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GCC Bugs
The latest version of this document is always available at
[1]http://gcc.gnu.org/bugs.html.
_________________________________________________________________
Table of Contents
* [2]Reporting Bugs
+ [3]What we need
+ [4]What we DON'T want
+ [5]Where to post it
+ [6]Detailed bug reporting instructions
+ [7]Detailed bug reporting instructions for GNAT
+ [8]Detailed bug reporting instructions when using a
precompiled header
* [9]Frequently Reported Bugs in GCC
+ [10]C++
o [11]Missing features
o [12]Bugs fixed in the upcoming 3.4 series
+ [13]Fortran
* [14]Non-bugs
+ [15]General
+ [16]C
+ [17]C++
o [18]Common problems when upgrading the compiler
_________________________________________________________________
Reporting Bugs
The main purpose of a bug report is to enable us to fix the bug. The
most important prerequisite for this is that the report must be
complete and self-contained, which we explain in detail below.
Before you report a bug, please check the [19]list of well-known bugs
and, if possible in any way, try a current development snapshot. If
you want to report a bug with versions of GCC before 3.1 we strongly
recommend upgrading to the current release first.
Before reporting that GCC compiles your code incorrectly, please
compile it with gcc -Wall and see whether this shows anything wrong
with your code that could be the cause instead of a bug in GCC.
Summarized bug reporting instructions
After this summary, you'll find detailed bug reporting instructions,
that explain how to obtain some of the information requested in this
summary.
What we need
Please include in your bug report all of the following items, the
first three of which can be obtained from the output of gcc -v:
* the exact version of GCC;
* the system type;
* the options given when GCC was configured/built;
* the complete command line that triggers the bug;
* the compiler output (error messages, warnings, etc.); and
* the preprocessed file (*.i*) that triggers the bug, generated by
adding -save-temps to the complete compilation command, or, in the
case of a bug report for the GNAT front end, a complete set of
source files (see below).
What we do not want
* A source file that #includes header files that are left out of the
bug report (see above)
* That source file and a collection of header files.
* An attached archive (tar, zip, shar, whatever) containing all (or
some :-) of the above.
* A code snippet that won't cause the compiler to produce the exact
output mentioned in the bug report (e.g., a snippet with just a
few lines around the one that apparently triggers the bug, with
some pieces replaced with ellipses or comments for extra
obfuscation :-)
* The location (URL) of the package that failed to build (we won't
download it, anyway, since you've already given us what we need to
duplicate the bug, haven't you? :-)
* An error that occurs only some of the times a certain file is
compiled, such that retrying a sufficient number of times results
in a successful compilation; this is a symptom of a hardware
problem, not of a compiler bug (sorry)
* E-mail messages that complement previous, incomplete bug reports.
Post a new, self-contained, full bug report instead, if possible
as a follow-up to the original bug report
* Assembly files (*.s) produced by the compiler, or any binary
files, such as object files, executables, core files, or
precompiled header files
* Duplicate bug reports, or reports of bugs already fixed in the
development tree, especially those that have already been reported
as fixed last week :-)
* Bugs in the assembler, the linker or the C library. These are
separate projects, with separate mailing lists and different bug
reporting procedures
* Bugs in releases or snapshots of GCC not issued by the GNU
Project. Report them to whoever provided you with the release
* Questions about the correctness or the expected behavior of
certain constructs that are not GCC extensions. Ask them in forums
dedicated to the discussion of the programming language
Where to post it
Please submit your bug report directly to the [20]GCC bug database.
Alternatively, you can use the gccbug script that mails your bug
report to the bug database.
Only if all this is absolutely impossible, mail all information to
[21]gcc-bugs@gcc.gnu.org.
Detailed bug reporting instructions
Please refer to the [22]next section when reporting bugs in GNAT, the
Ada compiler, or to the [23]one after that when reporting bugs that
appear when using a precompiled header.
