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From sl...@apache.org
Subject cvs commit: httpd-2.0/docs/manual dso.xml dso.html.en
Date Fri, 16 Aug 2002 01:19:16 GMT
slive       2002/08/15 18:19:16

  Modified:    docs/manual dso.html.en
  Added:       docs/manual dso.xml
  Log:
  New XML
  
  Revision  Changes    Path
  1.23      +40 -114   httpd-2.0/docs/manual/dso.html.en
  
  Index: dso.html.en
  ===================================================================
  RCS file: /home/cvs/httpd-2.0/docs/manual/dso.html.en,v
  retrieving revision 1.22
  retrieving revision 1.23
  diff -u -d -b -u -r1.22 -r1.23
  --- dso.html.en	31 May 2002 10:53:54 -0000	1.22
  +++ dso.html.en	16 Aug 2002 01:19:16 -0000	1.23
  @@ -1,20 +1,8 @@
  -<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
  -    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
  -
  -<html xmlns="http://www.w3.org/1999/xhtml">
  -  <head>
  -    <meta name="generator" content="HTML Tidy, see www.w3.org" />
  -
  -    <title>Dynamic Shared Object (DSO) support</title>
  -  </head>
  -  <!-- Background white, links blue (unvisited), navy (visited), red (active) -->
  -
  -  <body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
  -  vlink="#000080" alink="#FF0000">
  -    <!--#include virtual="header.html" -->
  -
  -    <h1 align="center">Dynamic Shared Object (DSO) Support</h1>
  -
  +<html><head><META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><!--
  +        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/manual.css" type="text/css" rel="stylesheet"></head><body><blockquote><div
align="center"><img src="./images/sub.gif" alt="[APACHE DOCUMENTATION]"><h3>Apache
HTTP Server Version 2.0</h3></div><h1 align="center">Dynamic Shared Object
(DSO) Support</h1>
       <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
  @@ -28,52 +16,23 @@
   
       <p>This document describes how to use DSO modules as well as
       the theory behind their use.</p>
  +  <ul><li><a href="#implementation">Implementation</a></li><li><a
href="#usage">Usage Summary</a></li><li><a href="#background">Background</a></li><li><a
href="#advantages">Advantages and Disadvantages</a></li></ul><hr><h2><a
name="implementation">Implementation</a></h2>
   
  -    <ul>
  -      <li><a href="#implementation">Implementation</a></li>
  -
  -      <li><a href="#usage">Usage Summary</a></li>
  -
  -      <li><a href="#background">Background</a></li>
  -
  -      <li><a href="#advantages">Advantages and
  -      Disadvantages</a></li>
  -    </ul>
  -    <hr />
  -
  -    <table border="1">
  -      <tr>
  -        <td valign="top"><strong>Related Modules</strong><br />
  -         <br />
  -         <a href="mod/mod_so.html">mod_so</a><br />
  -         </td>
  -
  -        <td valign="top"><strong>Related Directives</strong><br />
  -         <br />
  -         <a href="mod/mod_so.html#loadmodule">LoadModule</a><br />
  -         </td>
  -      </tr>
  -    </table>
  -
  -    <h2><a id="implementation"
  -    name="implementation">Implementation</a></h2>
  +<table border="1"><tr><td valign="top"><strong>Related Modules</strong><br><br><code><a
href="./mod/mod_so.html">mod_so</a></code><br></td><td valign="top"><strong>Related
Directives</strong><br><br><a href="./mod/mod_so.html#loadmodule" class="directive"><code
class="directive">LoadModule</code></a><br></td></tr></table>
   
