rse 98/04/13 08:40:18
Added: src README.DSO
Log:
Here it comes: My first cut for an overview README.DSO file describing the
relevant aspects of dynamic shared object (DSO) support in Apache 1.3. My
hope is that this information makes the DSO stuff more clear to those not very
familiar with it. And I hope my written down information is correct... ;-)
Revision Changes Path
1.1 apache-1.3/src/README.DSO
Index: README.DSO
===================================================================
Apache 1.3 Dynamic Shared Object (DSO) support
==============================================
Ralf S. Engelschall, April 1998
Background
----------
On modern Unix derivates there exists a nifty mechanism usually named Dynamic
Shared Object (DSO) which provides a way to build a piece of program code in
a special format to be able to load it under run-time into the address space
of an executable program.
This loading can usually be done in two ways: Automatically by a system
program named ld.so when the Unix loader has to start an executable program
or manually from within the executing program via a pragmatic 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 stay inside a system
directory (usually /usr/lib) and the link to the executable program is
established under link-time by specifying -lfoo to the linker command. This
hardcodes library references into the executable program file therewith under
start-time the Unix loader is able to lookup libfoo.so from /usr/lib or from
paths configured via the environment variable LD_LIBRARY_PATH. It then
resolves any (still unresolved) symbols in the executable program which are
defined and exported in the DSO. Symbols in the executable program are
usually not used inside the DSO (because its a reuseable library of general
code) and hence no resolving this way has to be done. The executable program
has no to do anything to be able to use the symbols from the DSO because the
complete resolving is done by the Unix loader.
In the second way the DSO's are usually called "shared objects" or "DSO
files" and can be arbitrarily named (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 under run-time manually loads the DSO into his
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 (still unresolved) symbols in the DSO which are defined and
exported in the executable program. This way the DSO gets knowledge of
the executable program as it would have been statically linked to it
under program link-time. Finally to make the DSO accessible to the
executable program it resolves particular symbols from the DSO via dlsym()
for later use inside dispatch tables, etc. In other words: The executable
program has no to manually resolve anything to be able to use it.
Although this DSO mechanism sounds straight foreward 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
this resolving is against the library design (where the library has no
knowledge of any program it is used for) and is neither available under all
platforms nor standardized. In practice only global symbols from the
executable program are available to the DSO which are explicitly marked as
exported. And forcing this exportation of global symbols is the main problem
one has to solve when using DSO for extending a program under run-time.
Practical Usage
---------------
The shared library approach is the typical one, because this is the way the
DSO mechanism was designed for, hence it is used for mostly 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 package available which use the DSO
mechanism to actually extend their functionality under run-time: Perl 5 (via
it's XS mechanism and the DynaLoader module), GIMP, Netscape Server, etc.
But Apache 1.3 now is also one of these, because Apache already uses a module
concept to extend its functionality and really uses a dispatch-list-based
approach to link these modules into the Apache core functionality. So, Apache
is really predestinated for using DSO to load it's modules under run-time.
The idea now is to provide two optional features for Apache 1.3: To compile
and place the Apache core program into a DSO library for shared usage and to
compile and place Apache modules into DSO files for explicit loading under
run-time.
Implementation
--------------
To place the complete Apache core program into a DSO library the rule
SHARED_CORE has to be enabled via APACI's --enable-rule=SHARED_CORE option
(see ../INSTALL file) or by changing the Rule command in
src/Configuration.tmpl to "Rule SHARED_CORE=yes" (see ./INSTALL file) the
Apache core code then is placed into a DSO library named libhttpd.so. Because
one cannot link a DSO against static libraries, an additional executable
program named libhttpd.ep is created which both ties those static code and
provides a stub for the main() function. Finally the httpd executable program
itself is replaced by a bootstrapping code which automatically makes sure the
Unix loader is able to load and start libhttpd.ep by providing the
LD_LIBRARY_PATH to libhttpd.so.
The DSO support for loading Apache modules is implemented completely
different: Here a module named mod_so.c is used which has to be statically
compiled into the Apache core. It is the only module besides http_core.c
which cannot be put into a DSO itself (bootstrapping!). Mostly all other
distributed Apache modules then can be placed into a DSO by individually
enabling the DSO build for them via APACI's --enable-shared option (see
../INSTALL file) or by changing the `AddModule' command in
src/Configuration.tmpl into a `SharedModule' command (see ./INSTALL file).
