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| 2 |
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| 3 | <head>
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| 4 | <title>GCC Frequently Asked Questions</title>
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| 5 | </head>
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| 6 |
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| 7 | <body>
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| 8 |
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| 9 | <h1>GCC Frequently Asked Questions</h1>
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| 10 |
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| 11 | <p>The latest version of this document is always available at
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| 12 | <a href="http://gcc.gnu.org/faq.html">http://gcc.gnu.org/faq.html</a>.</p>
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| 13 |
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| 14 | <p>This FAQ tries to answer specific questions concerning GCC. For
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| 15 | general information regarding C, C++, resp. Fortran please check the
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| 16 | <a href="http://www.eskimo.com/~scs/C-faq/top.html">comp.lang.c FAQ</a>,
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| 17 | <a href="http://www.jamesd.demon.co.uk/csc/faq.html">comp.std.c++
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| 18 | FAQ</a>,
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| 19 | and the <a href="http://www.fortran.com/fortran/info.html">Fortran
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| 20 | Information page</a>.</p>
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| 21 |
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| 22 | <p>Other GCC-related FAQs:
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| 23 | <a href="http://gcc.gnu.org/onlinedocs/libstdc++/faq/index.html">
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| 24 | libstdc++-v3</a>, and
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| 25 | <a href="http://gcc.gnu.org/java/faq.html">GCJ</a>.</p>
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| 26 |
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| 27 | <hr />
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| 28 | <h1>Questions</h1>
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| 29 | <ol>
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| 30 | <li><a href="#general">General information</a>
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| 31 | <ol>
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| 32 | <li><a href="#gcc">What is the relationship between GCC and EGCS?</a></li>
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| 33 | <li><a href="#open-development">What is an open development model?</a></li>
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| 34 | <li><a href="#support">How do I get a bug fixed or a feature added?</a></li>
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| 35 | <li><a href="#platforms">Does GCC work on my platform?</a></li>
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| 36 | </ol></li>
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| 37 |
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| 38 | <li><a href="#installation">Installation</a>
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| 39 | <ol>
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| 40 | <li><a href="#multiple">How to install multiple versions of GCC</a></li>
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| 41 | <li><a href="#rpath">Dynamic linker is unable to find GCC libraries</a></li>
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| 42 | <li><a href="#rpath">libstdc++/libio tests fail badly with --enable-shared</a></li>
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| 43 | <li><a href="#gas">GCC can not find GNU as/GNU ld</a></li>
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| 44 | <li><a href="#environ">cpp: Usage:... Error</a></li>
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| 45 | <li><a href="#optimizing">Optimizing the compiler itself</a></li>
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| 46 | <li><a href="#iconv">Why does <code>libiconv</code> get linked into <code>jc1</code> on Solaris?</a></li>
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| 47 | </ol></li>
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| 48 |
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| 49 | <li><a href="#testsuite">Testsuite problems</a>
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| 50 | <ol>
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| 51 | <li><a href="#testoptions">How do I pass flags like
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| 52 | <code>-fnew-abi</code> to the testsuite?</a></li>
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| 53 | <li><a href="#multipletests">How can I run the test suite with multiple options?</a></li>
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| 54 | </ol></li>
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| 55 |
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| 56 | <li><a href="#old">Older versions of GCC</a>
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| 57 | <ol>
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| 58 | <li><a href="#2.95sstream">Is there a stringstream / sstream for GCC 2.95.2?</a></li>
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| 59 | </ol></li>
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| 60 |
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| 61 | <li><a href="#misc">Miscellaneous</a>
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| 62 | <ol>
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| 63 | <li><a href="#friend">Friend Templates</a></li>
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| 64 | <li><a href="#dso"><code>dynamic_cast</code>, <code>throw</code>, <code>typeid</code> don't work with shared libraries</a></li>
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| 65 | <li><a href="#generated_files">Why do I need autoconf, bison, xgettext, automake, etc?</a></li>
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| 66 | <li><a href="#picflag-needed">Why can't I build a shared library?</a></li>
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| 67 | <li><a href="#vtables">When building C++, the linker says my constructors, destructors or virtual tables are undefined, but I defined them</a></li>
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| 68 | <li><a href="#incremental">Will GCC someday include an incremental linker?</a></li>
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| 69 | </ol></li>
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| 70 | </ol>
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| 71 |
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| 72 |
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| 73 | <hr />
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| 74 | <a name="general"></a>
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| 75 | <h1>General information</h1>
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| 76 |
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| 77 | <h2><a name="gcc">What is the relationship between GCC and EGCS?</a></h2>
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| 78 |
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| 79 | <p>In 1990/1991 gcc version 1 had reached a point of stability. For the
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| 80 | targets it could support, it worked well. It had limitations inherent in
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| 81 | its design that would be difficult to resolve, so a major effort was made
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| 82 | to resolve those limitations and gcc version 2 was the result.</p>
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| 83 |
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| 84 | <p>When we had gcc2 in a useful state, development efforts on gcc1 stopped
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| 85 | and we all concentrated on making gcc2 better than gcc1 could ever be. This
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| 86 | is the kind of step forward we wanted to make with the EGCS project when it
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| 87 | was formed in 1997.</p>
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| 88 |
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| 89 | <p>In April 1999 the Free Software Foundation officially halted
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| 90 | development on the gcc2 compiler and appointed the EGCS project as the
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| 91 | official GCC maintainers. The net result was a single project which
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| 92 | carries forward GCC development under the ultimate control of the
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| 93 | <a href="steering.html">GCC Steering Committee</a>.</p>
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| 94 |
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| 95 |
