2003-07-23 06:40:42 +04:00
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<html>
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<head>
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<title>GCC Bugs</title>
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</head>
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<body>
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<h1>GCC Bugs</h1>
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<p>The latest version of this document is always available at
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<a href="http://gcc.gnu.org/bugs.html">http://gcc.gnu.org/bugs.html</a>.</p>
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<hr />
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<h2>Table of Contents</h2>
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<ul>
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<li><a href="#report">Reporting Bugs</a>
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<ul>
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<li><a href="#need">What we need</a></li>
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<li><a href="#dontwant">What we DON'T want</a></li>
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<li><a href="#where">Where to post it</a></li>
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<li><a href="#detailed">Detailed bug reporting instructions</a></li>
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<li><a href="#gnat">Detailed bug reporting instructions for GNAT</a></li>
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<li><a href="#pch">Detailed bug reporting instructions when using a precompiled header</a></li>
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</ul>
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</li>
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<li><a href="#known">Frequently Reported Bugs in GCC</a>
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<ul>
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<li><a href="#cxx">C++</a>
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<ul>
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<li><a href="#cxx-abi">ABI bugs</a></li>
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<li><a href="#missing">Missing features</a></li>
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<li><a href="#parsing">Parse errors for "simple" code</a></li>
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</ul>
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</li>
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<li><a href="#fortran">Fortran</a></li>
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</ul>
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</li>
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<li><a href="#nonbugs">Non-bugs</a>
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<ul>
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<li><a href="#nonbugs_general">General</a></li>
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<li><a href="#nonbugs_c">C</a></li>
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<li><a href="#nonbugs_cxx">C++</a>
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<ul>
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<li><a href="#updating">Common problems updating from G++ 2.95 to
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G++ 3.0</a></li>
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</ul>
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</li>
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2003-10-18 15:03:09 +04:00
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</ul>
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</li>
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2003-07-23 06:40:42 +04:00
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</ul>
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<hr />
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<h1><a name="report">Reporting Bugs</a></h1>
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<p>The main purpose of a bug report is to enable us to fix the bug. The
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most important prerequisite for this is that the report must be complete and
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self-contained, which we explain in detail below.</p>
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<p>Before you report a bug, please check the
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<a href="#known">list of well-known bugs</a> and, <strong>if possible
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in any way, try a current development snapshot</strong>.
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If you want to report a bug with versions of GCC before 3.1 we strongly
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recommend upgrading to the current release first.</p>
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<p>Before reporting that GCC compiles your code incorrectly, please
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compile it with <code>gcc -Wall</code> and see whether this shows
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anything wrong with your code that could be the cause instead of a bug
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in GCC.</p>
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<h2>Summarized bug reporting instructions</h2>
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<p>After this summary, you'll find detailed bug reporting
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instructions, that explain how to obtain some of the information
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requested in this summary.</p>
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<h3><a name="need">What we need</a></h3>
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2003-10-18 15:03:09 +04:00
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<p>Please include in your bug report all of the following items, the first
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three of which can be obtained from the output of <code>gcc -v</code>:</p>
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<ul>
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<li>the exact version of GCC;</li>
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<li>the system type;</li>
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<li>the options given when GCC was configured/built;</li>
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<li>the complete command line that triggers the bug;</li>
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<li>the compiler output (error messages, warnings, etc.); and</li>
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<li>the <em>preprocessed</em> file (<code>*.i*</code>) that triggers the
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bug, generated by adding <code>-save-temps</code> to the complete
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compilation command, or, in the case of a bug report for the GNAT front end,
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a complete set of source files (see below).</li>
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</ul>
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<h3><a name="dontwant">What we do <strong>not</strong> want</a></h3>
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<ul>
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<li>A source file that <code>#include</code>s header files that are left
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out of the bug report (see above)</li>
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<li>That source file and a collection of header files.</li>
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<li>An attached archive (tar, zip, shar, whatever) containing all
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(or some :-) of the above.</li>
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<li>A code snippet that won't cause the compiler to produce the
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exact output mentioned in the bug report (e.g., a snippet with just
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a few lines around the one that <b>apparently</b> triggers the bug,
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with some pieces replaced with ellipses or comments for extra
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obfuscation :-)</li>
