mirror of
https://github.com/apache/httpd.git
synced 2025-11-03 17:53:20 +03:00
5d3d7c8875
git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1226563 13f79535-47bb-0310-9956-ffa450edef68
306 lines
17 KiB
XML
306 lines
17 KiB
XML
<?xml version="1.0" encoding="ISO-8859-1"?>
|
|
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
|
|
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!--
|
|
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
|
|
This file is generated from xml source: DO NOT EDIT
|
|
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
|
|
-->
|
|
<title>Dynamic Shared Object (DSO) Support - Apache HTTP Server</title>
|
|
<link href="./style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
|
|
<link href="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
|
|
<link href="./style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" />
|
|
<link href="./images/favicon.ico" rel="shortcut icon" /></head>
|
|
<body id="manual-page"><div id="page-header">
|
|
<p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p>
|
|
<p class="apache">Apache HTTP Server Version 2.5</p>
|
|
<img alt="" src="./images/feather.gif" /></div>
|
|
<div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
|
|
<div id="path">
|
|
<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="./">Version 2.5</a></div><div id="page-content"><div id="preamble"><h1>Dynamic Shared Object (DSO) Support</h1>
|
|
<div class="toplang">
|
|
<p><span>Available Languages: </span><a href="./en/dso.html" title="English"> en </a> |
|
|
<a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
|
|
<a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
|
|
<a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
|
|
<a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
|
|
</div>
|
|
|
|
<p>The Apache HTTP Server is a modular program where the
|
|
administrator can choose the functionality to include in the
|
|
server by selecting a set of modules.
|
|
Modules will be compiled as Dynamic Shared Objects (DSOs)
|
|
that exist separately from the main <code class="program"><a href="./programs/httpd.html">httpd</a></code>
|
|
binary file. DSO modules may be compiled at the time the server
|
|
is built, or they may be compiled and added at a later time
|
|
using the Apache Extension Tool (<code class="program"><a href="./programs/apxs.html">apxs</a></code>).</p>
|
|
<p>Alternatively, the modules can be statically compiled into
|
|
the <code class="program"><a href="./programs/httpd.html">httpd</a></code> binary when the server is built.</p>
|
|
|
|
<p>This document describes how to use DSO modules as well as
|
|
the theory behind their use.</p>
|
|
</div>
|
|
<div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#implementation">Implementation</a></li>
|
|
<li><img alt="" src="./images/down.gif" /> <a href="#usage">Usage Summary</a></li>
|
|
<li><img alt="" src="./images/down.gif" /> <a href="#background">Background</a></li>
|
|
<li><img alt="" src="./images/down.gif" /> <a href="#advantages">Advantages and Disadvantages</a></li>
|
|
</ul></div>
|
|
<div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
|
|
<div class="section">
|
|
<h2><a name="implementation" id="implementation">Implementation</a></h2>
|
|
|
|
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_so.html">mod_so</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code></li></ul></td></tr></table>
|
|
|
|
<p>The DSO support for loading individual Apache httpd modules is based
|
|
on a module named <code class="module"><a href="./mod/mod_so.html">mod_so</a></code> which must be statically
|
|
compiled into the Apache httpd core. It is the only module besides
|
|
<code class="module"><a href="./mod/core.html">core</a></code> which cannot be put into a DSO
|
|
itself. Practically all other distributed Apache httpd modules will then
|
|