In general, all the information we need can be obtained by collecting
the command line below, as well as its output and the preprocessed
file it generates.
gcc -v -save-temps all-your-options source-file
Typically the preprocessed file (extension .i for C or .ii for C++,
and .f if the preprocessor is used on Fortran files) will be large, so
please compress the resulting file with one of the popular compression
programs such as bzip2, gzip, zip or compress (in decreasing order of
preference). Use maximum compression (-9) if available. Please include
the compressed preprocessor output in your bug report, even if the
source code is freely available elsewhere; it makes the job of our
volunteer testers much easier.
The only excuses to not send us the preprocessed sources are (i) if
you've found a bug in the preprocessor, (ii) if you've reduced the
testcase to a small file that doesn't include any other file or (iii)
if the bug appears only when using precompiled headers. If you can't
post the preprocessed sources because they're proprietary code, then
try to create a small file that triggers the same problem.
Since we're supposed to be able to re-create the assembly output
(extension .s), you usually should not include it in the bug report,
although you may want to post parts of it to point out assembly code
you consider to be wrong.
Whether to use MIME attachments or uuencode is up to you. In any case,
make sure the compiler command line, version and error output are in
plain text, so that we don't have to decode the bug report in order to
tell who should take care of it. A meaningful subject indicating
language and platform also helps.
Please avoid posting an archive (.tar, .shar or .zip); we generally
need just a single file to reproduce the bug (the .i/.ii/.f
preprocessed file), and, by storing it in an archive, you're just
making our volunteers' jobs harder. Only when your bug report requires
multiple source files to be reproduced should you use an archive. This
is, for example, the case if you are using INCLUDE directives in
Fortran code, which are not processed by the preprocessor, but the
compiler. In that case, we need the main file and all INCLUDEd files.
In any case, make sure the compiler version, error message, etc, are
included in the body of your bug report as plain text, even if
needlessly duplicated as part of an archive.
If you fail to supply enough information for a bug report to be
reproduced, someone will probably ask you to post additional
information (or just ignore your bug report, if they're in a bad day,
so try to get it right on the first posting :-). In this case, please
post the additional information to the bug reporting mailing list, not
just to the person who requested it, unless explicitly told so. If
possible, please include in this follow-up all the information you had
supplied in the incomplete bug report (including the preprocessor
output), so that the new bug report is self-contained.
Detailed bug reporting instructions for GNAT
See the [24]previous section for bug reporting instructions for GCC
language implementations other than Ada.
Bug reports have to contain at least the following information in
order to be useful:
* the exact version of GCC, as shown by "gcc -v";
* the system type;
* the options when GCC was configured/built;
* the exact command line passed to the gcc program triggering the
bug (not just the flags passed to gnatmake, but gnatmake prints
the parameters it passed to gcc)
* a collection of source files for reproducing the bug, preferably a
minimal set (see below);
* a description of the expected behavior;
* a description of actual behavior.
If your code depends on additional source files (usually package
specifications), submit the source code for these compilation units in
a single file that is acceptable input to gnatchop, i.e. contains no
non-Ada text. If the compilation terminated normally, you can usually
obtain a list of dependencies using the "gnatls -d main_unit" command,
where main_unit is the file name of the main compilation unit (which
is also passed to gcc).
If you report a bug which causes the compiler to print a bug box,
include that bug box in your report, and do not forget to send all the
source files listed after the bug box along with your report.
If you use gnatprep, be sure to send in preprocessed sources (unless
you have to report a bug in gnatprep).
When you have checked that your report meets these criteria, please
submit it according to our [25]generic instructions. (If you use a
mailing list for reporting, please include an "[Ada]" tag in the
subject.)
Detailed bug reporting instructions when using a precompiled header
If you're encountering a bug when using a precompiled header, the
first thing to do is to delete the precompiled header, and try running
the same GCC command again. If the bug happens again, the bug doesn't
really involve precompiled headers, please report it without using
them by following the instructions [26]above.
If you've found a bug while building a precompiled header (for
instance, the compiler crashes), follow the usual instructions
[27]above.
