The latest version of this document is always available at http://gcc.gnu.org/bugs.html.
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 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.
After this summary, you'll find detailed bug reporting instructions, that explain how to obtain some of the information requested in this summary.
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:
*.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).#includes header files that are left
out of the bug report (see above)*.s) produced by the compiler, or any
binary files, such as object files, executables, core files, or
precompiled header filesPlease submit your bug report directly to the
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
gcc-bugs@gcc.gnu.org, but note that
such reports are often overlooked since they are not permanently recorded into
the database for later processing.
Please refer to the next section when reporting bugs in GNAT, the Ada compiler, or to the 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.
See the 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:
gcc -v";gcc program
triggering the bug
(not just the flags passed to gnatmake, but
gnatmake prints the parameters it passed to gcc)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 generic instructions.
(If you use a mailing list for reporting, please include an
"[Ada]" tag in the subject.)
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 above.
If you've found a bug while building a precompiled header (for instance, the compiler crashes), follow the usual instructions 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.
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.
GCC 3.0 had a new ABI, which affected class layout, function mangling and calling conventions. We had intended it to be complete, but unfortunately some issues came to light, too late to fix in the 3.0 series. The ABI should not change in dot releases, so we addressed most issues in GCC 3.1.
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.
Up to and including GCC 3.0, the compiler will give "parse error" for seemingly simple code, such as
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.
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 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.
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 this paper for more information.
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 this article.
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.
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 GNU libc web pages for details.
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.
In general there are three types of constructors (and destructors).
The first two are different, when virtual base classes are involved.
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).
[15.4]/1 tells you that you cannot have an incomplete type, or
pointer to incomplete (other than cv void *) in
an exception specification.
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.
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.
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.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.This means you may get lots of errors about things like
strcmp 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,
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.In addition to the problems listed above, the manual contains a section on Common Misunderstandings with GNU C++.