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
httpd
binary when the server is built. Alternatively,
modules can be compiled as Dynamic Shared Objects (DSOs) that exist
separately from the main httpd
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 (apxs).
This document describes how to use DSO modules as well as the theory behind their use.
Related Modules mod_so |
Related Directives LoadModule |
The DSO support for loading individual Apache modules is based on a module
named mod_so.c
which must be
statically compiled into the Apache core. It is the only module besides
core.c
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
configure
's --enable-module=shared
option
(see top-level INSTALL
file). After a module is compiled into
a DSO named mod_foo.so
you can use mod_so
's LoadModule
command in your
httpd.conf
file to load this module at server startup or restart.
To simplify this creation of DSO files for Apache modules
(especially for third-party modules) a new support program named apxs (APache eXtenSion) is
available. It can be used to build DSO based modules outside
of the Apache source tree. The idea is simple: When installing
Apache the configure
's make install
procedure installs the Apache C header files and puts the
platform-dependent compiler and linker flags for building DSO files
into the apxs
program. This way the user can use
apxs
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.
To give you an overview of the DSO features of Apache 2.0, here is a short and concise summary:
mod_foo.c
, into its own DSO mod_foo.so
:
$ ./configure --prefix=/path/to/install --enable-foo=shared $ make install |
mod_foo.c
, into its own DSO mod_foo.so
:
$ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c --enable-foo=shared $ make install |
$ ./configure --enable-so $ make install |
mod_foo.c
, into its own DSO mod_foo.so
outside
of the Apache source tree using apxs:
$ cd /path/to/3rdparty $ apxs -c mod_foo.c $ apxs -i -a -n foo mod_foo.so |
In all cases, once the shared module is compiled, you must use a LoadModule
directive
in httpd.conf
to tell Apache to activate the module.
On modern Unix derivatives there exists a nifty mechanism usually called dynamic linking/loading of Dynamic Shared Objects (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.
This loading can usually be done in two ways: Automatically by a system
program called ld.so
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 dlopen()/dlsym()
.
In the first way the DSO's are usually called shared libraries or
DSO libraries and named libfoo.so
or
libfoo.so.1.2
. They reside in a system directory (usually
/usr/lib
) and the link to the executable program is established
at build-time by specifying -lfoo
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 libfoo.so
in
/usr/lib
, in paths hard-coded via linker-options like
-R
or in paths configured via the environment variable
LD_LIBRARY_PATH
. It then resolves any (yet unresolved) symbols in
the executable program which are available in the DSO.
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 ld.so
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 libc.so
, saving disk space for every program.
In the second way the DSO's are usually called shared objects or
DSO files and can be named with an arbitrary extension (although the
canonical name is foo.so
). 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
dlopen()
. 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 libc.so
). 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.
Finally, to take advantage of the DSO's API the executable program has to
resolve particular symbols from the DSO via dlsym()
for later use
inside dispatch tables etc. 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.
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.
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.
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, etc. 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.
The above DSO based features have the following advantages:
LoadModule
httpd.conf
configuration commands instead of
configure
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], etc.) with only one Apache installation.
apxs
pair you can both work outside the Apache source tree and only need an
apxs -i
command followed by an apachectl
restart
to bring a new version of your currently developed module
into the running Apache server.
DSO has the following disadvantages:
ld -lfoo
) 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 (libc
)
and all other dynamic or static libraries used by the Apache core, or
from static library archives (libfoo.a
) 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 dlopen()
.