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-
- <h1 align="center">Dynamic Shared Object (DSO) Support</h1>
-
+<html><head><META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><!--
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+ --><title>Dynamic Shared Object (DSO) Support - Apache HTTP Server</title><link href="./style/manual.css" type="text/css" rel="stylesheet"></head><body><blockquote><div align="center"><img src="./images/sub.gif" alt="[APACHE DOCUMENTATION]"><h3>Apache HTTP Server Version 2.0</h3></div><h1 align="center">Dynamic Shared Object (DSO) Support</h1>
<p>The Apache HTTP Server is a modular program where the
administrator can choose the functionality to include in the
server by selecting a set of modules. The modules can be
<p>This document describes how to use DSO modules as well as
the theory behind their use.</p>
-
- <ul>
- <li><a href="#implementation">Implementation</a></li>
-
- <li><a href="#usage">Usage Summary</a></li>
-
- <li><a href="#background">Background</a></li>
-
- <li><a href="#advantages">Advantages and
- Disadvantages</a></li>
- </ul>
- <hr />
-
- <table border="1">
- <tr>
- <td valign="top"><strong>Related Modules</strong><br />
- <br />
- <a href="mod/mod_so.html">mod_so</a><br />
- </td>
-
- <td valign="top"><strong>Related Directives</strong><br />
- <br />
- <a href="mod/mod_so.html#loadmodule">LoadModule</a><br />
- </td>
- </tr>
- </table>
-
- <h2><a id="implementation"
- name="implementation">Implementation</a></h2>
-
- <p>The DSO support for loading individual Apache modules is
- based on a module named <a
- href="mod/mod_so.html"><code>mod_so.c</code></a> which must be
- statically compiled into the Apache core. It is the only module
- besides <code>core.c</code> which cannot be put into a DSO
- itself. Practically all other distributed Apache modules
- can then be placed into a DSO by individually enabling the DSO
- build for them via <code>configure</code>'s
- <code>--enable-<i>module</i>=shared</code> option as disucussed
- in the <a href="install.html">install documentation</a>. After
- a module is compiled into a DSO named <code>mod_foo.so</code>
- you can use <a href="mod/mod_so.html"><code>mod_so</code></a>'s
- <a
- href="mod/mod_so.html#loadmodule"><code>LoadModule</code></a>
- command in your <code>httpd.conf</code> file to load this
- module at server startup or restart.</p>
+ <ul><li><a href="#implementation">Implementation</a></li><li><a href="#usage">Usage Summary</a></li><li><a href="#background">Background</a></li><li><a href="#advantages">Advantages and Disadvantages</a></li></ul><hr><h2><a name="implementation">Implementation</a></h2>
+
+<table border="1"><tr><td valign="top"><strong>Related Modules</strong><br><br><code><a href="./mod/mod_so.html">mod_so</a></code><br></td><td valign="top"><strong>Related Directives</strong><br><br><a href="./mod/mod_so.html#loadmodule" class="directive"><code class="directive">LoadModule</code></a><br></td></tr></table>
+
+ <p>The DSO support for loading individual Apache modules is based
+ on a module named <code><a href="./mod/mod_so.html">mod_so</a></code> which must be statically
+ compiled into the Apache core. It is the only module besides
+ <code><a href="./mod/core.html">core</a></code> which cannot be put into a DSO
+ itself. Practically all other distributed Apache modules can then
+ be placed into a DSO by individually enabling the DSO build for
+ them via <code>configure</code>'s
+ <code>--enable-<em>module</em>=shared</code> option as disucussed
+ in the <a href="install.html">install documentation</a>. After a
+ module is compiled into a DSO named <code>mod_foo.so</code> you
+ can use <code><a href="./mod/mod_so.html">mod_so</a></code>'s <a href="./mod/mod_so.html#loadmodule" class="directive"><code class="directive">LoadModule</code></a> command in your
+ <code>httpd.conf</code> file to load this module at server startup
