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8 <title>Dynamic Shared Object (DSO) Support - Apache HTTP Server Version 2.5</title>
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18 <p class="apache">Apache HTTP Server Version 2.5</p>
19 <img alt="" src="./images/feather.gif" /></div>
20 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
22 <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 2.5</a></div><div id="page-content"><div id="preamble"><h1>Dynamic Shared Object (DSO) Support</h1>
24 <p><span>Available Languages: </span><a href="./en/dso.html" title="English"> en </a> |
25 <a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
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31 <p>The Apache HTTP Server is a modular program where the
32 administrator can choose the functionality to include in the
33 server by selecting a set of modules.
34 Modules will be compiled as Dynamic Shared Objects (DSOs)
35 that exist separately from the main <code class="program"><a href="./programs/httpd.html">httpd</a></code>
36 binary file. DSO modules may be compiled at the time the server
37 is built, or they may be compiled and added at a later time
38 using the Apache Extension Tool (<code class="program"><a href="./programs/apxs.html">apxs</a></code>).</p>
39 <p>Alternatively, the modules can be statically compiled into
40 the <code class="program"><a href="./programs/httpd.html">httpd</a></code> binary when the server is built.</p>
42 <p>This document describes how to use DSO modules as well as
43 the theory behind their use.</p>
45 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#implementation">Implementation</a></li>
46 <li><img alt="" src="./images/down.gif" /> <a href="#usage">Usage Summary</a></li>
47 <li><img alt="" src="./images/down.gif" /> <a href="#background">Background</a></li>
48 <li><img alt="" src="./images/down.gif" /> <a href="#advantages">Advantages and Disadvantages</a></li>
49 </ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
50 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
52 <h2><a name="implementation" id="implementation">Implementation</a></h2>
54 <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>
56 <p>The DSO support for loading individual Apache httpd modules is based
57 on a module named <code class="module"><a href="./mod/mod_so.html">mod_so</a></code> which must be statically
58 compiled into the Apache httpd core. It is the only module besides
59 <code class="module"><a href="./mod/core.html">core</a></code> which cannot be put into a DSO
60 itself. Practically all other distributed Apache httpd modules will then
61 be placed into a DSO. After a module is compiled into a DSO named
62 <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
63 <code>httpd.conf</code> file to load this module at server startup
65 <p>The DSO builds for individual modules can be disabled via
66 <code class="program"><a href="./programs/configure.html">configure</a></code>'s <code>--enable-mods-static</code>
67 option as discussed in the <a href="install.html">install
68 documentation</a>.</p>
70 <p>To simplify this creation of DSO files for Apache httpd modules
71 (especially for third-party modules) a support program
72 named <code class="program"><a href="./programs/apxs.html">apxs</a></code> (<dfn>APache
73 eXtenSion</dfn>) is available. It can be used to build DSO based
74 modules <em>outside of</em> the Apache httpd source tree. The idea is
75 simple: When installing Apache HTTP Server the <code class="program"><a href="./programs/configure.html">configure</a></code>'s
76 <code>make install</code> procedure installs the Apache httpd C
77 header files and puts the platform-dependent compiler and
78 linker flags for building DSO files into the <code class="program"><a href="./programs/apxs.html">apxs</a></code>
79 program. This way the user can use <code class="program"><a href="./programs/apxs.html">apxs</a></code> to compile
80 his Apache httpd module sources without the Apache httpd distribution
81 source tree and without having to fiddle with the
82 platform-dependent compiler and linker flags for DSO
84 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
86 <h2><a name="usage" id="usage">Usage Summary</a></h2>
88 <p>To give you an overview of the DSO features of Apache HTTP Server 2.x,
89 here is a short and concise summary:</p>
93 <p>Build and install a <em>distributed</em> Apache httpd module, say
94 <code>mod_foo.c</code>, into its own DSO
95 <code>mod_foo.so</code>:</p>
97 <div class="example"><p><code>
98 $ ./configure --prefix=/path/to/install --enable-foo<br />
104 <p>Configure Apache HTTP Server with all modules enabled. Only a basic
105 set will be loaded during server startup. You can change the set of loaded
106 modules by activating or deactivating the <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> directives in
107 <code>httpd.conf</code>.</p>
109 <div class="example"><p><code>
110 $ ./configure --enable-mods-shared=all<br />
116 <p>Some modules are only useful for developers and will not be build.
