docs/manual/dso.html.en

   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" />
  12 <link href="./images/favicon.ico" rel="shortcut icon" /></head>
  13 <body id="manual-page"><div id="page-header">
  14 <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>
  15 <p class="apache">Apache HTTP Server Version 2.3</p>
  16 <img alt="" src="./images/feather.gif" /></div>
  17 <div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="./images/left.gif" /></a></div>
  18 <div id="path">
  19 <a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="./">Version 2.3</a></div><div id="page-content"><div id="preamble"><h1>Dynamic Shared Object (DSO) Support</h1>
  20 <div class="toplang">
  21 <p><span>Available Languages: </span><a href="./en/dso.html" title="English">&nbsp;en&nbsp;</a> |
  22 <a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a> |
  23 <a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a> |
  24 <a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a> |
  25 <a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe">&nbsp;tr&nbsp;</a></p>
  26 </div>
  27
  28     <p>The Apache HTTP Server is a modular program where the
  29     administrator can choose the functionality to include in the
  30     server by selecting a set of modules. The modules can be
  31     statically compiled into the <code class="program"><a href="./programs/httpd.html">httpd</a></code> binary when the
  32     server is built. Alternatively, modules can be compiled as
  33     Dynamic Shared Objects (DSOs) that exist separately from the
  34     main <code class="program"><a href="./programs/httpd.html">httpd</a></code> binary file. DSO modules may be
  35     compiled at the time the server is built, or they may be
  36     compiled and added at a later time using the Apache Extension
  37     Tool (<code class="program"><a href="./programs/apxs.html">apxs</a></code>).</p>
  38
  39     <p>This document describes how to use DSO modules as well as
  40     the theory behind their use.</p>
  41   </div>
  42 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#implementation">Implementation</a></li>
  43 <li><img alt="" src="./images/down.gif" /> <a href="#usage">Usage Summary</a></li>
  44 <li><img alt="" src="./images/down.gif" /> <a href="#background">Background</a></li>
  45 <li><img alt="" src="./images/down.gif" /> <a href="#advantages">Advantages and Disadvantages</a></li>
  46 </ul></div>
  47 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
  48 <div class="section">
  49 <h2><a name="implementation" id="implementation">Implementation</a></h2>
  50
  51 <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>
  52
  53     <p>The DSO support for loading individual Apache modules is based
  54     on a module named <code class="module"><a href="./mod/mod_so.html">mod_so</a></code> which must be statically
  55     compiled into the Apache core. It is the only module besides
  56     <code class="module"><a href="./mod/core.html">core</a></code> which cannot be put into a DSO
  57     itself. Practically all other distributed Apache modules can then
  58     be placed into a DSO by individually enabling the DSO build for
  59     them via <code class="program"><a href="./programs/configure.html">configure</a></code>'s
  60     <code>--enable-<em>module</em>=shared</code> option as discussed
  61     in the <a href="install.html">install documentation</a>. After a
  62     module is compiled into a DSO named <code>mod_foo.so</code> you
  63     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
  64     <code>httpd.conf</code> file to load this module at server startup
  65     or restart.</p>
  66
  67     <p>To simplify this creation of DSO files for Apache modules
  68     (especially for third-party modules) a new support program
  69     named <code class="program"><a href="./programs/apxs.html">apxs</a></code> (<dfn>APache
  70     eXtenSion</dfn>) is available. It can be used to build DSO based
  71     modules <em>outside of</em> the Apache source tree. The idea is
  72     simple: When installing Apache the <code class="program"><a href="./programs/configure.html">configure</a></code>'s
  73     <code>make install</code> procedure installs the Apache C
  74     header files and puts the platform-dependent compiler and
  75     linker flags for building DSO files into the <code class="program"><a href="./programs/apxs.html">apxs</a></code>
  76     program. This way the user can use <code class="program"><a href="./programs/apxs.html">apxs</a></code> to compile
  77     his Apache module sources without the Apache distribution
  78     source tree and without having to fiddle with the
  79     platform-dependent compiler and linker flags for DSO
  80     support.</p>
  81 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
  82 <div class="section">
  83 <h2><a name="usage" id="usage">Usage Summary</a></h2>
  84
  85     <p>To give you an overview of the DSO features of Apache 2.x,
  86     here is a short and concise summary:</p>
  87
  88     <ol>
  89       <li>
  90         Build and install a <em>distributed</em> Apache module, say
  91         <code>mod_foo.c</code>, into its own DSO
  92         <code>mod_foo.so</code>:
  93
  94 <div class="example"><p><code>
  95 $ ./configure --prefix=/path/to/install --enable-foo=shared<br />
  96 $ make install
  97 </code></p></div>
  98       </li>
  99
 100       <li>
