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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-27 06:33:50 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-27 06:33:50 +0000
commitfe39ffb8b90ae4e002ed73fe98617cd590abb467 (patch)
treeb80e5956907d8aeaaffe4e4f0c068c0e6157ce8b /docs/manual/dso.html.en
parentInitial commit. (diff)
downloadapache2-fe39ffb8b90ae4e002ed73fe98617cd590abb467.tar.xz
apache2-fe39ffb8b90ae4e002ed73fe98617cd590abb467.zip
Adding upstream version 2.4.56.upstream/2.4.56
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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+<?xml version="1.0" encoding="UTF-8"?>
+<!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>
+<meta content="text/html; charset=UTF-8" http-equiv="Content-Type" />
+<!--
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ This file is generated from xml source: DO NOT EDIT
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ -->
+<title>Dynamic Shared Object (DSO) Support - Apache HTTP Server Version 2.4</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 rel="stylesheet" type="text/css" href="./style/css/prettify.css" />
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+</script>
+
+<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="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p>
+<p class="apache">Apache HTTP Server Version 2.4</p>
+<img alt="" src="./images/feather.png" /></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 2.4</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="./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.
+ 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"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><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><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</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 &amp; SSL version, minimalistic &amp; 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">&nbsp;en&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 class="top"><a href="#page-header"><img src="./images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&amp;A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Libera.chat, or sent to our <a href="https://httpd.apache.org/lists.html">mailing lists</a>.</div>
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