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| 96 | <hr />
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| 97 | <h2><a name="open-development">What is an open development model?</a></h2>
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| 98 |
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| 99 | <p>We are using a bazaar style
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| 100 | <a href="#cathedral-vs-bazaar"><b>[1]</b></a>
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| 101 | approach to GCC development: we make snapshots publicly available to
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| 102 | anyone who wants to try them; we welcome anyone to join
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| 103 | the development mailing list. All of the discussions on the
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| 104 | development mailing list are available via the web. We're going to be
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| 105 | making releases with a much higher frequency than they have been made
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| 106 | in the past.</p>
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| 107 |
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| 108 | <p>In addition to weekly snapshots of the GCC development sources, we
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| 109 | have the sources readable from a CVS server by anyone. Furthermore we
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| 110 | are using remote CVS to allow remote maintainers write access to the
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| 111 | sources.</p>
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| 112 |
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| 113 | <p>There have been many potential GCC developers who were not able to
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| 114 | participate in GCC development in the past. We want these people to
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| 115 | help in any way they can; we ultimately want GCC to be the best compiler
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| 116 | in the world.</p>
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| 117 |
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| 118 | <p>A compiler is a complicated piece of software, there will still be
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| 119 | strong central maintainers who will reject patches, who will demand
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| 120 | documentation of implementations, and who will keep the level of
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| 121 | quality as high as it is today. Code that could use wider testing may
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| 122 | be integrated--code that is simply ill-conceived won't be.</p>
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| 123 |
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| 124 | <p>GCC is not the first piece of software to use this open development
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| 125 | process; FreeBSD, the Emacs lisp repository, and the Linux kernel are
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| 126 | a few examples of the bazaar style of development.</p>
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| 127 |
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| 128 | <p>With GCC, we are adding new features and optimizations at a
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| 129 | rate that has not been done since the creation of gcc2; these
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| 130 | additions inevitably have a temporarily destabilizing effect.
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| 131 | With the help of developers working together with this bazaar style
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| 132 | development, the resulting stability and quality levels will be better
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| 133 | than we've had before.</p>
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| 134 |
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| 135 | <blockquote>
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| 136 | <a name="cathedral-vs-bazaar"><b>[1]</b></a>
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| 137 | We've been discussing different development models a lot over the
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| 138 | past few months. The paper which started all of this introduced two
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| 139 | terms: A <b>cathedral</b> development model versus a <b>bazaar</b>
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| 140 | development model. The paper is written by Eric S. Raymond, it is
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| 141 | called ``The Cathedral and the Bazaar''.
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| 142 | The paper is a useful starting point for discussions.
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| 143 | </blockquote>
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| 144 |
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| 145 | <hr />
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| 146 | <!-- The "bugreport" anchor was used in ICE messages of GCC < 2.95.3. -->
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| 147 | <h2 id="bugreport"><a name="support">How do I get a bug fixed or
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| 148 | a feature added?</a></h2>
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| 149 |
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| 150 | <p>There are lots of ways to get something fixed. The list below may be
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| 151 | incomplete, but it covers many of the common cases. These are listed
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| 152 | roughly in order of decreasing difficulty for the average GCC user,
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| 153 | meaning someone who is not skilled in the internals of GCC, and where
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| 154 | difficulty is measured in terms of the time required to fix the bug.
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| 155 | No alternative is better than any other; each has its benefits and
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| 156 | disadvantages.</p>
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| 157 |
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| 158 | <ul>
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| 159 | <li>Fix it yourself. This alternative will probably bring results,
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| 160 | if you work hard enough, but will probably take a lot of time,
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| 161 | and, depending on the quality of your work and the perceived
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| 162 | benefits of your changes, your code may or may not ever make it
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| 163 | into an official release of GCC.</li>
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| 164 |
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| 165 | <li><a href="bugs.html">Report the problem to the GCC bug tracking system</a>
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| 166 | and hope that someone will be kind
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| 167 | enough to fix it for you. While this is certainly possible, and
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| 168 | often happens, there is no guarantee that it will. You should
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| 169 | not expect the same response from this method that you would see
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| 170 | from a commercial support organization since the people who read
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| 171 | GCC bug reports, if they choose to help you, will be volunteering their
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| 172 | time.</li>
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| 173 |
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| 174 | <li>Hire someone to fix it for you. There are various companies and
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| 175 | individuals providing support for GCC. This alternative costs
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| 176 | money, but is relatively likely to get results.</li>
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| 177 | </ul>
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| 178 |
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| 179 | <hr />
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| 180 |
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| 181 | <h2><a name="platforms">Does GCC work on my platform?</a></h2>
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| 182 |
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| 183 | <p>The host/target specific installation notes for GCC include information
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| 184 | about known problems with installing or using GCC on particular platforms.