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<li>The location (URL) of the package that failed to build (we won't
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download it, anyway, since you've already given us what we need to
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duplicate the bug, haven't you? :-)</li>
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<li>An error that occurs only some of the times a certain file is
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compiled, such that retrying a sufficient number of times results in
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a successful compilation; this is a symptom of a hardware problem,
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not of a compiler bug (sorry)</li>
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<li>E-mail messages that complement previous, incomplete bug
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reports. Post a new, self-contained, full bug report instead, if
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possible as a follow-up to the original bug report</li>
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<li>Assembly files (<code>*.s</code>) produced by the compiler, or any
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binary files, such as object files, executables, core files, or
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precompiled header files</li>
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<li>Duplicate bug reports, or reports of bugs already fixed in the
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development tree, especially those that have already been reported
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as fixed last week :-)</li>
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<li>Bugs in the assembler, the linker or the C library. These are
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separate projects, with separate mailing lists and different bug
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reporting procedures</li>
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<li>Bugs in releases or snapshots of GCC not issued by the GNU
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Project. Report them to whoever provided you with the release</li>
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<li>Questions about the correctness or the expected behavior of
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certain constructs that are not GCC extensions. Ask them in forums
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dedicated to the discussion of the programming language</li>
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</ul>
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<h3><a name="where">Where to post it</a></h3>
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<p>Please submit your bug report directly to the
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<a href="http://gcc.gnu.org/bugzilla/">GCC bug database</a>.
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Alternatively, you can use the <code>gccbug</code> script that mails your bug
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report to the bug database.
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Only if all this is absolutely impossible, mail all information to
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<a href="mailto:gcc-bugs@gcc.gnu.org">gcc-bugs@gcc.gnu.org</a>, but note that
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such reports are often overlooked since they are not permanently recorded into
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the database for later processing.</p>
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<h2><a name="detailed">Detailed bug reporting instructions</a></h2>
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<p>Please refer to the <a href="#gnat">next section</a> when reporting
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bugs in GNAT, the Ada compiler, or to the <a href="#pch">one after
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that</a> when reporting bugs that appear when using a precompiled header.</p>
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<p>In general, all the information we need can be obtained by
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collecting the command line below, as well as its output and the
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preprocessed file it generates.</p>
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2003-10-18 15:03:09 +04:00
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<blockquote><p><code>gcc -v -save-temps <i>all-your-options
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source-file</i></code></p></blockquote>
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<p>Typically the preprocessed file (extension <code>.i</code> for C or
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<code>.ii</code> for C++, and <code>.f</code> if the preprocessor is used on
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Fortran files) will be large, so please compress the
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resulting file with one of the popular compression programs such as
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bzip2, gzip, zip or compress (in
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decreasing order of preference). Use maximum compression
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(<code>-9</code>) if available. Please include the compressed
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preprocessor output in your bug report, even if the source code is
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freely available elsewhere; it makes the job of our volunteer testers
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much easier.</p>
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<p>The <b>only</b> excuses to not send us the preprocessed sources are
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(i) if you've found a bug in the preprocessor, (ii) if you've reduced
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the testcase to a small file that doesn't include any other file or
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(iii) if the bug appears only when using precompiled headers. If you
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can't post the preprocessed sources because they're proprietary code,
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then try to create a small file that triggers the same problem.</p>
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<p>Since we're supposed to be able to re-create the assembly output
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(extension <code>.s</code>), you usually should not include
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it in the bug report, although you may want to post parts of it to
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point out assembly code you consider to be wrong.</p>
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<p>Whether to use MIME attachments or <code>uuencode</code> is up to
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you. In any case, make sure the compiler command line, version and
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error output are in plain text, so that we don't have to decode the
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bug report in order to tell who should take care of it. A meaningful
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subject indicating language and platform also helps.</p>
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<p>Please avoid posting an archive (.tar, .shar or .zip); we generally
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need just a single file to reproduce the bug (the .i/.ii/.f preprocessed
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file), and, by storing it in an archive, you're just making our
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volunteers' jobs harder. Only when your bug report requires multiple
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source files to be reproduced should you use an archive. This is, for example,
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the case if you are using <code>INCLUDE</code> directives in Fortran code,
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which are not processed by the preprocessor, but the compiler. In that case,
