).
@item
@opindex fshort-enums
Making @option{-fshort-enums} the default.
This would cause storage layout to be incompatible with most other C
compilers. And it doesn't seem very important, given that you can get
the same result in other ways. The case where it matters most is when
the enumeration-valued object is inside a structure, and in that case
you can specify a field width explicitly.
@item
Making bit-fields unsigned by default on particular machines where ``the
ABI standard'' says to do so.
The ISO C standard leaves it up to the implementation whether a bit-field
declared plain @code{int} is signed or not. This in effect creates two
alternative dialects of C@.
@opindex fsigned-bitfields
@opindex funsigned-bitfields
The GNU C compiler supports both dialects; you can specify the signed
dialect with @option{-fsigned-bitfields} and the unsigned dialect with
@option{-funsigned-bitfields}. However, this leaves open the question of
which dialect to use by default.
Currently, the preferred dialect makes plain bit-fields signed, because
this is simplest. Since @code{int} is the same as @code{signed int} in
every other context, it is cleanest for them to be the same in bit-fields
as well.
Some computer manufacturers have published Application Binary Interface
standards which specify that plain bit-fields should be unsigned. It is
a mistake, however, to say anything about this issue in an ABI@. This is
because the handling of plain bit-fields distinguishes two dialects of C@.
Both dialects are meaningful on every type of machine. Whether a
particular object file was compiled using signed bit-fields or unsigned
is of no concern to other object files, even if they access the same
bit-fields in the same data structures.
A given program is written in one or the other of these two dialects.
The program stands a chance to work on most any machine if it is
compiled with the proper dialect. It is unlikely to work at all if
compiled with the wrong dialect.
Many users appreciate the GNU C compiler because it provides an
environment that is uniform across machines. These users would be
inconvenienced if the compiler treated plain bit-fields differently on
certain machines.
Occasionally users write programs intended only for a particular machine
type. On these occasions, the users would benefit if the GNU C compiler
were to support by default the same dialect as the other compilers on
that machine. But such applications are rare. And users writing a
program to run on more than one type of machine cannot possibly benefit
from this kind of compatibility.
This is why GCC does and will treat plain bit-fields in the same
fashion on all types of machines (by default).
There are some arguments for making bit-fields unsigned by default on all
machines. If, for example, this becomes a universal de facto standard,
it would make sense for GCC to go along with it. This is something
to be considered in the future.
(Of course, users strongly concerned about portability should indicate
explicitly in each bit-field whether it is signed or not. In this way,
they write programs which have the same meaning in both C dialects.)
@item
@opindex ansi
@opindex std
Undefining @code{__STDC__} when @option{-ansi} is not used.
Currently, GCC defines @code{__STDC__} unconditionally. This provides
good results in practice.
Programmers normally use conditionals on @code{__STDC__} to ask whether
it is safe to use certain features of ISO C, such as function
prototypes or ISO token concatenation. Since plain @command{gcc} supports
all the features of ISO C, the correct answer to these questions is
``yes''.
Some users try to use @code{__STDC__} to check for the availability of
certain library facilities. This is actually incorrect usage in an ISO
C program, because the ISO C standard says that a conforming
freestanding implementation should define @code{__STDC__} even though it
does not have the library facilities. @samp{gcc -ansi -pedantic} is a
conforming freestanding implementation, and it is therefore required to
define @code{__STDC__}, even though it does not come with an ISO C
library.
Sometimes people say that defining @code{__STDC__} in a compiler that
does not completely conform to the ISO C standard somehow violates the
standard. This is illogical. The standard is a standard for compilers
that claim to support ISO C, such as @samp{gcc -ansi}---not for other
compilers such as plain @command{gcc}. Whatever the ISO C standard says
is relevant to the design of plain @command{gcc} without @option{-ansi} only
for pragmatic reasons, not as a requirement.
GCC normally defines @code{__STDC__} to be 1, and in addition
defines @code{__STRICT_ANSI__} if you specify the @option{-ansi} option,
or a @option{-std} option for strict conformance to some version of ISO C@.
On some hosts, system include files use a different convention, where
@code{__STDC__} is normally 0, but is 1 if the user specifies strict
conformance to the C Standard. GCC follows the host convention when
processing system include files, but when processing user files it follows
the usual GNU C convention.
@item
Undefining @code{__STDC__} in C++.
