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.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 2.2</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 2.2</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="/ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | 26<a href="/ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | 27<a href="/tr/dso.html" hreflang="tr" rel="alternate" title="T�rk�e"> tr </a></p> 28</div> 29 30 <p>The Apache HTTP Server is a modular program where the 31 administrator can choose the functionality to include in the 32 server by selecting a set of modules. The modules can be 33 statically compiled into the <code class="program"><a href="/programs/httpd.html">httpd</a></code> binary when the 34 server is built. Alternatively, modules can be compiled as 35 Dynamic Shared Objects (DSOs) that exist separately from the 36 main <code class="program"><a href="/programs/httpd.html">httpd</a></code> binary file. DSO modules may be 37 compiled at the time the server is built, or they may be 38 compiled and added at a later time using the Apache Extension 39 Tool (<code class="program"><a href="/programs/apxs.html">apxs</a></code>).</p> 40 41 <p>This document describes how to use DSO modules as well as 42 the theory behind their use.</p> 43 </div> 44<div id="quickview"><ul id="toc"><li><img alt="" src="/images/down.gif" /> <a href="#implementation">Implementation</a></li> 45<li><img alt="" src="/images/down.gif" /> <a href="#usage">Usage Summary</a></li> 46<li><img alt="" src="/images/down.gif" /> <a href="#background">Background</a></li> 47<li><img alt="" src="/images/down.gif" /> <a href="#advantages">Advantages and Disadvantages</a></li> 48</ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div> 49<div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 50<div class="section"> 51<h2><a name="implementation" id="implementation">Implementation</a></h2> 52 53<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> 54 55 <p>The DSO support for loading individual Apache modules is based 56 on a module named <code class="module"><a href="/mod/mod_so.html">mod_so</a></code> which must be statically 57 compiled into the Apache core. It is the only module besides 58 <code class="module"><a href="/mod/core.html">core</a></code> which cannot be put into a DSO 59 itself. Practically all other distributed Apache modules can then 60 be placed into a DSO by individually enabling the DSO build for 61 them via <code class="program"><a href="/programs/configure.html">configure</a></code>'s 62 <code>--enable-<em>module</em>=shared</code> option as discussed 63 in the <a href="install.html">install documentation</a>. After a 64 module is compiled into a DSO named <code>mod_foo.so</code> you 65 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 66 <code>httpd.conf</code> file to load this module at server startup 67 or restart.</p> 68 69 <p>To simplify this creation of DSO files for Apache modules 70 (especially for third-party modules) a new support program 71 named <code class="program"><a href="/programs/apxs.html">apxs</a></code> (<dfn>APache 72 eXtenSion</dfn>) is available. It can be used to build DSO based 73 modules <em>outside of</em> the Apache source tree. The idea is 74 simple: When installing Apache the <code class="program"><a href="/programs/configure.html">configure</a></code>'s 75 <code>make install</code> procedure installs the Apache C 76 header files and puts the platform-dependent compiler and 77 linker flags for building DSO files into the <code class="program"><a href="/programs/apxs.html">apxs</a></code> 78 program. This way the user can use <code class="program"><a href="/programs/apxs.html">apxs</a></code> to compile 79 his Apache module sources without the Apache distribution 80 source tree and without having to fiddle with the 81 platform-dependent compiler and linker flags for DSO 82 support.</p> 83</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 84<div class="section"> 85<h2><a name="usage" id="usage">Usage Summary</a></h2> 86 87 <p>To give you an overview of the DSO features of Apache 2.x, 88 here is a short and concise summary:</p> 89 90 <ol> 91 <li> 92 Build and install a <em>distributed</em> Apache module, say 93 <code>mod_foo.c</code>, into its own DSO 94 <code>mod_foo.so</code>: 95 96<div class="example"><p><code> 97$ /configure --prefix=/path/to/install --enable-foo=shared<br /> 98$ make install 99</code></p></div> 100 </li> 101 102 <li> 103 Build and install a <em>third-party</em> Apache module, say 104 <code>mod_foo.c</code>, into its own DSO 105 <code>mod_foo.so</code>: 106 107<div class="example"><p><code> 108$ /configure --add-module=<var>module_type</var>:/path/to/3rdparty/mod_foo.c \<br /> 109<span class="indent"> 110 --enable-foo=shared<br /> 111</span> 112$ make install 113</code></p></div> 114 </li> 115 116 <li> 117 Configure Apache for <em>later installation</em> of shared 118 modules: 119 120<div class="example"><p><code> 121$ /configure --enable-so<br /> 122$ make install 123</code></p></div> 124 </li> 125 126 <li> 127 Build and install a <em>third-party</em> Apache module, say 128 <code>mod_foo.c</code>, into its own DSO 129 <code>mod_foo.so</code> <em>outside of</em> the Apache 130 source tree using <code class="program"><a href="/programs/apxs.html">apxs</a></code>: 131 132<div class="example"><p><code> 133$ cd /path/to/3rdparty<br /> 134$ apxs -c mod_foo.c<br /> 135$ apxs -i -a -n foo mod_foo.la 136</code></p></div> 137 </li> 138 </ol> 139 140 <p>In all cases, once the shared module is compiled, you must 141 use a <code class="directive"><a href="/mod/mod_so.html#loadmodule">LoadModule</a></code> 142 directive in <code>httpd.conf</code> to tell Apache to activate 143 the module.</p> 144</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 145<div class="section"> 146<h2><a name="background" id="background">Background</a></h2> 147 148 <p>On modern Unix derivatives there exists a nifty mechanism 149 usually called dynamic linking/loading of <em>Dynamic Shared 150 Objects</em> (DSO) which provides a way to build a piece of 151 program code in a special format for loading it at run-time 152 into the address space of an executable program.</p> 153 154 <p>This loading can usually be done in two ways: Automatically 155 by a system program called <code>ld.so</code> when an 156 executable program is started or manually from within the 157 executing program via a programmatic system interface to the 158 Unix loader through the system calls 159 <code>dlopen()/dlsym()</code>.</p> 160 161 <p>In the first way the DSO's are usually called <em>shared 162 libraries</em> or <em>DSO libraries</em> and named 163 <code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They 164 reside in a system directory (usually <code>/usr/lib</code>) 165 and the link to the executable program is established at 166 build-time by specifying <code>-lfoo</code> to the linker 167 command. This hard-codes library references into the executable 168 program file so that at start-time the Unix loader is able to 169 locate <code>libfoo.so</code> in <code>/usr/lib</code>, in 170 paths hard-coded via linker-options like <code>-R</code> or in 171 paths configured via the environment variable 172 <code>LD_LIBRARY_PATH</code>. It then resolves any (yet 173 unresolved) symbols in the executable program which are 174 available in the DSO.</p> 175 176 <p>Symbols in the executable program are usually not referenced 177 by the DSO (because it's a reusable library of general code) 178 and hence no further resolving has to be done. The executable 179 program has no need to do anything on its own to use the 180 symbols from the DSO because the complete resolving is done by 181 the Unix loader. (In fact, the code to invoke 182 <code>ld.so</code> is part of the run-time startup code which 183 is linked into every executable program which has been bound 184 non-static). The advantage of dynamic loading of common library 185 code is obvious: the library code needs to be stored only once, 186 in a system library like <code>libc.so</code>, saving disk 187 space for every program.</p> 188 189 <p>In the second way the DSO's are usually called <em>shared 190 objects</em> or <em>DSO files</em> and can be named with an 191 arbitrary extension (although the canonical name is 192 <code>foo.so</code>). These files usually stay inside a 193 program-specific directory and there is no automatically 194 established link to the executable program where they are used. 195 Instead the executable program manually loads the DSO at 196 run-time into its address space via <code>dlopen()</code>. At 197 this time no resolving of symbols from the DSO for the 198 executable program is done. But instead the Unix loader 199 automatically resolves any (yet unresolved) symbols in the DSO 200 from the set of symbols exported by the executable program and 201 its already loaded DSO libraries (especially all symbols from 202 the ubiquitous <code>libc.so</code>). This way the DSO gets 203 knowledge of the executable program's symbol set as if it had 204 been statically linked with it in the first place.</p> 205 206 <p>Finally, to take advantage of the DSO's API the executable 207 program has to resolve particular symbols from the DSO via 208 <code>dlsym()</code> for later use inside dispatch tables 209 <em>etc.</em> In other words: The executable program has to 210 manually resolve every symbol it needs to be able to use it. 211 The advantage of such a mechanism is that optional program 212 parts need not be loaded (and thus do not spend memory) until 213 they are needed by the program in question. When required, 214 these program parts can be loaded dynamically to extend the 215 base program's functionality.