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we need the main file and all <code>INCLUDE</code>d files. In any case,
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make sure the compiler version, error message, etc, are included in
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the body of your bug report as plain text, even if needlessly
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duplicated as part of an archive.</p>
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<p>If you fail to supply enough information for a bug report to be
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reproduced, someone will probably ask you to post additional
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information (or just ignore your bug report, if they're in a bad day,
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so try to get it right on the first posting :-). In this case, please
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post the additional information to the bug reporting mailing list, not
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just to the person who requested it, unless explicitly told so. If
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possible, please include in this follow-up all the information you had
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supplied in the incomplete bug report (including the preprocessor
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output), so that the new bug report is self-contained.</p>
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<h2><a name="gnat">Detailed bug reporting instructions for GNAT</a></h2>
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<p>See the <a href="#detailed">previous section</a> for bug reporting
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instructions for GCC language implementations other than Ada.</p>
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<p>Bug reports have to contain at least the following information in
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order to be useful:</p>
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<ul>
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<li>the exact version of GCC, as shown by "<code>gcc -v</code>";</li>
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<li>the system type;</li>
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<li>the options when GCC was configured/built;</li>
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<li>the exact command line passed to the <code>gcc</code> program
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triggering the bug
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(not just the flags passed to <code>gnatmake</code>, but
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<code>gnatmake</code> prints the parameters it passed to <code>gcc</code>)</li>
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<li>a collection of source files for reproducing the bug,
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preferably a minimal set (see below);</li>
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<li>a description of the expected behavior;</li>
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<li>a description of actual behavior.</li>
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</ul>
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<p>If your code depends on additional source files (usually package
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specifications), submit the source code for these compilation units in
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a single file that is acceptable input to <code>gnatchop</code>,
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i.e. contains no non-Ada text. If the compilation terminated
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normally, you can usually obtain a list of dependencies using the
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"<code>gnatls -d <i>main_unit</i></code>" command, where
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<code><i>main_unit</i></code> is the file name of the main compilation
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unit (which is also passed to <code>gcc</code>).</p>
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<p>If you report a bug which causes the compiler to print a bug box,
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include that bug box in your report, and do not forget to send all the
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source files listed after the bug box along with your report.</p>
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<p>If you use <code>gnatprep</code>, be sure to send in preprocessed
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sources (unless you have to report a bug in <code>gnatprep</code>).</p>
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<p>When you have checked that your report meets these criteria, please
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submit it according to our <a href="#where">generic instructions</a>.
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(If you use a mailing list for reporting, please include an
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"<code>[Ada]</code>" tag in the subject.)</p>
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<h2><a name="pch">Detailed bug reporting instructions when using a
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precompiled header</a></h2>
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<p>If you're encountering a bug when using a precompiled header, the
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first thing to do is to delete the precompiled header, and try running
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the same GCC command again. If the bug happens again, the bug doesn't
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really involve precompiled headers, please report it without using
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them by following the instructions <a href="#detailed">above</a>.</p>
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<p>If you've found a bug while <i>building</i> a precompiled header
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(for instance, the compiler crashes), follow the usual instructions
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<a href="#detailed">above</a>.</p>
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<p>If you've found a real precompiled header bug, what we'll need to
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reproduce it is the sources to build the precompiled header (as a
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single <code>.i</code> file), the source file that uses the
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precompiled header, any other headers that source file includes, and
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the command lines that you used to build the precompiled header and to
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use it.</p>
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<p>Please <strong>don't</strong> send us the actual precompiled
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header. It is likely to be very large and we can't use it to
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reproduce the problem.</p>
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<hr />
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<h1><a name="known">Frequently Reported Bugs in GCC</a></h1>
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2003-10-18 15:03:09 +04:00
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<p>This is a list of bugs in GCC that are reported very often, but not
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yet fixed. While it is certainly better to fix bugs instead of documenting
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them, this document might save people the effort of writing a bug report
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when the bug is already well-known.</p>
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<p>There are many reasons why a reported bug doesn't get fixed.
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It might be difficult to fix, or fixing it might break compatibility.
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Often, reports get a low priority when there is a simple work-around.
