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 7 --> 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" /><link rel="stylesheet" type="text/css" href="/style/css/prettify.css" /> 12<script src="/style/scripts/prettify.min.js" type="text/javascript"> 13</script> 14 15<link href="/images/favicon.ico" rel="shortcut icon" /></head> 16<body id="manual-page"><div id="page-header"> 17<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> 18<p class="apache">Apache HTTP Server Version 2.4</p> 19<img alt="" src="/images/feather.gif" /></div> 20<div class="up"><a href="./"><img title="<-" alt="<-" src="/images/left.gif" /></a></div> 21<div id="path"> 22<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> 23<div class="toplang"> 24<p><span>Available Languages: </span><a href="/en/dso.html" title="English"> en </a> | 25<a href="/fr/dso.html" hreflang="fr" rel="alternate" title="Fran�ais"> fr </a> | 26<a href="/ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | 27<a href="/ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | 28<a href="/tr/dso.html" hreflang="tr" rel="alternate" title="T�rk�e"> tr </a></p> 29</div> 30 31 <p>The Apache HTTP Server is a modular program where the 32 administrator can choose the functionality to include in the 33 server by selecting a set of modules. 34 Modules will be compiled as Dynamic Shared Objects (DSOs) 35 that exist separately from the main <code class="program"><a href="/programs/httpd.html">httpd</a></code> 36 binary file. DSO modules may be compiled at the time the server 37 is built, or they may be compiled and added at a later time 38 using the Apache Extension Tool (<code class="program"><a href="/programs/apxs.html">apxs</a></code>).</p> 39 <p>Alternatively, the modules can be statically compiled into 40 the <code class="program"><a href="/programs/httpd.html">httpd</a></code> binary when the server is built.</p> 41 42 <p>This document describes how to use DSO modules as well as 43 the theory behind their use.</p> 44 </div> 45<div id="quickview"><ul id="toc"><li><img alt="" src="/images/down.gif" /> <a href="#implementation">Implementation</a></li> 46<li><img alt="" src="/images/down.gif" /> <a href="#usage">Usage Summary</a></li> 47<li><img alt="" src="/images/down.gif" /> <a href="#background">Background</a></li> 48<li><img alt="" src="/images/down.gif" /> <a href="#advantages">Advantages and Disadvantages</a></li> 49</ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div> 50<div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 51<div class="section"> 52<h2><a name="implementation" id="implementation">Implementation</a></h2> 53 54<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> 55 56 <p>The DSO support for loading individual Apache httpd modules is based 57 on a module named <code class="module"><a href="/mod/mod_so.html">mod_so</a></code> which must be statically 58 compiled into the Apache httpd core. It is the only module besides 59 <code class="module"><a href="/mod/core.html">core</a></code> which cannot be put into a DSO 60 itself. Practically all other distributed Apache httpd modules will then 61 be placed into a DSO. After a module is compiled into a DSO named 62 <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 63 <code>httpd.conf</code> file to load this module at server startup 64 or restart.</p> 65 <p>The DSO builds for individual modules can be disabled via 66 <code class="program"><a href="/programs/configure.html">configure</a></code>'s <code>--enable-mods-static</code> 67 option as discussed in the <a href="install.html">install 68 documentation</a>.</p> 69 70 <p>To simplify this creation of DSO files for Apache httpd modules 71 (especially for third-party modules) a support program 72 named <code class="program"><a href="/programs/apxs.html">apxs</a></code> (<dfn>APache 73 eXtenSion</dfn>) is available. It can be used to build DSO based 74 modules <em>outside of</em> the Apache httpd source tree. The idea is 75 simple: When installing Apache HTTP Server the <code class="program"><a href="/programs/configure.html">configure</a></code>'s 76 <code>make install</code> procedure installs the Apache httpd C 77 header files and puts the platform-dependent compiler and 78 linker flags for building DSO files into the <code class="program"><a href="/programs/apxs.html">apxs</a></code> 79 program. This way the user can use <code class="program"><a href="/programs/apxs.html">apxs</a></code> to compile 80 his Apache httpd module sources without the Apache httpd distribution 81 source tree and without having to fiddle with the 82 platform-dependent compiler and linker flags for DSO 83 support.</p> 84</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 85<div class="section"> 86<h2><a name="usage" id="usage">Usage Summary</a></h2> 87 88 <p>To give you an overview of the DSO features of Apache HTTP Server 2.x, 89 here is a short and concise summary:</p> 90 91 <ol> 92 <li> 93 <p>Build and install a <em>distributed</em> Apache httpd module, say 94 <code>mod_foo.c</code>, into its own DSO 95 <code>mod_foo.so</code>:</p> 96 97<div class="example"><p><code> 98$ /configure --prefix=/path/to/install --enable-foo<br /> 99$ make install 100</code></p></div> 101 </li> 102 103 <li> 104 <p>Configure Apache HTTP Server with all modules enabled. Only a basic 105 set will be loaded during server startup. You can change the set of loaded 106 modules by activating or deactivating the <code class="directive"><a href="/mod/mod_so.html#loadmodule">LoadModule</a></code> directives in 107 <code>httpd.conf</code>.