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-<?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">
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- XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
- This file is generated from xml source: DO NOT EDIT
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- -->
-<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" />
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-<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 2.0</p>
-<img alt="" src="./images/feather.gif" /></div>
-<div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="./images/left.gif" /></a></div>
-<div id="path">
-<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="./">Version 2.0</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">&nbsp;en&nbsp;</a> |
-<a href="./es/dso.html" hreflang="es" rel="alternate" title="Español">&nbsp;es&nbsp;</a> |
-<a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a> |
-<a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a> |
-<a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a> |
-<a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe">&nbsp;tr&nbsp;</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. 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.x,
- 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
- 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. On the other hand using shared objects for extending
- a program is not used by a lot of programs.</p>
-
- <p>As of 1998 there are only a few software packages available
- which use the DSO mechanism to actually extend their
- functionality at run-time: Perl 5 (via its XS mechanism and the
- DynaLoader module), Netscape Server, <em>etc.</em> Starting
- with version 1.3, Apache joined the crew, because Apache
- already uses a module concept to extend its functionality and
- internally uses a dispatch-list-based approach to link external
- modules into the Apache core functionality. So, Apache is
- really predestined for using DSO to load its modules at
- run-time.</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 actual 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 commands 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 &amp; SSL version, minimalistic &amp; powered up
- version [mod_perl, PHP3], <em>etc.</em>) with only one Apache
- installation.</li>
-
- <li>The server package can be easily extended with
- third-party modules even after installation. This is at least
- a great benefit for vendor package maintainers who can create
- a Apache core package and additional packages containing
- extensions like PHP3, mod_perl, mod_fastcgi,
- <em>etc.</em></li>
-
- <li>Easier Apache 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 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 server.</li>
- </ul>
-
- <p>DSO has the following disadvantages:</p>
-
- <ul>
- <li>The DSO mechanism cannot be used on every platform
- because not all operating systems support dynamic loading of
- code into the address space of a program.</li>
-
- <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 core, from the C library
- (<code>libc</code>) and all other dynamic or static libraries
- used by the Apache 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
- Apache 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">&nbsp;en&nbsp;</a> |
-<a href="./es/dso.html" hreflang="es" rel="alternate" title="Español">&nbsp;es&nbsp;</a> |
-<a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a> |
-<a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a> |
-<a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a> |
-<a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe">&nbsp;tr&nbsp;</a></p>
-</div><div id="footer">
-<p class="apache">Copyright 2009 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>
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