117 when using the module set <em>all</em>. To build all available modules
118 including developer modules use <em>reallyall</em>. In addition the
119 <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> directives for all
120 built modules can be activated via the configure option
121 <code>--enable-load-all-modules</code>.</p>
123 <div class="example"><p><code>
124 $ ./configure --enable-mods-shared=reallyall --enable-load-all-modules<br />
130 Build and install a <em>third-party</em> Apache httpd module, say
131 <code>mod_foo.c</code>, into its own DSO
132 <code>mod_foo.so</code> <em>outside of</em> the Apache httpd
133 source tree using <code class="program"><a href="./programs/apxs.html">apxs</a></code>:
135 <div class="example"><p><code>
136 $ cd /path/to/3rdparty<br />
137 $ apxs -cia mod_foo.c
142 <p>In all cases, once the shared module is compiled, you must
143 use a <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
144 directive in <code>httpd.conf</code> to tell Apache httpd to activate
147 <p>See the <a href="programs/apxs.html">apxs documentation</a> for more details.</p>
148 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
149 <div class="section">
150 <h2><a name="background" id="background">Background</a></h2>
152 <p>On modern Unix derivatives there exists a mechanism
153 called dynamic linking/loading of <em>Dynamic Shared
154 Objects</em> (DSO) which provides a way to build a piece of
155 program code in a special format for loading it at run-time
156 into the address space of an executable program.</p>
158 <p>This loading can usually be done in two ways: automatically
159 by a system program called <code>ld.so</code> when an
160 executable program is started or manually from within the
161 executing program via a programmatic system interface to the
162 Unix loader through the system calls
163 <code>dlopen()/dlsym()</code>.</p>
165 <p>In the first way the DSO's are usually called <em>shared
166 libraries</em> or <em>DSO libraries</em> and named
167 <code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They
168 reside in a system directory (usually <code>/usr/lib</code>)
169 and the link to the executable program is established at
170 build-time by specifying <code>-lfoo</code> to the linker
171 command. This hard-codes library references into the executable
172 program file so that at start-time the Unix loader is able to
173 locate <code>libfoo.so</code> in <code>/usr/lib</code>, in
174 paths hard-coded via linker-options like <code>-R</code> or in
175 paths configured via the environment variable
176 <code>LD_LIBRARY_PATH</code>. It then resolves any (yet
177 unresolved) symbols in the executable program which are
178 available in the DSO.</p>
180 <p>Symbols in the executable program are usually not referenced
181 by the DSO (because it's a reusable library of general code)
182 and hence no further resolving has to be done. The executable
183 program has no need to do anything on its own to use the
184 symbols from the DSO because the complete resolving is done by
185 the Unix loader. (In fact, the code to invoke
186 <code>ld.so</code> is part of the run-time startup code which
187 is linked into every executable program which has been bound
188 non-static). The advantage of dynamic loading of common library
189 code is obvious: the library code needs to be stored only once,
190 in a system library like <code>libc.so</code>, saving disk
191 space for every program.</p>
193 <p>In the second way the DSO's are usually called <em>shared
194 objects</em> or <em>DSO files</em> and can be named with an
195 arbitrary extension (although the canonical name is
196 <code>foo.so</code>). These files usually stay inside a
197 program-specific directory and there is no automatically
198 established link to the executable program where they are used.