</p> 108 109<div class="example"><p><code> 110$ /configure --enable-mods-shared=all<br /> 111$ make install 112</code></p></div> 113 </li> 114 115 <li> 116 <p>Some modules are only useful for developers and will not be build. 117 when using the module set <em>all</em>. To build all available modules 118 including developer modules use <em>reallyall</em>. In addition the 119 <code class="directive"><a href="/mod/mod_so.html#loadmodule">LoadModule</a></code> directives for all 120 built modules can be activated via the configure option 121 <code>--enable-load-all-modules</code>.</p> 122 123<div class="example"><p><code> 124$ /configure --enable-mods-shared=reallyall --enable-load-all-modules<br /> 125$ make install 126</code></p></div> 127 </li> 128 129 <li> 130 Build and install a <em>third-party</em> Apache httpd module, say 131 <code>mod_foo.c</code>, into its own DSO 132 <code>mod_foo.so</code> <em>outside of</em> the Apache httpd 133 source tree using <code class="program"><a href="/programs/apxs.html">apxs</a></code>: 134 135<div class="example"><p><code> 136$ cd /path/to/3rdparty<br /> 137$ apxs -cia mod_foo.c 138</code></p></div> 139 </li> 140 </ol> 141 142 <p>In all cases, once the shared module is compiled, you must 143 use a <code class="directive"><a href="/mod/mod_so.html#loadmodule">LoadModule</a></code> 144 directive in <code>httpd.conf</code> to tell Apache httpd to activate 145 the module.</p> 146 147 <p>See the <a href="programs/apxs.html">apxs documentation</a> for more details.</p> 148</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 149<div class="section"> 150<h2><a name="background" id="background">Background</a></h2> 151 152 <p>On modern Unix derivatives there exists a mechanism 153 called dynamic linking/loading of <em>Dynamic Shared 154 Objects</em> (DSO) which provides a way to build a piece of 155 program code in a special format for loading it at run-time 156 into the address space of an executable program.</p> 157 158 <p>This loading can usually be done in two ways: automatically 159 by a system program called <code>ld.so</code> when an 160 executable program is started or manually from within the 161 executing program via a programmatic system interface to the 162 Unix loader through the system calls 163 <code>dlopen()/dlsym()</code>.</p> 164 165 <p>In the first way the DSO's are usually called <em>shared 166 libraries</em> or <em>DSO libraries</em> and named 167 <code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They 168 reside in a system directory (usually <code>/usr/lib</code>) 169 and the link to the executable program is established at 170 build-time by specifying <code>-lfoo</code> to the linker 171 command. This hard-codes library references into the executable 172 program file so that at start-time the Unix loader is able to 173 locate <code>libfoo.so</code> in <code>/usr/lib</code>, in 174 paths hard-coded via linker-options like <code>-R</code> or in 175 paths configured via the environment variable 176 <code>LD_LIBRARY_PATH</code>. It then resolves any (yet 177 unresolved) symbols in the executable program which are 178 available in the DSO.</p> 179 180 <p>Symbols in the executable program are usually not referenced 181 by the DSO (because it's a reusable library of general code) 182 and hence no further resolving has to be done. The executable 183 program has no need to do anything on its own to use the 184 symbols from the DSO because the complete resolving is done by 185 the Unix loader. (In fact, the code to invoke 186 <code>ld.so</code> is part of the run-time startup code which 187 is linked into every executable program which has been bound 188 non-static). The advantage of dynamic loading of common library 189 code is obvious: the library code needs to be stored only once, 190 in a system library like <code>libc.so</code>, saving disk 191 space for every program.</p> 192 193 <p>In the second way the DSO's are usually called <em>shared 194 objects</em> or <em>DSO files</em> and can be named with an 195 arbitrary extension (although the canonical name is 196 <code>foo.so</code>). These files usually stay inside a 197 program-specific directory and there is no automatically 198 established link to the executable program where they are used. 199 Instead the executable program manually loads the DSO at 200 run-time into its address space via <code>dlopen()</code>. At 201 this time no resolving of symbols from the DSO for the 202 executable program is done. But instead the Unix loader 203 automatically resolves any (yet unresolved) symbols in the DSO 204 from the set of symbols exported by the executable program and 205 its already loaded DSO libraries (especially all symbols from 206 the ubiquitous <code>libc.so</code>). This way the DSO gets 207 knowledge of the executable program's symbol set as if it had 208 been statically linked with it in the first place.