Programs written to compile with C++-to-C translators get the
value of @code{__STDC__} that goes with the C compiler that is
subsequently used. These programs must test @code{__STDC__}
to determine what kind of C preprocessor that compiler uses:
whether they should concatenate tokens in the ISO C fashion
or in the traditional fashion.
These programs work properly with GNU C++ if @code{__STDC__} is defined.
They would not work otherwise.
In addition, many header files are written to provide prototypes in ISO
C but not in traditional C@. Many of these header files can work without
change in C++ provided @code{__STDC__} is defined. If @code{__STDC__}
is not defined, they will all fail, and will all need to be changed to
test explicitly for C++ as well.
@item
Deleting ``empty'' loops.
Historically, GCC has not deleted ``empty'' loops under the
assumption that the most likely reason you would put one in a program is
to have a delay, so deleting them will not make real programs run any
faster.
However, the rationale here is that optimization of a nonempty loop
cannot produce an empty one. This held for carefully written C compiled
with less powerful optimizers but is not always the case for carefully
written C++ or with more powerful optimizers.
Thus GCC will remove operations from loops whenever it can determine
those operations are not externally visible (apart from the time taken
to execute them, of course). In case the loop can be proved to be finite,
GCC will also remove the loop itself.
Be aware of this when performing timing tests, for instance the
following loop can be completely removed, provided
@code{some_expression} can provably not change any global state.
@smallexample
@{
int sum = 0;
int ix;
for (ix = 0; ix != 10000; ix++)
sum += some_expression;
@}
@end smallexample
Even though @code{sum} is accumulated in the loop, no use is made of
that summation, so the accumulation can be removed.
@item
Making side effects happen in the same order as in some other compiler.
@cindex side effects, order of evaluation
@cindex order of evaluation, side effects
It is never safe to depend on the order of evaluation of side effects.
For example, a function call like this may very well behave differently
from one compiler to another:
@smallexample
void func (int, int);
int i = 2;
func (i++, i++);
@end smallexample
There is no guarantee (in either the C or the C++ standard language
definitions) that the increments will be evaluated in any particular
order. Either increment might happen first. @code{func} might get the
arguments @samp{2, 3}, or it might get @samp{3, 2}, or even @samp{2, 2}.
@item
Making certain warnings into errors by default.
Some ISO C testsuites report failure when the compiler does not produce
an error message for a certain program.
@opindex pedantic-errors
ISO C requires a ``diagnostic'' message for certain kinds of invalid
programs, but a warning is defined by GCC to count as a diagnostic. If
GCC produces a warning but not an error, that is correct ISO C support.
If testsuites call this ``failure'', they should be run with the GCC
option @option{-pedantic-errors}, which will turn these warnings into
errors.
@end itemize
@node Warnings and Errors
@section Warning Messages and Error Messages
@cindex error messages
@cindex warnings vs errors
@cindex messages, warning and error
The GNU compiler can produce two kinds of diagnostics: errors and
warnings. Each kind has a different purpose:
@itemize @w{}
@item
@dfn{Errors} report problems that make it impossible to compile your
program. GCC reports errors with the source file name and line
number where the problem is apparent.
@item
@dfn{Warnings} report other unusual conditions in your code that
@emph{may} indicate a problem, although compilation can (and does)
proceed. Warning messages also report the source file name and line
number, but include the text @samp{warning:} to distinguish them
from error messages.
@end itemize
Warnings may indicate danger points where you should check to make sure
that your program really does what you intend; or the use of obsolete
features; or the use of nonstandard features of GNU C or C++. Many
warnings are issued only if you ask for them, with one of the @option{-W}
options (for instance, @option{-Wall} requests a variety of useful
warnings).
@opindex pedantic
@opindex pedantic-errors
GCC always tries to compile your program if possible; it never
gratuitously rejects a program whose meaning is clear merely because
(for instance) it fails to conform to a standard. In some cases,
however, the C and C++ standards specify that certain extensions are
forbidden, and a diagnostic @emph{must} be issued by a conforming
compiler. The @option{-pedantic} option tells GCC to issue warnings in
such cases; @option{-pedantic-errors} says to make them errors instead.
This does not mean that @emph{all} non-ISO constructs get warnings
or errors.
@xref{Warning Options,,Options to Request or Suppress Warnings}, for
more detail on these and related command-line options.
|