If you've found a real precompiled header bug, what we'll need to
reproduce it is the sources to build the precompiled header (as a
single .i file), the source file that uses the precompiled header, any
other headers that source file includes, and the command lines that
you used to build the precompiled header and to use it.
Please don't send us the actual precompiled header. It is likely to be
very large and we can't use it to reproduce the problem.
_________________________________________________________________
Frequently Reported Bugs in GCC
This is a list of bugs in GCC that are reported very often, but not
yet fixed. While it is certainly better to fix bugs instead of
documenting them, this document might save people the effort of
writing a bug report when the bug is already well-known.
There are many reasons why a reported bug doesn't get fixed. It might
be difficult to fix, or fixing it might break compatibility. Often,
reports get a low priority when there is a simple work-around. In
particular, bugs caused by invalid code have a simple work-around: fix
the code.
_________________________________________________________________
C++
Missing features
The export keyword is not implemented.
Most C++ compilers (G++ included) do not yet implement export,
which is necessary for separate compilation of template
declarations and definitions. Without export, a template
definition must be in scope to be used. The obvious workaround
is simply to place all definitions in the header itself.
Alternatively, the compilation unit containing template
definitions may be included from the header.
Bugs fixed in the upcoming 3.4 series
The following bugs are present up to (and including) GCC 3.3.x. They
have been fixed in 3.4.0.
Two-stage name-lookup.
GCC did not implement two-stage name-lookup (also see
[28]below).
Covariant return types.
GCC did not implement non-trivial covariant returns.
Parse errors for "simple" code.
GCC gave parse errors for seemingly simple code, such as
struct A
{
A();
A(int);
};
struct B
{
B(A);
B(A,A);
void foo();
};
A bar()
{
B b(A(),A(1)); // Variable b, initialized with two temporaries
B(A(2)).foo(); // B temporary, initialized with A temporary
return (A()); // return A temporary
}
Although being valid code, each of the three lines with a
comment was rejected by GCC. The work-arounds for older
compiler versions proposed below do not change the semantics of
the programs at all.
The problem in the first case was that GCC started to parse the
declaration of b as a function called b returning B, taking a
function returning A as an argument. When it encountered the 1,
it was too late. To show the compiler that this should be
really an expression, a comma operator with a dummy argument
could be used:
B b((0,A()),A(1));
The work-around for simpler cases like the second one was to
add additional parentheses around the expressions that were
mistaken as declarations:
(B(A(2))).foo();
In the third case, however, additional parentheses were causing
the problems: The compiler interpreted A() as a function
(taking no arguments, returning A), and (A()) as a cast lacking
an expression to be casted, hence the parse error. The
work-around was to omit the parentheses:
return A();
This problem occured in a number of variants; in throw
statements, people also frequently put the object in
parentheses.
_________________________________________________________________
Fortran
Fortran bugs are documented in the G77 manual rather than explicitly
listed here. Please see [29]Known Causes of Trouble with GNU Fortran
in the G77 manual.
_________________________________________________________________
Non-bugs
The following are not actually bugs, but are reported often enough to
warrant a mention here.
It is not always a bug in the compiler, if code which "worked" in a
previous version, is now rejected. Earlier versions of GCC sometimes
were less picky about standard conformance and accepted invalid source
code. In addition, programming languages themselves change, rendering
code invalid that used to be conforming (this holds especially for
C++). In either case, you should update your code to match recent
language standards.
_________________________________________________________________
General
Problems with floating point numbers - the [30]most often reported
non-bug.
In a number of cases, GCC appears to perform floating point
computations incorrectly. For example, the C++ program
#include <iostream>
int main()
{
double a = 0.5;
double b = 0.01;
std::cout << (int)(a / b) << std::endl;
return 0;
}
might print 50 on some systems and optimization levels, and 49
on others.
The is the result of rounding: The computer cannot represent
all real numbers exactly, so it has to use approximations. When
computing with approximation, the computer needs to round to
the nearest representable number.
This is not a bug in the compiler, but an inherent limitation
of the floating point types. Please study [31]this paper for
more information.