be placed into a DSO. After a module is compiled into a DSO named
|
|
<code>mod_foo.so</code> you can use <code class="module"><a href="./mod/mod_so.html">mod_so</a></code>'s <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> directive in your
|
|
<code>httpd.conf</code> file to load this module at server startup
|
|
or restart.</p>
|
|
<p>The DSO builds for individual modules can be disabled via
|
|
<code class="program"><a href="./programs/configure.html">configure</a></code>'s <code>--enable-mods-static</code>
|
|
option as discussed in the <a href="install.html">install
|
|
documentation</a>.</p>
|
|
|
|
<p>To simplify this creation of DSO files for Apache httpd modules
|
|
(especially for third-party modules) a support program
|
|
named <code class="program"><a href="./programs/apxs.html">apxs</a></code> (<dfn>APache
|
|
eXtenSion</dfn>) is available. It can be used to build DSO based
|
|
modules <em>outside of</em> the Apache httpd source tree. The idea is
|
|
simple: When installing Apache HTTP Server the <code class="program"><a href="./programs/configure.html">configure</a></code>'s
|
|
<code>make install</code> procedure installs the Apache httpd C
|
|
header files and puts the platform-dependent compiler and
|
|
linker flags for building DSO files into the <code class="program"><a href="./programs/apxs.html">apxs</a></code>
|
|
program. This way the user can use <code class="program"><a href="./programs/apxs.html">apxs</a></code> to compile
|
|
his Apache httpd module sources without the Apache httpd distribution
|
|
source tree and without having to fiddle with the
|
|
platform-dependent compiler and linker flags for DSO
|
|
support.</p>
|
|
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
|
|
<div class="section">
|
|
<h2><a name="usage" id="usage">Usage Summary</a></h2>
|
|
|
|
<p>To give you an overview of the DSO features of Apache HTTP Server 2.x,
|
|
here is a short and concise summary:</p>
|
|
|
|
<ol>
|
|
<li>
|
|
<p>Build and install a <em>distributed</em> Apache httpd module, say
|
|
<code>mod_foo.c</code>, into its own DSO
|
|
<code>mod_foo.so</code>:</p>
|
|
|
|
<div class="example"><p><code>
|
|
$ ./configure --prefix=/path/to/install --enable-foo<br />
|
|
$ make install
|
|
</code></p></div>
|
|
</li>
|
|
|
|
<li>
|
|
<p>Configure Apache HTTP Server with all modules enabled. Only a basic
|
|
set will be loaded during server startup. You can change the set of loaded
|
|
modules by activating or deactivating the <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> directives in
|
|
<code>httpd.conf</code>.</p>
|
|
|
|
<div class="example"><p><code>
|
|
$ ./configure --enable-mods-shared=all<br />
|
|
$ make install
|
|
</code></p></div>
|
|
</li>
|
|
|
|
<li>
|
|
<p>Some modules are only useful for developers and will not be build.
|
|
when using the module set <em>all</em>. To build all available modules
|
|
including developer modules use <em>reallyall</em>. In addition the
|
|
<code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> directives for all
|
|
built modules can be activated via the configure option
|
|
<code>--enable-load-all-modules</code>.</p>
|
|
|
|
<div class="example"><p><code>
|
|
$ ./configure --enable-mods-shared=reallyall --enable-load-all-modules<br />
|
|
$ make install
|
|
</code></p></div>
|
|
</li>
|
|
|
|
<li>
|
|
Build and install a <em>third-party</em> Apache httpd module, say
|
|
<code>mod_foo.c</code>, into its own DSO
|
|
<code>mod_foo.so</code> <em>outside of</em> the Apache httpd
|
|
source tree using <code class="program"><a href="./programs/apxs.html">apxs</a></code>:
|
|
|
|
<div class="example"><p><code>
|
|
$ cd /path/to/3rdparty<br />
|
|
$ apxs -cia mod_foo.c
|
|
</code></p></div>
|
|
</li>
|
|
</ol>
|
|
|
|
<p>In all cases, once the shared module is compiled, you must
|
|
use a <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
|
|
