1 <?xml version="1.0" encoding="ISO-8859-1"?>
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
3 <html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!--
4 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
5 This file is generated from xml source: DO NOT EDIT
6 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
8 <title>Dynamic Shared Object (DSO) Support - Apache HTTP Server</title>
9 <link href="./style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
10 <link href="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
11 <link href="./style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" />
12 <link href="./images/favicon.ico" rel="shortcut icon" /></head>
13 <body id="manual-page"><div id="page-header">
14 <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>
15 <p class="apache">Apache HTTP Server Version 2.3</p>
16 <img alt="" src="./images/feather.gif" /></div>
17 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
19 <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 2.3</a></div><div id="page-content"><div id="preamble"><h1>Dynamic Shared Object (DSO) Support</h1>
21 <p><span>Available Languages: </span><a href="./en/dso.html" title="English"> en </a> |
22 <a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
23 <a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
24 <a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
25 <a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
28 <p>The Apache HTTP Server is a modular program where the
29 administrator can choose the functionality to include in the
30 server by selecting a set of modules.
31 Modules will be compiled as Dynamic Shared Objects (DSOs)
32 that exist separately from the main <code class="program"><a href="./programs/httpd.html">httpd</a></code>
33 binary file. DSO modules may be compiled at the time the server
34 is built, or they may be compiled and added at a later time
35 using the Apache Extension Tool (<code class="program"><a href="./programs/apxs.html">apxs</a></code>).</p>
36 <p>Alternatively, the modules can be statically compiled into
37 the <code class="program"><a href="./programs/httpd.html">httpd</a></code> binary when the server is built.</p>
39 <p>This document describes how to use DSO modules as well as
40 the theory behind their use.</p>
42 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#implementation">Implementation</a></li>
43 <li><img alt="" src="./images/down.gif" /> <a href="#usage">Usage Summary</a></li>
44 <li><img alt="" src="./images/down.gif" /> <a href="#background">Background</a></li>
45 <li><img alt="" src="./images/down.gif" /> <a href="#advantages">Advantages and Disadvantages</a></li>
47 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
49 <h2><a name="implementation" id="implementation">Implementation</a></h2>
51 <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>
53 <p>The DSO support for loading individual Apache httpd modules is based
54 on a module named <code class="module"><a href="./mod/mod_so.html">mod_so</a></code> which must be statically
55 compiled into the Apache httpd core. It is the only module besides
56 <code class="module"><a href="./mod/core.html">core</a></code> which cannot be put into a DSO
57 itself. Practically all other distributed Apache httpd modules will then
58 be placed into a DSO. After a module is compiled into a DSO named
59 <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
60 <code>httpd.conf</code> file to load this module at server startup
62 <p>The DSO builds for individual modules can be disabled via
63 <code class="program"><a href="./programs/configure.html">configure</a></code>'s <code>--enable-mods-static</code>
64 option as discussed in the <a href="install.html">install
65 documentation</a>.</p>
67 <p>To simplify this creation of DSO files for Apache httpd modules
68 (especially for third-party modules) a support program
69 named <code class="program"><a href="./programs/apxs.html">apxs</a></code> (<dfn>APache
70 eXtenSion</dfn>) is available. It can be used to build DSO based
71 modules <em>outside of</em> the Apache httpd source tree. The idea is
72 simple: When installing Apache HTTP Server the <code class="program"><a href="./programs/configure.html">configure</a></code>'s
73 <code>make install</code> procedure installs the Apache httpd C
74 header files and puts the platform-dependent compiler and
75 linker flags for building DSO files into the <code class="program"><a href="./programs/apxs.html">apxs</a></code>
76 program. This way the user can use <code class="program"><a href="./programs/apxs.html">apxs</a></code> to compile
77 his Apache httpd module sources without the Apache httpd distribution
78 source tree and without having to fiddle with the
79 platform-dependent compiler and linker flags for DSO
81 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
83 <h2><a name="usage" id="usage">Usage Summary</a></h2>
85 <p>To give you an overview of the DSO features of Apache HTTP Server 2.x,
