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|
<html>
<head>
<title>GCC Bugs</title>
</head>
<body>
<h1>GCC Bugs</h1>
<p>The latest version of this document is always available at
<a href="http://www.gnu.org/software/gcc/bugs.html">http://www.gnu.org/software/gcc/bugs.html</a>.</p>
<hr />
<h2>Table of Contents</h2>
<ul>
<li><a href="#report">Reporting Bugs</a>
<ul>
<li><a href="#need">What we need</a></li>
<li><a href="#dontwant">What we DON'T want</a></li>
<li><a href="#where">Where to post it</a></li>
<li><a href="#detailed">Detailed bug reporting instructions</a></li>
<li><a href="#gnat">Detailed bug reporting instructions for GNAT</a></li>
</ul>
</li>
<li><a href="#manage">Managing Bugs (GNATS and the test-suite)</a></li>
<li><a href="#known">Frequently Reported Bugs in GCC</a>
<ul>
<li><a href="#general">General</a></li>
<li><a href="#fortran">Fortran</a></li>
<li><a href="#c">C</a></li>
<li><a href="#cplusplus">C++</a>
<ul>
<li><a href="#updating">Common problems updating from G++ 2.95 to
G++ 3.0</a></li>
<li><a href="#nonbugs">Non-bugs</a></li>
<li><a href="#missing">Missing features</a></li>
<li><a href="#parsing">Parse errors for "simple" code</a></li>
<li><a href="#-O3">Optimization at <code>-O3</code> takes a
very long time</a></li>
</ul>
</li>
</ul>
</li>
</ul>
<hr />
<h1><a name="report">Reporting Bugs</a></h1>
<p>Our preferred way of receiving bugs is via the
<a href="gnats.html">GCC GNATS bug reporting system</a>.</p>
<p>Before you report a bug, please check the
<a href="#known">list of well-known bugs</a> and, <strong>if possible
in any way, try a current development snapshot</strong>.
If you want to report a bug with versions of GCC before 3.1 we strongly
recommend upgrading to the current release first.</p>
<p>Before reporting that GCC compiles your code incorrectly, please
compile it with <code>gcc -Wall</code> and see whether this shows
anything wrong with your code that could be the cause instead of a bug
in GCC.</p>
<h2>Summarized bug reporting instructions</h2>
<p>After this summary, you'll find detailed bug reporting
instructions, that explain how to obtain some of the information
requested in this summary.</p>
<h3><a name="need">What we need</a></h3>
Please include in your bug report all of the following items, the first
three of which can be obtained from the output of <code>gcc -v</code>:
<ul>
<li>the exact version of GCC;</li>
<li>the system type;</li>
<li>the options given when GCC was configured/built;</li>
<li>the complete command line that triggers the bug;</li>
<li>the compiler output (error messages, warnings, etc.); and</li>
<li>the <em>preprocessed</em> file (<code>*.i*</code>) that triggers the
bug, generated by adding <code>-save-temps</code> 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).</li>
</ul>
<h3><a name="dontwant">What we do <strong>not</strong> want</a></h3>
<ul>
<li>A source file that <code>#include</code>s header files that are left
out of the bug report (see above)</li>
<li>That source file and a collection of header files.</li>
<li>An attached archive (tar, zip, shar, whatever) containing all
(or some :-) of the above.</li>
<li>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 <b>apparently</b> triggers the bug,
with some pieces replaced with ellipses or comments for extra
obfuscation :-)</li>
<li>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? :-)</li>
<li>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)</li>
<li>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</li>
<li>Assembly files (<code>*.s</code>) produced by the compiler, or any
binary files, such as object files, executables or core files</li>
<li>Duplicate bug reports, or reports of bugs already fixed in the
development tree, especially those that have already been reported
as fixed last week :-)</li>
<li>Bugs in the assembler, the linker or the C library. These are
separate projects, with separate mailing lists and different bug
reporting procedures</li>
<li>Bugs in releases or snapshots of GCC not issued by the GNU
Project. Report them to whoever provided you with the release</li>
<li>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</li>
</ul>
<h3><a name="where">Where to post it</a></h3>
<p>Please submit your bug report directly to the
<a href="gnats.html">GCC GNATS bug database</a>.