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| 185 | These are included in the sources for a release in INSTALL/specific.html,
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| 186 | and the <a href="http://gcc.gnu.org/install/specific.html">latest version</a>
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| 187 | is always available at the GCC web site.
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| 188 | Reports of <a href="http://gcc.gnu.org/buildstat.html">successful builds</a>
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| 189 | for several versions of GCC are also available at the web site.</p>
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| 190 |
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| 191 | <hr />
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| 192 | <a name="installation"></a>
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| 193 | <h1>Installation</h1>
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| 194 |
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| 195 | <h2><a name="multiple">How to install multiple versions of GCC</a></h2>
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| 196 |
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| 197 | <p>It may be desirable to install multiple versions of the compiler on
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| 198 | the same system. This can be done by using different prefix paths at
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| 199 | configure time and a few symlinks.</p>
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| 200 |
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| 201 | <p>Basically, configure the two compilers with different --prefix options,
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| 202 | then build and install each compiler. Assume you want "gcc" to be the latest
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| 203 | compiler and available in /usr/local/bin; also assume that you want "gcc2"
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| 204 | to be the older gcc2 compiler and also available in /usr/local/bin.</p>
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| 205 |
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| 206 | <p>The easiest way to do this is to configure the new GCC with
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| 207 | <code>--prefix=/usr/local/gcc</code> and the older gcc2 with
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| 208 | <code>--prefix=/usr/local/gcc2</code>. Build and install both
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| 209 | compilers. Then make a symlink from <code>/usr/local/bin/gcc</code>
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| 210 | to <code>/usr/local/gcc/bin/gcc</code> and from
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| 211 | <code>/usr/local/bin/gcc2</code> to
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| 212 | <code>/usr/local/gcc2/bin/gcc</code>. Create similar links for the
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| 213 | "g++", "c++" and "g77" compiler drivers.</p>
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| 214 |
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| 215 | <p>An alternative to using symlinks is to configure with a
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| 216 | <code>--program-transform-name</code> option. This option specifies a
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| 217 | sed command to process installed program names with. Using it you can,
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| 218 | for instance, have all the new GCC programs installed as "new-gcc" and
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| 219 | the like. You will still have to specify different
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| 220 | <code>--prefix</code> options for new GCC and old GCC, because it is
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| 221 | only the executable program names that are transformed. The difference
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| 222 | is that you (as administrator) do not have to set up symlinks, but
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| 223 | must specify additional directories in your (as a user) PATH. A
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| 224 | complication with <code>--program-transform-name</code> is that the
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| 225 | sed command invariably contains characters significant to the shell,
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| 226 | and these have to be escaped correctly, also it is not possible to use
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| 227 | "^" or "$" in the command. Here is the option to prefix "new-" to the
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| 228 | new GCC installed programs:</p>
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| 229 | <blockquote><code>
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| 230 | --program-transform-name='s,\\\\(.*\\\\),new-\\\\1,'
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| 231 | </code></blockquote>
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| 232 | <p>With the above <code>--prefix</code> option, that will install the new
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| 233 | GCC programs into <code>/usr/local/gcc/bin</code> with names prefixed
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| 234 | by "new-". You can use <code>--program-transform-name</code> if you
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| 235 | have multiple versions of GCC, and wish to be sure about which version
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| 236 | you are invoking.</p>
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| 237 |
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| 238 | <p>If you use <code>--prefix</code>, GCC may have difficulty locating a GNU
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| 239 | assembler or linker on your system, <a href="#gas">GCC can not find GNU
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| 240 | as/GNU ld</a> explains how to deal with this.</p>
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| 241 |
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| 242 | <p>Another option that may be easier is to use the
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| 243 | <code>--program-prefix=</code> or <code>--program-suffix=</code>
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| 244 | options to configure. So if you're installing GCC 2.95.2 and don't
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| 245 | want to disturb the current version of GCC in
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| 246 | <code>/usr/local/bin/</code>, you could do</p>
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| 247 | <blockquote><code>
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| 248 | configure --program-suffix=-2.95.2 <other configure options>
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| 249 | </code></blockquote>
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| 250 | <p>This should result in GCC being installed as
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| 251 | <code>/usr/local/bin/gcc-2.95.2</code> instead of
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| 252 | <code>/usr/local/bin/gcc</code>.</p>
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| 253 |
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| 254 | <hr />
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| 255 | <h2><a name="rpath">Dynamic linker is unable to find GCC libraries</a></h2>
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| 256 |
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| 257 | <p>This problem manifests itself by programs not finding shared
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| 258 | libraries they depend on when the programs are started. Note this
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| 259 | problem often manifests itself with failures in the libio/libstdc++
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| 260 | tests after configuring with <code>--enable-shared</code> and building GCC.</p>
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| 261 |
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| 262 | <p>GCC does not specify a runpath so that the dynamic linker can find
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| 263 | dynamic libraries at runtime.</p>
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| 264 |
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| 265 | <p>The short explanation is that if you always pass a -R option to the
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| 266 | linker, then your programs become dependent on directories which
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| 267 | may be NFS mounted, and programs may hang unnecessarily when an
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| 268 | NFS server goes down.</p>
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| 269 |
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| 270 | <p>The problem is not programs that do require the directories; those
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| 271 | programs are going to hang no matter what you do. The problem is
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| 272 | programs that do not require the directories.</p>
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| 273 |
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| 274 | <p>SunOS effectively always passed a <code>-R</code> option for every
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| 275 | <code>-L</code> option; this was a bad idea, and so it was removed for
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| 276 | Solaris. We should not recreate it.</p>
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| 277 |
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| 278 | <p>However, if you feel you really need such an option to be passed
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| 279 | automatically to the linker, you may add it to the GCC specs file.
