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23 <manualpage metafile="dso.xml.meta">
25 <title>Dynamic Shared Object (DSO) Support</title>
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 <program>httpd</program>
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 (<program>apxs</program>).</p>
36 <p>Alternatively, the modules can be statically compiled into
37 the <program>httpd</program> 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>
44 <section id="implementation"><title>Implementation</title>
48 <module>mod_so</module>
51 <directive module="mod_so">LoadModule</directive>
55 <p>The DSO support for loading individual Apache httpd modules is based
56 on a module named <module>mod_so</module> which must be statically
57 compiled into the Apache httpd core. It is the only module besides
58 <module>core</module> which cannot be put into a DSO
59 itself. Practically all other distributed Apache httpd modules will then
60 be placed into a DSO. After a module is compiled into a DSO named
61 <code>mod_foo.so</code> you can use <module>mod_so</module>'s <directive
62 module="mod_so">LoadModule</directive> 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 <program>configure</program>'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 <program>apxs</program> (<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 <program>configure</program>'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 <program>apxs</program>
79 program. This way the user can use <program>apxs</program> 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
86 <section id="usage"><title>Usage Summary</title>
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>
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 <directive
107 module="mod_so">LoadModule</directive> directives in
108 <code>httpd.conf</code>.</p>
111 $ ./configure --enable-mods-shared=all<br />
117 <p>Some modules are only useful for developers and will not be build.
118 when using the module set <em>all</em>. To build all available modules
119 including developer modules use <em>reallyall</em>. In addition the
120 <directive module="mod_so">LoadModule</directive> directives for all
121 built modules can be activated via the configure option
122 <code>--enable-load-all-modules</code>.</p>
125 $ ./configure --enable-mods-shared=reallyall --enable-load-all-modules<br />
131 Build and install a <em>third-party</em> Apache httpd module, say
132 <code>mod_foo.c</code>, into its own DSO
133 <code>mod_foo.so</code> <em>outside of</em> the Apache httpd
134 source tree using <program>apxs</program>:
137 $ cd /path/to/3rdparty<br />
138 $ apxs -cia mod_foo.c
143 <p>In all cases, once the shared module is compiled, you must
144 use a <directive module="mod_so">LoadModule</directive>
145 directive in <code>httpd.conf</code> to tell Apache httpd to activate
148 <p>See the <a href="programs/apxs.html">apxs documentation</a> for more details.</p>
151 <section id="background"><title>Background</title>
153 <p>On modern Unix derivatives there exists a mechanism
154 called dynamic linking/loading of <em>Dynamic Shared
155 Objects</em> (DSO) which provides a way to build a piece of
156 program code in a special format for loading it at run-time
157 into the address space of an executable program.</p>
159 <p>This loading can usually be done in two ways: automatically
160 by a system program called <code>ld.so</code> when an
161 executable program is started or manually from within the
162 executing program via a programmatic system interface to the
163 Unix loader through the system calls
164 <code>dlopen()/dlsym()</code>.</p>
166 <p>In the first way the DSO's are usually called <em>shared
167 libraries</em> or <em>DSO libraries</em> and named
168 <code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They
169 reside in a system directory (usually <code>/usr/lib</code>)
170 and the link to the executable program is established at
171 build-time by specifying <code>-lfoo</code> to the linker
172 command. This hard-codes library references into the executable
173 program file so that at start-time the Unix loader is able to
174 locate <code>libfoo.so</code> in <code>/usr/lib</code>, in
175 paths hard-coded via linker-options like <code>-R</code> or in
176 paths configured via the environment variable
177 <code>LD_LIBRARY_PATH</code>. It then resolves any (yet
178 unresolved) symbols in the executable program which are
179 available in the DSO.</p>
181 <p>Symbols in the executable program are usually not referenced
182 by the DSO (because it's a reusable library of general code)
183 and hence no further resolving has to be done. The executable
184 program has no need to do anything on its own to use the
185 symbols from the DSO because the complete resolving is done by
186 the Unix loader. (In fact, the code to invoke
187 <code>ld.so</code> is part of the run-time startup code which
188 is linked into every executable program which has been bound
189 non-static). The advantage of dynamic loading of common library
190 code is obvious: the library code needs to be stored only once,
191 in a system library like <code>libc.so</code>, saving disk
192 space for every program.</p>
194 <p>In the second way the DSO's are usually called <em>shared
195 objects</em> or <em>DSO files</em> and can be named with an
196 arbitrary extension (although the canonical name is
197 <code>foo.so</code>). These files usually stay inside a
198 program-specific directory and there is no automatically
199 established link to the executable program where they are used.