Only if this is not possible, mail all information to
<a href="mailto:bug-gcc@gnu.org">bug-gcc@gnu.org</a> or
<a href="mailto:gcc-bugs@gcc.gnu.org">gcc-bugs@gcc.gnu.org</a>.</p>
<p>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 <a href="gnats.html">GCC GNATS bug
database</a>.</p>
<h2><a name="detailed">Detailed bug reporting instructions</a></h2>
<p>Please refer to the <a href="#gnat">next section</a> when reporting
bugs in GNAT, the Ada compiler.</p>
<p>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.</p>
<blockquote><code>gcc -v -save-temps <i>all-your-options
source-file</i></code></blockquote>
<p>Typically the preprocessed file (extension <code>.i</code> for C or
<code>.ii</code> 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
(<code>-9</code>) 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.</p>
<p>The <b>only</b> 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.</p>
<p>Since we're supposed to be able to re-create the assembly output
(extension <code>.s</code>), 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.</p>
<p>Whether to use MIME attachments or <code>uuencode</code> 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.</p>
<p>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.</p>
<p>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.</p>
<h2><a name="gnat">Detailed bug reporting instructions for GNAT</a></h2>
<p>See the <a href="#detailed">previous section</a> for bug reporting
instructions for GCC language implementations other than Ada.</p>
<p>Bug reports have to contain at least the following information in
order to be useful:</p>
<ul>
<li>the exact version of GCC, as shown by "<code>gcc -v</code>";</li>
<li>the system type;</li>
<li>the options when GCC was configured/built;</li>
<li>the exact command line passed to the <code>gcc</code> program
triggering the bug
(not just the flags passed to <code>gnatmake</code>, but
<code>gnatmake</code> prints the parameters it passed to <code>gcc</code>)</li>
<li>a collection of source files for reproducing the bug,
preferably a minimal set (see below);</li>
<li>a description of the expected behavior;</li>
<li>a description of actual behavior.</li>
</ul>
<p>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 <code>gnatchop</code>,
i.e. contains no non-Ada text. If the compilation terminated
normally, you can usually obtain a list of dependencies using the
"<code>gnatls -d <i>main_unit</i></code>" command, where
<code><i>main_unit</i></code> is the file name of the main compilation
unit (which is also passed to <code>gcc</code>).</p>
<p>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.</p>
<p>If you use <code>gnatprep</code>, be sure to send in preprocessed
sources (unless you have to report a bug in <code>gnatprep</code>).</p>
<p>When you have checked that your report meets these criteria, please
submit it accoding to our <a href="#where">generic instructions</a>.
(If you use a mailing list for reporting, please include an
"<code>[Ada]</code>" tag in the subject.)</p>
<h1><a name="manage">Managing Bugs (GNATS and the test-suite)</a></h1>
<p>This section contains information mostly intended for GCC
contributors.</p>
<p>If you find a bug, but you are not fixing it (yet):</p>
<ol>
<li>Create a (minimal) test-case.</li>
<li>Add the test-case to our test-suite, marking it as XFAIL unless
the bug is a regression.</li>
<li>Add a bug report referencing the test-case to GNATS.</li>
</ol>
<p>If you fix a bug for which there is already a GNATS entry:</p>
<ol>
<li>Remove the XFAIL on the test-case.</li>
<li>Close the bug report in GNATS.</li>
</ol>
<p>If you find a bug, and you are fixing it right then:</p>
<ol>
<li>Create a (minimal) test-case.</li>
<li>Add the test-case to our test-suite, marking it as PASS.</li>
<li>Check in your fixes.</li>
</ol>
<hr />
<h1><a name="known">Frequently Reported Bugs in GCC</a></h1>
<h2><a name="fortran">Fortran</a></h2>
<p>Fortran bugs are documented in the G77 manual rather than
explicitly listed here. Please see
<a href="http://gcc.gnu.org/onlinedocs/g77/Trouble.html">Known Causes of
Trouble with GNU Fortran</a> in the G77 manual.</p>
<hr />
<h2><a name="c">C</a></h2>
<p>The following are not bugs in the C compiler, but are reported
often enough to warrant a mention here.</p>
<dl>
<dt>Cannot initialize a static variable with <code>stdin</code>.</dt>
<dd><p>This has nothing to do with GCC, but people ask us about it a
lot. Code like this:</p>
<blockquote><pre>
#include <stdio.h>
FILE *yyin = stdin;
</pre></blockquote>
<p>will not compile with GNU libc (GNU/Linux libc6), because
<code>stdin</code> is not a constant. This was done deliberately, to make
it easier to maintain binary compatibility when the type <code>FILE</code>
needs to be changed. It is surprising for people used to traditional Unix
C libraries, but it is permitted by the C standard.</p>
<p>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.</p>
<p>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
<a href="http://www.gnu.org/software/glibc/">GNU libc web pages</a>
for details.