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| 280 | This file can be found in the same directory that contains cc1 (run
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| 281 | <code>gcc -print-prog-name=cc1</code> to find it). You may add linker
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| 282 | flags such as <code>-R</code> or <code>-rpath</code>, depending on
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| 283 | platform and linker, to the <code>*link</code> or <code>*lib</code>
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| 284 | specs.</p>
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| 285 |
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| 286 | <p>Another alternative is to install a wrapper script around gcc, g++
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| 287 | or ld that adds the appropriate directory to the environment variable
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| 288 | <code>LD_RUN_PATH</code> or equivalent (again, it's
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| 289 | platform-dependent).</p>
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| 290 |
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| 291 | <p>Yet another option, that works on a few platforms, is to hard-code
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| 292 | the full pathname of the library into its soname. This can only be
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| 293 | accomplished by modifying the appropriate <tt>.ml</tt> file within
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| 294 | <tt>libstdc++/config</tt> (and also <tt>libg++/config</tt>, if you are
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| 295 | building libg++), so that <code>$(libdir)/</code> appears just before
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| 296 | the library name in <code>-soname</code> or <code>-h</code> options.</p>
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| 297 |
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| 298 | <hr />
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| 299 | <h2><a name="gas">GCC can not find GNU as/GNU ld</a></h2>
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| 300 | <p>GCC searches the PATH for an assembler and a loader, but it only
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| 301 | does so after searching a directory list hard-coded in the GCC
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| 302 | executables. Since, on most platforms, the hard-coded list includes
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| 303 | directories in which the system assembler and loader can be found, you
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| 304 | may have to take one of the following actions to arrange that GCC uses
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| 305 | the GNU versions of those programs.</p>
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| 306 |
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| 307 | <p>To ensure that GCC finds the GNU assembler (the GNU loader), which
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| 308 | are required by <a href="http://gcc.gnu.org/install/specific.html">some
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| 309 | configurations</a>,
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| 310 | you should configure these with the same --prefix option as you used
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| 311 | for GCC. Then build & install GNU as (GNU ld) and proceed with
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| 312 | building GCC.</p>
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| 313 |
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| 314 | <p>Another alternative is to create links to GNU as and ld in any of
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| 315 | the directories printed by the command `<tt>gcc -print-search-dirs |
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| 316 | grep '^programs:'</tt>'. The link to `<tt>ld</tt>' should be named
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| 317 | `<tt>real-ld</tt>' if `<tt>ld</tt>' already exists. If such links do
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| 318 | not exist while you're compiling GCC, you may have to create them in
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| 319 | the build directories too, within the <tt>gcc</tt> directory
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| 320 | <em>and</em> in all the <tt>gcc/stage*</tt> subdirectories.</p>
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| 321 |
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| 322 | <p>GCC 2.95 allows you to specify the full pathname of the assembler
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| 323 | and the linker to use. The configure flags are
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| 324 | `<tt>--with-as=/path/to/as</tt>' and `<tt>--with-ld=/path/to/ld</tt>'.