+ or restart.</p>
<p>To simplify this creation of DSO files for Apache modules
(especially for third-party modules) a new support program
source tree and without having to fiddle with the
platform-dependent compiler and linker flags for DSO
support.</p>
-
- <h2><a id="usage" name="usage">Usage Summary</a></h2>
+<h2><a name="usage">Usage Summary</a></h2>
<p>To give you an overview of the DSO features of Apache 2.0,
here is a short and concise summary:</p>
<code>mod_foo.c</code>, into its own DSO
<code>mod_foo.so</code>:
- <table bgcolor="#f0f0f0" cellpadding="10">
- <tr>
- <td>
-<pre>
-$ ./configure --prefix=/path/to/install
- --enable-foo=shared
+<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>
+$ ./configure --prefix=/path/to/install --enable-foo=shared<br>
$ make install
-</pre>
- </td>
- </tr>
- </table>
+</code></td></tr></table></blockquote>
</li>
<li>
<code>mod_foo.c</code>, into its own DSO
<code>mod_foo.so</code>:
- <table bgcolor="#f0f0f0" cellpadding="10">
- <tr>
- <td>
-<pre>
-$ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c
- --enable-foo=shared
+<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>
+$ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c --enable-foo=shared<br>
$ make install
-</pre>
- </td>
- </tr>
- </table>
+</code></td></tr></table></blockquote>
</li>
<li>
Configure Apache for <em>later installation</em> of shared
modules:
- <table bgcolor="#f0f0f0" cellpadding="10">
- <tr>
- <td>
-<pre>
-$ ./configure --enable-so
+<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>
+$ ./configure --enable-so<br>
$ make install
-</pre>
- </td>
- </tr>
- </table>
+</code></td></tr></table></blockquote>
</li>
<li>
<code>mod_foo.so</code> <em>outside of</em> the Apache
source tree using <a href="programs/apxs.html">apxs</a>:
- <table bgcolor="#f0f0f0" cellpadding="10">
- <tr>
- <td>
-<pre>
-$ cd /path/to/3rdparty
-$ apxs -c mod_foo.c
+<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>
+$ cd /path/to/3rdparty<br>
+$ apxs -c mod_foo.c<br>
$ apxs -i -a -n foo mod_foo.so
-</pre>
- </td>
- </tr>
- </table>
+</code></td></tr></table></blockquote>
</li>
</ol>
<p>In all cases, once the shared module is compiled, you must
- use a <a
- href="mod/mod_so.html#loadmodule"><code>LoadModule</code></a>
+ use a <a href="./mod/mod_so.html#loadmodule" class="directive"><code class="directive">LoadModule</code></a>
directive in <code>httpd.conf</code> to tell Apache to activate
the module.</p>
-
- <h2><a id="background" name="background">Background</a></h2>
+<h2><a name="background">Background</a></h2>
<p>On modern Unix derivatives there exists a nifty mechanism
usually called dynamic linking/loading of <em>Dynamic Shared
modules into the Apache core functionality. So, Apache is
really predestined for using DSO to load its modules at
run-time.</p>
-
- <h2><a id="advantages" name="advantages">Advantages and
- Disadvantages</a></h2>
+<h2><a name="advantages">Advantages and Disadvantages</a></h2>
<p>The above DSO based features have the following
advantages:</p>
<ul>
<li>The server package is more flexible at run-time because
- the actual server process can be assembled at run-time via <a
- href="mod/mod_so.html#loadmodule"><code>LoadModule</code></a>
+ the actual server process can be assembled at run-time via
+ <a href="./mod/mod_so.html#loadmodule" class="directive"><code class="directive">LoadModule</code></a>
<code>httpd.conf</code> configuration commands instead of
<code>configure</code> options at build-time. For instance
this way one is able to run different server instances
Apache core itself already contains a reference to it or
loading the code yourself via <code>dlopen()</code>.</li>
</ul>
- <!--#include virtual="footer.html" -->
- </body>
-</html>
+<hr></blockquote><h3 align="center">Apache HTTP Server Version 2.0</h3><a href="./"><img src="./images/index.gif" alt="Index"></a><a href="./"><img src="./images/home.gif" alt="Home"></a></body></html>
