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7 --><title>Dynamic Shared Object (DSO) Support - Apache HTTP Server</title><link href="./style/css/manual-sbar-right-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="right sidebar - blue (font 100%)" /><link href="./style/css/manual-sbar-right-90pc.css" rel="alternate stylesheet" media="all" type="text/css" title="right sidebar - blue (font 90%)" /><link href="./style/css/manual-sbar-right-fix15.css" rel="stylesheet" media="all" type="text/css" title="right sidebar - blue (font fix 15)" /><link href="./style/css/manual-sbar-right-fix13.css" rel="alternate stylesheet" media="all" type="text/css" title="right sidebar - blue (font fix 13)" /><link href="./style/css/manual-sbar-right-100pc-b.css" rel="alternate stylesheet" media="all" type="text/css" title="right sidebar - black (font 100%)" /><link href="./style/css/manual-sbar-right-90pc-b.css" rel="alternate stylesheet" media="all" type="text/css" title="right sidebar - black (font 90%)" /><link href="./style/css/manual-sbar-right-fix15-b.css" rel="alternate stylesheet" media="all" type="text/css" title="right sidebar - black (font fix 15)" /><link href="./style/css/manual-sbar-right-fix13-b.css" rel="alternate stylesheet" media="all" type="text/css" title="right sidebar - black (font fix 13)" /><link href="./style/css/manual-sbar-left-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="left sidebar - blue (font 100%)" /><link href="./style/css/manual-sbar-left-90pc.css" rel="alternate stylesheet" media="all" type="text/css" title="left sidebar - blue (font 90%)" /><link href="./style/css/manual-sbar-left-fix15.css" rel="alternate stylesheet" media="all" type="text/css" title="left sidebar - blue (font fix 15)" /><link href="./style/css/manual-sbar-left-fix13.css" rel="alternate stylesheet" media="all" type="text/css" title="left sidebar - blue (font fix 13)" /><link href="./style/css/manual-sbar-left-100pc-b.css" rel="alternate stylesheet" media="all" type="text/css" title="left sidebar - black (font 100%)" /><link href="./style/css/manual-sbar-left-90pc-b.css" rel="alternate stylesheet" media="all" type="text/css" title="left sidebar - black (font 90%)" /><link href="./style/css/manual-sbar-left-fix15-b.css" rel="alternate stylesheet" media="all" type="text/css" title="left sidebar - black (font fix 15)" /><link href="./style/css/manual-sbar-left-fix13-b.css" rel="alternate stylesheet" media="all" type="text/css" title="left sidebar - black (font fix 13)" /><link href="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="loose style - blue (font 100%)" /><link href="./style/css/manual-loose-90pc.css" rel="alternate stylesheet" media="all" type="text/css" title="loose style - blue (font 90%)" /><link href="./style/css/manual-loose-fix15.css" rel="alternate stylesheet" media="all" type="text/css" title="loose style - blue (font fix 15)" /><link href="./style/css/manual-loose-fix13.css" rel="alternate stylesheet" media="all" type="text/css" title="loose style - blue (font fix 13)" /><link href="./style/css/manual-loose-100pc-b.css" rel="alternate stylesheet" media="all" type="text/css" title="loose style - black (font 100%)" /><link href="./style/css/manual-loose-90pc-b.css" rel="alternate stylesheet" media="all" type="text/css" title="loose style - black (font 90%)" /><link href="./style/css/manual-loose-fix15-b.css" rel="alternate stylesheet" media="all" type="text/css" title="loose style - black (font fix 15)" /><link href="./style/css/manual-loose-fix13-b.css" rel="alternate stylesheet" media="all" type="text/css" title="loose style - black (font fix 13)" /><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="./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="<-" alt="<-" src="./images/left.gif" /></a></div><div id="path"><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 2.0</a></div><div id="page-content"><div id="preamble"><h1>Dynamic Shared Object (DSO) Support</h1>
8 <p>The Apache HTTP Server is a modular program where the
9 administrator can choose the functionality to include in the
10 server by selecting a set of modules. The modules can be
11 statically compiled into the <code>httpd</code> binary when the
12 server is built. Alternatively, modules can be compiled as
13 Dynamic Shared Objects (DSOs) that exist separately from the
14 main <code>httpd</code> binary file. DSO modules may be
15 compiled at the time the server is built, or they may be
16 compiled and added at a later time using the Apache Extension
17 Tool (<a href="programs/apxs.html">apxs</a>).</p>
19 <p>This document describes how to use DSO modules as well as
20 the theory behind their use.</p>
21 </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>
23 <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>
25 <p>The DSO support for loading individual Apache modules is based
26 on a module named <code class="module"><a href="./mod/mod_so.html">mod_so</a></code> which must be statically
27 compiled into the Apache core. It is the only module besides
28 <code class="module"><a href="./mod/core.html">core</a></code> which cannot be put into a DSO
29 itself. Practically all other distributed Apache modules can then
