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8 <title>Dynamic Shared Object (DSO) Support - Apache HTTP Server</title>
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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 2.1</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-project/">Documentation</a> > <a href="./">Version 2.1</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="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
23 <a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
26 <p>The Apache HTTP Server is a modular program where the
27 administrator can choose the functionality to include in the
28 server by selecting a set of modules. The modules can be
29 statically compiled into the <code class="program"><a href="./programs/httpd.html">httpd</a></code> binary when the
30 server is built. Alternatively, modules can be compiled as
31 Dynamic Shared Objects (DSOs) that exist separately from the
32 main <code class="program"><a href="./programs/httpd.html">httpd</a></code> binary file. DSO modules may be
33 compiled at the time the server is built, or they may be
34 compiled and added at a later time using the Apache Extension
35 Tool (<code class="program"><a href="./programs/apxs.html">apxs</a></code>).</p>
37 <p>This document describes how to use DSO modules as well as
38 the theory behind their use.</p>
40 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#implementation">Implementation</a></li>
41 <li><img alt="" src="./images/down.gif" /> <a href="#usage">Usage Summary</a></li>
42 <li><img alt="" src="./images/down.gif" /> <a href="#background">Background</a></li>
43 <li><img alt="" src="./images/down.gif" /> <a href="#advantages">Advantages and Disadvantages</a></li>
45 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
47 <h2><a name="implementation" id="implementation">Implementation</a></h2>
49 <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>
51 <p>The DSO support for loading individual Apache modules is based
52 on a module named <code class="module"><a href="./mod/mod_so.html">mod_so</a></code> which must be statically
53 compiled into the Apache core. It is the only module besides
54 <code class="module"><a href="./mod/core.html">core</a></code> which cannot be put into a DSO
55 itself. Practically all other distributed Apache modules can then
56 be placed into a DSO by individually enabling the DSO build for
57 them via <code class="program"><a href="./programs/configure.html">configure</a></code>'s
58 <code>--enable-<em>module</em>=shared</code> option as discussed
59 in the <a href="install.html">install documentation</a>. After a
60 module is compiled into a DSO named <code>mod_foo.so</code> you
61 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
62 <code>httpd.conf</code> file to load this module at server startup
65 <p>To simplify this creation of DSO files for Apache modules
66 (especially for third-party modules) a new support program
67 named <code class="program"><a href="./programs/apxs.html">apxs</a></code> (<dfn>APache
68 eXtenSion</dfn>) is available. It can be used to build DSO based
69 modules <em>outside of</em> the Apache source tree. The idea is
70 simple: When installing Apache the <code class="program"><a href="./programs/configure.html">configure</a></code>'s
71 <code>make install</code> procedure installs the Apache C
72 header files and puts the platform-dependent compiler and
73 linker flags for building DSO files into the <code class="program"><a href="./programs/apxs.html">apxs</a></code>
74 program. This way the user can use <code class="program"><a href="./programs/apxs.html">apxs</a></code> to compile
75 his Apache module sources without the Apache distribution
76 source tree and without having to fiddle with the
77 platform-dependent compiler and linker flags for DSO
79 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
81 <h2><a name="usage" id="usage">Usage Summary</a></h2>
83 <p>To give you an overview of the DSO features of Apache 2.0,
84 here is a short and concise summary:</p>
88 Build and install a <em>distributed</em> Apache module, say
89 <code>mod_foo.c</code>, into its own DSO
90 <code>mod_foo.so</code>:
92 <div class="example"><p><code>
93 $ ./configure --prefix=/path/to/install --enable-foo=shared<br />
99 Build and install a <em>third-party</em> Apache module, say
100 <code>mod_foo.c</code>, into its own DSO
101 <code>mod_foo.so</code>:
103 <div class="example"><p><code>
104 $ ./configure --add-module=<var>module_type</var>:/path/to/3rdparty/mod_foo.c \<br />