</p></dd>
<dt>Cannot use preprocessor directive in macro arguments.</dt>
<dd><p>Let me guess... you wrote code that looks something like this:</p>
<blockquote><pre>
memcpy(dest, src,
#ifdef PLATFORM1
12
#else
24
#endif
);
</pre></blockquote>
<p>and you got a whole pile of error messages:</p>
<blockquote><code>
test.c:11: warning: preprocessing directive not recognized within
macro arg<br />
test.c:11: warning: preprocessing directive not recognized within
macro arg<br />
test.c:11: warning: preprocessing directive not recognized within
macro arg<br />
test.c: In function `foo':<br />
test.c:6: undefined or invalid # directive<br />
test.c:8: undefined or invalid # directive<br />
test.c:9: parse error before `24'<br />
test.c:10: undefined or invalid # directive<br />
test.c:11: parse error before `#'<br />
</code></blockquote>
<p><strong>Update:</strong> As of GCC 3.2 this kind of construct is
always accepted and CPP will probably do what you expect, but see the
manual for detailed semantics.</p>
<p>However, versions of GCC prior to 3.2 did not allow you to put
<code>#ifdef</code> (or any other directive) inside the arguments of a
macro. Your C library's <code><string.h></code> happens to
define <code>memcpy</code> as a macro - this is perfectly legitimate.
The code therefore would not compile.</p>
<p>This kind of code is not portable. It is "undefined behavior"
according to the C standard; that means different compilers will 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</p>
<blockquote><pre>
#ifdef PLATFORM1
memcpy(dest, src, 12);
#else
memcpy(dest, src, 24);
#endif
</pre></blockquote>
<p>This is a bit more typing, but I personally think it's better style
in addition to being more portable.</p>
<p>In recent versions of glibc, <code>printf</code> is among the
functions which are implemented as macros.</p></dd>
</dl>
<hr />
<h2><a name="cplusplus">C++</a></h2>
<p>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. <a href="#report">How to report bugs</a> tells you how to
report a bug.</p>
<p>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,
<em>fix the code</em>. 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 <em>no longer</em>
C++. This means that code which might have `worked' in a previous
version, is now rejected. You should update your code to be C++.</p>
<p>You should try to use the latest stable release of the GNU C++
compiler.</p>
<h3><a name="updating">Common problems updating from G++ 2.95 to G++
3.0</a></h3>
<p>G++ 3.0 conforms much closer to the ISO C++ standard (available at
<a href="http://www.ncits.org/cplusplus.htm">http://www.ncits.org/cplusplus.htm</a>).</p>
<p>We have also implemented some of the core and library defect reports
(available at
<a href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_defects.html">http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_defects.html</a>
&
<a href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html">
http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html</a>
respectively).</p>
<ul>
<li>The ABI has changed. This means that both class layout and name
mangling is different. You <em>must</em> recompile all c++ libraries (if
you don't you will get link errors).</li>
<li>The standard library is much more conformant, and uses the
<code>std::</code> namespace.</li>
<li><code>std::</code> is now a real namespace, not an alias for
<code>::</code>.</li>
<li>The standard header files for the c library don't end with
<code>.h</code>, but begin with <code>c</code> (i.e.
<code><cstdlib></code> rather than <code><stdlib.h></code>).
The <code>.h</code> names are still available, but are deprecated.</li>
<li><code><strstream></code> is deprecated, use
<code><sstream></code> instead.</li>
<li><code>streambuf::seekoff</code> &
<code>streambuf::seekpos</code> are private, instead use
<code>streambuf::pubseekoff</code> &
<code>streambuf::pubseekpos</code> respectively.</li>
<li>If <code>std::operator << (std::ostream &, long long)</code>
doesn't exist, you need to recompile libstdc++ with
<code>--enable-long-long</code>.</li>
</ul>
This means you may get lots of errors about things like
<code>strcmp</code> not being found. You've most likely forgotton to
tell the compiler to look in the <code>std::</code> namespace. There are
several ways to do this,
<ul>
<li>Say, <code>std::strcmp</code> at the call. This is the most explicit
way of saying what you mean.</li>
<li>Say, <code>using std::strcmp;</code> somewhere before the call. You
will need to do this for each function or type you wish to use from the
standard library.</li>
<li>Say, <code>using namespace std;</code> somewhere before the call.
This is the quick-but-dirty fix. This brings the <em>whole</em> of the
<code>std::</code> namespace into scope. <em>Never</em> do this in a
header file, as you will be forcing users of your header file to do the
same.</li>
</ul>
<h3><a name="abi">ABI bugs</a></h3>
<p>3.0 had a new ABI, which affected class layout, function mangling and
calling conventions. We had intended it to be complete, 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.