  -    <p>The DSO support for loading individual Apache modules is
  -    based on a module named <a
  -    href="mod/mod_so.html"><code>mod_so.c</code></a> which must be
  -    statically compiled into the Apache core. It is the only module
  -    besides <code>core.c</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>configure</code>'s
  -    <code>--enable-<i>module</i>=shared</code> option as disucussed
  -    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 <a href="mod/mod_so.html"><code>mod_so</code></a>'s
  -    <a
  -    href="mod/mod_so.html#loadmodule"><code>LoadModule</code></a>
  -    command in your <code>httpd.conf</code> file to load this
  -    module at server startup or restart.</p>
  +    <p>The DSO support for loading individual Apache modules is based
  +    on a module named <code><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><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>configure</code>'s
  +    <code>--enable-<em>module</em>=shared</code> option as disucussed
  +    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><a href="./mod/mod_so.html">mod_so</a></code>'s
<a href="./mod/mod_so.html#loadmodule" class="directive"><code class="directive">LoadModule</code></a>
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
  @@ -89,8 +48,7 @@
       source tree and without having to fiddle with the
       platform-dependent compiler and linker flags for DSO
       support.</p>
  -
  -    <h2><a id="usage" name="usage">Usage Summary</a></h2>
  +<h2><a name="usage">Usage Summary</a></h2>
   
       <p>To give you an overview of the DSO features of Apache 2.0,
       here is a short and concise summary:</p>
  @@ -101,17 +59,10 @@
           <code>mod_foo.c</code>, into its own DSO
           <code>mod_foo.so</code>: 
   
  -        <table bgcolor="#f0f0f0" cellpadding="10">
  -          <tr>
  -            <td>
  -<pre>
  -$ ./configure --prefix=/path/to/install
  -        --enable-foo=shared
  +<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>
  +$ ./configure --prefix=/path/to/install --enable-foo=shared<br>
   $ make install
  -</pre>
  -            </td>
  -          </tr>
  -        </table>
  +</code></td></tr></table></blockquote>
         </li>
   
         <li>
  @@ -119,33 +70,20 @@
           <code>mod_foo.c</code>, into its own DSO
           <code>mod_foo.so</code>: 
   
  -        <table bgcolor="#f0f0f0" cellpadding="10">
  -          <tr>
  -            <td>
  -<pre>
  -$ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c 
  -        --enable-foo=shared
  +<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>
  +$ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c --enable-foo=shared<br>
   $ make install
  -</pre>
  -            </td>
  -          </tr>
  -        </table>
  +</code></td></tr></table></blockquote>
         </li>
   
         <li>
           Configure Apache for <em>later installation</em> of shared
           modules: 
   
  -        <table bgcolor="#f0f0f0" cellpadding="10">
  -          <tr>
  -            <td>
  -<pre>
  -$ ./configure --enable-so
  +<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>
  +$ ./configure --enable-so<br>
   $ make install
  -</pre>
  -            </td>
  -          </tr>
  -        </table>
  +</code></td></tr></table></blockquote>
         </li>
   
         <li>
  @@ -154,27 +92,19 @@
           <code>mod_foo.so</code> <em>outside of</em> the Apache
           source tree using <a href="programs/apxs.html">apxs</a>: 
   
  -        <table bgcolor="#f0f0f0" cellpadding="10">
  -          <tr>
  -            <td>
  -<pre>
  -$ cd /path/to/3rdparty
  -$ apxs -c mod_foo.c
  +<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>
  +$ cd /path/to/3rdparty<br>
  +$ apxs -c mod_foo.c<br>
   $ apxs -i -a -n foo mod_foo.so
  -</pre>
  -            </td>
  -          </tr>
  -        </table>
  +</code></td></tr></table></blockquote>
         </li>
       </ol>
   
       <p>In all cases, once the shared module is compiled, you must
  -    use a <a
  -    href="mod/mod_so.html#loadmodule"><code>LoadModule</code></a>
  +    use a <a href="./mod/mod_so.html#loadmodule" class="directive"><code class="directive">LoadModule</code></a>
       directive in <code>httpd.conf</code> to tell Apache to activate
       the module.</p>
  -
  -    <h2><a id="background" name="background">Background</a></h2>
  +<h2><a name="background">Background</a></h2>
   
