brian 98/05/11 21:00:47
Added: htdocs/manual sharedobjects.html
Log:
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
===================================================================
<!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>
Added: htdocs/manual sharedobjects.html
Log:
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
===================================================================
<!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>
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