</p>
<dl>
<dt>Covariant return types</dt>
<dd>We do not implement non-trivial covariant returns. We also generate
incorrect virtual function tables for trivial covariance. Although
trivial covariance will work, it is incompatible with the ABI. GNATS PR
3706 tracks this problem.</dd>
</dl>
<h3><a name="nonbugs">Non-bugs</a></h3>
<p>Here are some features that have been reported as bugs, but are
not.</p>
<dl>
<dt>Nested classes can access private types of the containing
class.</dt>
<dd><p>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.</p></dd>
<dt>Classes in exception specifiers must be complete types.</dt>
<dd><p>[15.4]/1 tells you that you cannot have an incomplete type, or
pointer to incomplete (other than <code><i>cv</i> void *</code>) in
an exception specification.</p></dd>
<dt>G++ emits two copies of constructors and destructors.</dt>
<dd><p>In general there are <em>three</em> types of constructors (and
destructors).</p>
<ol>
<li>The complete object constructor/destructor.</li>
<li>The base object constructor/destructor.</li>
<li>The allocating destructor/deallocating destructor.</li>
</ol>
<p>The first two are different, when virtual base classes are involved.
In some cases we can do better, and this is logged in GNATS.</p></dd>
<dt>Exceptions don't work in multithreaded applications.</dt>
<dd><p>You need to rebuild g++ and libstdc++ with
<code>--enable-threads</code>. 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.</p></dd>
<dt>Global destructors are not run in the correct order.</dt>
<dd><p>Global destructors should be run in the reverse order of their
constructors <em>completing</em>. In most cases this is the same as
the reverse order of constructors <em>starting</em>, but sometimes it
is different, and that is important. You need to compile and link your
programs with <code>--use-cxa-atexit</code>. We have not turned this
switch on by default, as it requires a <code>cxa</code> aware runtime
library (<code>libc</code>, <code>glibc</code>, or
equivalent).</p></dd>
<dt>Problems with floating point computations.</dt>
<dd><p>In a number of cases, GCC appears to perform floating point
computations incorrectly. For example, the program</p>
<blockquote><code>
#include <iostream><br />
<br />
int main() {<br />
<br />
double min = 0.0;<br />
double max = 0.5;<br />
double width = 0.01;<br />
std::cout << (int)(((max - min) / width) - 1) <<
std::endl;<br />
<br />
}<br />
</code></blockquote>
<p>might print 50 on some systems and optimization levels, and 51 on
others.</p>
<p>The is the result of <em>rounding</em>: 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.</p>
<p>This is not a bug in the compiler, but an inherent limitation of
the float and double types. Please study
<a href="http://www.validlab.com/goldberg/paper.ps">this paper</a>
for more information.</p></dd>
<dt>Templates, scoping, and digraphs.</dt>
<dd><p>If you have a class in global namespace, say named
<code>X</code>, and want to give it as a template argument to some
other class, say <code>std::vector</code>, then this here fails with a
parser error: <code>std::vector<::X></code>.
</p>
<p>
The reason is that the standard mandates that the sequence
<code><:</code> is treated as if it were the token
<code>[</code>, and the parser then reports a parse error before the
character <code>:</code> (by which it means the second
colon). There are several such combinations of characters, and
they are called <em>digraphs</em>.
</p>
<p>
The simplest way to avoid this is to write <code>std::vector<
::X></code>, i.e. place a space between the opening angle bracket
and the scope operator.
</p></dd>
</dl>
<h3><a name="missing">Missing features</a></h3>
<p>We know some things are missing from G++.</p>
<dl>
<dt>The <code>export</code> keyword is not implemented.</dt>
<dd><p>Most C++ compilers (G++ included) do not yet implement
<code>export</code>, which is necessary for separate compilation of
template declarations and definitions. Without <code>export</code>, 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.</p></dd>
<dt>Two stage lookup in templates is not implemented.</dt>
<dd><p>[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.</p></dd>
</dl>
<h3><a name="parsing">Parse errors for "simple" code</a></h3>
Up to and including GCC 3.0, the compiler will give "parse error" for
seemingly simple code, such as
<pre>
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
}
</pre>
The problem is that GCC starts to parse the declaration of
<code>b</code> as a function <code>b</code> returning <code>B</code>,
taking a function returning <code>A</code> 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:
<pre>
(B(A(2))).func();
</pre>
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:
<pre>
B b((0,A()),A(1));
</pre>
<p>
Another example is the parse error for the <code>return</code>
statement in</p>
<pre>
struct A{};
struct B{
A a;
A f1(bool);
};
A B::f1(bool b)
{
if (b)
return (A());
return a;
}
</pre>
<p>The problem is that the compiler interprets <code>A()</code> as a
function (taking no arguments, returning <code>A</code>), and
<code>(A()</code>) as a cast - with a missing expression, hence the
parse error. The work-around is to omit the parentheses:</p>
<pre>
if (b)
return A();
</pre>
<p>This problem occurs in a number of variants; in <code>throw</code>
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.</p>
<h3><a name="-O3">Optimization at <code>-O3</code> takes a
very long time</a></h3>
<p>At <code>-O3</code>, 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.</p>
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