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355b4669e025ff377602b6fc7caaf30dbc218371jacobs<H1 align="center">
355b4669e025ff377602b6fc7caaf30dbc218371jacobsDynamic Shared Object (DSO) Support
355b4669e025ff377602b6fc7caaf30dbc218371jacobs</H1>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<p>The Apache HTTP Server is a modular program where the administrator
355b4669e025ff377602b6fc7caaf30dbc218371jacobscan choose the functionality to include in the server by selecting a
355b4669e025ff377602b6fc7caaf30dbc218371jacobsset of modules. The modules can be statically compiled into the
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<code>httpd</code> binary when the server is built. Alternatively,
355b4669e025ff377602b6fc7caaf30dbc218371jacobsmodules can be compiled as Dynamic Shared Objects (DSOs) that exist
355b4669e025ff377602b6fc7caaf30dbc218371jacobsseparately from the main <code>httpd</code> binary file. DSO modules
355b4669e025ff377602b6fc7caaf30dbc218371jacobsmay be compiled at the time the server is built, or they may be
355b4669e025ff377602b6fc7caaf30dbc218371jacobscompiled and added at a later time using the Apache Extension Tool (<a
355b4669e025ff377602b6fc7caaf30dbc218371jacobshref="programs/apxs.html">apxs</a>).</p>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<p>This document describes how to use DSO modules as well as the theory
355b4669e025ff377602b6fc7caaf30dbc218371jacobsbehind their use.</p>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<ul>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<li><a href="#implementation">Implementation</a></li>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<li><a href="#usage">Usage Summary</a></li>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<li><a href="#background">Background</a></li>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<li><a href="#advantages">Advantages and Disadvantages</a></li>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs</ul>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<hr>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<table border="1">
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<tr><td valign="top">
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<strong>Related Modules</strong><br><br>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<a href="mod/mod_so.html">mod_so</a><br>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs</td><td valign="top">
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<strong>Related Directives</strong><br><br>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<A HREF="mod/mod_so.html#loadmodule">LoadModule</A><br>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs</td></tr></table>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<H2><a name="implementation">Implementation</a></H2>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<P>The DSO support for loading individual Apache modules is based on a
355b4669e025ff377602b6fc7caaf30dbc218371jacobsmodule named <A HREF="mod/mod_so.html"><CODE>mod_so.c</CODE></A> which
355b4669e025ff377602b6fc7caaf30dbc218371jacobsmust be statically compiled into the Apache core. It is the only
355b4669e025ff377602b6fc7caaf30dbc218371jacobsmodule besides <CODE>core.c</CODE> which cannot be put into a DSO
355b4669e025ff377602b6fc7caaf30dbc218371jacobsitself. Practically all other distributed Apache modules then can
355b4669e025ff377602b6fc7caaf30dbc218371jacobsthen be placed into a DSO by individually enabling the DSO build for
355b4669e025ff377602b6fc7caaf30dbc218371jacobsthem via <CODE>configure</CODE>'s
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<CODE>--enable-<i>module</i>=shared</CODE> option as disucussed in the
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<a href="install.html">install documentation</a>. After a module is
355b4669e025ff377602b6fc7caaf30dbc218371jacobscompiled into a DSO named <CODE>mod_foo.so</CODE> you can use <A
355b4669e025ff377602b6fc7caaf30dbc218371jacobsHREF="mod/mod_so.html"><CODE>mod_so</CODE></A>'s <A
355b4669e025ff377602b6fc7caaf30dbc218371jacobsHREF="mod/mod_so.html#loadmodule"><CODE>LoadModule</CODE></A> command
355b4669e025ff377602b6fc7caaf30dbc218371jacobsin your <CODE>httpd.conf</CODE> file to load this module at server
355b4669e025ff377602b6fc7caaf30dbc218371jacobsstartup or restart.
