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<title>Dynamic Shared Object (DSO) Support - Apache HTTP Server</title>
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<p class="apache">Apache HTTP Server Version 2.0</p>
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<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 2.0</a></div><div id="page-content"><div id="preamble"><h1>Dynamic Shared Object (DSO) Support</h1>
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<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. 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. Alternatively, modules can 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>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 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 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 modules can then
be placed into a DSO by individually enabling the DSO build for
them via <code class="program"><a href="./programs/configure.html">configure</a></code>'s
<code>--enable-<em>module</em>=shared</code> option as discussed
in the <a href="install.html">install documentation</a>. 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> command in your
<code>httpd.conf</code> file to load this module at server startup
or restart.</p>
<p>To simplify this creation of DSO files for Apache modules
(especially for third-party modules) a new 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 source tree. The idea is
simple: When installing Apache the <code class="program"><a href="./programs/configure.html">configure</a></code>'s
<code>make install</code> procedure installs the Apache 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 module sources without the Apache 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 2.0,
here is a short and concise summary:</p>
<ol>
<li>
Build and install a <em>distributed</em> Apache module, say
<code>mod_foo.c</code>, into its own DSO
<code>mod_foo.so</code>:
<div class="example"><p><code>
$ ./configure --prefix=/path/to/install --enable-foo=shared<br />
$ make install
</code></p></div>
</li>
<li>
Build and install a <em>third-party</em> Apache module, say
<code>mod_foo.c</code>, into its own DSO
<code>mod_foo.so</code>:
<div class="example"><p><code>
$ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c --enable-foo=shared<br />
$ make install
</code></p></div>
</li>
<li>
Configure Apache for <em>later installation</em> of shared
modules:
<div class="example"><p><code>
$ ./configure --enable-so<br />
$ make install
</code></p></div>
</li>
<li>
Build and install a <em>third-party</em> Apache module, say
<code>mod_foo.c</code>, into its own DSO
<code>mod_foo.so</code> <em>outside of</em> the Apache
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 -c mod_foo.c<br />
$ apxs -i -a -n foo mod_foo.la
</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 to activate
the module.</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 nifty mechanism
usually 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
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