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In particular, bugs caused by invalid code have a simple work-around:
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<em>fix the code</em>.</p>
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<hr />
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<h2><a name="cxx">C++</a></h2>
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<h3><a name="cxx-abi">ABI bugs</a></h3>
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<p>GCC 3.0 had a new ABI, which affected class layout, function mangling and
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calling conventions. We had intended it to be complete, but unfortunately
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some issues came to light, too late to fix in the 3.0 series.
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The ABI should not change in dot releases, so we addressed most issues
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in GCC 3.1.</p>
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<dl>
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<dt>Covariant return types</dt>
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<dd>Up to (and including) GCC 3.3 we did not implement non-trivial
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covariant returns. This has been addressed for GCC 3.4.</dd>
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</dl>
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<h3><a name="missing">Missing features</a></h3>
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<p>We know some things are missing from G++.</p>
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<dl>
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<dt>The <code>export</code> keyword is not implemented.</dt>
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<dd><p>Most C++ compilers (G++ included) do not yet implement
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<code>export</code>, which is necessary for separate compilation of
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template declarations and definitions. Without <code>export</code>, a
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template definition must be in scope to be used. The obvious
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workaround is simply to place all definitions in the header
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itself. Alternatively, the compilation unit containing template
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definitions may be included from the header.</p></dd>
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<dt>Two stage lookup in templates is not implemented.</dt>
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<dd><p>[14.6] specifies how names are looked up inside a template. G++
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does not do this correctly, but for most templates this will not be
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noticeable.</p></dd>
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</dl>
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<h3><a name="parsing">Parse errors for "simple" code</a></h3>
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<p>Up to and including GCC 3.0, the compiler will give "parse error" for
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seemingly simple code, such as</p>
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<pre>
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struct A{
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A();
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A(int);
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void func();
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};
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struct B{
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B(A);
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B(A,A);
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void func();
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};
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void foo(){
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B b(A(),A(1)); //Variable b, initialized with two temporaries
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B(A(2)).func(); //B temporary, initialized with A temporary
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}
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</pre>
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<p>The problem is that GCC starts to parse the declaration of
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<code>b</code> as a function <code>b</code> returning <code>B</code>,
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taking a function returning <code>A</code> as an argument. When it
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sees the 1, it is too late. The work-around in these cases is to add
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additional parentheses around the expressions that are mistaken as
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declarations:</p>
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<pre>
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(B(A(2))).func();
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</pre>
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<p>Sometimes, even that is not enough; to show the compiler that this
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should be really an expression, a comma operator with a dummy argument
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can be used:</p>
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<pre>
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B b((0,A()),A(1));
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</pre>
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<p>Another example is the parse error for the <code>return</code>
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statement in</p>
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<pre>
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struct A{};
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struct B{
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A a;
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A f1(bool);
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};
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|
A B::f1(bool b)
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|
{
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if (b)
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|
return (A());
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return a;
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}
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</pre>
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<p>The problem is that the compiler interprets <code>A()</code> as a
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function (taking no arguments, returning <code>A</code>), and
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<code>(A()</code>) as a cast - with a missing expression, hence the
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parse error. The work-around is to omit the parentheses:</p>
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<pre>
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if (b)
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return A();
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</pre>
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<p>This problem occurs in a number of variants; in <code>throw</code>
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|
statements, people also frequently put the object in parentheses. The
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exact error also somewhat varies with the compiler version. The
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work-arounds proposed do not change the semantics of the program at
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all; they make them perhaps less readable.</p>
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|
<hr />
|
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|
2003-07-23 06:40:42 +04:00
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|
<h2><a name="fortran">Fortran</a></h2>
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|
<p>Fortran bugs are documented in the G77 manual rather than
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|
explicitly listed here. Please see
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|
<a href="http://gcc.gnu.org/onlinedocs/g77/Trouble.html">Known Causes of
|
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|
|
Trouble with GNU Fortran</a> in the G77 manual.</p>
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|
<hr />
|
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|
|
2003-10-18 15:03:09 +04:00
|
|
|
<h1><a name="nonbugs">Non-bugs</a></h1>
|
2003-07-23 06:40:42 +04:00
|
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|
|
2003-10-18 15:03:09 +04:00
|
|
|
<p>The following are not actually bugs, but are reported often
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|
enough to warrant a mention here.</p>
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|
<p>It is not always a bug in the compiler, if code which "worked" in a
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|
previous version, is now rejected. Earlier versions of GCC sometimes were
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|
less picky about standard conformance and accepted invalid source code.