       <p>On modern Unix derivatives there exists a nifty mechanism
       usually called dynamic linking/loading of <em>Dynamic Shared
  @@ -274,17 +204,15 @@
       modules into the Apache core functionality. So, Apache is
       really predestined for using DSO to load its modules at
       run-time.</p>
  -
  -    <h2><a id="advantages" name="advantages">Advantages and
  -    Disadvantages</a></h2>
  +<h2><a name="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 <a
  -      href="mod/mod_so.html#loadmodule"><code>LoadModule</code></a>
  +      the actual server process can be assembled at run-time via
  +      <a href="./mod/mod_so.html#loadmodule" class="directive"><code class="directive">LoadModule</code></a>
         <code>httpd.conf</code> configuration commands instead of
         <code>configure</code> options at build-time. For instance
         this way one is able to run different server instances
  @@ -338,7 +266,5 @@
         Apache core itself already contains a reference to it or
         loading the code yourself via <code>dlopen()</code>.</li>
       </ul>
  -    <!--#include virtual="footer.html" -->
  -  </body>
  -</html>
   
  +<hr></blockquote><h3 align="center">Apache HTTP Server Version 2.0</h3><a
href="./"><img src="./images/index.gif" alt="Index"></a><a href="./"><img
src="./images/home.gif" alt="Home"></a></body></html>
  \ No newline at end of file
  
  
  
  1.1                  httpd-2.0/docs/manual/dso.xml
  
  Index: dso.xml
  ===================================================================
  <?xml version="1.0" encoding="UTF-8" ?>
  <!DOCTYPE manualpage SYSTEM "./style/manualpage.dtd">
  <?xml-stylesheet type="text/xsl" href="./style/manual.en.xsl"?>
  
  <manualpage>
    <relativepath href="."/>
  
    <title>Dynamic Shared Object (DSO) Support</title>
  
    <summary>
      <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>httpd</code> binary when the
      server is built. Alternatively, modules can be compiled as
      Dynamic Shared Objects (DSOs) that exist separately from the
      main <code>httpd</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 (<a href="programs/apxs.html">apxs</a>).</p>
  
      <p>This document describes how to use DSO modules as well as
      the theory behind their use.</p>
    </summary>
  
  
  <section id="implementation"><title>Implementation</title>
  
  <related>
  <modulelist>
  <module>mod_so</module>
  </modulelist>
  <directivelist>
  <directive module="mod_so">LoadModule</directive>
  </directivelist>
  </related>
  
      <p>The DSO support for loading individual Apache modules is based
      on a module named <module>mod_so</module> which must be statically
      compiled into the Apache core. It is the only module besides
      <module>core</module> 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>configure</code>'s
      <code>--enable-<em>module</em>=shared</code> option as disucussed
      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 <module>mod_so</module>'s <directive
      module="mod_so">LoadModule</directive> 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 <a href="programs/apxs.html">apxs</a> (<em>APache
      eXtenSion</em>) 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>configure</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>apxs</code>
      program. This way the user can use <code>apxs</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>
  </section>
  
  <section id="usage"><title>Usage Summary</title>
  
      <p>To give you an overview of the DSO features of Apache 2.0,
      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>: 
  
  <example>
  $ ./configure --prefix=/path/to/install --enable-foo=shared<br />
  $ make install
  </example>
        </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>: 
  
  <example>
  $ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c --enable-foo=shared<br
/>
  $ make install
  </example>
        </li>
  
        <li>
          Configure Apache for <em>later installation</em> of shared
          modules: 
  
  <example>
  $ ./configure --enable-so<br />
  $ make install
  </example>
        </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 <a href="programs/apxs.html">apxs</a>: 
  
  <example>
  $ cd /path/to/3rdparty<br />
  $ apxs -c mod_foo.c<br />
  $ apxs -i -a -n foo mod_foo.so
  </example>
        </li>
      </ol>
  
      <p>In all cases, once the shared module is compiled, you must
      use a <directive module="mod_so">LoadModule</directive>
      directive in <code>httpd.conf</code> to tell Apache to activate
      the module.</p>
  </section>
  
  <section id="background"><title>Background</title>
  
      <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>
  </section>
  
  <section id="advantages"><title>Advantages and Disadvantages</title>
  
      <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
        <directive module="mod_so">LoadModule</directive>
        <code>httpd.conf</code> configuration commands instead of
        <code>configure</code> options at build-time. For instance
        this way one is able to run different server instances
        (standard &amp; SSL version, minimalistic &amp; 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>apxs</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>
  
  </section>
  
  </manualpage>
  
  

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