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 2.2</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 2.2</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 2006 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>
314
<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>
b'\\ No newline at end of file'
added
removed
docs/manual/dso.html.en