</p> 216 217 <p>Although this DSO mechanism sounds straightforward there is 218 at least one difficult step here: The resolving of symbols from 219 the executable program for the DSO when using a DSO to extend a 220 program (the second way). Why? Because "reverse resolving" DSO 221 symbols from the executable program's symbol set is against the 222 library design (where the library has no knowledge about the 223 programs it is used by) and is neither available under all 224 platforms nor standardized. In practice the executable 225 program's global symbols are often not re-exported and thus not 226 available for use in a DSO. Finding a way to force the linker 227 to export all global symbols is the main problem one has to 228 solve when using DSO for extending a program at run-time.</p> 229 230 <p>The shared library approach is the typical one, because it 231 is what the DSO mechanism was designed for, hence it is used 232 for nearly all types of libraries the operating system 233 provides. On the other hand using shared objects for extending 234 a program is not used by a lot of programs.</p> 235 236 <p>As of 1998 there are only a few software packages available 237 which use the DSO mechanism to actually extend their 238 functionality at run-time: Perl 5 (via its XS mechanism and the 239 DynaLoader module), Netscape Server, <em>etc.</em> Starting 240 with version 1.3, Apache joined the crew, because Apache 241 already uses a module concept to extend its functionality and 242 internally uses a dispatch-list-based approach to link external 243 modules into the Apache core functionality. So, Apache is 244 really predestined for using DSO to load its modules at 245 run-time.</p> 246</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 247<div class="section"> 248<h2><a name="advantages" id="advantages">Advantages and Disadvantages</a></h2> 249 250 <p>The above DSO based features have the following 251 advantages:</p> 252 253 <ul> 254 <li>The server package is more flexible at run-time because 255 the actual server process can be assembled at run-time via 256 <code class="directive"><a href="/mod/mod_so.html#loadmodule">LoadModule</a></code> 257 <code>httpd.conf</code> configuration commands instead of 258 <code class="program"><a href="/programs/configure.html">configure</a></code> options at build-time. For instance 259 this way one is able to run different server instances 260 (standard & SSL version, minimalistic & powered up 261 version [mod_perl, PHP3], <em>etc.</em>) with only one Apache 262 installation.</li> 263 264 <li>The server package can be easily extended with 265 third-party modules even after installation. This is at least 266 a great benefit for vendor package maintainers who can create 267 a Apache core package and additional packages containing 268 extensions like PHP3, mod_perl, mod_fastcgi, 269 <em>etc.</em></li> 270 271 <li>Easier Apache module prototyping because with the 272 DSO/<code class="program"><a href="/programs/apxs.html">apxs</a></code> pair you can both work outside the 273 Apache source tree and only need an <code>apxs -i</code> 274 command followed by an <code>apachectl restart</code> to 275 bring a new version of your currently developed module into 276 the running Apache server.</li> 277 </ul> 278 279 <p>DSO has the following disadvantages:</p> 280 281 <ul> 282 <li>The DSO mechanism cannot be used on every platform 283 because not all operating systems support dynamic loading of 284 code into the address space of a program.</li> 285 286 <li>The server is approximately 20% slower at startup time 287 because of the symbol resolving overhead the Unix loader now 288 has to do.</li> 289 290 <li>The server is approximately 5% slower at execution time 291 under some platforms because position independent code (PIC) 292 sometimes needs complicated assembler tricks for relative 293 addressing which are not necessarily as fast as absolute 294 addressing.</li> 295 296 <li>Because DSO modules cannot be linked against other 297 DSO-based libraries (<code>ld -lfoo</code>) on all platforms 298 (for instance a.out-based platforms usually don't provide 299 this functionality while ELF-based platforms do) you cannot 300 use the DSO mechanism for all types of modules. Or in other 301 words, modules compiled as DSO files are restricted to only 302 use symbols from the Apache core, from the C library 303 (<code>libc</code>) and all other dynamic or static libraries 304 used by the Apache core, or from static library archives 305 (<code>libfoo.a</code>) containing position independent code. 306 The only chances to use other code is to either make sure the 307 Apache core itself already contains a reference to it or 308 loading the code yourself via <code>dlopen()</code>.</li> 309 </ul> 310 311</div></div> 312<div class="bottomlang"> 313<p><span>Available Languages: </span><a href="/en/dso.html" title="English"> en </a> | 314<a href="/ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | 315<a href="/ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | 316<a href="/tr/dso.html" hreflang="tr" rel="alternate" title="T�rk�e"> tr </a></p> 317</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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