diff options
Diffstat (limited to 'rubbos/app/apache2/manual/dso.html.en')
| -rw-r--r-- | rubbos/app/apache2/manual/dso.html.en | 318 |
1 files changed, 318 insertions, 0 deletions
diff --git a/rubbos/app/apache2/manual/dso.html.en b/rubbos/app/apache2/manual/dso.html.en new file mode 100644 index 00000000..f37d73a0 --- /dev/null +++ b/rubbos/app/apache2/manual/dso.html.en @@ -0,0 +1,318 @@ +<?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 2.0</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/">Documentation</a> > <a href="./">Version 2.0</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="./es/dso.html" hreflang="es" rel="alternate" title="Español"> es </a> | +<a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français"> fr </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> | +<a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </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.x, + 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 +</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=module_type:/path/to/3rdparty/mod_foo.c --enable-foo=shared<br /> +$ make install +</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 +</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="./es/dso.html" hreflang="es" rel="alternate" title="Español"> es </a> | +<a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français"> fr </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> | +<a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p> +</div><div id="footer"> +<p class="apache">Copyright 2009 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>
\ No newline at end of file |