105 <span class="indent">
106 --enable-foo=shared<br />
113 Configure Apache for <em>later installation</em> of shared
116 <div class="example"><p><code>
117 $ ./configure --enable-so<br />
123 Build and install a <em>third-party</em> Apache module, say
124 <code>mod_foo.c</code>, into its own DSO
125 <code>mod_foo.so</code> <em>outside of</em> the Apache
126 source tree using <code class="program"><a href="./programs/apxs.html">apxs</a></code>:
128 <div class="example"><p><code>
129 $ cd /path/to/3rdparty<br />
130 $ apxs -c mod_foo.c<br />
131 $ apxs -i -a -n foo mod_foo.la
136 <p>In all cases, once the shared module is compiled, you must
137 use a <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
138 directive in <code>httpd.conf</code> to tell Apache to activate
140 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
141 <div class="section">
142 <h2><a name="background" id="background">Background</a></h2>
144 <p>On modern Unix derivatives there exists a nifty mechanism
145 usually called dynamic linking/loading of <em>Dynamic Shared
146 Objects</em> (DSO) which provides a way to build a piece of
147 program code in a special format for loading it at run-time
148 into the address space of an executable program.</p>
150 <p>This loading can usually be done in two ways: Automatically
151 by a system program called <code>ld.so</code> when an
152 executable program is started or manually from within the
153 executing program via a programmatic system interface to the
154 Unix loader through the system calls
155 <code>dlopen()/dlsym()</code>.</p>
157 <p>In the first way the DSO's are usually called <em>shared
158 libraries</em> or <em>DSO libraries</em> and named
159 <code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They
160 reside in a system directory (usually <code>/usr/lib</code>)
161 and the link to the executable program is established at
162 build-time by specifying <code>-lfoo</code> to the linker
163 command. This hard-codes library references into the executable
164 program file so that at start-time the Unix loader is able to
165 locate <code>libfoo.so</code> in <code>/usr/lib</code>, in
166 paths hard-coded via linker-options like <code>-R</code> or in
167 paths configured via the environment variable
168 <code>LD_LIBRARY_PATH</code>. It then resolves any (yet
169 unresolved) symbols in the executable program which are
170 available in the DSO.</p>
172 <p>Symbols in the executable program are usually not referenced
173 by the DSO (because it's a reusable library of general code)
174 and hence no further resolving has to be done. The executable
175 program has no need to do anything on its own to use the
176 symbols from the DSO because the complete resolving is done by
177 the Unix loader. (In fact, the code to invoke
178 <code>ld.so</code> is part of the run-time startup code which
179 is linked into every executable program which has been bound
180 non-static). The advantage of dynamic loading of common library
181 code is obvious: the library code needs to be stored only once,
182 in a system library like <code>libc.so</code>, saving disk
183 space for every program.</p>
185 <p>In the second way the DSO's are usually called <em>shared
186 objects</em> or <em>DSO files</em> and can be named with an
187 arbitrary extension (although the canonical name is
188 <code>foo.so</code>). These files usually stay inside a
189 program-specific directory and there is no automatically
190 established link to the executable program where they are used.
191 Instead the executable program manually loads the DSO at
192 run-time into its address space via <code>dlopen()</code>. At
193 this time no resolving of symbols from the DSO for the
194 executable program is done. But instead the Unix loader
195 automatically resolves any (yet unresolved) symbols in the DSO
196 from the set of symbols exported by the executable program and
197 its already loaded DSO libraries (especially all symbols from
198 the ubiquitous <code>libc.so</code>). This way the DSO gets
199 knowledge of the executable program's symbol set as if it had
200 been statically linked with it in the first place.</p>
202 <p>Finally, to take advantage of the DSO's API the executable
203 program has to resolve particular symbols from the DSO via
204 <code>dlsym()</code> for later use inside dispatch tables
205 <em>etc.</em> In other words: The executable program has to
206 manually resolve every symbol it needs to be able to use it.