30 be placed into a DSO by individually enabling the DSO build for
31 them via <code>configure</code>'s
32 <code>--enable-<em>module</em>=shared</code> option as disucussed
33 in the <a href="install.html">install documentation</a>. After a
34 module is compiled into a DSO named <code>mod_foo.so</code> you
35 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
36 <code>httpd.conf</code> file to load this module at server startup
39 <p>To simplify this creation of DSO files for Apache modules
40 (especially for third-party modules) a new support program
41 named <a href="programs/apxs.html">apxs</a> (<em>APache
42 eXtenSion</em>) is available. It can be used to build DSO based
43 modules <em>outside of</em> the Apache source tree. The idea is
44 simple: When installing Apache the <code>configure</code>'s
45 <code>make install</code> procedure installs the Apache C
46 header files and puts the platform-dependent compiler and
47 linker flags for building DSO files into the <code>apxs</code>
48 program. This way the user can use <code>apxs</code> to compile
49 his Apache module sources without the Apache distribution
50 source tree and without having to fiddle with the
51 platform-dependent compiler and linker flags for DSO
53 </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>
55 <p>To give you an overview of the DSO features of Apache 2.0,
56 here is a short and concise summary:</p>
60 Build and install a <em>distributed</em> Apache module, say
61 <code>mod_foo.c</code>, into its own DSO
62 <code>mod_foo.so</code>:
64 <div class="example"><p><code>
65 $ ./configure --prefix=/path/to/install --enable-foo=shared<br />
71 Build and install a <em>third-party</em> Apache module, say
72 <code>mod_foo.c</code>, into its own DSO
73 <code>mod_foo.so</code>:
75 <div class="example"><p><code>
76 $ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c --enable-foo=shared<br />
82 Configure Apache for <em>later installation</em> of shared
85 <div class="example"><p><code>
86 $ ./configure --enable-so<br />
92 Build and install a <em>third-party</em> Apache module, say
93 <code>mod_foo.c</code>, into its own DSO
94 <code>mod_foo.so</code> <em>outside of</em> the Apache
95 source tree using <a href="programs/apxs.html">apxs</a>:
97 <div class="example"><p><code>
98 $ cd /path/to/3rdparty<br />
99 $ apxs -c mod_foo.c<br />
100 $ apxs -i -a -n foo mod_foo.la
105 <p>In all cases, once the shared module is compiled, you must
106 use a <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
107 directive in <code>httpd.conf</code> to tell Apache to activate
109 </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>
111 <p>On modern Unix derivatives there exists a nifty mechanism
112 usually called dynamic linking/loading of <em>Dynamic Shared
113 Objects</em> (DSO) which provides a way to build a piece of
114 program code in a special format for loading it at run-time
115 into the address space of an executable program.</p>
117 <p>This loading can usually be done in two ways: Automatically
118 by a system program called <code>ld.so</code> when an
119 executable program is started or manually from within the
120 executing program via a programmatic system interface to the
121 Unix loader through the system calls
122 <code>dlopen()/dlsym()</code>.</p>
124 <p>In the first way the DSO's are usually called <em>shared
125 libraries</em> or <em>DSO libraries</em> and named
126 <code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They
127 reside in a system directory (usually <code>/usr/lib</code>)
128 and the link to the executable program is established at
129 build-time by specifying <code>-lfoo</code> to the linker
130 command. This hard-codes library references into the executable
131 program file so that at start-time the Unix loader is able to
132 locate <code>libfoo.so</code> in <code>/usr/lib</code>, in
133 paths hard-coded via linker-options like <code>-R</code> or in
134 paths configured via the environment variable
135 <code>LD_LIBRARY_PATH</code>. It then resolves any (yet
136 unresolved) symbols in the executable program which are
137 available in the DSO.</p>
139 <p>Symbols in the executable program are usually not referenced
140 by the DSO (because it's a reusable library of general code)
141 and hence no further resolving has to be done. The executable
142 program has no need to do anything on its own to use the
143 symbols from the DSO because the complete resolving is done by
144 the Unix loader. (In fact, the code to invoke
145 <code>ld.so</code> is part of the run-time startup code which
146 is linked into every executable program which has been bound
147 non-static). The advantage of dynamic loading of common library
148 code is obvious: the library code needs to be stored only once,
149 in a system library like <code>libc.so</code>, saving disk
150 space for every program.</p>
152 <p>In the second way the DSO's are usually called <em>shared
153 objects</em> or <em>DSO files</em> and can be named with an
154 arbitrary extension (although the canonical name is
155 <code>foo.so</code>). These files usually stay inside a
156 program-specific directory and there is no automatically
157 established link to the executable program where they are used.