directive in <code>httpd.conf</code> to tell Apache httpd to activate
|
|
the module.</p>
|
|
|
|
<p>See the <a href="programs/apxs.html">apxs documentation</a> for more details.</p>
|
|
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
|
|
<div class="section">
|
|
<h2><a name="background" id="background">Background</a></h2>
|
|
|
|
<p>On modern Unix derivatives there exists a mechanism
|
|
called dynamic linking/loading of <em>Dynamic Shared
|
|
Objects</em> (DSO) which provides a way to build a piece of
|
|
program code in a special format for loading it at run-time
|
|
into the address space of an executable program.</p>
|
|
|
|
<p>This loading can usually be done in two ways: automatically
|
|
by a system program called <code>ld.so</code> when an
|
|
executable program is started or manually from within the
|
|
executing program via a programmatic system interface to the
|
|
Unix loader through the system calls
|
|
<code>dlopen()/dlsym()</code>.</p>
|
|
|
|
<p>In the first way the DSO's are usually called <em>shared
|
|
libraries</em> or <em>DSO libraries</em> and named
|
|
<code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They
|
|
reside in a system directory (usually <code>/usr/lib</code>)
|
|
and the link to the executable program is established at
|
|
build-time by specifying <code>-lfoo</code> to the linker
|
|
command. This hard-codes library references into the executable
|
|
program file so that at start-time the Unix loader is able to
|
|
locate <code>libfoo.so</code> in <code>/usr/lib</code>, in
|
|
paths hard-coded via linker-options like <code>-R</code> or in
|
|
paths configured via the environment variable
|
|
<code>LD_LIBRARY_PATH</code>. It then resolves any (yet
|
|
unresolved) symbols in the executable program which are
|
|
available in the DSO.</p>
|
|
|
|
<p>Symbols in the executable program are usually not referenced
|
|
by the DSO (because it's a reusable library of general code)
|
|
and hence no further resolving has to be done. The executable
|
|
program has no need to do anything on its own to use the
|
|
symbols from the DSO because the complete resolving is done by
|
|
the Unix loader. (In fact, the code to invoke
|
|
<code>ld.so</code> is part of the run-time startup code which
|
|
is linked into every executable program which has been bound
|
|
non-static). The advantage of dynamic loading of common library
|
|
code is obvious: the library code needs to be stored only once,
|
|
in a system library like <code>libc.so</code>, saving disk
|
|
space for every program.</p>
|
|
|
|
<p>In the second way the DSO's are usually called <em>shared
|
|
objects</em> or <em>DSO files</em> and can be named with an
|
|
arbitrary extension (although the canonical name is
|
|
<code>foo.so</code>). These files usually stay inside a
|
|
program-specific directory and there is no automatically
|
|
established link to the executable program where they are used.
|
|
Instead the executable program manually loads the DSO at
|
|
run-time into its address space via <code>dlopen()</code>. At
|
|
this time no resolving of symbols from the DSO for the
|
|
executable program is done. But instead the Unix loader
|
|
automatically resolves any (yet unresolved) symbols in the DSO
|
|
from the set of symbols exported by the executable program and
|
|
its already loaded DSO libraries (especially all symbols from
|
|
the ubiquitous <code>libc.so</code>). This way the DSO gets
|
|
knowledge of the executable program's symbol set as if it had
|
|
been statically linked with it in the first place.</p>
|
|
|
|
<p>Finally, to take advantage of the DSO's API the executable
|
|
program has to resolve particular symbols from the DSO via
|
|
<code>dlsym()</code> for later use inside dispatch tables
|
|
<em>etc.</em> In other words: The executable program has to
|
|
manually resolve every symbol it needs to be able to use it.
