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Subject cvs commit: apache-1.3/src README.DSO
Date Mon, 13 Apr 1998 15:40:19 GMT
rse         98/04/13 08:40:18

  Added:       src      README.DSO
  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
   Apache 1.3 Dynamic Shared Object (DSO) support
   Ralf S. Engelschall, April 1998
   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 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 or 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 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
   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
   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 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
   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 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, 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/* /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
        o Build and install via APACI (preferred):
          $ ./configure --prefix=/path/to/install
          $ make install
        o Build and install manually: 
          - Edit src/Configuration:
            << "AddModule    modules/xxxx/mod_foo.o" 
            >> "SharedModule modules/xxxx/"
          $ make
          $ cp src/xxxx/ /path/to/install/libexec
          - Edit /path/to/install/etc/httpd.conf
            >> "LoadModule foo_module /path/to/install/libexec/"
   3. Build and install a third-party Apache module, say mod_foo.c, 
      into its own DSO
        o Build and install via APACI (preferred):
          $ ./configure --add-module=/path/to/3rdparty/mod_foo.c 
          $ 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/"
          $ make
          $ cp src/xxxx/ /path/to/install/libexec
          - Edit /path/to/install/etc/httpd.conf
            >> "LoadModule foo_module /path/to/install/libexec/"
   4. Build and install a third-party Apache module, say mod_foo.c, 
      into its own DSO _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
   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
     + 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

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