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<P>To simplify this creation of DSO files for Apache modules
355b4669e025ff377602b6fc7caaf30dbc218371jacobs(especially for third-party modules) a new support program named <a
355b4669e025ff377602b6fc7caaf30dbc218371jacobshref="programs/apxs.html">apxs</a> (<EM>APache eXtenSion</EM>) is
355b4669e025ff377602b6fc7caaf30dbc218371jacobsavailable. It can be used to build DSO based modules <EM>outside
355b4669e025ff377602b6fc7caaf30dbc218371jacobsof</EM> the Apache source tree. The idea is simple: When installing
355b4669e025ff377602b6fc7caaf30dbc218371jacobsApache the <CODE>configure</CODE>'s <CODE>make install</CODE>
355b4669e025ff377602b6fc7caaf30dbc218371jacobsprocedure installs the Apache C header files and puts the
355b4669e025ff377602b6fc7caaf30dbc218371jacobsplatform-dependent compiler and linker flags for building DSO files
355b4669e025ff377602b6fc7caaf30dbc218371jacobsinto the <CODE>apxs</CODE> program. This way the user can use
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<CODE>apxs</CODE> to compile his Apache module sources without the
355b4669e025ff377602b6fc7caaf30dbc218371jacobsApache distribution source tree and without having to fiddle with the
355b4669e025ff377602b6fc7caaf30dbc218371jacobsplatform-dependent compiler and linker flags for DSO support.
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<H2><a name="usage">Usage Summary</a></H2>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<P>To give you an overview of the DSO features of Apache 2.0, here is a short
355b4669e025ff377602b6fc7caaf30dbc218371jacobsand concise summary:
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<OL>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<LI>Build and install a <EM>distributed</EM> Apache module, say
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<CODE>mod_foo.c</CODE>, into its own DSO <CODE>mod_foo.so</CODE>:
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<PRE>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs$ /configure --prefix=/path/to/install
355b4669e025ff377602b6fc7caaf30dbc218371jacobs --enable-foo=shared
355b4669e025ff377602b6fc7caaf30dbc218371jacobs$ make install
355b4669e025ff377602b6fc7caaf30dbc218371jacobs</PRE>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs</TD></TR></TABLE>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<LI>Build and install a <EM>third-party</EM> Apache module, say
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<CODE>mod_foo.c</CODE>, into its own DSO <CODE>mod_foo.so</CODE>:
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<PRE>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs$ /configure --add-module=module_type:/path/to/3rdparty/mod_foo.c
355b4669e025ff377602b6fc7caaf30dbc218371jacobs --enable-foo=shared
355b4669e025ff377602b6fc7caaf30dbc218371jacobs$ make install
355b4669e025ff377602b6fc7caaf30dbc218371jacobs</PRE>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs</TD></TR></TABLE>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<LI>Configure Apache for <EM>later installation</EM> of shared modules:
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<PRE>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs$ /configure --enable-so
355b4669e025ff377602b6fc7caaf30dbc218371jacobs$ make install
355b4669e025ff377602b6fc7caaf30dbc218371jacobs</PRE>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs</TD></TR></TABLE>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<LI>Build and install a <EM>third-party</EM> Apache module, say
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<CODE>mod_foo.c</CODE>, into its own DSO <CODE>mod_foo.so</CODE> <EM>outside
355b4669e025ff377602b6fc7caaf30dbc218371jacobsof</EM> the Apache source tree using <a href="programs/apxs.html">apxs</a>:
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs<PRE>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs$ cd /path/to/3rdparty
355b4669e025ff377602b6fc7caaf30dbc218371jacobs$ apxs -c mod_foo.c
355b4669e025ff377602b6fc7caaf30dbc218371jacobs$ apxs -i -a -n foo mod_foo.so
355b4669e025ff377602b6fc7caaf30dbc218371jacobs</PRE>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs</TD></TR></TABLE>
355b4669e025ff377602b6fc7caaf30dbc218371jacobs
0a44ef6d9afbfe052a7e975f55ea0d2954b62a82jacobs</OL>
0a44ef6d9afbfe052a7e975f55ea0d2954b62a82jacobs
0a44ef6d9afbfe052a7e975f55ea0d2954b62a82jacobs<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.
<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 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>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>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>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>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>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>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>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>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.
<H2><a name="advantages">Advantages and Disadvantages</a></H2>
<P>The above DSO based features have the following advantages:
<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 &amp; SSL version, minimalistic &amp; powered up version
[mod_perl, PHP3], <EM>etc.</EM>) with only one Apache installation.
<P>
<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>
<P>
<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.
</UL>
<P>DSO has the following disadvantages:
<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.
<P>
<LI> The server is approximately 20% slower at startup time because of the
symbol resolving overhead the Unix loader now has to do.
<P>
<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.
<P>
<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>.
<P>
</UL>
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