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| 325 | GCC will try to use these pathnames before looking for `<tt>as</tt>'
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| 326 | or `<tt>(real-)ld</tt>' in the standard search dirs. If, at
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| 327 | configure-time, the specified programs are found to be GNU utilities,
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| 328 | `<tt>--with-gnu-as</tt>' and `<tt>--with-gnu-ld</tt>' need not be
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| 329 | used; these flags will be auto-detected. One drawback of this option
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| 330 | is that it won't allow you to override the search path for assembler
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| 331 | and linker with command-line options <tt>-B/path/</tt> if the
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| 332 | specified filenames exist.</p>
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| 333 |
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| 334 | <hr />
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| 335 | <h2><a name="environ">cpp: Usage:... Error</a></h2>
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| 336 |
|
|---|
| 337 | <p>If you get an error like this when building GCC (particularly when building
|
|---|
| 338 | __mulsi3), then you likely have a problem with your environment variables.</p>
|
|---|
| 339 | <pre>
|
|---|
| 340 | cpp: Usage: /usr/lib/gcc-lib/i586-unknown-linux-gnulibc1/2.7.2.3/cpp
|
|---|
| 341 | [switches] input output
|
|---|
| 342 | </pre>
|
|---|
| 343 | <p>First look for an explicit '.' in either LIBRARY_PATH or GCC_EXEC_PREFIX
|
|---|
| 344 | from your environment. If you do not find an explicit '.', look for
|
|---|
| 345 | an empty pathname in those variables. Note that ':' at either the start
|
|---|
| 346 | or end of these variables is an implicit '.' and will cause problems.</p>
|
|---|
| 347 |
|
|---|
| 348 | <p>Also note '::' in these paths will also cause similar problems.</p>
|
|---|
| 349 |
|
|---|
| 350 |
|
|---|
| 351 | <hr />
|
|---|
| 352 | <h2><a name="optimizing">Optimizing the compiler itself</a></h2>
|
|---|
| 353 |
|
|---|
| 354 | <p>If you want to test a particular optimization option, it's useful to try
|
|---|
| 355 | bootstrapping the compiler with that option turned on. For example, to
|
|---|
| 356 | test the <code>-fssa</code> option, you could bootstrap like this:</p>
|
|---|
| 357 |
|
|---|
| 358 | <pre>make BOOT_CFLAGS="-O2 -fssa" bootstrap</pre>
|
|---|
| 359 |
|
|---|
| 360 | <hr />
|
|---|
| 361 | <h2><a name="iconv">Why does <code>libiconv</code> get linked into <code>jc1</code> on Solaris?</a></h2>
|
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| 362 |
|
|---|
| 363 | <p>The Java front end requires <code>iconv</code>. If the compiler
|
|---|
| 364 | used to bootstrap GCC finds <code>libiconv</code> (because the GNU
|
|---|
| 365 | version of <code>libiconv</code> has been installed in the same prefix
|
|---|
| 366 | as the bootstrap compiler), but the newly built GCC does not find the
|
|---|
| 367 | library (because it will be installed with a different prefix), then a
|
|---|
| 368 | link-time error will occur when building <code>jc1</code>. This
|
|---|
| 369 | problem does not show up so often on platforms that have
|
|---|
| 370 | <code>libiconv</code> in a default location (like
|
|---|
| 371 | <code>/usr/lib</code>) because then both compilers can find a library
|
|---|
| 372 | named <code>libiconv</code>, even though it is a different
|
|---|
| 373 | library.</p>
|
|---|
| 374 |
|
|---|
| 375 | <p>Using <code>--disable-nls</code> at configure-time does not
|
|---|
| 376 | prevent this problem because <code>jc1</code> uses
|
|---|
| 377 | <code>iconv</code> even in that case. Solutions include temporarily
|
|---|
| 378 | removing the GNU <code>libiconv</code>, copying it to a default
|
|---|
| 379 | location such as <code>/usr/lib/</code>, and using
|
|---|
| 380 | <code>--enable-languages</code> at configure-time to disable Java.</p>
|
|---|
| 381 |
|
|---|
| 382 | <hr />
|
|---|
| 383 | <a name="testsuite"></a>
|
|---|
| 384 | <h1>Testsuite problems</h1>
|
|---|
| 385 |
|
|---|
| 386 | <h2><a name="testoptions">How do I pass flags like
|
|---|
| 387 | <code>-fnew-abi</code> to the testsuite?</a></h2>
|
|---|
| 388 |
|
|---|
| 389 | <p>If you invoke <code>runtest</code> directly, you can use the
|
|---|
| 390 | <code>--tool_opts</code> option, e.g:</p>
|
|---|
| 391 | <pre>
|
|---|
| 392 | runtest --tool_opts "-fnew-abi -fno-honor-std" <other options>
|
|---|
| 393 | </pre>
|
|---|
| 394 | <p>Or, if you use <code>make check</code> you can use the
|
|---|