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|
In addition, programming languages themselves change, rendering code
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|
invalid that used to be conforming (this holds especially for C++).
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In either case, you should update your code to match recent language
|
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|
|
standards.</p>
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|
<hr />
|
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|
|
<h2><a name="nonbugs_general">General</a></h2>
|
2003-07-23 06:40:42 +04:00
|
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|
<dl>
|
2003-10-18 15:03:09 +04:00
|
|
|
<dt>Problems with floating point numbers - the
|
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|
|
<a href="http://gcc.gnu.org/PR323">most often reported non-bug</a>.</dt>
|
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|
|
<dd><p>In a number of cases, GCC appears to perform floating point
|
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|
|
computations incorrectly. For example, the C++ program</p>
|
|
|
|
|
<blockquote><pre>
|
|
|
|
|
#include <iostream>
|
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|
|
|
|
|
|
|
int main()
|
|
|
|
|
{
|
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|
|
|
double a = 0.5;
|
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|
|
|
double b = 0.01;
|
|
|
|
|
std::cout << (int)(a / b) << std::endl;
|
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|
|
return 0;
|
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|
|
|
}
|
|
|
|
|
</pre></blockquote>
|
|
|
|
|
<p>might print 50 on some systems and optimization levels, and 49 on
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|
|
others.</p>
|
|
|
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|
|
|
|
|
|
<p>The is the result of <em>rounding</em>: The computer cannot
|
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|
|
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 floating point types. Please study
|
|
|
|
|
<a href="http://www.validlab.com/goldberg/paper.ps">this paper</a>
|
|
|
|
|
for more information.</p></dd>
|
|
|
|
|
</dl>
|
2003-07-23 06:40:42 +04:00
|
|
|
|
2003-10-18 15:03:09 +04:00
|
|
|
<hr />
|
|
|
|
|
|
|
|
|
|
<h2><a name="nonbugs_c">C</a></h2>
|
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|
|
|
|
|
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|
|
<dl>
|
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|
|
<dt>Casting does not work as expected when optimization is turned on.</dt>
|
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|
|
|
|
|
|
|
|
<dd><p>This is often caused by a violation of aliasing rules, which are part
|
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|
|
|
of the ISO C standard. These rules say that a program is invalid if you try
|
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|
|
|
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 <code>short</code>s and 32-bit
|
|
|
|
|
<code>int</code>s):</p>
|
2003-07-23 06:40:42 +04:00
|
|
|
<blockquote><pre>
|
|
|
|
|
#include <stdio.h>
|
|
|
|
|
|
2003-10-18 15:03:09 +04:00
|
|
|
int main()
|
|
|
|
|
{
|
|
|
|
|
short a[2];
|
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|
|
|
|
|
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|
|
a[0]=0x1111;
|
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|
|
|
a[1]=0x1111;
|
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|
|
|
|
|
|
|
|
*(int *)a = 0x22222222; /* violation of aliasing rules */
|
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|
|
|
|
|
|
|
|
printf("%x %x\n", a[0], a[1]);
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
2003-07-23 06:40:42 +04:00
|
|
|
</pre></blockquote>
|
2003-10-18 15:03:09 +04:00
|
|
|
<p>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.</p>
|
|
|
|
|
|
|
|
|
|
<p>In the case above, the compiler may assume that no access through an
|
|
|
|
|
integer pointer can change the array <code>a</code>, consisting of shorts.
|
|
|
|
|
Thus, <code>printf</code> may be called with the original values of
|
|
|
|
|
<code>a[0]</code> and <code>a[1]</code>. What really happens is up to
|
|
|
|
|
the compiler and may change with architecture and optimization level.</p>
|
|
|
|
|
|
|
|
|
|
<p>Recent versions of GCC turn on the option <code>-fstrict-aliasing</code>
|
|
|
|
|
(which allows alias-based optimizations) by default with <code>-O2</code>.