 101         Build and install a <em>third-party</em> Apache module, say
 102         <code>mod_foo.c</code>, into its own DSO
 103         <code>mod_foo.so</code>:
 104
 105 <div class="example"><p><code>
 106 $ ./configure --add-module=<var>module_type</var>:/path/to/3rdparty/mod_foo.c \<br />
 107 <span class="indent">
 108   --enable-foo=shared<br />
 109 </span>
 110 $ make install
 111 </code></p></div>
 112       </li>
 113
 114       <li>
 115         Configure Apache for <em>later installation</em> of shared
 116         modules:
 117
 118 <div class="example"><p><code>
 119 $ ./configure --enable-so<br />
 120 $ make install
 121 </code></p></div>
 122       </li>
 123
 124       <li>
 125         Build and install a <em>third-party</em> Apache module, say
 126         <code>mod_foo.c</code>, into its own DSO
 127         <code>mod_foo.so</code> <em>outside of</em> the Apache
 128         source tree using <code class="program"><a href="./programs/apxs.html">apxs</a></code>:
 129
 130 <div class="example"><p><code>
 131 $ cd /path/to/3rdparty<br />
 132 $ apxs -c mod_foo.c<br />
 133 $ apxs -i -a -n foo mod_foo.la
 134 </code></p></div>
 135       </li>
 136     </ol>
 137
 138     <p>In all cases, once the shared module is compiled, you must
 139     use a <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
 140     directive in <code>httpd.conf</code> to tell Apache to activate
 141     the module.</p>
 142 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
 143 <div class="section">
 144 <h2><a name="background" id="background">Background</a></h2>
 145
 146     <p>On modern Unix derivatives there exists a nifty mechanism
 147     usually called dynamic linking/loading of <em>Dynamic Shared
 148     Objects</em> (DSO) which provides a way to build a piece of
 149     program code in a special format for loading it at run-time
 150     into the address space of an executable program.</p>
 151
 152     <p>This loading can usually be done in two ways: Automatically
 153     by a system program called <code>ld.so</code> when an
 154     executable program is started or manually from within the
 155     executing program via a programmatic system interface to the
 156     Unix loader through the system calls
 157     <code>dlopen()/dlsym()</code>.</p>
 158
 159     <p>In the first way the DSO's are usually called <em>shared
 160     libraries</em> or <em>DSO libraries</em> and named
 161     <code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They
 162     reside in a system directory (usually <code>/usr/lib</code>)
 163     and the link to the executable program is established at
 164     build-time by specifying <code>-lfoo</code> to the linker
 165     command. This hard-codes library references into the executable
 166     program file so that at start-time the Unix loader is able to
 167     locate <code>libfoo.so</code> in <code>/usr/lib</code>, in
 168     paths hard-coded via linker-options like <code>-R</code> or in
 169     paths configured via the environment variable
 170     <code>LD_LIBRARY_PATH</code>. It then resolves any (yet
 171     unresolved) symbols in the executable program which are
 172     available in the DSO.</p>
 173
 174     <p>Symbols in the executable program are usually not referenced
 175     by the DSO (because it's a reusable library of general code)
 176     and hence no further resolving has to be done. The executable
 177     program has no need to do anything on its own to use the
 178     symbols from the DSO because the complete resolving is done by
 179     the Unix loader. (In fact, the code to invoke
 180     <code>ld.so</code> is part of the run-time startup code which
 181     is linked into every executable program which has been bound
 182     non-static). The advantage of dynamic loading of common library
 183     code is obvious: the library code needs to be stored only once,
 184     in a system library like <code>libc.so</code>, saving disk
 185     space for every program.</p>
 186
 187     <p>In the second way the DSO's are usually called <em>shared
 188     objects</em> or <em>DSO files</em> and can be named with an
 189     arbitrary extension (although the canonical name is
 190     <code>foo.so</code>). These files usually stay inside a
 191     program-specific directory and there is no automatically
 192     established link to the executable program where they are used.
 193     Instead the executable program manually loads the DSO at
 194     run-time into its address space via <code>dlopen()</code>. At
 195     this time no resolving of symbols from the DSO for the
 196     executable program is done. But instead the Unix loader
 197     automatically resolves any (yet unresolved) symbols in the DSO
 198     from the set of symbols exported by the executable program and
 199     its already loaded DSO libraries (especially all symbols from
 200     the ubiquitous <code>libc.so</code>). This way the DSO gets
 201     knowledge of the executable program's symbol set as if it had
 202     been statically linked with it in the first place.</p>
 203
 204     <p>Finally, to take advantage of the DSO's API the executable
 205     program has to resolve particular symbols from the DSO via
 206     <code>dlsym()</code> for later use inside dispatch tables
 207     <em>etc.</em> In other words: The executable program has to
 208     manually resolve every symbol it needs to be able to use it.