200 Instead the executable program manually loads the DSO at
201 run-time into its address space via <code>dlopen()</code>. At
202 this time no resolving of symbols from the DSO for the
203 executable program is done. But instead the Unix loader
204 automatically resolves any (yet unresolved) symbols in the DSO
205 from the set of symbols exported by the executable program and
206 its already loaded DSO libraries (especially all symbols from
207 the ubiquitous <code>libc.so</code>). This way the DSO gets
208 knowledge of the executable program's symbol set as if it had
209 been statically linked with it in the first place.</p>
211 <p>Finally, to take advantage of the DSO's API the executable
212 program has to resolve particular symbols from the DSO via
213 <code>dlsym()</code> for later use inside dispatch tables
214 <em>etc.</em> In other words: The executable program has to
215 manually resolve every symbol it needs to be able to use it.
216 The advantage of such a mechanism is that optional program
217 parts need not be loaded (and thus do not spend memory) until
218 they are needed by the program in question. When required,
219 these program parts can be loaded dynamically to extend the
220 base program's functionality.</p>
222 <p>Although this DSO mechanism sounds straightforward there is
223 at least one difficult step here: The resolving of symbols from
224 the executable program for the DSO when using a DSO to extend a
225 program (the second way). Why? Because "reverse resolving" DSO
226 symbols from the executable program's symbol set is against the
227 library design (where the library has no knowledge about the
228 programs it is used by) and is neither available under all
229 platforms nor standardized. In practice the executable
230 program's global symbols are often not re-exported and thus not
231 available for use in a DSO. Finding a way to force the linker
232 to export all global symbols is the main problem one has to
233 solve when using DSO for extending a program at run-time.</p>
235 <p>The shared library approach is the typical one, because it
236 is what the DSO mechanism was designed for, hence it is used
237 for nearly all types of libraries the operating system
242 <section id="advantages"><title>Advantages and Disadvantages</title>
244 <p>The above DSO based features have the following
248 <li>The server package is more flexible at run-time because
249 the server process can be assembled at run-time via
250 <directive module="mod_so">LoadModule</directive>
251 <code>httpd.conf</code> configuration directives instead of
252 <program>configure</program> options at build-time. For instance,
253 this way one is able to run different server instances
254 (standard & SSL version, minimalistic & dynamic
255 version [mod_perl, mod_php], <em>etc.</em>) with only one Apache httpd
258 <li>The server package can be easily extended with
259 third-party modules even after installation. This is
260 a great benefit for vendor package maintainers, who can create
261 an Apache httpd core package and additional packages containing
262 extensions like PHP, mod_perl, mod_security, <em>etc.</em></li>
264 <li>Easier Apache httpd module prototyping, because with the
265 DSO/<program>apxs</program> pair you can both work outside the
266 Apache httpd source tree and only need an <code>apxs -i</code>
267 command followed by an <code>apachectl restart</code> to
268 bring a new version of your currently developed module into
269 the running Apache HTTP Server.</li>
272 <p>DSO has the following disadvantages:</p>
275 <li>The server is approximately 20% slower at startup time
276 because of the symbol resolving overhead the Unix loader now
279 <li>The server is approximately 5% slower at execution time
280 under some platforms, because position independent code (PIC)
281 sometimes needs complicated assembler tricks for relative
282 addressing, which are not necessarily as fast as absolute
285 <li>Because DSO modules cannot be linked against other
286 DSO-based libraries (<code>ld -lfoo</code>) on all platforms
287 (for instance a.out-based platforms usually don't provide
288 this functionality while ELF-based platforms do) you cannot
289 use the DSO mechanism for all types of modules. Or in other
290 words, modules compiled as DSO files are restricted to only
291 use symbols from the Apache httpd core, from the C library
292 (<code>libc</code>) and all other dynamic or static libraries
293 used by the Apache httpd core, or from static library archives
294 (<code>libfoo.a</code>) containing position independent code.
295 The only chances to use other code is to either make sure the
296 httpd core itself already contains a reference to it or
297 loading the code yourself via <code>dlopen()</code>.</li>