| 395 | <code>make</code> variable <code>RUNTESTFLAGS</code>, e.g:</p>
|
|---|
| 396 | <pre>
|
|---|
| 397 | make RUNTESTFLAGS="--tool_opts '-fnew-abi -fno-honor-std'" check-g++
|
|---|
| 398 | </pre>
|
|---|
| 399 |
|
|---|
| 400 | <hr />
|
|---|
| 401 | <h2><a name="multipletests"> How can I run the test suite with multiple options? </a></h2>
|
|---|
| 402 |
|
|---|
| 403 | <p>If you invoke <code>runtest</code> directly, you can use the
|
|---|
| 404 | <code>--target_board</code> option, e.g:</p>
|
|---|
| 405 | <pre>
|
|---|
| 406 | runtest --target_board "unix{-fPIC,-fpic,}" <other options>
|
|---|
| 407 | </pre>
|
|---|
| 408 | <p>Or, if you use <code>make check</code> you can use the
|
|---|
| 409 | <code>make</code> variable <code>RUNTESTFLAGS</code>, e.g:</p>
|
|---|
| 410 | <pre>
|
|---|
| 411 | make RUNTESTFLAGS="--target_board 'unix{-fPIC,-fpic,}'" check-gcc
|
|---|
| 412 | </pre>
|
|---|
| 413 | <p>Either of these examples will run the tests three times. Once
|
|---|
| 414 | with <code>-fPIC</code>, once with <code>-fpic</code>, and once with
|
|---|
| 415 | no additional flags.</p>
|
|---|
| 416 |
|
|---|
| 417 | <p>This technique is particularly useful on multilibbed targets.</p>
|
|---|
| 418 |
|
|---|
| 419 | <hr />
|
|---|
| 420 | <a name="old"></a>
|
|---|
| 421 | <h1>Older versions of GCC and EGCS</h1>
|
|---|
| 422 |
|
|---|
| 423 | <h2><a name="2.95sstream">Is there a stringstream / sstream for GCC 2.95.2?</a></h2>
|
|---|
| 424 |
|
|---|
| 425 | <p>Yes, it's at:
|
|---|
| 426 | <a href="http://gcc.gnu.org/ml/libstdc++/2000-q2/msg00700/sstream">
|
|---|
| 427 | http://gcc.gnu.org/ml/libstdc++/2000-q2/msg00700/sstream</a>.</p>
|
|---|
| 428 |
|
|---|
| 429 | <hr />
|
|---|
| 430 | <a name="misc"></a>
|
|---|
| 431 | <h1>Miscellaneous</h1>
|
|---|
| 432 |
|
|---|
| 433 |
|
|---|
| 434 | <h2><a name="friend">Friend Templates</a></h2>
|
|---|
| 435 |
|
|---|
| 436 | <p>In order to make a specialization of a template function a friend
|
|---|
| 437 | of a (possibly template) class, you must explicitly state that the
|
|---|
| 438 | friend function is a template, by appending angle brackets to its
|
|---|
| 439 | name, and this template function must have been declared already.
|
|---|
| 440 | Here's an example:</p>
|
|---|
| 441 | <pre>
|
|---|
| 442 | template <typename T> class foo {
|
|---|
| 443 | friend void bar(foo<T>);
|
|---|
| 444 | }
|
|---|
| 445 | </pre>
|
|---|
| 446 | <p>The above declaration declares a non-template function named
|
|---|
| 447 | <code>bar</code>, so it must be explicitly defined for <b>each</b>
|
|---|
| 448 | specialization of <code>foo</code>. A template definition of <code>bar</code>
|
|---|
| 449 | won't do, because it is unrelated with the non-template declaration
|
|---|
| 450 | above. So you'd have to end up writing:</p>
|
|---|
| 451 | <pre>
|
|---|
| 452 | void bar(foo<int>) { /* ... */ }
|
|---|
| 453 | void bar(foo<void>) { /* ... */ }
|
|---|
| 454 | </pre>
|
|---|
| 455 | <p>If you meant <code>bar</code> to be a template function, you should
|
|---|
| 456 | have forward-declared it as follows. Note that, since the template
|
|---|
| 457 | function declaration refers to the template class, the template class
|
|---|
| 458 | must be forward-declared too:</p>
|
|---|
| 459 | <pre>
|
|---|
| 460 | template <typename T>
|
|---|
| 461 | class foo;
|
|---|
| 462 |
|
|---|
| 463 | template <typename T>
|
|---|
| 464 | void bar(foo<T>);
|
|---|
| 465 |
|
|---|
| 466 | template <typename T>
|
|---|
| 467 | class foo {
|
|---|
| 468 | friend void bar<>(foo<T>);
|
|---|
| 469 | };
|
|---|
| 470 |
|
|---|
| 471 | template <typename T>
|
|---|
| 472 | void bar(foo<T>) { /* ... */ }
|
|---|
| 473 | </pre>
|
|---|
| 474 | <p>In this case, the template argument list could be left empty,
|
|---|
| 475 | because it can be implicitly deduced from the function arguments, but
|
|---|
| 476 | the angle brackets must be present, otherwise the declaration will be
|
|---|
| 477 | taken as a non-template function. Furthermore, in some cases, you may
|
|---|
| 478 | have to explicitly specify the template arguments, to remove
|
|---|
| 479 | ambiguity.</p>
|
|---|
| 480 |
|
|---|
| 481 | <p>An error in the last public comment draft of the ANSI/ISO C++
|
|---|
| 482 | Standard and the fact that previous releases of GCC would accept such
|
|---|
| 483 | friend declarations as template declarations has led people to believe
|
|---|
| 484 | that the forward declaration was not necessary, but, according to the
|
|---|
| 485 | final version of the Standard, it is.</p>
|
|---|
| 486 |
|
|---|
| 487 |
|
|---|
| 488 | <hr />
|
|---|
| 489 | <h2><a name="dso"><code>dynamic_cast</code>, <code>throw</code>, <code>typeid</code> don't work with shared libraries</a></h2>
|
|---|
| 490 |
|
|---|
