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-
-<H1>
-Apache 1.3<BR>
-Dynamic Shared Object (DSO)<BR>
-Support
-</H1>
-
-<ADDRESS>Originally written by<BR>
-Ralf S. Engelschall <rse@apache.org>, April 1998</ADDRESS>
-
-</DIV>
-
-<H3>Background</H3>
-
-<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>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>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>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>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>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 etc. 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>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.
-
-<H3>Practical Usage</H3>
-
-<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>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, etc. 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>As of Apache 1.3, the configuration system supports two optional features
-for taking advantage of the modular DSO approach: compilation of the Apache
-core program into a DSO library for shared usage and compilation of the
-Apache modules into DSO files for explicit loading at run-time.
-
-<H3>Implementation</H3>
-
-<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 has to be
-statically compiled into the Apache core. It is the only module besides
-<CODE>http_core.c</CODE> which cannot be put into a DSO itself
-(bootstrapping!). Practically all other distributed Apache modules then can
-then be placed into a DSO by individually enabling the DSO build for them via
-<CODE>configure</CODE>'s <CODE>--enable-shared</CODE> option (see top-level
-<CODE>INSTALL</CODE> file) or by changing the <CODE>AddModule</CODE> command
-in your <CODE>src/Configuration</CODE> into a <CODE>SharedModule</CODE>
-command (see <CODE>src/INSTALL</CODE> file). 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>To simplify this creation of DSO files for Apache modules (especially for
-third-party modules) a new support program named <CODE>apxs</CODE> (<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>To place the complete Apache core program into a DSO library (only required
-on some of the supported platforms to force the linker to export the apache
-core symbols -- a prerequisite for the DSO modularization) the rule
-<CODE>SHARED_CORE</CODE> has to be enabled via <CODE>configure</CODE>'s
-<CODE>--enable-rule=SHARED_CORE</CODE> option (see top-level
-<CODE>INSTALL</CODE> file) or by changing the <CODE>Rule</CODE> command in
-your <CODE>Configuration</CODE> file to <CODE>Rule SHARED_CORE=yes</CODE> (see
-<CODE>src/INSTALL</CODE> file). The Apache core code is then placed into a DSO
-library named <CODE>libhttpd.so</CODE>. Because one cannot link a DSO against
-static libraries on all platforms, an additional executable program named
-<CODE>libhttpd.ep</CODE> is created which both binds this static code and
-provides a stub for the <CODE>main()</CODE> function. Finally the
-<CODE>httpd</CODE> executable program itself is replaced by a bootstrapping
-code which automatically makes sure the Unix loader is able to load and start
-<CODE>libhttpd.ep</CODE> by providing the <CODE>LD_LIBRARY_PATH</CODE> to
-<CODE>libhttpd.so</CODE>.
-
-<H3>Supported Platforms</H3>
-
-<P>Apache's <CODE>src/Configure</CODE> script currently has only limited but
-adequate built-in knowledge on how to compile DSO files, because as already
-mentioned this is heavily platform-dependent. Nevertheless all major Unix
-platforms are supported. The definitive current state (May 1998) is this:
-
-<P>
-<UL>
-<LI>Out-of-the-box supported platforms:<BR>
-(actually tested versions in parenthesis)
-
-<PRE>
-o FreeBSD (2.1.5, 2.2.5, 2.2.6)
-o OpenBSD (2.x)
-o NetBSD (1.3.1)
-o Linux (Debian/1.3.1, RedHat/4.2)
-o Solaris (2.4, 2.5.1, 2.6)
-o SunOS (4.1.3)
-o OSF1 (4.0)
-o IRIX (6.2)
-o HP/UX (10.20)
-o UnixWare (2.01, 2.1.2)
-o SCO (5.0.4)
-o AIX (3.2, 4.1.5, 4.2, 4.3)
-o ReliantUNIX/SINIX (5.43)
-o SVR4 (-)
-</PRE>
-
-<P>
-<LI> Explicitly unsupported platforms:
-
-<PRE>
-o Ultrix (no dlopen-style interface under this platform)
-</PRE>
-
-</UL>
-
-<H3>Usage Summary</H3>
-
-<P>To give you an overview of the DSO features of Apache 1.3, here is a short
-and concise summary:
-
-<OL>
-
-<LI>Placing the Apache core code (all the stuff which usually forms the
-<CODE>httpd</CODE> binary) into a DSO <CODE>libhttpd.so</CODE>, an executable
-program <CODE>libhttpd.ep</CODE> and a bootstrapping executable program
-<CODE>httpd</CODE> (Notice: this is only required on some of the supported
-platforms to force the linker to export the Apache core symbols, which in turn
-is a prerequisite for the DSO modularization):
-
-<P>
-<UL>
-<LI>Build and install via <CODE>configure</CODE> (preferred):
-<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
-<PRE>
-$ ./configure --prefix=/path/to/install
- --enable-rule=SHARED_CORE ...