86 here is a short and concise summary:</p>
90 <p>Build and install a <em>distributed</em> Apache httpd module, say
91 <code>mod_foo.c</code>, into its own DSO
92 <code>mod_foo.so</code>:</p>
94 <div class="example"><p><code>
95 $ ./configure --prefix=/path/to/install --enable-foo<br />
101 <p>Configure Apache HTTP Server with all modules enabled, and loaded
102 as shared objects. You can then remove individual ones by
103 commenting out the <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> directives in
104 <code>httpd.conf</code>.</p>
106 <div class="example"><p><code>
107 $ ./configure --enable-mods-shared=all<br />
113 Build and install a <em>third-party</em> Apache httpd module, say
114 <code>mod_foo.c</code>, into its own DSO
115 <code>mod_foo.so</code> <em>outside of</em> the Apache httpd
116 source tree using <code class="program"><a href="./programs/apxs.html">apxs</a></code>:
118 <div class="example"><p><code>
119 $ cd /path/to/3rdparty<br />
120 $ apxs -cia mod_foo.c
125 <p>In all cases, once the shared module is compiled, you must
126 use a <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
127 directive in <code>httpd.conf</code> to tell Apache httpd to activate
130 <p>See the <a href="programs/apxs.html">apxs documentation</a> for more details.</p>
131 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
132 <div class="section">
133 <h2><a name="background" id="background">Background</a></h2>
135 <p>On modern Unix derivatives there exists a mechanism
136 called dynamic linking/loading of <em>Dynamic Shared
137 Objects</em> (DSO) which provides a way to build a piece of
138 program code in a special format for loading it at run-time
139 into the address space of an executable program.</p>
141 <p>This loading can usually be done in two ways: automatically
142 by a system program called <code>ld.so</code> when an
143 executable program is started or manually from within the
144 executing program via a programmatic system interface to the
145 Unix loader through the system calls
146 <code>dlopen()/dlsym()</code>.</p>
148 <p>In the first way the DSO's are usually called <em>shared
149 libraries</em> or <em>DSO libraries</em> and named
150 <code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They
151 reside in a system directory (usually <code>/usr/lib</code>)
152 and the link to the executable program is established at
153 build-time by specifying <code>-lfoo</code> to the linker
154 command. This hard-codes library references into the executable
155 program file so that at start-time the Unix loader is able to
156 locate <code>libfoo.so</code> in <code>/usr/lib</code>, in
157 paths hard-coded via linker-options like <code>-R</code> or in
158 paths configured via the environment variable
159 <code>LD_LIBRARY_PATH</code>. It then resolves any (yet
160 unresolved) symbols in the executable program which are
161 available in the DSO.</p>
163 <p>Symbols in the executable program are usually not referenced
164 by the DSO (because it's a reusable library of general code)
165 and hence no further resolving has to be done. The executable
166 program has no need to do anything on its own to use the
167 symbols from the DSO because the complete resolving is done by
168 the Unix loader. (In fact, the code to invoke
169 <code>ld.so</code> is part of the run-time startup code which
170 is linked into every executable program which has been bound
171 non-static). The advantage of dynamic loading of common library
172 code is obvious: the library code needs to be stored only once,
173 in a system library like <code>libc.so</code>, saving disk
174 space for every program.</p>
176 <p>In the second way the DSO's are usually called <em>shared
177 objects</em> or <em>DSO files</em> and can be named with an
178 arbitrary extension (although the canonical name is
179 <code>foo.so</code>). These files usually stay inside a
180 program-specific directory and there is no automatically
181 established link to the executable program where they are used.
182 Instead the executable program manually loads the DSO at
183 run-time into its address space via <code>dlopen()</code>. At
184 this time no resolving of symbols from the DSO for the
185 executable program is done. But instead the Unix loader
186 automatically resolves any (yet unresolved) symbols in the DSO
187 from the set of symbols exported by the executable program and
188 its already loaded DSO libraries (especially all symbols from
189 the ubiquitous <code>libc.so</code>). This way the DSO gets
190 knowledge of the executable program's symbol set as if it had
191 been statically linked with it in the first place.</p>
193 <p>Finally, to take advantage of the DSO's API the executable
194 program has to resolve particular symbols from the DSO via
195 <code>dlsym()</code> for later use inside dispatch tables
196 <em>etc.</em> In other words: The executable program has to
197 manually resolve every symbol it needs to be able to use it.