| 491 | <p>The new C++ ABI in the GCC 3.0 series uses address comparisons,
|
|---|
| 492 | rather than string compares, to determine type equality. This leads
|
|---|
| 493 | to better performance. Like other objects that have to be present in the
|
|---|
| 494 | final executable, these <code>std::type_info</code> objects have what
|
|---|
| 495 | is called vague linkage because they are not tightly bound to any one
|
|---|
| 496 | particular translation unit (object file). The compiler has to emit
|
|---|
| 497 | them in any translation unit that requires their presence, and then
|
|---|
| 498 | rely on the linking and loading process to make sure that only one of
|
|---|
| 499 | them is active in the final executable. With static linking all of
|
|---|
| 500 | these symbols are resolved at link time, but with dynamic linking,
|
|---|
| 501 | further resolution occurs at load time. You have to ensure that
|
|---|
| 502 | objects within a shared library are resolved against objects in the
|
|---|
| 503 | executable and other shared libraries.</p>
|
|---|
| 504 |
|
|---|
| 505 | <ul>
|
|---|
| 506 | <li>For a program which is linked against a shared library, no additional
|
|---|
| 507 | precautions are needed.</li>
|
|---|
| 508 |
|
|---|
| 509 | <li>You cannot create a shared library with the "<code>-Bsymbolic</code>"
|
|---|
| 510 | option, as that prevents the resolution described above.</li>
|
|---|
| 511 |
|
|---|
| 512 | <li>If you use <code>dlopen</code> to explicitly load code from a shared
|
|---|
| 513 | library, you must do several things. First, export global symbols from
|
|---|
| 514 | the executable by linking it with the "<code>-E</code>" flag (you will
|
|---|
| 515 | have to specify this as "<code>-Wl,-E</code>" if you are invoking
|
|---|
| 516 | the linker in the usual manner from the compiler driver, <code>g++</code>).
|
|---|
| 517 | You must also make the external symbols in the loaded library
|
|---|
| 518 | available for subsequent libraries by providing the <code>RTLD_GLOBAL</code>
|
|---|
| 519 | flag to <code>dlopen</code>. The symbol resolution can be immediate or
|
|---|
| 520 | lazy.</li>
|
|---|
| 521 |
|
|---|
| 522 | </ul>
|
|---|
| 523 |
|
|---|
| 524 | <p>Template instantiations are another, user visible, case of objects
|
|---|
| 525 | with vague linkage, which needs similar resolution. If you do not take
|
|---|
| 526 | the above precautions, you may discover that a template instantiation
|
|---|
| 527 | with the same argument list, but instantiated in multiple translation
|
|---|
| 528 | units, has several addresses, depending in which translation unit the
|
|---|
| 529 | address is taken. (This is <em>not</em> an exhaustive list of the kind
|
|---|
| 530 | of objects which have vague linkage and are expected to be resolved
|
|---|
| 531 | during linking & loading.)</p>
|
|---|
| 532 |
|
|---|
| 533 | <p>If you are worried about different objects with the same name
|
|---|
| 534 | colliding during the linking or loading process, then you should use
|
|---|
| 535 | namespaces to disambiguate them. Giving distinct objects with global
|
|---|
| 536 | linkage the same name is a violation of the One Definition Rule (ODR)
|
|---|
| 537 | [basic.def.odr].</p>
|
|---|
| 538 |
|
|---|
| 539 | <p>For more details about the way that GCC implements these and other
|
|---|
| 540 | C++ features, please read the <a
|
|---|
| 541 | href="http://www.codesourcery.com/cxx-abi/">ABI specification</a>.
|
|---|
| 542 | Note the <code>std::type_info</code> objects which <i>must</i> be
|
|---|
| 543 | resolved all begin with "_ZTS". Refer to <code>ld</code>'s
|
|---|
| 544 | documentation for a description of the "<code>-E</code>" &
|
|---|
| 545 | "<code>-Bsymbolic</code>" flags.</p>
|
|---|
| 546 |
|
|---|
| 547 | <hr />
|
|---|
| 548 | <h2><a name="generated_files">Why do I need autoconf, bison, xgettext, automake, etc?</a></h2>
|
|---|
| 549 |
|
|---|
| 550 | <p>If you're using diffs up dated from one snapshot to the next, or
|
|---|
| 551 | if you're using the CVS repository, you may need several additional programs
|
|---|
| 552 | to build GCC.</p>
|
|---|
| 553 |
|
|---|
| 554 | <p>These include, but are not necessarily limited to autoconf, automake,
|
|---|
| 555 | bison, and xgettext.</p>
|
|---|
| 556 |
|
|---|
| 557 | <p>This is necessary because neither diff nor cvs keep timestamps
|
|---|
| 558 | correct. This causes problems for generated files as "make" may think
|
|---|
| 559 | those generated files are out of date and try to regenerate them.</p>
|
|---|
| 560 |
|
|---|
| 561 | <p>An easy way to work around this problem is to use the <code>gcc_update
|
|---|
| 562 | </code> script in the contrib subdirectory of GCC, which handles this
|
|---|
| 563 | transparently without requiring installation of any additional tools.
