The latest version of this document is always available at http://www.gnu.org/software/gcc/bugs.html.
Our preferred way of receiving bugs is via the GCC GNATS bug reporting system.
Before you report a bug, please check the 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 3.0 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.
After this summary, you'll find detailed bug reporting instructions, that explain how to obtain some of the information requested in this summary.
gcc -v)gcc-lib in the
output of gcc -v-save-temps to the complete
compilation command (see below)Please submit your bug report directly to the GCC GNATS bug database. If this is not possible, please mail all information to gcc-bugs@gcc.gnu.org.
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.i for C or
.ii for C++) 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, or (ii) if you've reduced the testcase to a small file that doesn't include any other file. 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 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. 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.
The gcc lists have message size limits (200 kbytes) and bug reports over those limits will currently be bounced. If your bug is larger than that, please post it using the GCC GNATS bug database.
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.
This section contains information mostly intended for GCC contributors.
If you find a bug, but you are not fixing it (yet):
If you fix a bug for which there is already a GNATS entry:
If you find a bug, and you are fixing it right then:
The following bugs are very frequently reported.
mbx.c from the PINE
4.30 or IMAP2000 distribution on Sparc systems running Solaris.Fortran bugs are documented in the G77 manual rather than explicitly listed here. Please see Known Causes of Trouble with GNU Fortran in the G77 manual.
The following are not bugs in the C compiler, but are reported often enough to warrant a mention here.
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;
stdin is not a constant. This was done deliberately, in
order for there to be no limit on the number of open FILE
objects. 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 GNU libc web pages for details.
Let me guess... you wrote code that looks something like this:
memcpy(dest, src,
#ifdef PLATFORM1
12
#else
24
#endif
);
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
test.c:11: parse error before `#'
#ifdef (or any other directive) inside the arguments of
a macro. Your C library's <string.h> happens to
define memcpy as a macro - this is perfectly legitimate.
The code therefore will not compile.
We have two good reasons for not allowing directives inside macro arguments. First, it is not portable. It is "undefined behavior" according to the C standard; that means different compilers will do different things with it. Some will give you errors. Some will dump core. Some will silently mangle your code - you could get the equivalent of
memcpy(dest, src, 1224);
from the above example. A very few might do what you expected it to. We therefore feel it is most useful for GCC to reject this construct immediately so that it is found and fixed.
Second, it is extraordinarily difficult to implement the preprocessor such that it does what you would expect for every possible directive found inside a macro argument. The best example is perhaps
#define foo(arg) ... arg ...
foo(blah
#undef foo
blah)
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
In recent versions of glibc, printf is among the
functions which are implemented as macros.
This is the list of bugs (and non-bugs) in g++ (aka GNU C++) 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. How to report bugs tells you how to report a bug.
There are many reasons why reported bugs don'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 C++ code have a simple work-around, fix the code. Now that there is an agreed ISO/ANSI standard for C++, the compiler has a definitive document to adhere to. Earlier versions might have accepted source code that is no longer C++. This means that code which might have `worked' in a previous version, is now rejected. You should update your code to be C++.
You should try to use the latest stable release of the GNU C++ compiler. This is currently 3.0. Many commonly reported bugs in earlier releases are fixed in that version.
G++ 3.0 conforms much closer to the ISO C++ standard (available at http://www.ncits.org/cplusplus.htm).
We have also implemented some of the core and library defect reports (available at http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_defects.html & http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html respectively).
std:: namespace.
std:: is now a real namespace, not an alias for
::.
.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. (The 3.0 BUGS file says
<stringstream>, that was an error,
stringstream is the class defined in
<sstream>.)
streambuf::seekoff &
streambuf::seekpos are private, instead use
streambuf::pubseekoff &
streambuf::pubseekpos respectively.
std::operator << (std::ostream &, long long)
doesn't exist, you need to recompile libstdc++ with
--enable-long-long.
strcmp not being found. You've most likely forgotton to
tell the compiler to look in the std:: namespace. There are
several ways to do this,
std::strcmp at the call. This is the most explicit
way of saying what you mean.
using std::strcmp; somewhere before the call. You
will need to do this for each function or type you wish to use from the
standard library.
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 you will be forcing users of your header file to do the
same.
Here are some features that have been reported as bugs, but are not.
G++ now implements type access control on member types. 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.
[15.4]/1 tells you that you cannot have an incomplete type, or
pointer to incomplete (other than cv void *) in
an exception specification.
In general there are three types of constructors (and destructors).
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.
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).
In a number of cases, GCC appears to perform floating point computations incorrectly. For example, the program
#include <iostream>
int main() {
double min = 0.0;
double max = 0.5;
double width = 0.01;
std::cout << (int)(((max - min) / width) - 1) <<
std::endl;
}
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 float and double types. Please study this paper for more information.
We know some things are missing from G++.
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.
[14.6] specifies how names are looked up inside a template. G++ does not do this correctly, but for most templates this will not be noticeable.
struct A{
A();
A(int);
void func();
};
struct B{
B(A);
B(A,A);
void func();
};
void foo(){
B b(A(),A(1)); //Variable b, initialized with two temporaries
B(A(2)).func(); //B temporary, initialized with A temporary
}
The problem is that GCC starts to parse the declaration of
b as a function b returning B,
taking a function returning A as an argument. When it
sees the 1, it is too late. The work-around in these cases is to add
additional parentheses around the expressions that are mistaken as
declarations:
(B(A(2))).func();Sometimes, even that is not enough; to show the compiler that this should be really an expression, a comma operator with a dummy argument can be used:
B b((0,A()),A(1));
Another example is the parse error for the return
statement in
struct A{};
struct B{
A a;
A f1(bool);
};
A B::f1(bool b)
{
if (b)
return (A());
return a;
}
The problem is that the compiler interprets A() as a
function (taking no arguments, returning A), and
(A()) as a cast - with a missing expression, hence the
parse error. The work-around is to omit the parentheses:
if (b)
return A();
This problem occurs in a number of variants; in throw
statements, people also frequently put the object in parentheses. The
exact error also somewhat varies with the compiler version. The
work-arounds proposed do not change the semantics of the program at
all; they make them perhaps less readable.
-O3 takes a
very long timeAt -O3, all functions are candidates for inlining. The
heuristic used has some deficiencies which show up when allowed such
freedom. This is g++ specific, as it has an earlier inliner than
gcc.