</p> 209 210 <p>Finally, to take advantage of the DSO's API the executable 211 program has to resolve particular symbols from the DSO via 212 <code>dlsym()</code> for later use inside dispatch tables 213 <em>etc.</em> In other words: The executable program has to 214 manually resolve every symbol it needs to be able to use it. 215 The advantage of such a mechanism is that optional program 216 parts need not be loaded (and thus do not spend memory) until 217 they are needed by the program in question. When required, 218 these program parts can be loaded dynamically to extend the 219 base program's functionality.</p> 220 221 <p>Although this DSO mechanism sounds straightforward there is 222 at least one difficult step here: The resolving of symbols from 223 the executable program for the DSO when using a DSO to extend a 224 program (the second way). Why? Because "reverse resolving" DSO 225 symbols from the executable program's symbol set is against the 226 library design (where the library has no knowledge about the 227 programs it is used by) and is neither available under all 228 platforms nor standardized. In practice the executable 229 program's global symbols are often not re-exported and thus not 230 available for use in a DSO. Finding a way to force the linker 231 to export all global symbols is the main problem one has to 232 solve when using DSO for extending a program at run-time.</p> 233 234 <p>The shared library approach is the typical one, because it 235 is what the DSO mechanism was designed for, hence it is used 236 for nearly all types of libraries the operating system 237 provides.</p> 238 239</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 240<div class="section"> 241<h2><a name="advantages" id="advantages">Advantages and Disadvantages</a></h2> 242 243 <p>The above DSO based features have the following 244 advantages:</p> 245 246 <ul> 247 <li>The server package is more flexible at run-time because 248 the server process can be assembled at run-time via 249 <code class="directive"><a href="/mod/mod_so.html#loadmodule">LoadModule</a></code> 250 <code>httpd.conf</code> configuration directives instead of 251 <code class="program"><a href="/programs/configure.html">configure</a></code> options at build-time. For instance, 252 this way one is able to run different server instances 253 (standard & SSL version, minimalistic & dynamic 254 version [mod_perl, mod_php], <em>etc.</em>) with only one Apache httpd 255 installation.</li> 256 257 <li>The server package can be easily extended with 258 third-party modules even after installation. This is 259 a great benefit for vendor package maintainers, who can create 260 an Apache httpd core package and additional packages containing 261 extensions like PHP, mod_perl, mod_security, <em>etc.</em></li> 262 263 <li>Easier Apache httpd module prototyping, because with the 264 DSO/<code class="program"><a href="/programs/apxs.html">apxs</a></code> pair you can both work outside the 265 Apache httpd source tree and only need an <code>apxs -i</code> 266 command followed by an <code>apachectl restart</code> to 267 bring a new version of your currently developed module into 268 the running Apache HTTP Server.</li> 269 </ul> 270 271 <p>DSO has the following disadvantages:</p> 272 273 <ul> 274 <li>The server is approximately 20% slower at startup time 275 because of the symbol resolving overhead the Unix loader now 276 has to do.</li> 277 278 <li>The server is approximately 5% slower at execution time 279 under some platforms, because position independent code (PIC) 280 sometimes needs complicated assembler tricks for relative 281 addressing, which are not necessarily as fast as absolute 282 addressing.</li> 283 284 <li>Because DSO modules cannot be linked against other 285 DSO-based libraries (<code>ld -lfoo</code>) on all platforms 286 (for instance a.out-based platforms usually don't provide 287 this functionality while ELF-based platforms do) you cannot 288 use the DSO mechanism for all types of modules. Or in other 289 words, modules compiled as DSO files are restricted to only 290 use symbols from the Apache httpd core, from the C library 291 (<code>libc</code>) and all other dynamic or static libraries 292 used by the Apache httpd core, or from static library archives 293 (<code>libfoo.a</code>) containing position independent code. 294 The only chances to use other code is to either make sure the 295 httpd core itself already contains a reference to it or 296 loading the code yourself via <code>dlopen()</code>.</li> 297 </ul> 298 299</div></div> 300<div class="bottomlang"> 301<p><span>Available Languages: </span><a href="/en/dso.html" title="English"> en </a> | 302<a href="/fr/dso.html" hreflang="fr" rel="alternate" title="Fran�ais"> fr </a> | 303<a href="/ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | 304<a href="/ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | 305<a href="/tr/dso.html" hreflang="tr" rel="alternate" title="T�rk�e"> tr </a></p> 306</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. 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