 209     The advantage of such a mechanism is that optional program
 210     parts need not be loaded (and thus do not spend memory) until
 211     they are needed by the program in question. When required,
 212     these program parts can be loaded dynamically to extend the
 213     base program's functionality.</p>
 214
 215     <p>Although this DSO mechanism sounds straightforward there is
 216     at least one difficult step here: The resolving of symbols from
 217     the executable program for the DSO when using a DSO to extend a
 218     program (the second way). Why? Because "reverse resolving" DSO
 219     symbols from the executable program's symbol set is against the
 220     library design (where the library has no knowledge about the
 221     programs it is used by) and is neither available under all
 222     platforms nor standardized. In practice the executable
 223     program's global symbols are often not re-exported and thus not
 224     available for use in a DSO. Finding a way to force the linker
 225     to export all global symbols is the main problem one has to
 226     solve when using DSO for extending a program at run-time.</p>
 227
 228     <p>The shared library approach is the typical one, because it
 229     is what the DSO mechanism was designed for, hence it is used
 230     for nearly all types of libraries the operating system
 231     provides. On the other hand using shared objects for extending
 232     a program is not used by a lot of programs.</p>
 233
 234     <p>As of 1998 there are only a few software packages available
 235     which use the DSO mechanism to actually extend their
 236     functionality at run-time: Perl 5 (via its XS mechanism and the
 237     DynaLoader module), Netscape Server, <em>etc.</em> Starting
 238     with version 1.3, Apache joined the crew, because Apache
 239     already uses a module concept to extend its functionality and
 240     internally uses a dispatch-list-based approach to link external
 241     modules into the Apache core functionality. So, Apache is
 242     really predestined for using DSO to load its modules at
 243     run-time.</p>
 244 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
 245 <div class="section">
 246 <h2><a name="advantages" id="advantages">Advantages and Disadvantages</a></h2>
 247
 248     <p>The above DSO based features have the following
 249     advantages:</p>
 250
 251     <ul>
 252       <li>The server package is more flexible at run-time because
 253       the actual server process can be assembled at run-time via
 254       <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code>
 255       <code>httpd.conf</code> configuration commands instead of
 256       <code class="program"><a href="./programs/configure.html">configure</a></code> options at build-time. For instance
 257       this way one is able to run different server instances
 258       (standard &amp; SSL version, minimalistic &amp; powered up
 259       version [mod_perl, PHP3], <em>etc.</em>) with only one Apache
 260       installation.</li>
 261
 262       <li>The server package can be easily extended with
 263       third-party modules even after installation. This is at least
 264       a great benefit for vendor package maintainers who can create
 265       a Apache core package and additional packages containing
 266       extensions like PHP3, mod_perl, mod_fastcgi,
 267       <em>etc.</em></li>
 268
 269       <li>Easier Apache module prototyping because with the
 270       DSO/<code class="program"><a href="./programs/apxs.html">apxs</a></code> pair you can both work outside the
 271       Apache source tree and only need an <code>apxs -i</code>
 272       command followed by an <code>apachectl restart</code> to
 273       bring a new version of your currently developed module into
 274       the running Apache server.</li>
 275     </ul>
 276
 277     <p>DSO has the following disadvantages:</p>
 278
 279     <ul>
 280       <li>The DSO mechanism cannot be used on every platform
 281       because not all operating systems support dynamic loading of
 282       code into the address space of a program.</li>
 283
 284       <li>The server is approximately 20% slower at startup time
 285       because of the symbol resolving overhead the Unix loader now
 286       has to do.</li>
 287
 288       <li>The server is approximately 5% slower at execution time
 289       under some platforms because position independent code (PIC)
 290       sometimes needs complicated assembler tricks for relative
 291       addressing which are not necessarily as fast as absolute
 292       addressing.</li>
 293
 294       <li>Because DSO modules cannot be linked against other
 295       DSO-based libraries (<code>ld -lfoo</code>) on all platforms
 296       (for instance a.out-based platforms usually don't provide
 297       this functionality while ELF-based platforms do) you cannot
 298       use the DSO mechanism for all types of modules. Or in other
 299       words, modules compiled as DSO files are restricted to only
 300       use symbols from the Apache core, from the C library
 301       (<code>libc</code>) and all other dynamic or static libraries
 302       used by the Apache core, or from static library archives
 303       (<code>libfoo.a</code>) containing position independent code.
 304       The only chances to use other code is to either make sure the
 305       Apache core itself already contains a reference to it or
 306       loading the code yourself via <code>dlopen()</code>.</li>
 307     </ul>
 308
 309 </div></div>
 310 <div class="bottomlang">
 311 <p><span>Available Languages: </span><a href="./en/dso.html" title="English">&nbsp;en&nbsp;</a> |
 312 <a href="./fr/dso.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a> |
 313 <a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a> |
 314 <a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a> |
 315 <a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe">&nbsp;tr&nbsp;</a></p>
 316 </div><div id="footer">
 317 <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>
 318 <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>
 319 </body></html>