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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-07 15:01:30 +0000 |
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| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-07 15:01:30 +0000 |
| commit | 6beeb1b708550be0d4a53b272283e17e5e35fe17 (patch) | |
| tree | 1ce8673d4aaa948e5554000101f46536a1e4cc29 /docs/manual/dso.html.en | |
| parent | Initial commit. (diff) | |
| download | apache2-upstream/2.4.57.tar.xz apache2-upstream/2.4.57.zip | |
Adding upstream version 2.4.57.upstream/2.4.57
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'docs/manual/dso.html.en')
| -rw-r--r-- | docs/manual/dso.html.en | 332 |
1 files changed, 332 insertions, 0 deletions
diff --git a/docs/manual/dso.html.en b/docs/manual/dso.html.en new file mode 100644 index 0000000..85ffc30 --- /dev/null +++ b/docs/manual/dso.html.en @@ -0,0 +1,332 @@ +<?xml version="1.0" encoding="UTF-8"?> +<!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> +<meta content="text/html; charset=UTF-8" http-equiv="Content-Type" /> +<!-- + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + This file is generated from xml source: DO NOT EDIT + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + --> +<title>Dynamic Shared Object (DSO) Support - Apache HTTP Server Version 2.4</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 rel="stylesheet" type="text/css" href="./style/css/prettify.css" /> +<script src="./style/scripts/prettify.min.js" type="text/javascript"> +</script> + +<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="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p> +<p class="apache">Apache HTTP Server Version 2.4</p> +<img alt="" src="./images/feather.png" /></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 2.4</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="./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. + Modules will 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>Alternatively, 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.</p> + + <p>This document describes how to use DSO modules as well as + the theory behind their use.</p> + </div> +<div id="quickview"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><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><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</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 httpd 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 httpd 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 httpd modules will then + be placed into a DSO. 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> directive in your + <code>httpd.conf</code> file to load this module at server startup + or restart.</p> + <p>The DSO builds for individual modules can be disabled via + <code class="program"><a href="./programs/configure.html">configure</a></code>'s <code>--enable-mods-static</code> + option as discussed in the <a href="install.html">install + documentation</a>.</p> + + <p>To simplify this creation of DSO files for Apache httpd modules + (especially for third-party modules) a 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 httpd source tree. The idea is + simple: When installing Apache HTTP Server the <code class="program"><a href="./programs/configure.html">configure</a></code>'s + <code>make install</code> procedure installs the Apache httpd 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 httpd module sources without the Apache httpd 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 HTTP Server 2.x, + here is a short and concise summary:</p> + + <ol> + <li> + <p>Build and install a <em>distributed</em> Apache httpd module, say + <code>mod_foo.c</code>, into its own DSO + <code>mod_foo.so</code>:</p> + +<div class="example"><p><code> +$ ./configure --prefix=/path/to/install --enable-foo<br /> +$ make install +</code></p></div> + </li> + + <li> + <p>Configure Apache HTTP Server with all modules enabled. Only a basic + set will be loaded during server startup. You can change the set of loaded + modules by activating or deactivating the <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> directives in + <code>httpd.conf</code>.</p> + +<div class="example"><p><code> +$ ./configure --enable-mods-shared=all<br /> +$ make install +</code></p></div> + </li> + + <li> + <p>Some modules are only useful for developers and will not be build. + when using the module set <em>all</em>. To build all available modules + including developer modules use <em>reallyall</em>. In addition the + <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> directives for all + built modules can be activated via the configure option + <code>--enable-load-all-modules</code>.</p> + +<div class="example"><p><code> +$ ./configure --enable-mods-shared=reallyall --enable-load-all-modules<br /> +$ make install +</code></p></div> + </li> + + <li> + Build and install a <em>third-party</em> Apache httpd module, say + <code>mod_foo.c</code>, into its own DSO + <code>mod_foo.so</code> <em>outside of</em> the Apache httpd + 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 -cia mod_foo.c +</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 httpd to activate + the module.</p> + + <p>See the <a href="programs/apxs.html">apxs documentation</a> for more details.</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 mechanism + 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.</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 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 directives 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 & dynamic + version [mod_perl, mod_php], <em>etc.</em>) with only one Apache httpd + installation.</li> + + <li>The server package can be easily extended with + third-party modules even after installation. This is + a great benefit for vendor package maintainers, who can create + an Apache httpd core package and additional packages containing + extensions like PHP, mod_perl, mod_security, <em>etc.</em></li> + + <li>Easier Apache httpd 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 httpd 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 HTTP Server.</li> + </ul> + + <p>DSO has the following disadvantages:</p> + + <ul> + <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 httpd core, from the C library + (<code>libc</code>) and all other dynamic or static libraries + used by the Apache httpd 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 + httpd 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="./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 class="top"><a href="#page-header"><img src="./images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Libera.chat, or sent to our <a href="https://httpd.apache.org/lists.html">mailing lists</a>.</div> +<script type="text/javascript"><!--//--><![CDATA[//><!-- +var comments_shortname = 'httpd'; +var comments_identifier = 'http://httpd.apache.org/docs/2.4/dso.html'; +(function(w, d) { + if (w.location.hostname.toLowerCase() == "httpd.apache.org") { + d.write('<div id="comments_thread"><\/div>'); + var s = d.createElement('script'); + s.type = 'text/javascript'; + s.async = true; + s.src = 'https://comments.apache.org/show_comments.lua?site=' + comments_shortname + '&page=' + comments_identifier; + (d.getElementsByTagName('head')[0] || d.getElementsByTagName('body')[0]).appendChild(s); + } + else { + d.write('<div id="comments_thread">Comments are disabled for this page at the moment.<\/div>'); + } +})(window, document); +//--><!]]></script></div><div id="footer"> +<p class="apache">Copyright 2023 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="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p></div><script type="text/javascript"><!--//--><![CDATA[//><!-- +if (typeof(prettyPrint) !== 'undefined') { + prettyPrint(); +} +//--><!]]></script> +</body></html>
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