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From r..@hyperreal.org
Subject cvs commit: apache-1.3/src README.DSO
Date Tue, 14 Apr 1998 10:51:40 GMT
rse         98/04/14 03:51:40

  Modified:    src      README.DSO
  Log:
  Merge in a lot of fixes and enhancements from Martin.
  
  Revision  Changes    Path
  1.3       +139 -119  apache-1.3/src/README.DSO
  
  Index: README.DSO
  ===================================================================
  RCS file: /export/home/cvs/apache-1.3/src/README.DSO,v
  retrieving revision 1.2
  retrieving revision 1.3
  diff -u -r1.2 -r1.3
  --- README.DSO	1998/04/14 08:27:33	1.2
  +++ README.DSO	1998/04/14 10:51:39	1.3
  @@ -7,111 +7,127 @@
    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. 
  + 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 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(). 
  + 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 stay inside a system
  + 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 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
  + established at link-time by specifying -lfoo to the linker command. This
  + hardcodes library references into the executable program file so that at
  + start-time the Unix loader is able to locate libfoo.so in /usr/lib or in
    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.
  + 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 reuseable 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 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. 
  + 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 straight foreward there is at least one
  + 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
  - 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.
  + `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.
   
    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
  + 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 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.
  + 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), GIMP, 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.
  +
  + As of Apache 1.3, the configuration system supports two optional features for
  + taking advantage of the modular DSO approach: compilation of the Apache core
  + program into a DSO library for shared usage and compilation of the Apache
  + modules into DSO files for explicit loading at 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 place the complete Apache core program into a DSO library (only required
  + on some of the supported platforms to force the linker to export the apache
  + core symbols -- a prerequisite for the DSO modularization) the rule
  + SHARED_CORE has to be enabled via configure's --enable-rule=SHARED_CORE
  + option (see ../INSTALL file) or by changing the Rule command in
  + Configuration.tmpl to "Rule SHARED_CORE=yes" (see ./INSTALL file). The Apache
  + core code is then 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 binds this 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 individual Apache modules is based on a module
  + named mod_so.c 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!). 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-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 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 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
  + third-party modules) a new support program named `apxs' 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-dependend
  + 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-dependend compiler and linker flags for DSO support.
   
    Supported Platforms
  @@ -125,16 +141,16 @@
    Out-of-the-box supported platforms:
    (actually tested versions in parenthesis)
   
  -   o  FreeBSD   (2.1.5, 2.2.5, 2.2.6)
  -   o  Linux     (Debian/1.3.1, RedHat/4.2)
  -   o  Solaris   (2.4, 2.5.1, 2.6)
  -   o  SunOS     (4.1.3)
  -   o  OSF1      (4.0)
  -   o  IRIX      (6.2)
  -   o  HP/UX     (10.20)
  -   o  UnixWare  (2.01, 2.1.2)
  -   o  SINIX     (?)
  -   o  SVR4      (-)
  +   o  FreeBSD            (2.1.5, 2.2.5, 2.2.6)
  +   o  Linux              (Debian/1.3.1, RedHat/4.2)
  +   o  Solaris            (2.4, 2.5.1, 2.6)
  +   o  SunOS              (4.1.3)
  +   o  OSF1               (4.0)
  +   o  IRIX               (6.2)
  +   o  HP/UX              (10.20)
  +   o  UnixWare           (2.01, 2.1.2)
  +   o  ReliantUNIX/SINIX  (5.43)
  +   o  SVR4               (-)
   
    Explicitly unsupported platforms:
   
  @@ -148,13 +164,15 @@
    -------------
   
    To give you an overview of the DSO features of Apache 1.3, here is a short
  - and concrete summary:
  + and concise 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:
  + 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 (Notice: this is only required on
  +    some of the supported platforms to force the linker to export the Apache
  +    core symbols, which in turn is a prerequisite for the DSO modularization):
   
  -      o Build and install via APACI (preferred):
  +      o Build and install via configure (preferred):
           $ ./configure --prefix=/path/to/install
                         --enable-rule=SHARED_CORE ...
           $ make install
  @@ -173,7 +191,7 @@
    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):
  +      o Build and install via configure (preferred):
           $ ./configure --prefix=/path/to/install
                         --enable-shared=foo
           $ make install
  @@ -190,7 +208,7 @@
    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):
  +      o Build and install via configure (preferred):
           $ ./configure --add-module=/path/to/3rdparty/mod_foo.c 
                         --enable-shared=foo
           $ make install
  @@ -205,7 +223,7 @@
             >> "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:
  +    into its own DSO mod_foo.so _outside of_ the Apache source tree:
   
         o Build and install via APXS: 
           $ cd /path/to/3rdparty
  @@ -218,26 +236,25 @@
    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 is more flexible at run-time because the actual server
  +     process can be assembled at run-time via LoadModule httpd.conf
  +     configuration commands instead of Configuration AddModule commands 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.
   
      + 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'
  +   + Easier Apache module prototyping because with the DSO/APXS pair you can
  +     both work 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 DSO mechanism cannot be used on every platform because not all
  +     operating systems support dynamic loading.
   
      - The server is approximately 20% slower at startup time because of the
        symbol resolving overhead the Unix loader now has to do.
  @@ -250,19 +267,22 @@
      - 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
  +     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 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.
  +     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.
  +   - Under some platforms (many SVR4 systems) there is no way to force the
  +     linker to export all global symbols for use in DSO's when linking the
  +     Apache httpd executable program. But without the visibility of the Apache
  +     core symbols no standard Apache module could be used as a 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 enforces SHARED_CORE on these
  +     platforms when DSO features are used in the Configuration file or on the
  +     configure command line.
   
                                          Ralf S. Engelschall
                                          rse@engelschall.com
  
  
  

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