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+<?xml version="1.0" encoding="UTF-8" ?>
+<!DOCTYPE manualpage SYSTEM "./style/manualpage.dtd">
+<?xml-stylesheet type="text/xsl" href="./style/manual.en.xsl"?>
+
+<manualpage>
+ <relativepath href="."/>
+
+ <title>Dynamic Shared Object (DSO) Support</title>
+
+ <summary>
+ <p>The Apache HTTP Server is a modular program where the
+ administrator can choose the functionality to include in the
+ server by selecting a set of modules. The modules can be
+ statically compiled into the <code>httpd</code> binary when the
+ server is built. Alternatively, modules can be compiled as
+ Dynamic Shared Objects (DSOs) that exist separately from the
+ main <code>httpd</code> binary file. DSO modules may be
+ compiled at the time the server is built, or they may be
+ compiled and added at a later time using the Apache Extension
+ Tool (<a href="programs/apxs.html">apxs</a>).</p>
+
+ <p>This document describes how to use DSO modules as well as
+ the theory behind their use.</p>
+ </summary>
+
+
+<section id="implementation"><title>Implementation</title>
+
+<related>
+<modulelist>
+<module>mod_so</module>
+</modulelist>
+<directivelist>
+<directive module="mod_so">LoadModule</directive>
+</directivelist>
+</related>
+
+ <p>The DSO support for loading individual Apache modules is based
+ on a module named <module>mod_so</module> which must be statically
+ compiled into the Apache core. It is the only module besides
+ <module>core</module> which cannot be put into a DSO
+ itself. Practically all other distributed Apache modules can then
+ be placed into a DSO by individually enabling the DSO build for
+ them via <code>configure</code>'s
+ <code>--enable-<em>module</em>=shared</code> option as disucussed
+ in the <a href="install.html">install documentation</a>. After a
+ module is compiled into a DSO named <code>mod_foo.so</code> you
+ can use <module>mod_so</module>'s <directive
+ module="mod_so">LoadModule</directive> command in your
+ <code>httpd.conf</code> file to load this module at server startup
+ or restart.</p>
+
+ <p>To simplify this creation of DSO files for Apache modules
+ (especially for third-party modules) a new support program
+ named <a href="programs/apxs.html">apxs</a> (<em>APache
+ eXtenSion</em>) is available. It can be used to build DSO based
+ modules <em>outside of</em> the Apache source tree. The idea is
+ simple: When installing Apache the <code>configure</code>'s
+ <code>make install</code> procedure installs the Apache C
+ header files and puts the platform-dependent compiler and
+ linker flags for building DSO files into the <code>apxs</code>
+ program. This way the user can use <code>apxs</code> to compile
+ his Apache module sources without the Apache distribution
+ source tree and without having to fiddle with the
+ platform-dependent compiler and linker flags for DSO
+ support.</p>
+</section>
+
+<section id="usage"><title>Usage Summary</title>
+
+ <p>To give you an overview of the DSO features of Apache 2.0,
+ here is a short and concise summary:</p>
+
+ <ol>
+ <li>
+ Build and install a <em>distributed</em> Apache module, say
+ <code>mod_foo.c</code>, into its own DSO
+ <code>mod_foo.so</code>:
+
+<example>
+$ ./configure --prefix=/path/to/install --enable-foo=shared<br />
+$ make install
+</example>
+ </li>
+
+ <li>
+ Build and install a <em>third-party</em> Apache module, say
+ <code>mod_foo.c</code>, into its own DSO
+ <code>mod_foo.so</code>:
+
+<example>
+$ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c --enable-foo=shared<br />
+$ make install
+</example>
+ </li>
+
+ <li>
+ Configure Apache for <em>later installation</em> of shared
+ modules:
+
+<example>
+$ ./configure --enable-so<br />
+$ make install
+</example>
+ </li>
+
+ <li>
+ Build and install a <em>third-party</em> Apache module, say
+ <code>mod_foo.c</code>, into its own DSO
+ <code>mod_foo.so</code> <em>outside of</em> the Apache