After a module is placed 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 ones) a new support program named `apxs' is available. I can be
used to build DSO based modules _outside_ the Apache source tree. The idea is
simple: When installing Apache the APACI "make install" procedure installs
the Apache C header files and puts the platform-dependend compiler and linker
flags for building DSO files into the `apxs' program. This way the user can
use `apxs' to compile it's Apache module sources without the Apache
distribution source tree and without having to fiddle with the
platform-dependend compiler and linker flags for DSO support.
Usage Summary
-------------
To give you an overview of the DSO features of Apache 1.3, here is a short
and concrete summary:
1. Placing the Apache core code (all the stuff which usually forms
the httpd binary) into a DSO libhttpd.so, an executable program
libhttpd.ep and a bootstrapping executable program httpd:
o Build and install via APACI (preferred):
$ ./configure --prefix=/path/to/install
--enable-rule=SHARED_CORE ...
$ make install
o Build and install manually:
- Edit src/Configuration:
<< "Rule SHARED_CORE=default"
>> "Rule SHARED_CORE=yes"
<< "EXTRA_CFLAGS= "
>> "EXTRA_CFLAGS= -DSHARED_CORE_DIR=\"/path/to/install/libexec\"
$ make
$ cp src/libhttpd.so* /path/to/install/libexec/
$ cp src/libhttpd.ep /path/to/install/libexec/
$ cp src/httpd /path/to/install/bin/
2. Build and install a distributed Apache module, say mod_foo.c,
into its own DSO mod_foo.so:
o Build and install via APACI (preferred):
$ ./configure --prefix=/path/to/install
--enable-shared=foo
$ make install
o Build and install manually:
- Edit src/Configuration:
<< "AddModule modules/xxxx/mod_foo.o"
>> "SharedModule modules/xxxx/mod_foo.so"
$ make
$ cp src/xxxx/mod_foo.so /path/to/install/libexec
- Edit /path/to/install/etc/httpd.conf
>> "LoadModule foo_module /path/to/install/libexec/mod_foo.so"
3. Build and install a third-party Apache module, say mod_foo.c,
into its own DSO mod_foo.so
o Build and install via APACI (preferred):
$ ./configure --add-module=/path/to/3rdparty/mod_foo.c
--enable-shared=foo
$ make install
o Build and install manually:
$ cp /path/to/3rdparty/mod_foo.c
/path/to/apache-1.3/src/modules/extra/
- Edit src/Configuration:
>> "SharedModule modules/extra/mod_foo.so"
$ make
$ cp src/xxxx/mod_foo.so /path/to/install/libexec
- Edit /path/to/install/etc/httpd.conf
>> "LoadModule foo_module /path/to/install/libexec/mod_foo.so"
4. Build and install a third-party Apache module, say mod_foo.c,
into its own DSO mod_foo.so _outside_ the Apache source tree:
o Build and install via APXS:
$ cd /path/to/3rdparty
$ apxs -c mod_foo.c
$ apxs -i -a -n foo mod_foo.so
Advantages & Disadvantages
--------------------------
The above DSO based features of Apache 1.3 have the following advantages (+)
and disadvantages (-):
+ The server package is more flexible under run-time because the actual
used server process can be assembled under run-time via LoadModule
httpd.conf configuration commands instead of Configuration AddModule
commands under 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.
+ 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, etc.
+ Easier Apache module prototyping because with the DSO/APXS couple you can
both works outside the Apache source tree and only need an `apxs -i'
command followed by a `apachectl restart' to bring a new version of your
currently developed module into the running Apache server.
- The DSO mechanism cannot be used on any platform because not all
operating systems support this mechanism.
- 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 independed 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) 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) or from static
library archives (libfoo.a) containing position independend code. The
only chance 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.
- Because under some platforms like SVR4 there is no way to force the
linker to export the global symbols when linking the Apache httpd
executable program. This way these aren't available to modules built as
DSO. The only chance here is to use the SHARED_CORE feature because this
way the global symbols are forced to be exported. As a consequence the
Apache src/Configure script automatically forced SHARED_CORE under those
platforms when DSO should be used.
Ralf S. Engelschall
[EMAIL PROTECTED]
www.engelschall.com
