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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Appendix B. Porting and Maintenance</title><meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /><meta name="keywords" content="&#10; ISO C++&#10; , &#10; library&#10; " /><link rel="start" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="spine.html" title="The GNU C++ Library" /><link rel="prev" href="bk01apas05.html" title="Design Notes" /><link rel="next" href="internals.html" title="Porting to New Hardware or Operating Systems" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Appendix B. Porting and Maintenance</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bk01apas05.html">Prev</a> </td><th width="60%" align="center">The GNU C++ Library</th><td width="20%" align="right"> <a accesskey="n" href="internals.html">Next</a></td></tr></table><hr /></div><div class="appendix" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="appendix.porting"></a>Appendix B. Porting and Maintenance</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="appendix_porting.html#appendix.porting.build_hacking">Configure and Build Hacking</a></span></dt><dd><dl><dt><span class="sect2"><a href="appendix_porting.html#build_hacking.prereq">Prerequisites</a></span></dt><dt><span class="sect2"><a href="appendix_porting.html#build_hacking.map">Overview: What Comes from Where</a></span></dt><dt><span class="sect2"><a href="appendix_porting.html#build_hacking.scripts">Storing Information in non-AC files (like configure.host)</a></span></dt><dt><span class="sect2"><a href="appendix_porting.html#build_hacking.conventions">Coding and Commenting Conventions</a></span></dt><dt><span class="sect2"><a href="appendix_porting.html#build_hacking.acinclude">The acinclude.m4 layout</a></span></dt><dt><span class="sect2"><a href="appendix_porting.html#build_hacking.enable"><code class="constant">GLIBCXX_ENABLE</code>, the <code class="literal">--enable</code> maker</a></span></dt></dl></dd><dt><span class="sect1"><a href="internals.html">Porting to New Hardware or Operating Systems</a></span></dt><dd><dl><dt><span class="sect2"><a href="internals.html#internals.os">Operating System</a></span></dt><dt><span class="sect2"><a href="internals.html#internals.cpu">CPU</a></span></dt><dt><span class="sect2"><a href="internals.html#internals.char_types">Character Types</a></span></dt><dt><span class="sect2"><a href="internals.html#internals.thread_safety">Thread Safety</a></span></dt><dt><span class="sect2"><a href="internals.html#internals.numeric_limits">Numeric Limits</a></span></dt><dt><span class="sect2"><a href="internals.html#internals.libtool">Libtool</a></span></dt></dl></dd><dt><span class="sect1"><a href="abi.html">ABI Policy and Guidelines</a></span></dt><dd><dl><dt><span class="sect2"><a href="abi.html#abi.cxx_interface">The C++ Interface</a></span></dt><dt><span class="sect2"><a href="abi.html#abi.versioning">Versioning</a></span></dt><dt><span class="sect2"><a href="abi.html#abi.changes_allowed">Allowed Changes</a></span></dt><dt><span class="sect2"><a href="abi.html#abi.changes_no">Prohibited Changes</a></span></dt><dt><span class="sect2"><a href="abi.html#abi.impl">Implementation</a></span></dt><dt><span class="sect2"><a href="abi.html#abi.testing">Testing</a></span></dt><dt><span class="sect2"><a href="abi.html#abi.issues">Outstanding Issues</a></span></dt></dl></dd><dt><span class="sect1"><a href="api.html">API Evolution and Deprecation History</a></span></dt><dd><dl><dt><span class="sect2"><a href="api.html#api.rel_300"><code class="constant">3.0</code></a></span></dt><dt><span class="sect2"><a href="api.html#api.rel_310"><code class="constant">3.1</code></a></span></dt><dt><span class="sect2"><a href="api.html#api.rel_320"><code class="constant">3.2</code></a></span></dt><dt><span class="sect2"><a href="api.html#api.rel_330"><code class="constant">3.3</code></a></span></dt><dt><span class="sect2"><a href="api.html#api.rel_340"><code class="constant">3.4</code></a></span></dt><dt><span class="sect2"><a href="api.html#api.rel_400"><code class="constant">4.0</code></a></span></dt><dt><span class="sect2"><a href="api.html#api.rel_410"><code class="constant">4.1</code></a></span></dt><dt><span class="sect2"><a href="api.html#api.rel_420"><code