_________________________________________________________________
C
Casting does not work as expected when optimization is turned on.
This is often caused by a violation of aliasing rules, which
are part of the ISO C standard. These rules say that a program
is invalid if you try to access a variable through a pointer of
an incompatible type. This is happening in the following
example where a short is accessed through a pointer to integer
(the code assumes 16-bit shorts and 32-bit ints):
#include <stdio.h>
int main()
{
short a[2];
a[0]=0x1111;
a[1]=0x1111;
*(int *)a = 0x22222222; /* violation of aliasing rules */
printf("%x %x\n", a[0], a[1]);
return 0;
}
The aliasing rules were designed to allow compilers more
aggressive optimization. Basically, a compiler can assume that
all changes to variables happen through pointers or references
to variables of a type compatible to the accessed variable.
Dereferencing a pointer that violates the aliasing rules
results in undefined behavior.
In the case above, the compiler may assume that no access
through an integer pointer can change the array a, consisting
of shorts. Thus, printf may be called with the original values
of a[0] and a[1]. What really happens is up to the compiler and
may change with architecture and optimization level.
Recent versions of GCC turn on the option -fstrict-aliasing
(which allows alias-based optimizations) by default with -O2.
And some architectures then really print "1111 1111" as result.
Without optimization the executable will generate the
"expected" output "2222 2222".
To disable optimizations based on alias-analysis for faulty
legacy code, the option -fno-strict-aliasing can be used as a
work-around.
The option -Wstrict-aliasing (which is included in -Wall) warns
about some - but not all - cases of violation of aliasing rules
when -fstrict-aliasing is active.
To fix the code above, you can use a union instead of a cast
(note that this is a GCC extension which might not work with
other compilers):
#include <stdio.h>
int main()
{
union
{
short a[2];
int i;
} u;
u.a[0]=0x1111;
u.a[1]=0x1111;
u.i = 0x22222222;
printf("%x %x\n", u.a[0], u.a[1]);
return 0;
}
Now the result will always be "2222 2222".
For some more insight into the subject, please have a look at
[32]this article.
Cannot use preprocessor directive in macro arguments.
Let me guess... you used an older version of GCC to compile
code that looks something like this:
memcpy(dest, src,
#ifdef PLATFORM1
12
#else
24
#endif
);
and you got a whole pile of error messages:
test.c:11: warning: preprocessing directive not recognized within macro arg
test.c:11: warning: preprocessing directive not recognized within macro arg
test.c:11: warning: preprocessing directive not recognized within macro arg
test.c: In function `foo':
test.c:6: undefined or invalid # directive
test.c:8: undefined or invalid # directive
test.c:9: parse error before `24'
test.c:10: undefined or invalid # directive
This is because your C library's <string.h> happens to define
memcpy as a macro - which is perfectly legitimate. In recent
versions of glibc, for example, printf is among those functions
which are implemented as macros.
Versions of GCC prior to 3.3 did not allow you to put #ifdef
(or any other preprocessor directive) inside the arguments of a
macro. The code therefore would not compile.
As of GCC 3.3 this kind of construct is always accepted and the
preprocessor will probably do what you expect, but see the
manual for detailed semantics.
However, this kind of code is not portable. It is "undefined
behavior" according to the C standard; that means different
compilers may do different things with it. It is always
possible to rewrite code which uses conditionals inside macros
so that it doesn't. You could write the above example
#ifdef PLATFORM1
memcpy(dest, src, 12);
#else
memcpy(dest, src, 24);
#endif
This is a bit more typing, but I personally think it's better
style in addition to being more portable.
Cannot initialize a static variable with stdin.
This has nothing to do with GCC, but people ask us about it a
lot. Code like this:
#include <stdio.h>
FILE *yyin = stdin;
will not compile with GNU libc, because stdin is not a
constant. This was done deliberately, to make it easier to
maintain binary compatibility when the type FILE needs to be
changed. It is surprising for people used to traditional Unix C
libraries, but it is permitted by the C standard.
