PR:

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. git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@81249 13f79535-47bb-0310-9956-ffa450edef68
author: brian <brian@unknown> 1998-05-12 04:00:47 +0000
committer: brian <brian@unknown> 1998-05-12 04:00:47 +0000
commit: 89c64e6f484c824851944c39e30ef37cc76d7c68 (patch)
tree: 723fe69f92993548dc06ba420ba5668c4b2c2374 /docs/manual/dso.html
parent: 1087ddf63e9cd4e54899e78ee872d3c6f5ae581f (diff)
download: httpd-89c64e6f484c824851944c39e30ef37cc76d7c68.tar.gz
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
+<HTML><HEAD>
+<TITLE>Apache 1.3 Dynamic Shared Object (DSO) support</TITLE>
+</HEAD>
+
+<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
+<BODY
+ BGCOLOR="#FFFFFF"
+ TEXT="#000000"
+ LINK="#0000FF"
+ VLINK="#000080"
+ ALINK="#FF0000"
+>
+<!--#include virtual="header.html" -->
+
+<H1>Apache 1.3 Dynamic Shared Object (DSO) support</H1>
+
+<P><HR><P>
+
+<address>Originally written by Ralf S. Engelschall, April 1998</address>
+
+<H3>Background</H3>
+
+<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
+ program.
+
+<P>This loading can usually be done in two ways: Automatically by a
+ system program called <CODE>ld.so</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>libfoo.so</CODE> or
+ <CODE>libfoo.so.1.2</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>libfoo.so</CODE> in <CODE>/usr/lib</CODE> or in paths
+ 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>ld.so</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>libc.so</CODE>, saving disk space for every
+ program.
+
+<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>foo.so</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>libc.so</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.
+
+<H3>Implementation</H3>
+
+<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 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.
+
+<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 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.
+
+<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:
+
+<PRE>
+ 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.
+</PRE>
+
+
+<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:
+
+<OL>
+
+<LI>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):
+
+<PRE>
+   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/libhttpd.so* /path/to/install/libexec/
+     $ cp src/libhttpd.ep  /path/to/install/libexec/
+     $ cp src/httpd        /path/to/install/bin/
+</PRE>    
+
+<LI>Build and install a distributed Apache module, say mod_foo.c, 
+    into its own DSO mod_foo.so:
+
+<PRE>
+   o Build and install via configure (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"
+</PRE>
+
+<LI>Build and install a third-party Apache module, say mod_foo.c, 
+    into its own DSO mod_foo.so
+
+<PRE>
+   o Build and install via configure (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"
+</PRE>
+
+<LI>Build and install a third-party Apache module, say mod_foo.c, 
+    into its own DSO mod_foo.so _outside of_ the Apache source tree:
+
+<PRE>
+   o Build and install via APXS: 
+     $ cd /path/to/3rdparty
+     $ apxs -c mod_foo.c
+     $ apxs -i -a -n foo mod_foo.so
+</PRE>
+
+</UL>
+
+<H3>Advantages & Disadvantages</H3>
+
+<P>The above DSO based features of Apache 1.3 have the following advantages:
+
+<UL>
+<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.
+</UL>
+
+<P>DSO has the following disadvantages:
+
+<UL>
+<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.
+</UL>
+
+<PRE>
+                     Ralf S. Engelschall
+                     rse@engelschall.com
+                     www.engelschall.com
+</PRE>
+
+
+<!--#include virtual="footer.html" -->
+</BODY>
+</HTML>
+
+
author	brian <brian@unknown>	1998-05-12 04:00:47 +0000
committer	brian <brian@unknown>	1998-05-12 04:00:47 +0000
commit	89c64e6f484c824851944c39e30ef37cc76d7c68 (patch)
tree	723fe69f92993548dc06ba420ba5668c4b2c2374 /docs/manual/dso.html
parent	1087ddf63e9cd4e54899e78ee872d3c6f5ae581f (diff)
download	httpd-89c64e6f484c824851944c39e30ef37cc76d7c68.tar.gz