|
|---|
| 564 | (Note: Up to and including GCC 2.95 this script was called <code>egcs_update
|
|---|
| 565 | </code>.)</p>
|
|---|
| 566 |
|
|---|
| 567 |
|
|---|
| 568 | <p>When building from diffs or CVS or if you modified some sources,
|
|---|
| 569 | you may also need to obtain development versions of some GNU tools, as
|
|---|
| 570 | the production versions do not necessarily handle all features needed
|
|---|
| 571 | to rebuild GCC.</p>
|
|---|
| 572 |
|
|---|
| 573 | <p>In general, the current versions of these tools from <a
|
|---|
| 574 | href="ftp://ftp.gnu.org/gnu/">ftp://ftp.gnu.org/gnu/</a> will work.
|
|---|
| 575 | At present, Autoconf 2.50 is not supported, and you will need to use
|
|---|
| 576 | Autoconf 2.13; work is in progress to fix this problem. Also look at
|
|---|
| 577 | <a href="ftp://gcc.gnu.org/pub/gcc/infrastructure/">
|
|---|
| 578 | ftp://gcc.gnu.org/pub/gcc/infrastructure/</a> for any special versions
|
|---|
| 579 | of packages.</p>
|
|---|
| 580 |
|
|---|
| 581 |
|
|---|
| 582 | <hr />
|
|---|
| 583 | <h2><a name="picflag-needed">Why can't I build a shared library?</a></h2>
|
|---|
| 584 |
|
|---|
| 585 | <p>When building a shared library you may get an error message from the
|
|---|
| 586 | linker like `assert pure-text failed:' or `DP relative code in file'.</p>
|
|---|
| 587 |
|
|---|
| 588 | <p>This kind of error occurs when you've failed to provide proper flags
|
|---|
| 589 | to gcc when linking the shared library. </p>
|
|---|
| 590 |
|
|---|
| 591 | <p>You can get this error even if all the .o files for the shared library were
|
|---|
| 592 | compiled with the proper PIC option. When building a shared library, gcc will
|
|---|
| 593 | compile additional code to be included in the library. That additional code
|
|---|
| 594 | must also be compiled with the proper PIC option.</p>
|
|---|
| 595 |
|
|---|
| 596 | <p>Adding the proper PIC option (<tt>-fpic</tt> or <tt>-fPIC</tt>) to the link
|
|---|
| 597 | line which creates the shared library will fix this problem on targets that
|
|---|
| 598 | support PIC in this manner. For example:</p>
|
|---|
| 599 | <pre>
|
|---|
| 600 | gcc -c -fPIC myfile.c
|
|---|
| 601 | gcc -shared -o libmyfile.so -fPIC myfile.o
|
|---|
| 602 | </pre>
|
|---|
| 603 |
|
|---|
| 604 |
|
|---|
| 605 | <hr />
|
|---|
| 606 | <h2><a name="vtables">When building C++, the linker says my constructors, destructors or virtual tables are undefined, but I defined them</a></h2>
|
|---|
| 607 |
|
|---|
| 608 | <p>The ISO C++ Standard specifies that all virtual methods of a class
|
|---|
| 609 | that are not pure-virtual must be defined, but does not require any
|
|---|
| 610 | diagnostic for violations of this rule [class.virtual]/8. Based on
|
|---|
| 611 | this assumption, GCC will only emit the implicitly defined
|
|---|
| 612 | constructors, the assignment operator, the destructor and the virtual
|
|---|
| 613 | table of a class in the translation unit that defines its first such
|
|---|
| 614 | non-inline method.</p>
|
|---|
| 615 |
|
|---|
| 616 | <p>Therefore, if you fail to define this particular method, the linker
|
|---|
| 617 | may complain about the lack of definitions for apparently unrelated
|
|---|
| 618 | symbols. Unfortunately, in order to improve this error message, it
|
|---|
| 619 | might be necessary to change the linker, and this can't always be
|
|---|
| 620 | done.</p>
|
|---|
| 621 |
|
|---|
| 622 | <p>The solution is to ensure that all virtual methods that are not
|
|---|
| 623 | pure are defined. Note that a destructor must be defined even if it
|
|---|
| 624 | is declared pure-virtual [class.dtor]/7.</p>
|
|---|
| 625 |
|
|---|
| 626 |
|
|---|
| 627 | <hr />
|
|---|
| 628 | <h2><a name="incremental">Will GCC someday include an incremental linker?</a></h2>
|
|---|
| 629 |
|
|---|
| 630 | <p>Incremental linking is part of the linker, not the compiler. As
|
|---|
| 631 | such, GCC doesn't have anything to do with incremental linking.
|
|---|
| 632 | Depending on what platform you use, it may be possible to tell GCC to
|
|---|
| 633 | use the platform's native linker (e.g., Solaris' ild(1)).</p>
|
|---|
| 634 |
|
|---|
| 635 |
|
|---|
| 636 | </body>
|
|---|
| 637 | </html>
|
|---|