+ source tree using <a href="programs/apxs.html">apxs</a>:
+
+<example>
+$ cd /path/to/3rdparty<br />
+$ apxs -c mod_foo.c<br />
+$ apxs -i -a -n foo mod_foo.so
+</example>
+ </li>
+ </ol>
+
+ <p>In all cases, once the shared module is compiled, you must
+ use a <directive module="mod_so">LoadModule</directive>
+ directive in <code>httpd.conf</code> to tell Apache to activate
+ the module.</p>
+</section>
+
+<section id="background"><title>Background</title>
+
+ <p>On modern Unix derivatives there exists a nifty mechanism
+ usually called dynamic linking/loading of <em>Dynamic Shared
+ Objects</em> (DSO) which provides a way to build a piece of
+ program code in a special format for loading it at run-time
+ into the address space of an executable program.</p>
+
+ <p>This loading can usually be done in two ways: Automatically
+ by a system program called <code>ld.so</code> when an
+ executable program is started or manually from within the
+ executing program via a programmatic system interface to the
+ Unix loader through the system calls
+ <code>dlopen()/dlsym()</code>.</p>
+
+ <p>In the first way the DSO's are usually called <em>shared
+ libraries</em> or <em>DSO libraries</em> and named
+ <code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They
+ reside in a system directory (usually <code>/usr/lib</code>)
+ and the link to the executable program is established at
+ build-time by specifying <code>-lfoo</code> to the linker
+ command. This hard-codes library references into the executable
+ program file so that at start-time the Unix loader is able to
+ locate <code>libfoo.so</code> in <code>/usr/lib</code>, in
+ paths hard-coded via linker-options like <code>-R</code> or in
+ paths configured via the environment variable
+ <code>LD_LIBRARY_PATH</code>. It then resolves any (yet
+ unresolved) symbols in the executable program which are
+ available in the DSO.</p>
+
+ <p>Symbols in the executable program are usually not referenced
+ by the DSO (because it's a reusable library of general code)
+ and hence no further resolving has to be done. The executable
+ program has no need to do anything on its own to use the
+ symbols from the DSO because the complete resolving is done by
+ the Unix loader. (In fact, the code to invoke
+ <code>ld.so</code> is part of the run-time startup code which
+ is linked into every executable program which has been bound
+ non-static). The advantage of dynamic loading of common library
+ code is obvious: the library code needs to be stored only once,
+ in a system library like <code>libc.so</code>, saving disk
+ space for every program.</p>
+
+ <p>In the second way the DSO's are usually called <em>shared
+ objects</em> or <em>DSO files</em> and can be named with an
+ arbitrary extension (although the canonical name is
+ <code>foo.so</code>). These files usually stay inside a
+ program-specific directory and there is no automatically
+ established link to the executable program where they are used.
+ Instead the executable program manually loads the DSO at
+ run-time into its address space via <code>dlopen()</code>. At
+ this time no resolving of symbols from the DSO for the
+ executable program is done. But instead the Unix loader
+ automatically resolves any (yet unresolved) symbols in the DSO
+ from the set of symbols exported by the executable program and
+ its already loaded DSO libraries (especially all symbols from
+ the ubiquitous <code>libc.so</code>). This way the DSO gets
+ knowledge of the executable program's symbol set as if it had
+ been statically linked with it in the first place.</p>
+
+ <p>Finally, to take advantage of the DSO's API the executable
+ program has to resolve particular symbols from the DSO via
+ <code>dlsym()</code> for later use inside dispatch tables
+ <em>etc.</em> In other words: The executable program has to
+ manually resolve every symbol it needs to be able to use it.