+<?xml version="1.0" encoding="ISO-8859-1"?>
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!--
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ This file is generated from xml source: DO NOT EDIT
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ -->
+<title>Dynamic Shared Object (DSO) Support - Apache HTTP Server</title>
+<link href="./style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
+<link href="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
+<link href="./style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" />
+<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 2.3</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/">Documentation</a> > <a href="./">Version 2.3</a></div><div id="page-content"><div id="preamble"><h1>Dynamic Shared Object (DSO) Support</h1>
+<div class="toplang">
+<p><span>Available Languages: </span><a href="./en/dso.html" title="English"> en </a> |
+<a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
+<a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
+</div>
+
+ <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 class="program"><a href="./programs/httpd.html">httpd</a></code> binary when the
+ server is built. Alternatively, modules can be compiled as
+ Dynamic Shared Objects (DSOs) that exist separately from the
+ main <code class="program"><a href="./programs/httpd.html">httpd</a></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 (<code class="program"><a href="./programs/apxs.html">apxs</a></code>).</p>
+
+ <p>This document describes how to use DSO modules as well as
+ the theory behind their use.</p>
+ </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>
+
+<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>
+
+ <p>The DSO support for loading individual Apache modules is based
+ on a module named <code class="module"><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 class="module"><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 class="program"><a href="./programs/configure.html">configure</a></code>'s
+ <code>--enable-<em>module</em>=shared</code> option as discussed
+ 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 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
+ <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 <code class="program"><a href="./programs/apxs.html">apxs</a></code> (<dfn>APache
+ eXtenSion</dfn>) 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 class="program"><a href="./programs/configure.html">configure</a></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 class="program"><a href="./programs/apxs.html">apxs</a></code>
+ program. This way the user can use <code class="program"><a href="./programs/apxs.html">apxs</a></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>
+</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>
+
+ <p>To give you an overview of the DSO features of Apache 2.2,
+ 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>:
+
+<div class="example"><p><code>
+$ ./configure --prefix=/path/to/install --enable-foo=shared<br />
$ make install
-</PRE>
-</TD></TR></TABLE>
-
-<LI>Build and install manually:
-<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
-<PRE>
-- Edit src/Configuration:
- << Rule SHARED_CORE=default
- >> Rule SHARED_CORE=yes
- << EXTRA_CFLAGS=
- >> EXTRA_CFLAGS= -DSHARED_CORE_DIR=\"/path/to/install/libexec\"
-$ make
-$ cp src/libhttpd.so* /path/to/install/libexec/
-$ cp src/libhttpd.ep /path/to/install/libexec/
-$ cp src/httpd /path/to/install/bin/
-</PRE>
-</TD></TR></TABLE>
-</UL>
-
-<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>:
-
-<P>
-<UL>
-<LI>Build and install via <CODE>configure</CODE> (preferred):
-<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
-<PRE>
-$ ./configure --prefix=/path/to/install
- --enable-shared=foo
+</code></p></div>
+ </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>:
+
+<div class="example"><p><code>
+$ ./configure --add-module=<var>module_type</var>:/path/to/3rdparty/mod_foo.c \<br />
+<span class="indent">
+ --enable-foo=shared<br />
+</span>
$ make install
-</PRE>
-</TD></TR></TABLE>
-
-<LI>Build and install manually:
-<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
-<PRE>
-- Edit src/Configuration:
- << AddModule modules/xxxx/mod_foo.o
- >> SharedModule modules/xxxx/mod_foo.so
-$ make
-$ cp src/xxxx/mod_foo.so /path/to/install/libexec
-- Edit /path/to/install/etc/httpd.conf
- >> LoadModule foo_module /path/to/install/libexec/mod_foo.so
-</PRE>
-</TD></TR></TABLE>
-</UL>
-
-<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>
-
-<P>
-<UL>
