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<
p>This document describes how to use DSO modules as well as the theory
<
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>
<
strong>Related Modules</
strong><
br><
br>
<
strong>Related Directives</
strong><
br><
br>
<
H2><
a name="implementation">Implementation</
a></
H2>
<
P>The DSO support for loading individual Apache modules is based on a
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 then 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 <
Ain your <
CODE>
httpd.conf</
CODE> file to load this module at server
<
P>To simplify this creation of DSO files for Apache modules
(especially for third-party modules) a new support program named <
aavailable. 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.
<
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
<
LI>Build and install a <
EM>distributed</
EM> Apache module, say
<
TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><
TR><
TD>
<
LI>Build and install a <
EM>third-party</
EM> Apache module, say
<
TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><
TR><
TD>
<
LI>Configure Apache for <
EM>later installation</
EM> of shared modules:
<
TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><
TR><
TD>
<
LI>Build and install a <
EM>third-party</
EM> Apache module, say
<
TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><
TR><
TD>
<
p>In all cases, once the shared module is compiled, you must use a <
AHREF="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
<
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:
<
LI> The server package is more flexible at run-time because the actual server
process can be assembled at run-time via <
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> 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> 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.
<
P>DSO has the following disadvantages:
<
LI> The DSO mechanism cannot be used on every platform because not all
operating systems support dynamic loading of code into the address space
<
LI> The server is approximately 20% slower at startup time because of the
symbol resolving overhead the Unix loader now has to do.
<
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> 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>.