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Subject cvs commit: apache-1.3/htdocs/manual sharedobjects.html
Date Tue, 12 May 1998 04:00:48 GMT
brian       98/05/11 21:00:47

  Added:       htdocs/manual sharedobjects.html
  I took Ralf's README.DSO, HTMLized it, and put it here, a more appropriate
  place than in the root-level of the distribution.  Now we can link to it, too.
  Also, Ralf, I removed the section on execution order differences between
  static and dynamically linked modules, since you committed a patch which
  makes this behavios consistant with all-staticly-linked.
  Revision  Changes    Path
  1.1                  apache-1.3/htdocs/manual/sharedobjects.html
  Index: sharedobjects.html
  <TITLE>Apache 1.3 Dynamic Shared Object (DSO) support</TITLE>
  <!-- Background white, links blue (unvisited), navy (visited), red (active) -->
  <!--#include virtual="header.html" -->
  <H1>Apache 1.3 Dynamic Shared Object (DSO) support</H1>
  <address>Originally written by Ralf S. Engelschall, April 1998</address>
  <P>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
  <P>This loading can usually be done in two ways: Automatically by a
   system program called <CODE></CODE> 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 <CODE>dlopen()/dlsym()</CODE>.
  <P>In the first way the DSO's are usually called "shared libraries" or
   "DSO libraries" and named <CODE></CODE> or
   <CODE></CODE>. They reside in a system directory
   (usually <CODE>/usr/lib</CODE>) and the link to the executable
   program is established at link-time by specifying <CODE>-lfoo</CODE>
   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 <CODE></CODE> in <CODE>/usr/lib</CODE> or in
   configured via the environment variable
   <CODE>LD_LIBRARY_PATH</CODE>. It then resolves any (yet unresolved)
   symbols in the executable program which are available in the DSO.
  <P>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 <CODE></CODE> 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 <CODE></CODE>, saving disk space for every
  <P>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 <CODE></CODE>). 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 <CODE>dlopen()</CODE>. 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 <CODE></CODE>).
   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.
  <P>Finally, to take advantage of the DSO's API the executable program
   has to resolve particular symbols from the DSO via
   <CODE>dlsym()</CODE> 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.
  <P>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.
  <H3>Practical Usage</H3>
  <P>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.
  <P>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.
  <P>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.
  <P> 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
   you can use mod_so's `LoadModule' command in your httpd.conf file to load
   this module at server startup or restart.
  <P>To simplify this creation of DSO files for Apache modules (especially for
   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.
  <P>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 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
  <H3>Supported Platforms</H3>
  <P>Apache's src/Configure script currently has only limited built-in knowledge
   on how to compile DSO files because (as already mentioned) this is heavily
   platform-dependent. Nevertheless all major Unix platforms are supported.  The
   definitive current state (May 1998) is this:
   Out-of-the-box supported platforms:
   (actually tested versions in parenthesis)
     o  FreeBSD            (2.1.5, 2.2.5, 2.2.6)
     o  OpenBSD            (2.x)
     o  NetBSD             (1.3.1)
     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  AIX                (3.2, 4.1.5, 4.2, 4.3)
     o  ReliantUNIX/SINIX  (5.43)
     o  SVR4               (-)
   Explicitly unsupported platforms:
     o  Ultrix: There is no dlopen-style interface under this platform.
  <H3>Usage Summary</H3>
  <P>To give you an overview of the DSO features of Apache 1.3, here is
   a short and concise summary:
  <LI>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 (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 configure (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/
  <LI>Build and install a distributed Apache module, say mod_foo.c, 
      into its own DSO
     o Build and install via configure (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/"
  <LI>Build and install a third-party Apache module, say mod_foo.c, 
      into its own DSO
     o Build and install via configure (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/"
  <LI>Build and install a third-party Apache module, say mod_foo.c, 
      into its own DSO _outside of_ the Apache source tree:
     o Build and install via APXS: 
       $ cd /path/to/3rdparty
       $ apxs -c mod_foo.c
       $ apxs -i -a -n foo
  <H3>Advantages & Disadvantages</H3>
  <P>The above DSO based features of Apache 1.3 have the following advantages:
  <LI> 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.
  <LI> 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.
  <LI> 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.
  <P>DSO has the following disadvantages:
  <LI> The DSO mechanism cannot be used on every platform because not all
       operating systems support dynamic loading.
  <LI> The server is approximately 20% slower at startup time because of the
       symbol resolving overhead the Unix loader now has to do.
  <LI> 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.
  <LI> 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 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().
  <LI> 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
  <!--#include virtual="footer.html" -->

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