158 Instead the executable program manually loads the DSO at
159 run-time into its address space via <code>dlopen()</code>. At
160 this time no resolving of symbols from the DSO for the
161 executable program is done. But instead the Unix loader
162 automatically resolves any (yet unresolved) symbols in the DSO
163 from the set of symbols exported by the executable program and
164 its already loaded DSO libraries (especially all symbols from
165 the ubiquitous <code>libc.so</code>). This way the DSO gets
166 knowledge of the executable program's symbol set as if it had
167 been statically linked with it in the first place.</p>
169 <p>Finally, to take advantage of the DSO's API the executable
170 program has to resolve particular symbols from the DSO via
171 <code>dlsym()</code> for later use inside dispatch tables
172 <em>etc.</em> In other words: The executable program has to
173 manually resolve every symbol it needs to be able to use it.
174 The advantage of such a mechanism is that optional program
175 parts need not be loaded (and thus do not spend memory) until
176 they are needed by the program in question. When required,
177 these program parts can be loaded dynamically to extend the
178 base program's functionality.</p>
180 <p>Although this DSO mechanism sounds straightforward there is
181 at least one difficult step here: The resolving of symbols from
182 the executable program for the DSO when using a DSO to extend a
183 program (the second way). Why? Because "reverse resolving" DSO
184 symbols from the executable program's symbol set is against the
185 library design (where the library has no knowledge about the
186 programs it is used by) and is neither available under all
187 platforms nor standardized. In practice the executable
188 program's global symbols are often not re-exported and thus not
189 available for use in a DSO. Finding a way to force the linker
190 to export all global symbols is the main problem one has to
191 solve when using DSO for extending a program at run-time.</p>
193 <p>The shared library approach is the typical one, because it
194 is what the DSO mechanism was designed for, hence it is used
195 for nearly all types of libraries the operating system
196 provides. On the other hand using shared objects for extending
197 a program is not used by a lot of programs.</p>
199 <p>As of 1998 there are only a few software packages available
200 which use the DSO mechanism to actually extend their
201 functionality at run-time: Perl 5 (via its XS mechanism and the
202 DynaLoader module), Netscape Server, <em>etc.</em> Starting
203 with version 1.3, Apache joined the crew, because Apache
204 already uses a module concept to extend its functionality and
205 internally uses a dispatch-list-based approach to link external
206 modules into the Apache core functionality. So, Apache is
207 really predestined for using DSO to load its modules at
209 </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>
211 <p>The above DSO based features have the following
215 <li>The server package is more flexible at run-time because
216 the actual server process can be assembled at run-time via
217 <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
218 <code>httpd.conf</code> configuration commands instead of
219 <code>configure</code> options at build-time. For instance
220 this way one is able to run different server instances
221 (standard & SSL version, minimalistic & powered up
222 version [mod_perl, PHP3], <em>etc.</em>) with only one Apache
225 <li>The server package can be easily extended with
226 third-party modules even after installation. This is at least
227 a great benefit for vendor package maintainers who can create
228 a Apache core package and additional packages containing
229 extensions like PHP3, mod_perl, mod_fastcgi,
232 <li>Easier Apache module prototyping because with the
233 DSO/<code>apxs</code> pair you can both work outside the
234 Apache source tree and only need an <code>apxs -i</code>
235 command followed by an <code>apachectl restart</code> to
236 bring a new version of your currently developed module into
237 the running Apache server.</li>
240 <p>DSO has the following disadvantages:</p>
243 <li>The DSO mechanism cannot be used on every platform
244 because not all operating systems support dynamic loading of
245 code into the address space of a program.</li>
247 <li>The server is approximately 20% slower at startup time
248 because of the symbol resolving overhead the Unix loader now
251 <li>The server is approximately 5% slower at execution time
252 under some platforms because position independent code (PIC)
253 sometimes needs complicated assembler tricks for relative
254 addressing which are not necessarily as fast as absolute
257 <li>Because DSO modules cannot be linked against other
258 DSO-based libraries (<code>ld -lfoo</code>) on all platforms
259 (for instance a.out-based platforms usually don't provide
260 this functionality while ELF-based platforms do) you cannot
261 use the DSO mechanism for all types of modules. Or in other
262 words, modules compiled as DSO files are restricted to only
263 use symbols from the Apache core, from the C library
264 (<code>libc</code>) and all other dynamic or static libraries
265 used by the Apache core, or from static library archives
266 (<code>libfoo.a</code>) containing position independent code.
267 The only chances to use other code is to either make sure the
268 Apache core itself already contains a reference to it or
269 loading the code yourself via <code>dlopen()</code>.</li>
272 </div></div><div id="footer"><p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</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></body></html>