199 Instead the executable program manually loads the DSO at
200 run-time into its address space via <code>dlopen()</code>. At
201 this time no resolving of symbols from the DSO for the
202 executable program is done. But instead the Unix loader
203 automatically resolves any (yet unresolved) symbols in the DSO
204 from the set of symbols exported by the executable program and
205 its already loaded DSO libraries (especially all symbols from
206 the ubiquitous <code>libc.so</code>). This way the DSO gets
207 knowledge of the executable program's symbol set as if it had
208 been statically linked with it in the first place.</p>
210 <p>Finally, to take advantage of the DSO's API the executable
211 program has to resolve particular symbols from the DSO via
212 <code>dlsym()</code> for later use inside dispatch tables
213 <em>etc.</em> In other words: The executable program has to
214 manually resolve every symbol it needs to be able to use it.
215 The advantage of such a mechanism is that optional program
216 parts need not be loaded (and thus do not spend memory) until
217 they are needed by the program in question. When required,
218 these program parts can be loaded dynamically to extend the
219 base program's functionality.</p>
221 <p>Although this DSO mechanism sounds straightforward there is
222 at least one difficult step here: The resolving of symbols from
223 the executable program for the DSO when using a DSO to extend a
224 program (the second way). Why? Because "reverse resolving" DSO
225 symbols from the executable program's symbol set is against the
226 library design (where the library has no knowledge about the
227 programs it is used by) and is neither available under all
228 platforms nor standardized. In practice the executable
229 program's global symbols are often not re-exported and thus not
230 available for use in a DSO. Finding a way to force the linker
231 to export all global symbols is the main problem one has to
232 solve when using DSO for extending a program at run-time.</p>
234 <p>The shared library approach is the typical one, because it
235 is what the DSO mechanism was designed for, hence it is used
236 for nearly all types of libraries the operating system
239 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
240 <div class="section">
241 <h2><a name="advantages" id="advantages">Advantages and Disadvantages</a></h2>
243 <p>The above DSO based features have the following
247 <li>The server package is more flexible at run-time because
248 the server process can be assembled at run-time via
249 <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
250 <code>httpd.conf</code> configuration directives instead of
251 <code class="program"><a href="./programs/configure.html">configure</a></code> options at build-time. For instance,
252 this way one is able to run different server instances
253 (standard & SSL version, minimalistic & dynamic
254 version [mod_perl, mod_php], <em>etc.</em>) with only one Apache httpd
257 <li>The server package can be easily extended with
258 third-party modules even after installation. This is
259 a great benefit for vendor package maintainers, who can create
260 an Apache httpd core package and additional packages containing
261 extensions like PHP, mod_perl, mod_security, <em>etc.</em></li>
263 <li>Easier Apache httpd module prototyping, because with the
264 DSO/<code class="program"><a href="./programs/apxs.html">apxs</a></code> pair you can both work outside the
265 Apache httpd source tree and only need an <code>apxs -i</code>
266 command followed by an <code>apachectl restart</code> to
267 bring a new version of your currently developed module into
268 the running Apache HTTP Server.</li>
271 <p>DSO has the following disadvantages:</p>
274 <li>The server is approximately 20% slower at startup time
275 because of the symbol resolving overhead the Unix loader now
278 <li>The server is approximately 5% slower at execution time
279 under some platforms, because position independent code (PIC)
280 sometimes needs complicated assembler tricks for relative
281 addressing, which are not necessarily as fast as absolute
284 <li>Because DSO modules cannot be linked against other
285 DSO-based libraries (<code>ld -lfoo</code>) on all platforms
286 (for instance a.out-based platforms usually don't provide
287 this functionality while ELF-based platforms do) you cannot
288 use the DSO mechanism for all types of modules. Or in other
289 words, modules compiled as DSO files are restricted to only
290 use symbols from the Apache httpd core, from the C library
291 (<code>libc</code>) and all other dynamic or static libraries
292 used by the Apache httpd core, or from static library archives
293 (<code>libfoo.a</code>) containing position independent code.
294 The only chances to use other code is to either make sure the
295 httpd core itself already contains a reference to it or
296 loading the code yourself via <code>dlopen()</code>.</li>
300 <div class="bottomlang">
301 <p><span>Available Languages: </span><a href="./en/dso.html" title="English"> en </a> |
302 <a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
303 <a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
304 <a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
305 <a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
306 </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&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again 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 Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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