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diff --git a/docs/manual/dso.html.en b/docs/manual/dso.html.en deleted file mode 100644 index 41d965d671..0000000000 --- a/docs/manual/dso.html.en +++ /dev/null @@ -1,344 +0,0 @@ -<!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> - - <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> - - <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> - - <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>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> - - <h2><a id="usage" 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> - - <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>: - - <table bgcolor="#f0f0f0" cellpadding="10"> - <tr> - <td> -<pre> -$ ./configure --prefix=/path/to/install - --enable-foo=shared -$ make install -</pre> - </td> - </tr> - </table> - </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>: - - <table bgcolor="#f0f0f0" cellpadding="10"> - <tr> - <td> -<pre> -$ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c - --enable-foo=shared -$ make install -</pre> - </td> - </tr> - </table> - </li> - - <li> - Configure Apache for <em>later installation</em> of shared - modules: - - <table bgcolor="#f0f0f0" cellpadding="10"> - <tr> - <td> -<pre> -$ ./configure --enable-so -$ make install -</pre> - </td> - </tr> - </table> - </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>: - - <table bgcolor="#f0f0f0" cellpadding="10"> - <tr> - <td> -<pre> -$ cd /path/to/3rdparty -$ apxs -c mod_foo.c -$ apxs -i -a -n foo mod_foo.so -</pre> - </td> - </tr> - </table> - </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> - directive in <code>httpd.conf</code> to tell Apache to activate - the module.</p> - - <h2><a id="background" name="background">Background</a></h2> - - <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> - - <h2><a id="advantages" 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> - <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 & SSL version, minimalistic & 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> - <!--#include virtual="footer.html" --> - </body> -</html> - |