198 The advantage of such a mechanism is that optional program
199 parts need not be loaded (and thus do not spend memory) until
200 they are needed by the program in question. When required,
201 these program parts can be loaded dynamically to extend the
202 base program's functionality.</p>
204 <p>Although this DSO mechanism sounds straightforward there is
205 at least one difficult step here: The resolving of symbols from
206 the executable program for the DSO when using a DSO to extend a
207 program (the second way). Why? Because "reverse resolving" DSO
208 symbols from the executable program's symbol set is against the
209 library design (where the library has no knowledge about the
210 programs it is used by) and is neither available under all
211 platforms nor standardized. In practice the executable
212 program's global symbols are often not re-exported and thus not
213 available for use in a DSO. Finding a way to force the linker
214 to export all global symbols is the main problem one has to
215 solve when using DSO for extending a program at run-time.</p>
217 <p>The shared library approach is the typical one, because it
218 is what the DSO mechanism was designed for, hence it is used
219 for nearly all types of libraries the operating system
222 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
223 <div class="section">
224 <h2><a name="advantages" id="advantages">Advantages and Disadvantages</a></h2>
226 <p>The above DSO based features have the following
230 <li>The server package is more flexible at run-time because
231 the server process can be assembled at run-time via
232 <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
233 <code>httpd.conf</code> configuration directives instead of
234 <code class="program"><a href="./programs/configure.html">configure</a></code> options at build-time. For instance,
235 this way one is able to run different server instances
236 (standard & SSL version, minimalistic & dynamic
237 version [mod_perl, mod_php], <em>etc.</em>) with only one Apache httpd
240 <li>The server package can be easily extended with
241 third-party modules even after installation. This is
242 a great benefit for vendor package maintainers, who can create
243 an Apache httpd core package and additional packages containing
244 extensions like PHP, mod_perl, mod_security, <em>etc.</em></li>
246 <li>Easier Apache httpd module prototyping, because with the
247 DSO/<code class="program"><a href="./programs/apxs.html">apxs</a></code> pair you can both work outside the
248 Apache httpd source tree and only need an <code>apxs -i</code>
249 command followed by an <code>apachectl restart</code> to
250 bring a new version of your currently developed module into
251 the running Apache HTTP Server.</li>
254 <p>DSO has the following disadvantages:</p>
257 <li>The server is approximately 20% slower at startup time
258 because of the symbol resolving overhead the Unix loader now
261 <li>The server is approximately 5% slower at execution time
262 under some platforms, because position independent code (PIC)
263 sometimes needs complicated assembler tricks for relative
264 addressing, which are not necessarily as fast as absolute
267 <li>Because DSO modules cannot be linked against other
268 DSO-based libraries (<code>ld -lfoo</code>) on all platforms
269 (for instance a.out-based platforms usually don't provide
270 this functionality while ELF-based platforms do) you cannot
271 use the DSO mechanism for all types of modules. Or in other
272 words, modules compiled as DSO files are restricted to only
273 use symbols from the Apache httpd core, from the C library
274 (<code>libc</code>) and all other dynamic or static libraries
275 used by the Apache httpd core, or from static library archives
276 (<code>libfoo.a</code>) containing position independent code.
277 The only chances to use other code is to either make sure the
278 httpd core itself already contains a reference to it or
279 loading the code yourself via <code>dlopen()</code>.</li>
283 <div class="bottomlang">
284 <p><span>Available Languages: </span><a href="./en/dso.html" title="English"> en </a> |
285 <a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
286 <a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
287 <a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
288 <a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
289 </div><div id="footer">
290 <p class="apache">Copyright 2011 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>
291 <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>