|
|
The advantage of such a mechanism is that optional program
|
|
parts need not be loaded (and thus do not spend memory) until
|
|
they are needed by the program in question. When required,
|
|
these program parts can be loaded dynamically to extend the
|
|
base program's functionality.</p>
|
|
|
|
<p>Although this DSO mechanism sounds straightforward there is
|
|
at least one difficult step here: The resolving of symbols from
|
|
the executable program for the DSO when using a DSO to extend a
|
|
program (the second way). Why? Because "reverse resolving" DSO
|
|
symbols from the executable program's symbol set is against the
|
|
library design (where the library has no knowledge about the
|
|
programs it is used by) and is neither available under all
|
|
platforms nor standardized. In practice the executable
|
|
program's global symbols are often not re-exported and thus not
|
|
available for use in a DSO. Finding a way to force the linker
|
|
to export all global symbols is the main problem one has to
|
|
solve when using DSO for extending a program at run-time.</p>
|
|
|
|
<p>The shared library approach is the typical one, because it
|
|
is what the DSO mechanism was designed for, hence it is used
|
|
for nearly all types of libraries the operating system
|
|
provides.</p>
|
|
|
|
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
|
|
<div class="section">
|
|
<h2><a name="advantages" id="advantages">Advantages and Disadvantages</a></h2>
|
|
|
|
<p>The above DSO based features have the following
|
|
advantages:</p>
|
|
|
|
<ul>
|
|
<li>The server package is more flexible at run-time because
|
|
the server process can be assembled at run-time via
|
|
<code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
|
|
<code>httpd.conf</code> configuration directives instead of
|
|
<code class="program"><a href="./programs/configure.html">configure</a></code> options at build-time. For instance,
|
|
this way one is able to run different server instances
|
|
(standard & SSL version, minimalistic & dynamic
|
|
version [mod_perl, mod_php], <em>etc.</em>) with only one Apache httpd
|
|
installation.</li>
|
|
|
|
<li>The server package can be easily extended with
|
|
third-party modules even after installation. This is
|
|
a great benefit for vendor package maintainers, who can create
|
|
an Apache httpd core package and additional packages containing
|
|
extensions like PHP, mod_perl, mod_security, <em>etc.</em></li>
|
|
|
|
<li>Easier Apache httpd module prototyping, because with the
|
|
DSO/<code class="program"><a href="./programs/apxs.html">apxs</a></code> pair you can both work outside the
|
|
Apache httpd source tree and only need an <code>apxs -i</code>
|
|
command followed by an <code>apachectl restart</code> to
|
|
bring a new version of your currently developed module into
|
|
the running Apache HTTP Server.</li>
|
|
</ul>
|
|
|
|
<p>DSO has the following disadvantages:</p>
|
|
|
|
<ul>
|
|
<li>The server is approximately 20% slower at startup time
|
|
because of the symbol resolving overhead the Unix loader now
|
|
has to do.</li>
|
|
|
|
<li>The server is approximately 5% slower at execution time
|
|
under some platforms, because position independent code (PIC)
|
|
sometimes needs complicated assembler tricks for relative
|
|
addressing, which are not necessarily as fast as absolute
|
|
addressing.</li>
|
|
|
|
<li>Because DSO modules cannot be linked against other
|
|
DSO-based libraries (<code>ld -lfoo</code>) on all platforms
|
|
(for instance a.out-based platforms usually don't provide
|
|
this functionality while ELF-based platforms do) you cannot
|
|
use the DSO mechanism for all types of modules. Or in other
|
|
words, modules compiled as DSO files are restricted to only
|
|
use symbols from the Apache httpd core, from the C library
|
|
(<code>libc</code>) and all other dynamic or static libraries
|
|
used by the Apache httpd core, or from static library archives
|
|
(<code>libfoo.a</code>) containing position independent code.
|
|
The only chances to use other code is to either make sure the
|
|
httpd core itself already contains a reference to it or
|
|
loading the code yourself via <code>dlopen()</code>.</li>
|
|
</ul>
|
|
|
|
</div></div>
|
|
<div class="bottomlang">
|
|
<p><span>Available Languages: </span><a href="./en/dso.html" title="English"> en </a> |
|
|
<a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
|
|
<a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
|
|
<a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
|
|
<a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
|
|
</div><div id="footer">
|
|
<p class="apache">Copyright 2012 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
|
|
<p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p></div>
|
|
</body></html> |