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310 lines
13 KiB
XML
310 lines
13 KiB
XML
<?xml version="1.0" encoding="UTF-8" ?>
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<!DOCTYPE manualpage SYSTEM "./style/manualpage.dtd">
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<?xml-stylesheet type="text/xsl" href="./style/manual.en.xsl"?>
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<!--
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Copyright 2002-2004 The Apache Software Foundation
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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<manualpage metafile="dso.xml.meta">
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<title>Dynamic Shared Object (DSO) Support</title>
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<summary>
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<p>The Apache HTTP Server is a modular program where the
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administrator can choose the functionality to include in the
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server by selecting a set of modules. The modules can be
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statically compiled into the <code>httpd</code> binary when the
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server is built. Alternatively, modules can be compiled as
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Dynamic Shared Objects (DSOs) that exist separately from the
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main <code>httpd</code> binary file. DSO modules may be
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compiled at the time the server is built, or they may be
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compiled and added at a later time using the Apache Extension
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Tool (<a href="programs/apxs.html">apxs</a>).</p>
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<p>This document describes how to use DSO modules as well as
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the theory behind their use.</p>
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</summary>
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<section id="implementation"><title>Implementation</title>
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<related>
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<modulelist>
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<module>mod_so</module>
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</modulelist>
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<directivelist>
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<directive module="mod_so">LoadModule</directive>
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</directivelist>
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</related>
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<p>The DSO support for loading individual Apache modules is based
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on a module named <module>mod_so</module> which must be statically
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compiled into the Apache core. It is the only module besides
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<module>core</module> which cannot be put into a DSO
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itself. Practically all other distributed Apache modules can then
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be placed into a DSO by individually enabling the DSO build for
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them via <code>configure</code>'s
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<code>--enable-<em>module</em>=shared</code> option as discussed
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in the <a href="install.html">install documentation</a>. After a
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module is compiled into a DSO named <code>mod_foo.so</code> you
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can use <module>mod_so</module>'s <directive
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module="mod_so">LoadModule</directive> command in your
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<code>httpd.conf</code> file to load this module at server startup
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or restart.</p>
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<p>To simplify this creation of DSO files for Apache modules
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(especially for third-party modules) a new support program
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named <a href="programs/apxs.html">apxs</a> (<em>APache
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eXtenSion</em>) is available. It can be used to build DSO based
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modules <em>outside of</em> the Apache source tree. The idea is
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simple: When installing Apache the <code>configure</code>'s
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<code>make install</code> procedure installs the Apache C
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header files and puts the platform-dependent compiler and
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linker flags for building DSO files into the <code>apxs</code>
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program. This way the user can use <code>apxs</code> to compile
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his Apache module sources without the Apache distribution
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source tree and without having to fiddle with the
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platform-dependent compiler and linker flags for DSO
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support.</p>
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</section>
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<section id="usage"><title>Usage Summary</title>
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<p>To give you an overview of the DSO features of Apache 2.0,
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here is a short and concise summary:</p>
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<ol>
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<li>
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Build and install a <em>distributed</em> Apache module, say
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<code>mod_foo.c</code>, into its own DSO
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<code>mod_foo.so</code>:
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<example>
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$ ./configure --prefix=/path/to/install --enable-foo=shared<br />
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$ make install
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</example>
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</li>
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<li>
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Build and install a <em>third-party</em> Apache module, say
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<code>mod_foo.c</code>, into its own DSO
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<code>mod_foo.so</code>:
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<example>
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$ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c --enable-foo=shared<br />
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$ make install
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</example>
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</li>
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<li>
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Configure Apache for <em>later installation</em> of shared
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modules:
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<example>
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$ ./configure --enable-so<br />
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$ make install
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</example>
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</li>
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<li>
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Build and install a <em>third-party</em> Apache module, say
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<code>mod_foo.c</code>, into its own DSO
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<code>mod_foo.so</code> <em>outside of</em> the Apache
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source tree using <a href="programs/apxs.html">apxs</a>:
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<example>
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$ cd /path/to/3rdparty<br />
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$ apxs -c mod_foo.c<br />
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$ apxs -i -a -n foo mod_foo.la
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</example>
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</li>
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</ol>
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<p>In all cases, once the shared module is compiled, you must
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use a <directive module="mod_so">LoadModule</directive>
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directive in <code>httpd.conf</code> to tell Apache to activate
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the module.</p>
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</section>
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<section id="background"><title>Background</title>
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<p>On modern Unix derivatives there exists a nifty mechanism
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usually called dynamic linking/loading of <em>Dynamic Shared
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Objects</em> (DSO) which provides a way to build a piece of
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program code in a special format for loading it at run-time
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into the address space of an executable program.</p>
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<p>This loading can usually be done in two ways: Automatically
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by a system program called <code>ld.so</code> when an
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executable program is started or manually from within the
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executing program via a programmatic system interface to the
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Unix loader through the system calls
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<code>dlopen()/dlsym()</code>.</p>
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<p>In the first way the DSO's are usually called <em>shared
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libraries</em> or <em>DSO libraries</em> and named
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<code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They
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reside in a system directory (usually <code>/usr/lib</code>)
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and the link to the executable program is established at
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build-time by specifying <code>-lfoo</code> to the linker
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command. This hard-codes library references into the executable
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program file so that at start-time the Unix loader is able to
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locate <code>libfoo.so</code> in <code>/usr/lib</code>, in
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paths hard-coded via linker-options like <code>-R</code> or in
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paths configured via the environment variable
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<code>LD_LIBRARY_PATH</code>. It then resolves any (yet
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unresolved) symbols in the executable program which are
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available in the DSO.</p>
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<p>Symbols in the executable program are usually not referenced
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by the DSO (because it's a reusable library of general code)
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and hence no further resolving has to be done. The executable
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program has no need to do anything on its own to use the
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symbols from the DSO because the complete resolving is done by
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the Unix loader. (In fact, the code to invoke
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<code>ld.so</code> is part of the run-time startup code which
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is linked into every executable program which has been bound
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non-static). The advantage of dynamic loading of common library
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code is obvious: the library code needs to be stored only once,
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in a system library like <code>libc.so</code>, saving disk
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space for every program.</p>
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<p>In the second way the DSO's are usually called <em>shared
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objects</em> or <em>DSO files</em> and can be named with an
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arbitrary extension (although the canonical name is
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<code>foo.so</code>). These files usually stay inside a
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program-specific directory and there is no automatically
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established link to the executable program where they are used.