This construct commonly occurs in code generated by old
versions of lex or yacc. We suggest you try regenerating the
parser with a current version of flex or bison, respectively.
In your own code, the appropriate fix is to move the
initialization to the beginning of main.
There is a common misconception that the GCC developers are
responsible for GNU libc. These are in fact two entirely
separate projects; please check the [33]GNU libc web pages for
details.
_________________________________________________________________
C++
Nested classes can access private members and types of the containing
class.
Defect report 45 clarifies that nested classes are members of
the class they are nested in, and so are granted access to
private members of that class.
G++ emits two copies of constructors and destructors.
In general there are three types of constructors (and
destructors).
1. The complete object constructor/destructor.
2. The base object constructor/destructor.
3. The allocating constructor/deallocating destructor.
The first two are different, when virtual base classes are
involved.
Global destructors are not run in the correct order.
Global destructors should be run in the reverse order of their
constructors completing. In most cases this is the same as the
reverse order of constructors starting, but sometimes it is
different, and that is important. You need to compile and link
your programs with --use-cxa-atexit. We have not turned this
switch on by default, as it requires a cxa aware runtime
library (libc, glibc, or equivalent).
Classes in exception specifiers must be complete types.
[15.4]/1 tells you that you cannot have an incomplete type, or
pointer to incomplete (other than cv void *) in an exception
specification.
Exceptions don't work in multithreaded applications.
You need to rebuild g++ and libstdc++ with --enable-threads.
Remember, C++ exceptions are not like hardware interrupts. You
cannot throw an exception in one thread and catch it in
another. You cannot throw an exception from a signal handler
and catch it in the main thread.
Templates, scoping, and digraphs.
If you have a class in the global namespace, say named X, and
want to give it as a template argument to some other class, say
std::vector, then std::vector<::X> fails with a parser error.
The reason is that the standard mandates that the sequence <:
is treated as if it were the token [. (There are several such
combinations of characters - they are called digraphs.)
Depending on the version, the compiler then reports a parse
error before the character : (the colon before X) or a missing
closing bracket ].
The simplest way to avoid this is to write std::vector< ::X>,
i.e. place a space between the opening angle bracket and the
scope operator.
Common problems when upgrading the compiler
ABI changes
The application binary interface (ABI) defines how the elements of
classes are laid out, how functions are called, how function names are
mangled etc. It usually changes with each major release (i.e. when the
first or second part of the version number changes). You must
recompile all C++ libraries, or you risk linker errors or
malfunctioning programs. However, the ABI is not changed with bug-fix
releases (i.e. when the third part of the version number changes). The
code should be binary compatible among these versions.
Standard conformance
With each release, we try to make G++ conform closer to the ISO C++
standard (available at [34]http://www.ncits.org/cplusplus.htm). We
have also implemented some of the core and library defect reports
(available at
[35]http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_defects.html &
[36]http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html
respectively).
Non-conforming legacy code that worked with older versions of GCC may
be rejected by more recent compilers. There is no command-line switch
to ensure compatibility in general, because trying to parse
standard-conforming and old-style code at the same time would render
the C++ frontend unmaintainable. However, some non-conforming
constructs are allowed when the command-line option -fpermissive is
used.
Two milestones in standard conformance are GCC 3.0 (including a major
overhaul of the standard library) and the upcoming 3.4.0 version (with
its new C++ parser).
New in GCC 3.0
* The standard library is much more conformant, and uses the std::
namespace (which is now a real namespace, not an alias for ::).
* The standard header files for the c library don't end with .h, but
begin with c (i.e. <cstdlib> rather than <stdlib.h>). The .h names
are still available, but are deprecated.
* <strstream> is deprecated, use <sstream> instead.
* streambuf::seekoff & streambuf::seekpos are private, instead use
streambuf::pubseekoff & streambuf::pubseekpos respectively.
* If std::operator << (std::ostream &, long long) doesn't exist, you
need to recompile libstdc++ with --enable-long-long.
