
                                  GCC Bugs
                                      
   The latest version of this document is always available at
   [1]http://www.gnu.org/software/gcc/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]Managing Bugs (GNATS and the test-suite)
     * [8]Frequently Reported Bugs in GCC 3.0
          + [9]General
          + [10]Fortran
          + [11]C
          + [12]C++
               o [13]Common problems updating from G++ 2.95 to G++ 3.0
               o [14]Non-bugs
               o [15]Missing features
               o [16]Parse errors for "simple" code
               o [17]Optimization at -O3 takes a very long time
     _________________________________________________________________
   
                                Reporting Bugs
                                       
   Our preferred way of receiving bugs is via the [18]GCC GNATS bug
   reporting system.
   
   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 3.0 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.
   
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 GCC version (not just 3.xx; run gcc -v)
     * The system type (the dir name after gcc-lib in the output of gcc
       -v
     * The complete command line that triggers the bug
     * The compiler output (error messages, warnings, etc)
     * The preprocessed file (*.i*) that triggers the bug, generated by
       adding -save-temps to the complete compilation command (see below)
     * The options given when GCC was configured/built
       
  What we DON'T 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 (don't waste
       your time; the preprocessor will collect them for you! :-)
     * An attached archive (tar, zip, shar, whatever) containing all (or
       some :-) of the above, so that we can tell what the bug report is
       about without having to unarchive it
     * 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 (or reply) to the original bug report
     * Assembly files (*.s) produced by the compiler, or any binary
       files, such as object files, executables or core 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 GNATS bug
   database. If this is not possible, please mail all information to
   [21]gcc-bugs@gcc.gnu.org.
   
Detailed bug reporting instructions

   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++)
   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 [22]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.
   
                   Managing Bugs (GNATS and the test-suite)
                                       
   This section contains information mostly intended for GCC
   contributors.
   
   If you find a bug, but you are not fixing it (yet):
    1. Create a (minimal) test-case.
    2. Add the test-case to our test-suite, marking it as XFAIL.
    3. Add a bug report referencing the test-case to GNATS.
       
   If you fix a bug for which there is already a GNATS entry:
    1. Remove the XFAIL on the test-case.
    2. Close the bug report in GNATS.
       
   If you find a bug, and you are fixing it right then:
    1. Create a (minimal) test-case.
    2. Add the test-case to our test-suite, marking it as PASS.
    3. Check in your fixes.
     _________________________________________________________________
   
                      Frequently Reported Bugs in GCC 3.0
                                       
General

   The following bugs are very frequently reported.
     * GCC 2.95.2 does not build on GNU/Linux systems using glibc 2.2,
       such as Red Hat 7.0. This is fixed in GCC 2.95.3 and GCC 3.0.
     * GCC 2.95.2 crashes when compiling mbx.c from the PINE 4.30 or
       IMAP2000 distribution on Sparc systems running Solaris.
     _________________________________________________________________
   
Fortran

   Fortran bugs are documented in the G77 manual rather than explicitly
   listed here. Please see [23]Known Causes of Trouble with GNU Fortran
   in the G77 manual.
     _________________________________________________________________
   
C

   The following are not bugs in the C compiler, but are reported often
   enough to warrant a mention here.
   
   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 (GNU/Linux libc6), because 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 [24]GNU libc web pages for
          details.
          
   Cannot use preprocessor directive in macro arguments.
          Let me guess... you wrote 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
     test.c:11: parse error before `#'
     
          The problem, simply put, is that GCC's preprocessor does not
          allow you to put #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)
     
          which is impossible to implement in portable C without leaking
          memory. Allowing only a subset of directives would be
          confusing.
          
          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.
          
          In recent versions of glibc, printf is among the functions
          which are implemented as macros.
     _________________________________________________________________
   
C++

   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. [25]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.
   
  Common problems updating from G++ 2.95 to G++ 3.0
  
   G++ 3.0 conforms much closer to the ISO C++ standard (available at
   [26]http://www.ncits.org/cplusplus.htm).
   
   We have also implemented some of the core and library defect reports
   (available at
   [27]http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_defects.html &
   [28]http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html
   respectively).
     * The ABI has changed. This means that both class layout and name
       mangling is different. You must recompile all c++ libraries (if
       you don't you will get link errors).
     * The standard library is much more conformant, and uses the std::
       namespace.
     * std:: 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. (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.
     * If 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 forgotton to tell the compiler to look
   in the std:: namespace. There are several ways to do this,
     * Say, std::strcmp at the call. This is the most explicit way of
       saying what you mean.
     * Say, 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.
     * 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 you will be forcing
       users of your header file to do the same.
       
  Non-bugs
  
   Here are some features that have been reported as bugs, but are not.
   
   Nested classes can access private types of the containing class.
          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.
          
   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.
          
   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 destructor/deallocating destructor.
            
          The first two are different, when virtual base classes are
          involved. In some cases we can do better, and this is logged in
          GNATS.
          
   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.
          
   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).
          
   Problems with floating point computations.
          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;
     }
     
          might print 50 on some systems and optimization levels, and 51
          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 float and double types. Please study [29]this paper for
          more information.
          
  Missing features
  
   We know some things are missing from G++.
   
   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.
          
   Two stage lookup in templates is not implemented.
          [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.
          
  Parse errors for "simple" code
  
   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.
   
  Optimization at -O3 takes a very long time
  
   At -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.

References

   1. http://www.gnu.org/software/gcc/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#manage
   8. http://gcc.gnu.org/bugs.html#known
   9. http://gcc.gnu.org/bugs.html#general
  10. http://gcc.gnu.org/bugs.html#fortran
  11. http://gcc.gnu.org/bugs.html#c
  12. http://gcc.gnu.org/bugs.html#cplusplus
  13. http://gcc.gnu.org/bugs.html#updating
  14. http://gcc.gnu.org/bugs.html#nonbugs
  15. http://gcc.gnu.org/bugs.html#missing
  16. http://gcc.gnu.org/bugs.html#parsing
  17. http://gcc.gnu.org/bugs.html#-O3
  18. http://gcc.gnu.org/gnats.html
  19. http://gcc.gnu.org/bugs.html#known
  20. http://gcc.gnu.org/gnats.html
  21. mailto:gcc-bugs@gcc.gnu.org
  22. http://gcc.gnu.org/gnats.html
  23. http://gcc.gnu.org/onlinedocs/g77_bugs.html
  24. http://www.gnu.org/software/glibc/
  25. http://gcc.gnu.org/bugs.html#report
  26. http://www.ncits.org/cplusplus.htm
  27. http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_defects.html
  28. http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html
  29. http://www.validlab.com/goldberg/paper.ps