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Instead the executable program manually loads the DSO at
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run-time into its address space via <code>dlopen()</code>. At
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this time no resolving of symbols from the DSO for the
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executable program is done. But instead the Unix loader
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automatically resolves any (yet unresolved) symbols in the DSO
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from the set of symbols exported by the executable program and
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its already loaded DSO libraries (especially all symbols from
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the ubiquitous <code>libc.so</code>). This way the DSO gets
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knowledge of the executable program's symbol set as if it had
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been statically linked with it in the first place.</p>
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<p>Finally, to take advantage of the DSO's API the executable
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program has to resolve particular symbols from the DSO via
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<code>dlsym()</code> for later use inside dispatch tables
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<em>etc.</em> In other words: The executable program has to
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manually resolve every symbol it needs to be able to use it.
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The advantage of such a mechanism is that optional program
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parts need not be loaded (and thus do not spend memory) until
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they are needed by the program in question. When required,
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these program parts can be loaded dynamically to extend the
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base program's functionality.</p>
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<p>Although this DSO mechanism sounds straightforward there is
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at least one difficult step here: The resolving of symbols from
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the executable program for the DSO when using a DSO to extend a
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program (the second way). Why? Because "reverse resolving" DSO
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symbols from the executable program's symbol set is against the
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library design (where the library has no knowledge about the
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programs it is used by) and is neither available under all
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platforms nor standardized. In practice the executable
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program's global symbols are often not re-exported and thus not
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available for use in a DSO. Finding a way to force the linker
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to export all global symbols is the main problem one has to
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solve when using DSO for extending a program at run-time.</p>
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<p>The shared library approach is the typical one, because it
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is what the DSO mechanism was designed for, hence it is used
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for nearly all types of libraries the operating system
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provides. On the other hand using shared objects for extending
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a program is not used by a lot of programs.</p>
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<p>As of 1998 there are only a few software packages available
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which use the DSO mechanism to actually extend their
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functionality at run-time: Perl 5 (via its XS mechanism and the
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DynaLoader module), Netscape Server, <em>etc.</em> Starting
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with version 1.3, Apache joined the crew, because Apache
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already uses a module concept to extend its functionality and
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internally uses a dispatch-list-based approach to link external
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modules into the Apache core functionality. So, Apache is
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really predestined for using DSO to load its modules at
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run-time.</p>
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</section>
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<section id="advantages"><title>Advantages and Disadvantages</title>
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<p>The above DSO based features have the following
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advantages:</p>
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<ul>
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<li>The server package is more flexible at run-time because
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the actual server process can be assembled at run-time via
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<directive module="mod_so">LoadModule</directive>
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<code>httpd.conf</code> configuration commands instead of
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<code>configure</code> options at build-time. For instance
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this way one is able to run different server instances
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(standard & SSL version, minimalistic & powered up
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version [mod_perl, PHP3], <em>etc.</em>) with only one Apache
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installation.</li>
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<li>The server package can be easily extended with
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third-party modules even after installation. This is at least
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a great benefit for vendor package maintainers who can create
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a Apache core package and additional packages containing
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extensions like PHP3, mod_perl, mod_fastcgi,
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<em>etc.</em></li>
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<li>Easier Apache module prototyping because with the
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DSO/<code>apxs</code> pair you can both work outside the
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Apache source tree and only need an <code>apxs -i</code>
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command followed by an <code>apachectl restart</code> to
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bring a new version of your currently developed module into
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the running Apache server.</li>
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</ul>
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<p>DSO has the following disadvantages:</p>
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<ul>
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<li>The DSO mechanism cannot be used on every platform
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because not all operating systems support dynamic loading of
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code into the address space of a program.</li>
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<li>The server is approximately 20% slower at startup time
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because of the symbol resolving overhead the Unix loader now
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has to do.</li>
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<li>The server is approximately 5% slower at execution time
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under some platforms because position independent code (PIC)
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sometimes needs complicated assembler tricks for relative
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addressing which are not necessarily as fast as absolute
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addressing.</li>
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<li>Because DSO modules cannot be linked against other
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DSO-based libraries (<code>ld -lfoo</code>) on all platforms
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(for instance a.out-based platforms usually don't provide
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this functionality while ELF-based platforms do) you cannot
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use the DSO mechanism for all types of modules. Or in other
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words, modules compiled as DSO files are restricted to only
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use symbols from the Apache core, from the C library
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(<code>libc</code>) and all other dynamic or static libraries
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used by the Apache core, or from static library archives
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(<code>libfoo.a</code>) containing position independent code.
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The only chances to use other code is to either make sure the
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Apache core itself already contains a reference to it or
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loading the code yourself via <code>dlopen()</code>.</li>
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</ul>
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</section>
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</manualpage>
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