-<LI>Build and install via <CODE>configure</CODE> (preferred):
-<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
-<PRE>
-$ ./configure --add-module=/path/to/3rdparty/mod_foo.c
- --enable-shared=foo
+</code></p></div>
+ </li>
+
+ <li>
+ Configure Apache for <em>later installation</em> of shared
+ modules:
+
+<div class="example"><p><code>
+$ ./configure --enable-so<br />
$ make install
-</PRE>
-</TD></TR></TABLE>
-
-<LI>Build and install manually:
-<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
-<PRE>
-$ cp /path/to/3rdparty/mod_foo.c /path/to/apache-1.3/src/modules/extra/
-- Edit src/Configuration:
- >> SharedModule modules/extra/mod_foo.so
-$ make
-$ cp src/xxxx/mod_foo.so /path/to/install/libexec
-- Edit /path/to/install/etc/httpd.conf
- >> LoadModule foo_module /path/to/install/libexec/mod_foo.so
-</PRE>
-</TD></TR></TABLE>
-</UL>
-
-<P>
-<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:
-
-<P>
-<UL>
-<LI>Build and install via <CODE>apxs</CODE>:
-<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
-<PRE>
-$ cd /path/to/3rdparty
-$ apxs -c mod_foo.c
-$ apxs -i -a -n foo mod_foo.so
-</PRE>
-</TD></TR></TABLE>
-</UL>
-
-</OL>
-
-<H3>Advantages & Disadvantages</H3>
-
-<P>The above DSO based features of Apache 1.3 have the following advantages:
-
-<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>
- <CODE>httpd.conf</CODE> configuration commands instead of
- <CODE>Configuration</CODE> <CODE>AddModule</CODE> commands 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], etc.) with only one Apache installation.
-<P>
-<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, etc.
-<P>
-<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.
-</UL>
-
-<P>DSO has the following disadvantages:
-
-<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.
-<P>
-<LI> The server is approximately 20% slower at startup time because of the
- symbol resolving overhead the Unix loader now has to do.
-<P>
-<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.
-<P>
-<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, loading
- the code yourself via <CODE>dlopen()</CODE> or enabling the
- <CODE>SHARED_CHAIN</CODE> rule while building Apache when your platform
- supports linking DSO files against DSO libraries.
-<P>
-<LI> Under some platforms (many SVR4 systems) there is no way to force the
- linker to export all global symbols for use in DSO's when linking the
- Apache httpd executable program. But without the visibility of the Apache
- core symbols no standard Apache module could be used as a DSO. The only
- chance here is to use the <CODE>SHARED_CORE</CODE> feature because this
- way the global symbols are forced to be exported. As a consequence the
- Apache <CODE>src/Configure</CODE> script automatically enforces
- <CODE>SHARED_CORE</CODE> on these platforms when DSO features are used in
- the <CODE>Configuration</CODE> file or on the configure command line.
-</UL>
-
-<!--#include virtual="footer.html" -->
-</BLOCKQUOTE>
-</BODY>
-</HTML>
+</code></p></div>
+ </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 <code class="program"><a href="./programs/apxs.html">apxs</a></code>:
+
+<div class="example"><p><code>
+$ cd /path/to/3rdparty<br />
+$ apxs -c mod_foo.c<br />
+$ apxs -i -a -n foo mod_foo.la
+</code></p></div>
+ </li>
+ </ol>
+
+ <p>In all cases, once the shared module is compiled, you must
+ use a <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
+ directive in <code>httpd.conf</code> to tell Apache to activate
+ the module.</p>
+</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>
+
+ <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>
+</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>
+
+ <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
+ <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
+ <code>httpd.conf</code> configuration commands instead of
+ <code class="program"><a href="./programs/configure.html">configure</a></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 class="program"><a href="./programs/apxs.html">apxs</a></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>
+
+</div></div>
+<div class="bottomlang">
+<p><span>Available Languages: </span><a href="./en/dso.html" title="English"> en </a> |
+<a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
+<a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
+</div><div id="footer">
+<p class="apache">Copyright 2007 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>
+<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>
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