If you get lots of errors about things like cout not being found,
you've most likely forgotten to tell the compiler to look in the std::
namespace. There are several ways to do this:
* Say std::cout at the call. This is the most explicit way of saying
what you mean.
* Say using std::cout; somewhere before the call. You will need to
do this for each function or type you wish to use from the
standard library.
* Say using namespace std; somewhere before the call. This is the
quick-but-dirty fix. This brings the whole of the std:: namespace
into scope. Never do this in a header file, as every user of your
header file will be affected by this decision.
New in GCC 3.4.0
The new parser brings a lot of improvements, especially concerning
name-lookup.
* The "implicit typename" extension got removed (it was already
deprecated since GCC 3.1), so that the following code is now
rejected, see [14.6]:
template <typename> struct A
{
typedef int X;
};
template <typename T> struct B
{
A<T>::X x; // error
typename A<T>::X y; // OK
};
B<void> b;
* For similar reasons, the following code now requires the template
keyword, see [14.2]:
template <typename> struct A
{
template <int> struct X {};
};
template <typename T> struct B
{
typename A<T>::X<0> x; // error
typename A<T>::template X<0> y; // OK
};
B<void> b;
* We now have two-stage name-lookup, so that the following code is
rejected, see [14.6]/9:
template <typename T> int foo()
{
return i; // error
}
* This also affects members of base classes, see [14.6.2]:
template <typename> struct A
{
int i, j;
};
template <typename T> struct B : A<T>
{
int foo1() { return i; } // error
int foo2() { return this->i; } // OK
int foo3() { return B<T>::i; } // OK
int foo4() { return A<T>::i; } // OK
using A<T>::j;
int foo5() { return j; } // OK
};
In addition to the problems listed above, the manual contains a
section on [37]Common Misunderstandings with GNU C++.
References
1. http://gcc.gnu.org/bugs.html
2. http://gcc.gnu.org/bugs.html#report
3. http://gcc.gnu.org/bugs.html#need
4. http://gcc.gnu.org/bugs.html#dontwant
5. http://gcc.gnu.org/bugs.html#where
6. http://gcc.gnu.org/bugs.html#detailed
7. http://gcc.gnu.org/bugs.html#gnat
8. http://gcc.gnu.org/bugs.html#pch
9. http://gcc.gnu.org/bugs.html#known
10. http://gcc.gnu.org/bugs.html#cxx
11. http://gcc.gnu.org/bugs.html#missing
12. http://gcc.gnu.org/bugs.html#fixed34
13. http://gcc.gnu.org/bugs.html#fortran
14. http://gcc.gnu.org/bugs.html#nonbugs
15. http://gcc.gnu.org/bugs.html#nonbugs_general
16. http://gcc.gnu.org/bugs.html#nonbugs_c
17. http://gcc.gnu.org/bugs.html#nonbugs_cxx
18. http://gcc.gnu.org/bugs.html#upgrading
19. http://gcc.gnu.org/bugs.html#known
20. http://gcc.gnu.org/bugzilla/
21. mailto:gcc-bugs@gcc.gnu.org
22. http://gcc.gnu.org/bugs.html#gnat
23. http://gcc.gnu.org/bugs.html#pch
24. http://gcc.gnu.org/bugs.html#detailed
25. http://gcc.gnu.org/bugs.html#where
26. http://gcc.gnu.org/bugs.html#detailed
27. http://gcc.gnu.org/bugs.html#detailed
28. http://gcc.gnu.org/bugs.html#new34
29. http://gcc.gnu.org/onlinedocs/g77/Trouble.html
30. http://gcc.gnu.org/PR323
31. http://www.validlab.com/goldberg/paper.ps
32. http://mail-index.NetBSD.org/tech-kern/2003/08/11/0001.html
33. http://www.gnu.org/software/libc/
34. http://www.ncits.org/cplusplus.htm
35. http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_defects.html
36. http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html
37. http://gcc.gnu.org/onlinedocs/gcc/C---Misunderstandings.html
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