+ The advantage of such a mechanism is that optional program
+ parts need not be loaded (and thus do not spend memory) until
+ they are needed by the program in question. When required,
+ these program parts can be loaded dynamically to extend the
+ base program's functionality.</p>
+
+ <p>Although this DSO mechanism sounds straightforward there is
+ at least one difficult step here: The resolving of symbols from
+ the executable program for the DSO when using a DSO to extend a
+ program (the second way). Why? Because "reverse resolving" DSO
+ symbols from the executable program's symbol set is against the
+ library design (where the library has no knowledge about the
+ programs it is used by) and is neither available under all
+ platforms nor standardized. In practice the executable
+ program's global symbols are often not re-exported and thus not
+ available for use in a DSO. Finding a way to force the linker
+ to export all global symbols is the main problem one has to
+ solve when using DSO for extending a program at run-time.</p>
+
+ <p>The shared library approach is the typical one, because it
+ is what the DSO mechanism was designed for, hence it is used
+ for nearly all types of libraries the operating system
+ provides. On the other hand using shared objects for extending
+ a program is not used by a lot of programs.</p>
+
+ <p>As of 1998 there are only a few software packages available
+ which use the DSO mechanism to actually extend their
+ functionality at run-time: Perl 5 (via its XS mechanism and the
+ DynaLoader module), Netscape Server, <em>etc.</em> Starting
+ with version 1.3, Apache joined the crew, because Apache
+ already uses a module concept to extend its functionality and
+ internally uses a dispatch-list-based approach to link external
+ modules into the Apache core functionality. So, Apache is
+ really predestined for using DSO to load its modules at
+ run-time.</p>
+</section>
+
+<section id="advantages"><title>Advantages and Disadvantages</title>
+
+ <p>The above DSO based features have the following
+ advantages:</p>
+
+ <ul>
+ <li>The server package is more flexible at run-time because
+ the actual server process can be assembled at run-time via
+ <directive module="mod_so">LoadModule</directive>
+ <code>httpd.conf</code> configuration commands instead of
+ <code>configure</code> options at build-time. For instance
+ this way one is able to run different server instances
+ (standard & SSL version, minimalistic & powered up
+ version [mod_perl, PHP3], <em>etc.</em>) with only one Apache
+ installation.</li>
+
+ <li>The server package can be easily extended with
+ third-party modules even after installation. This is at least
+ a great benefit for vendor package maintainers who can create
+ a Apache core package and additional packages containing
+ extensions like PHP3, mod_perl, mod_fastcgi,
+ <em>etc.</em></li>
+
+ <li>Easier Apache module prototyping because with the
+ DSO/<code>apxs</code> pair you can both work outside the
+ Apache source tree and only need an <code>apxs -i</code>
+ command followed by an <code>apachectl restart</code> to
+ bring a new version of your currently developed module into
+ the running Apache server.</li>
+ </ul>
+
+ <p>DSO has the following disadvantages:</p>
+
+ <ul>
+ <li>The DSO mechanism cannot be used on every platform
+ because not all operating systems support dynamic loading of
+ code into the address space of a program.</li>
+
+ <li>The server is approximately 20% slower at startup time
+ because of the symbol resolving overhead the Unix loader now
+ has to do.</li>
+
+ <li>The server is approximately 5% slower at execution time
+ under some platforms because position independent code (PIC)
+ sometimes needs complicated assembler tricks for relative
+ addressing which are not necessarily as fast as absolute
+ addressing.</li>
+
+ <li>Because DSO modules cannot be linked against other
+ DSO-based libraries (<code>ld -lfoo</code>) on all platforms
+ (for instance a.out-based platforms usually don't provide
+ this functionality while ELF-based platforms do) you cannot
+ use the DSO mechanism for all types of modules. Or in other
+ words, modules compiled as DSO files are restricted to only
+ use symbols from the Apache core, from the C library
+ (<code>libc</code>) and all other dynamic or static libraries
+ used by the Apache core, or from static library archives
+ (<code>libfoo.a</code>) containing position independent code.
+ The only chances to use other code is to either make sure the
+ Apache core itself already contains a reference to it or
+ loading the code yourself via <code>dlopen()</code>.</li>
+ </ul>
+
+</section>
+
+</manualpage>
\ No newline at end of file
projects / apache / commitdiff