Dynamic Shared Object (DSO) Support
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.
Modules will be compiled as Dynamic Shared Objects (DSOs)
that exist separately from the main
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 (
Alternatively, the modules can be statically compiled into
httpd binary when the server is built.
This document describes how to use DSO modules as well as the theory behind their use.
|Related Modules||Related Directives|
The DSO support for loading individual Apache httpd modules is based
on a module named
mod_so which must be statically
compiled into the Apache httpd core. It is the only module besides
core which cannot be put into a DSO
itself. Practically all other distributed Apache httpd modules will then
be placed into a DSO. After a module is compiled into a DSO named
mod_foo.so you can use
LoadModule directive in your
httpd.conf file to load this module at server startup
To simplify this creation of DSO files for Apache httpd modules
(especially for third-party modules) a support program
eXtenSion) is available. It can be used to build DSO based
modules outside of the Apache httpd source tree. The idea is
simple: When installing Apache HTTP Server the
make install procedure installs the Apache httpd C
header files and puts the platform-dependent compiler and
linker flags for building DSO files into the
program. This way the user can use
apxs to compile
his Apache httpd module sources without the Apache httpd distribution
source tree and without having to fiddle with the
platform-dependent compiler and linker flags for DSO
To give you an overview of the DSO features of Apache HTTP Server 2.x, here is a short and concise summary:
Build and install a distributed Apache httpd module, say
mod_foo.c, into its own DSO
$ ./configure --prefix=/path/to/install --enable-foo
$ make install
Configure Apache HTTP Server with all modules enabled. Only a basic set will be loaded during server startup. You can change the set of loaded modules by activating or deactivating the
$ ./configure --enable-mods-shared=all
$ make install
Some modules are only useful for developers and will not be build. when using the module set all. To build all available modules including developer modules use reallyall. In addition the
LoadModuledirectives for all built modules can be activated via the configure option
$ ./configure --enable-mods-shared=reallyall --enable-load-all-modules
$ make install
Build and install a third-party Apache httpd module, say
mod_foo.c, into its own DSO
mod_foo.sooutside of the Apache httpd source tree using
$ cd /path/to/3rdparty
$ apxs -cia mod_foo.c
In all cases, once the shared module is compiled, you must
httpd.conf to tell Apache httpd to activate
See the apxs documentation for more details.
On modern Unix derivatives there exists a mechanism 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
In the first way the DSO's are usually called shared
libraries or DSO libraries and named
reside in a system directory (usually
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
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
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
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.
The above DSO based features have the following advantages:
- The server package is more flexible at run-time because
the server process can be assembled at run-time via
httpd.confconfiguration directives instead of
configureoptions at build-time. For instance, this way one is able to run different server instances (standard & SSL version, minimalistic & dynamic version [mod_perl, mod_php], etc.) with only one Apache httpd installation.
- The server package can be easily extended with third-party modules even after installation. This is a great benefit for vendor package maintainers, who can create an Apache httpd core package and additional packages containing extensions like PHP, mod_perl, mod_security, etc.
- Easier Apache httpd module prototyping, because with the
apxspair you can both work outside the Apache httpd source tree and only need an
apxs -icommand followed by an
apachectl restartto bring a new version of your currently developed module into the running Apache HTTP Server.
DSO has the following disadvantages:
- The server is approximately 20% slower at startup time because of the symbol resolving overhead the Unix loader now has to do.
- 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.
- Because DSO modules cannot be linked against other
DSO-based libraries (
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 httpd core, from the C library (
libc) and all other dynamic or static libraries used by the Apache httpd 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 httpd core itself already contains a reference to it or loading the code yourself via