# GCC Bugs

The latest version of this document is always available at http://gcc.gnu.org/bugs.html.

## Table of Contents

- Reporting Bugs
  - What we need
  - What we DON'T want
  - Where to post it
  - Detailed bug reporting instructions
  - Detailed bug reporting instructions for GNAT
  - Detailed bug reporting instructions when using a precompiled header
- Frequently Reported Bugs in GCC
  - Fortran
- Non-bugs
  - General
  - C
  - C++
    - 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.

Before you report a bug, please check the list of well-known bugs and, if possible, try a current development snapshot. If you want to report a bug with versions of GCC before 3.4 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 `#include`s 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)
- 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 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.

## Detailed bug reporting instructions

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`

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.

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 `INCLUDE`d 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.

## Detailed bug reporting instructions for GNAT

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:

- 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 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 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.

# 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++

## Fortran

G77 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.

# 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 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.

  This 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.

## C

Increment/decrement operator (`++`/`--`) not working as expected - a problem with many variations.

  The following expressions have unpredictable results:

  >     x[i]=++i
  >     foo(i,++i)
  >     i*(++i)                 /* special case with foo=="operator*" */
  >     std::cout << i << ++i   /* foo(foo(std::cout,i),++i)          */

  since the `i` without increment can be evaluated before or after `++i`.

  The C and C++ standards have the notion of "sequence points". Everything that happens between two sequence points happens in an unspecified order, but it has to happen after the first and before the second sequence point. The end of a statement and a function call are examples for sequence points, whereas assignments and the comma between function arguments are not.

  Modifying a value twice between two sequence points as shown in the following examples is even worse:

  >     i=++i
  >     foo(++i,++i)
  >     (++i)*(++i)               /* special case with foo=="operator*" */
  >     std::cout << ++i << ++i   /* foo(foo(std::cout,++i),++i)        */

  This leads to undefined behavior (i.e. the compiler can do anything).

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 `short`s and 32-bit `int`s):

  >     #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.

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 GNU libc web pages for details.

## C++

  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.

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.

Copy constructor access check while initializing a reference.

  Consider this code:

  >     class A
  >     {
  >     public:
  >       A();

  >     private:
  >       A(const A&);   // private copy ctor
  >     };

  >     A makeA(void);
  >     void foo(const A&);

  >     void bar(void)
  >     {
  >       foo(A());       // error, copy ctor is not accessible
  >       foo(makeA());   // error, copy ctor is not accessible

  >       A a1;
  >       foo(a1);        // OK, a1 is a lvalue
  >     }

  Starting with GCC 3.4.0, binding an rvalue to a const reference requires an accessible copy constructor. This might be surprising at first sight, especially since most popular compilers do not correctly implement this rule.

  The C++ Standard says that a temporary object should be created in this context and its contents filled with a copy of the object we are trying to bind to the reference; it also says that the temporary copy can be elided, but the semantic constraints (eg. accessibility) of the copy constructor still have to be checked.

  For further information, you can consult the following paragraphs of the C++ standard: [dcl.init.ref]/5, bullet 2, sub-bullet 1, and [class.temporary]/2.

### Common problems when upgrading the compiler

#### ABI changes

The C++ application binary interface (ABI) consists of two components: the first defines how the elements of classes are laid out, how functions are called, how function names are mangled, etc; the second part deals with the internals of the objects in libstdc++. Although we strive for a non-changing ABI, so far we have had to modify it with each major release. If you change your compiler to a different major release you must recompile all libraries that contain C++ code. If you fail to do so you risk getting linker errors or malfunctioning programs. Some of our Java support libraries also contain C++ code, so you might want to recompile all libraries to be safe. It should not be necessary to recompile if you have changed to a bug-fix release of the same version of the compiler; bug-fix releases are careful to avoid ABI changes. See also the compatibility section of the GCC manual.

Remark: A major release is designated by a change to the first or second component of the two- or three-part version number. A minor (bug-fix) release is designated by a change to the third component only. Thus GCC 3.2 and 3.3 are major releases, while 3.3.1 and 3.3.2 are bug-fix releases for GCC 3.3. With the 3.4 series we are introducing a new naming scheme; the first release of this series is 3.4.0 instead of just 3.4.

#### Standard conformance

With each release, we try to make G++ conform 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://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html & http://www.open-std.org/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 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 Common Misunderstandings with GNU C++.