|
|
|
|
|
And some architectures then really print "1111 1111" as result. Without
|
|
|
|
|
optimization the executable will generate the "expected" output
|
|
|
|
|
"2222 2222".</p>
|
|
|
|
|
|
|
|
|
|
<p>To disable optimizations based on alias-analysis for faulty legacy code,
|
|
|
|
|
the option <code>-fno-strict-aliasing</code> can be used as a work-around.</p>
|
|
|
|
|
|
|
|
|
|
<p>The option <code>-Wstrict-aliasing</code> (which is included in
|
|
|
|
|
<code>-Wall</code>) warns about some - but not all - cases of violation
|
|
|
|
|
of aliasing rules when <code>-fstrict-aliasing</code> is active.</p>
|
|
|
|
|
|
|
|
|
|
<p>To fix the code above, you can use a <code>union</code> instead of a
|
|
|
|
|
cast (note that this is a GCC extension which might not work with other
|
|
|
|
|
compilers):</p>
|
|
|
|
|
<blockquote><pre>
|
|
|
|
|
#include <stdio.h>
|
2003-07-23 06:40:42 +04:00
|
|
|
|
2003-10-18 15:03:09 +04:00
|
|
|
int main()
|
|
|
|
|
{
|
|
|
|
|
union
|
|
|
|
|
{
|
|
|
|
|
short a[2];
|
|
|
|
|
int i;
|
|
|
|
|
} u;
|
2003-07-23 06:40:42 +04:00
|
|
|
|
2003-10-18 15:03:09 +04:00
|
|
|
u.a[0]=0x1111;
|
|
|
|
|
u.a[1]=0x1111;
|
|
|
|
|
|
|
|
|
|
u.i = 0x22222222;
|
|
|
|
|
|
|
|
|
|
printf("%x %x\n", u.a[0], u.a[1]);
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
</pre></blockquote>
|
|
|
|
|
<p>Now the result will always be "2222 2222".</p>
|
|
|
|
|
|
|
|
|
|
<p>For some more insight into the subject, please have a look at
|
|
|
|
|
<a href="http://mail-index.NetBSD.org/tech-kern/2003/08/11/0001.html">this
|
|
|
|
|
article</a>.</p></dd>
|
2003-07-23 06:40:42 +04:00
|
|
|
|
|
|
|
|
|
|
|
|
|
<dt>Cannot use preprocessor directive in macro arguments.</dt>
|
2003-10-18 15:03:09 +04:00
|
|
|
<dd><p>Let me guess... you used an older version of GCC to compile code
|
|
|
|
|
that looks something like this:</p>
|
2003-07-23 06:40:42 +04:00
|
|
|
<blockquote><pre>
|
|
|
|
|
memcpy(dest, src,
|
|
|
|
|
#ifdef PLATFORM1
|
|
|
|
|
12
|
|
|
|
|
#else
|
|
|
|
|
24
|
|
|
|
|
#endif
|
|
|
|
|
);
|
|
|
|
|
</pre></blockquote>
|
|
|
|
|
<p>and you got a whole pile of error messages:</p>
|
2003-10-18 15:03:09 +04:00
|
|
|
<blockquote><pre>
|
|
|
|
|
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
|
|
|
|
|
</pre></blockquote>
|
|
|
|
|
|
|
|
|
|
<p>This is because your C library's <code><string.h></code> happens
|
|
|
|
|
to define <code>memcpy</code> as a macro - which is perfectly legitimate.