class="constant">4.2</code></a></span></dt><dt><span class="sect2"><a href="api.html#api.rel_430"><code class="constant">4.3</code></a></span></dt></dl></dd><dt><span class="sect1"><a href="backwards.html">Backwards Compatibility</a></span></dt><dd><dl><dt><span class="sect2"><a href="backwards.html#backwards.first">First</a></span></dt><dt><span class="sect2"><a href="backwards.html#backwards.second">Second</a></span></dt><dt><span class="sect2"><a href="backwards.html#backwards.third">Third</a></span></dt></dl></dd></dl></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="appendix.porting.build_hacking"></a>Configure and Build Hacking</h2></div></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="build_hacking.prereq"></a>Prerequisites</h3></div></div></div><p>
As noted <a class="ulink" href="http://gcc.gnu.org/install/prerequisites.html" target="_top">previously</a>,
certain other tools are necessary for hacking on files that
control configure (<code class="code">configure.ac</code>,
<code class="code">acinclude.m4</code>) and make
(<code class="code">Makefile.am</code>). These additional tools
(<code class="code">automake</code>, and <code class="code">autoconf</code>) are further
described in detail in their respective manuals. All the libraries
in GCC try to stay in sync with each other in terms of versions of
the auto-tools used, so please try to play nicely with the
neighbors.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="build_hacking.map"></a>Overview: What Comes from Where</h3></div></div></div><pre class="screen">
<img src="../images/confdeps.png" alt="Dependency Graph Configure to Build Files" />
</pre><p>
Regenerate all generated files by using the command sequence
<code class="code">"autoreconf"</code> at the top level of the libstdc++ source
directory. The following will also work, but is much more complex:
<code class="code">"aclocal-1.7 &amp;&amp; autoconf-2.59 &amp;&amp;
autoheader-2.59 &amp;&amp; automake-1.7"</code> The version
numbers may be absent entirely or otherwise vary depending on
<a class="ulink" href="http://gcc.gnu.org/install/prerequisites.html" target="_top">the
current requirements</a> and your vendor's choice of
installation names.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="build_hacking.scripts"></a>Storing Information in non-AC files (like configure.host)</h3></div></div></div><p>
Until that glorious day when we can use AC_TRY_LINK with a
cross-compiler, we have to hardcode the results of what the tests
would have shown if they could be run. So we have an inflexible
mess like crossconfig.m4.
</p><p>
Wouldn't it be nice if we could store that information in files
like configure.host, which can be modified without needing to
regenerate anything, and can even be tweaked without really
knowing how the configury all works? Perhaps break the pieces of
crossconfig.m4 out and place them in their appropriate
config/{cpu,os} directory.
</p><p>
Alas, writing macros like
"<code class="code">AC_DEFINE(HAVE_A_NICE_DAY)</code>" can only be done inside
files which are passed through autoconf. Files which are pure
shell script can be source'd at configure time. Files which
contain autoconf macros must be processed with autoconf. We could
still try breaking the pieces out into "config/*/cross.m4" bits,
for instance, but then we would need arguments to aclocal/autoconf
to properly find them all when generating configure. I would
discourage that.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="build_hacking.conventions"></a>Coding and Commenting Conventions</h3></div></div></div><p>
Most comments should use {octothorpes, shibboleths, hash marks,
pound signs, whatevers} rather than "dnl". Nearly all comments in
configure.ac should. Comments inside macros written in ancilliary
.m4 files should. About the only comments which should
<span class="emphasis"><em>not</em></span> use #, but use dnl instead, are comments
<span class="emphasis"><em>outside</em></span> our own macros in the ancilliary
files. The difference is that # comments show up in
<code class="code">configure</code> (which is most helpful for debugging),
while dnl'd lines just vanish. Since the macros in ancilliary
files generate code which appears in odd places, their "outside"
comments tend to not be useful while reading
<code class="code">configure</code>.