207 The advantage of such a mechanism is that optional program
208 parts need not be loaded (and thus do not spend memory) until
209 they are needed by the program in question. When required,
210 these program parts can be loaded dynamically to extend the
211 base program's functionality.</p>
213 <p>Although this DSO mechanism sounds straightforward there is
214 at least one difficult step here: The resolving of symbols from
215 the executable program for the DSO when using a DSO to extend a
216 program (the second way). Why? Because "reverse resolving" DSO
217 symbols from the executable program's symbol set is against the
218 library design (where the library has no knowledge about the
219 programs it is used by) and is neither available under all
220 platforms nor standardized. In practice the executable
221 program's global symbols are often not re-exported and thus not
222 available for use in a DSO. Finding a way to force the linker
223 to export all global symbols is the main problem one has to
224 solve when using DSO for extending a program at run-time.</p>
226 <p>The shared library approach is the typical one, because it
227 is what the DSO mechanism was designed for, hence it is used
228 for nearly all types of libraries the operating system
229 provides. On the other hand using shared objects for extending
230 a program is not used by a lot of programs.</p>
232 <p>As of 1998 there are only a few software packages available
233 which use the DSO mechanism to actually extend their
234 functionality at run-time: Perl 5 (via its XS mechanism and the
235 DynaLoader module), Netscape Server, <em>etc.</em> Starting
236 with version 1.3, Apache joined the crew, because Apache
237 already uses a module concept to extend its functionality and
238 internally uses a dispatch-list-based approach to link external
239 modules into the Apache core functionality. So, Apache is
240 really predestined for using DSO to load its modules at
242 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
243 <div class="section">
244 <h2><a name="advantages" id="advantages">Advantages and Disadvantages</a></h2>
246 <p>The above DSO based features have the following
250 <li>The server package is more flexible at run-time because
251 the actual server process can be assembled at run-time via
252 <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
253 <code>httpd.conf</code> configuration commands instead of
254 <code class="program"><a href="./programs/configure.html">configure</a></code> options at build-time. For instance
255 this way one is able to run different server instances
256 (standard & SSL version, minimalistic & powered up
257 version [mod_perl, PHP3], <em>etc.</em>) with only one Apache
260 <li>The server package can be easily extended with
261 third-party modules even after installation. This is at least
262 a great benefit for vendor package maintainers who can create
263 a Apache core package and additional packages containing
264 extensions like PHP3, mod_perl, mod_fastcgi,
267 <li>Easier Apache module prototyping because with the
268 DSO/<code class="program"><a href="./programs/apxs.html">apxs</a></code> pair you can both work outside the
269 Apache source tree and only need an <code>apxs -i</code>
270 command followed by an <code>apachectl restart</code> to
271 bring a new version of your currently developed module into
272 the running Apache server.</li>
275 <p>DSO has the following disadvantages:</p>
278 <li>The DSO mechanism cannot be used on every platform
279 because not all operating systems support dynamic loading of
280 code into the address space of a program.</li>
282 <li>The server is approximately 20% slower at startup time
283 because of the symbol resolving overhead the Unix loader now
286 <li>The server is approximately 5% slower at execution time
287 under some platforms because position independent code (PIC)
288 sometimes needs complicated assembler tricks for relative
289 addressing which are not necessarily as fast as absolute
292 <li>Because DSO modules cannot be linked against other
293 DSO-based libraries (<code>ld -lfoo</code>) on all platforms
294 (for instance a.out-based platforms usually don't provide
295 this functionality while ELF-based platforms do) you cannot
296 use the DSO mechanism for all types of modules. Or in other
297 words, modules compiled as DSO files are restricted to only
298 use symbols from the Apache core, from the C library
299 (<code>libc</code>) and all other dynamic or static libraries
300 used by the Apache core, or from static library archives
301 (<code>libfoo.a</code>) containing position independent code.
302 The only chances to use other code is to either make sure the
303 Apache core itself already contains a reference to it or
304 loading the code yourself via <code>dlopen()</code>.</li>
308 <div class="bottomlang">
309 <p><span>Available Languages: </span><a href="./en/dso.html" title="English"> en </a> |
310 <a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
311 <a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
312 </div><div id="footer">
313 <p class="apache">Copyright 1995-2005 The Apache Software Foundation or its licensors, as applicable.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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