|
|
|
|
|
In recent versions of glibc, for example, <code>printf</code> is among those
|
|
|
|
|
functions which are implemented as macros.</p>
|
|
|
|
|
|
|
|
|
|
<p>Versions of GCC prior to 3.3 did not allow you to put <code>#ifdef</code>
|
|
|
|
|
(or any other preprocessor directive) inside the arguments of a macro. The
|
|
|
|
|
code therefore would not compile.</p>
|
|
|
|
|
|
|
|
|
|
<p>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.</p>
|
|
|
|
|
|
|
|
|
|
<p>However, this kind of code is not portable. It is "undefined behavior"
|
|
|
|
|
according to the C standard; that means different compilers may do
|
2003-07-23 06:40:42 +04:00
|
|
|
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
|
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|
|
memcpy(dest, src, 12);
|
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|
|
#else
|
|
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|
memcpy(dest, src, 24);
|
|
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|
#endif
|
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|
|
</pre></blockquote>
|
|
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|
|
<p>This is a bit more typing, but I personally think it's better style
|
2003-10-18 15:03:09 +04:00
|
|
|
in addition to being more portable.</p></dd>
|
|
|
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|
|
|
|
|
|
|
|
|
|
|
<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
|
|
|
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|
lot. Code like this:</p>
|
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|
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|
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|
|
<blockquote><pre>
|
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|
#include <stdio.h>
|
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|
|
|
|
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|
FILE *yyin = stdin;
|
|
|
|
|
</pre></blockquote>
|
|
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|
|
|
|
|
|
|
<p>will not compile with GNU libc, 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>
|
2003-07-23 06:40:42 +04:00
|
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|
|
2003-10-18 15:03:09 +04:00
|
|
|
<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>
|
|
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|
|
for details.
|
|
|
|
|
</p></dd>
|
2003-07-23 06:40:42 +04:00
|
|
|
</dl>
|
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|
|
|
|
<hr />
|
|
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|
|
2003-10-18 15:03:09 +04:00
|
|
|
<h2><a name="nonbugs_cxx">C++</a></h2>
|
2003-07-23 06:40:42 +04:00
|
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|
|
2003-10-18 15:03:09 +04:00
|
|
|
<dl>
|
|
|
|
|
<dt>Nested classes can access private members and types of the containing
|
|
|
|
|
class.</dt>
|
2003-07-23 06:40:42 +04:00
|
|
|
|
2003-10-18 15:03:09 +04:00
|
|
|
<dd><p>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>
|
2003-07-23 06:40:42 +04:00
|
|
|
|
2003-10-18 15:03:09 +04:00
|
|
|
<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 constructor/deallocating destructor.</li>
|
|
|
|
|
</ol>
|
|
|
|
|
<p>The first two are different, when virtual base classes are involved.
|
|
|
|
|
</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>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>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>Templates, scoping, and digraphs.</dt>
|
|
|
|
|
|
|
|
|
|
<dd><p>If you have a class in the 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 <code>std::vector<::X></code>
|
|
|
|
|
fails with a parser error.</p>
|
|
|
|
|
|
|
|
|
|
<p>The reason is that the standard mandates that the sequence
|
|
|
|
|
<code><:</code> is treated as if it were the token <code>[</code>.
|
|
|
|
|
(There are several such combinations of characters - they are called
|
|
|
|
|
<em>digraphs</em>.) Depending on the version, the compiler then reports
|
|
|
|
|
a parse error before the character <code>:</code> (the colon before
|
|
|
|
|
<code>X</code>) or a missing closing bracket <code>]</code>.</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>
|
2003-07-23 06:40:42 +04:00
|
|
|
|
|
|
|
|
<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>
|
2003-10-18 15:03:09 +04:00
|
|
|
&
|
2003-07-23 06:40:42 +04:00
|
|
|
<a href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html">
|
2003-10-18 15:03:09 +04:00
|
|
|
http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html</a>
|
2003-07-23 06:40:42 +04:00
|
|
|
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>
|
|
|
|
|
|
2003-10-18 15:03:09 +04:00
|
|
|
<p>This means you may get lots of errors about things like
|
2003-07-23 06:40:42 +04:00
|
|
|
<code>strcmp</code> not being found. You've most likely forgotten to
|
|
|
|
|
tell the compiler to look in the <code>std::</code> namespace. There are
|
2003-10-18 15:03:09 +04:00
|
|
|
several ways to do this,</p>
|
2003-07-23 06:40:42 +04:00
|
|
|
|
|
|
|
|
<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>
|
|
|
|
|
|
|
|
|
|
|
2003-10-18 15:03:09 +04:00
|
|
|
<p>In addition to the problems listed above, the manual contains a section on
|
2003-08-09 15:11:09 +04:00
|
|
|
<a href="http://gcc.gnu.org/onlinedocs/gcc/C---Misunderstandings.html">Common
|
|
|
|
|
Misunderstandings with GNU C++</a>.</p>
|
2003-07-23 06:40:42 +04:00
|
|
|
|
|
|
|
|
</body>
|
|
|
|
|
</html>
|