</p><p>
Do not use any <code class="code">$target*</code> variables, such as
<code class="code">$target_alias</code>. The single exception is in
configure.ac, for automake+dejagnu's sake.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="build_hacking.acinclude"></a>The acinclude.m4 layout</h3></div></div></div><p>
The nice thing about acinclude.m4/aclocal.m4 is that macros aren't
actually performed/called/expanded/whatever here, just loaded. So
we can arrange the contents however we like. As of this writing,
acinclude.m4 is arranged as follows:
</p><pre class="programlisting">
GLIBCXX_CHECK_HOST
GLIBCXX_TOPREL_CONFIGURE
GLIBCXX_CONFIGURE
</pre><p>
All the major variable "discovery" is done here. CXX, multilibs,
etc.
</p><pre class="programlisting">
fragments included from elsewhere
</pre><p>
Right now, "fragments" == "the math/linkage bits".
</p><pre class="programlisting">
GLIBCXX_CHECK_COMPILER_FEATURES
GLIBCXX_CHECK_LINKER_FEATURES
GLIBCXX_CHECK_WCHAR_T_SUPPORT
</pre><p>
Next come extra compiler/linker feature tests. Wide character
support was placed here because I couldn't think of another place
for it. It will probably get broken apart like the math tests,
because we're still disabling wchars on systems which could actually
support them.
</p><pre class="programlisting">
GLIBCXX_CHECK_SETRLIMIT_ancilliary
GLIBCXX_CHECK_SETRLIMIT
GLIBCXX_CHECK_S_ISREG_OR_S_IFREG
GLIBCXX_CHECK_POLL
GLIBCXX_CHECK_WRITEV
GLIBCXX_CONFIGURE_TESTSUITE
</pre><p>
Feature tests which only get used in one place. Here, things used
only in the testsuite, plus a couple bits used in the guts of I/O.
</p><pre class="programlisting">
GLIBCXX_EXPORT_INCLUDES
GLIBCXX_EXPORT_FLAGS
GLIBCXX_EXPORT_INSTALL_INFO
</pre><p>
Installation variables, multilibs, working with the rest of the
compiler. Many of the critical variables used in the makefiles are
set here.
</p><pre class="programlisting">
GLIBGCC_ENABLE
GLIBCXX_ENABLE_C99
GLIBCXX_ENABLE_CHEADERS
GLIBCXX_ENABLE_CLOCALE
GLIBCXX_ENABLE_CONCEPT_CHECKS
GLIBCXX_ENABLE_CSTDIO
GLIBCXX_ENABLE_CXX_FLAGS
GLIBCXX_ENABLE_C_MBCHAR
GLIBCXX_ENABLE_DEBUG
GLIBCXX_ENABLE_DEBUG_FLAGS
GLIBCXX_ENABLE_LONG_LONG
GLIBCXX_ENABLE_PCH
GLIBCXX_ENABLE_SJLJ_EXCEPTIONS
GLIBCXX_ENABLE_SYMVERS
GLIBCXX_ENABLE_THREADS
</pre><p>
All the features which can be controlled with enable/disable
configure options. Note how they're alphabetized now? Keep them
like that. :-)
</p><pre class="programlisting">
AC_LC_MESSAGES
libtool bits
</pre><p>
Things which we don't seem to use directly, but just has to be
present otherwise stuff magically goes wonky.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="build_hacking.enable"></a><code class="constant">GLIBCXX_ENABLE</code>, the <code class="literal">--enable</code> maker</h3></div></div></div><p>
All the GLIBCXX_ENABLE_FOO macros use a common helper,
GLIBCXX_ENABLE. (You don't have to use it, but it's easy.) The
helper does two things for us:
</p><div class="orderedlist"><ol type="1"><li><p>
Builds the call to the AC_ARG_ENABLE macro, with --help text
properly quoted and aligned. (Death to changequote!)
</p></li><li><p>
Checks the result against a list of allowed possibilities, and
signals a fatal error if there's no match. This means that the
rest of the GLIBCXX_ENABLE_FOO macro doesn't need to test for
strange arguments, nor do we need to protect against
empty/whitespace strings with the <code class="code">"x$foo" = "xbar"</code>
idiom.
</p></li></ol></div><p>Doing these things correctly takes some extra autoconf/autom4te code,
which made our macros nearly illegible. So all the ugliness is factored
out into this one helper macro.
</p><p>Many of the macros take an argument, passed from when they are expanded
in configure.ac. The argument controls the default value of the
enable/disable switch. Previously, the arguments themselves had defaults.
Now they don't, because that's extra complexity with zero gain for us.
</p><p>There are three "overloaded signatures". When reading the descriptions
below, keep in mind that the brackets are autoconf's quotation characters,
and that they will be stripped. Examples of just about everything occur
in acinclude.m4, if you want to look.
</p><pre class="programlisting">
GLIBCXX_ENABLE (FEATURE, DEFAULT, HELP-ARG, HELP-STRING)
GLIBCXX_ENABLE (FEATURE, DEFAULT, HELP-ARG, HELP-STRING, permit a|b|c)
GLIBCXX_ENABLE (FEATURE, DEFAULT, HELP-ARG, HELP-STRING, SHELL-CODE-HANDLER)
</pre><div class="itemizedlist"><ul type="disc"><li><p>
FEATURE is the string that follows --enable. The results of the
test (such as it is) will be in the variable $enable_FEATURE,
where FEATURE has been squashed. Example:
<code class="code">[extra-foo]</code>, controlled by the --enable-extra-foo
option and stored in $enable_extra_foo.
</p></li><li><p>
DEFAULT is the value to store in $enable_FEATURE if the user does
not pass --enable/--disable. It should be one of the permitted
values passed later. Examples: <code class="code">[yes]</code>, or
<code class="code">[bar]</code>, or <code class="code">[$1]</code> (which passes the
argument given to the GLIBCXX_ENABLE_FOO macro as the
default).
</p><p>
For cases where we need to probe for particular models of things,
it is useful to have an undocumented "auto" value here (see
GLIBCXX_ENABLE_CLOCALE for an example).
</p></li><li><p>
HELP-ARG is any text to append to the option string itself in the
--help output. Examples: <code class="code">[]</code> (i.e., an empty string,
which appends nothing), <code class="code">[=BAR]</code>, which produces
<code class="code">--enable-extra-foo=BAR</code>, and
<code class="code">[@&lt;:@=BAR@:&gt;@]</code>, which produces
<code class="code">--enable-extra-foo[=BAR]</code>. See the difference? See
what it implies to the user?
</p><p>
If you're wondering what that line noise in the last example was,
that's how you embed autoconf special characters in output text.
They're called <a class="ulink" href="http://www.gnu.org/software/autoconf/manual/autoconf-2.57/html_node/autoconf_95.html#SEC95" target="_top"><span class="emphasis"><em>quadrigraphs</em></span></a>
and you should use them whenever necessary.
</p></li><li><p>HELP-STRING is what you think it is. Do not include the
"default" text like we used to do; it will be done for you by
GLIBCXX_ENABLE. By convention, these are not full English
sentences. Example: [turn on extra foo]
</p></li></ul></div><p>
With no other arguments, only the standard autoconf patterns are
allowed: "<code class="code">--{enable,disable}-foo[={yes,no}]</code>" The
$enable_FEATURE variable is guaranteed to equal either "yes" or "no"
after the macro. If the user tries to pass something else, an
explanatory error message will be given, and configure will halt.
</p><p>
The second signature takes a fifth argument, "<code class="code">[permit
a | b | c | ...]</code>"
This allows <span class="emphasis"><em>a</em></span> or <span class="emphasis"><em>b</em></span> or
... after the equals sign in the option, and $enable_FEATURE is
guaranteed to equal one of them after the macro. Note that if you
want to allow plain --enable/--disable with no "=whatever", you must
include "yes" and "no" in the list of permitted values. Also note
that whatever you passed as DEFAULT must be in the list. If the
user tries to pass something not on the list, a semi-explanatory
error message will be given, and configure will halt. Example:
<code class="code">[permit generic|gnu|ieee_1003.1-2001|yes|no|auto]</code>
</p><p>
The third signature takes a fifth argument. It is arbitrary shell
code to execute if the user actually passes the enable/disable
option. (If the user does not, the default is used. Duh.) No
argument checking at all is done in this signature. See
GLIBCXX_ENABLE_CXX_FLAGS for an example of handling, and an error
message.
</p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk01apas05.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="spine.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="internals.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Design Notes </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Porting to New Hardware or Operating Systems</td></tr></table></div></body></html>
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