Mailing List Archive: cvs commit: apache-1.3/htdocs/manual sharedobjects.html

rse 98/05/12 04:12:48

Modified: htdocs/manual sharedobjects.html
Log:
A lot of cosmetic stuff: more <CODE>, less <PRE>, etc. pp.
Thanks to Brian for the initial ASCII->HTML translation.

Revision Changes Path
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Index: sharedobjects.html
===================================================================
RCS file: /export/home/cvs/apache-1.3/htdocs/manual/sharedobjects.html,v
retrieving revision 1.1
retrieving revision 1.2
diff -u -r1.1 -r1.2
--- sharedobjects.html 1998/05/12 04:00:47 1.1
+++ sharedobjects.html 1998/05/12 11:12:47 1.2
@@ -11,327 +11,372 @@
VLINK="#000080"
ALINK="#FF0000"
>
+<BLOCKQUOTE>


-<H1>Apache 1.3 Dynamic Shared Object (DSO) support</H1>
+<DIV ALIGN=CENTER>

-<P><HR><P>
+<H1>
+Apache 1.3<BR>
+Dynamic Shared Object (DSO)<BR>
+Support
+</H1>

-<address>Originally written by Ralf S. Engelschall, April 1998</address>
+<ADDRESS>Originally written by<BR>
+Ralf S. Engelschall <rse@apache.org&gt, April 1998</ADDRESS>

+</DIV>
+
<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>On modern Unix derivatives there exists a nifty mechanism usually called
+dynamic linking/loading of <EM>Dynamic Shared Objects</EM> (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>In the first way the DSO's are usually called <EM>shared libraries</EM> or
+<EM>DSO libraries</EM> 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 build-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>, in paths hard-coded via linker-options like
+<CODE>-R</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>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 <EM>shared objects</EM> or
+<EM>DSO files</EM> 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>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.
+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.
+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), 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>The DSO support for loading individual Apache modules is based on a module
+named <CODE>mod_so.c</CODE> which has to be statically compiled into the
+Apache core. It is the only module besides <CODE>http_core.c</CODE> 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 <CODE>configure</CODE>'s
+<CODE>--enable-shared</CODE> option (see top-level <CODE>INSTALL</CODE> file)
+or by changing the <CODE>AddModule</CODE> command in your
+<CODE>src/Configuration</CODE> into a <CODE>SharedModule</CODE> command (see
+<CODE>src/INSTALL</CODE> file). After a module is compiled into a DSO named
+<CODE>mod_foo.so</CODE> you can use <CODE>mod_so</CODE>'s
+<CODE>LoadModule</CODE> command in your <CODE>httpd.conf</CODE> 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.
+third-party modules) a new support program named <CODE>apxs</CODE> (<EM>APache
+eXtenSion</EM>) is available. It can be used to build DSO based modules
+<EM>outside of</EM> the Apache source tree. The idea is simple: When
+installing Apache the <CODE>configure</CODE>'s <CODE>make install</CODE>
+procedure installs the Apache C header files and puts the platform-dependend
+compiler and linker flags for building DSO files into the <CODE>apxs</CODE>
+program. This way the user can use <CODE>apxs</CODE> 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.
+on some of the supported platforms to force the linker to export the apache
+core symbols -- a prerequisite for the DSO modularization) the rule
+<CODE>SHARED_CORE</CODE> has to be enabled via <CODE>configure</CODE>'s
+<CODE>--enable-rule=SHARED_CORE</CODE> option (see top-level
+<CODE>INSTALL</CODE> file) or by changing the <CODE>Rule</CODE> command in
+your <CODE>Configuration</CODE> file to <CODE>Rule SHARED_CORE=yes</CODE> (see
+<CODE>src/INSTALL</CODE> file). The Apache core code is then placed into a DSO
+library named <CODE>libhttpd.so</CODE>. Because one cannot link a DSO against
+static libraries on all platforms, an additional executable program named
+<CODE>libhttpd.ep</CODE> is created which both binds this static code and
+provides a stub for the <CODE>main()</CODE> function. Finally the
+<CODE>httpd</CODE> executable program itself is replaced by a bootstrapping
+code which automatically makes sure the Unix loader is able to load and start
+<CODE>libhttpd.ep</CODE> by providing the <CODE>LD_LIBRARY_PATH</CODE> to
+<CODE>libhttpd.so</CODE>.

<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:
+<P>Apache's <CODE>src/Configure</CODE> script currently has only limited but
+adequate 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)
+<P>
+<UL>
+<LI>Out-of-the-box supported platforms:<BR>
+(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 (-)
+<PRE>
+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 (-)
+</PRE>

- Explicitly unsupported platforms:
+<P>
+<LI> Explicitly unsupported platforms:

- o Ultrix: There is no dlopen-style interface under this platform.
+<PRE>
+o Ultrix (no dlopen-style interface under this platform)
</PRE>

+</UL>

<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:
+<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):
+<LI>Placing the Apache core code (all the stuff which usually forms the
+<CODE>httpd</CODE> binary) into a DSO <CODE>libhttpd.so</CODE>, an executable
+program <CODE>libhttpd.ep</CODE> and a bootstrapping executable program
+<CODE>httpd</CODE> (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):

+<P>
+<UL>
+<LI>Build and install via <CODE>configure</CODE> (preferred):
+<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
<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/
+$ ./configure --prefix=/path/to/install
+ --enable-rule=SHARED_CORE ...
+$ make install
+</PRE>
+</TD></TR></TABLE>
+
+<LI>Build and install manually:
+<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
+<PRE>
+- Edit src/Configuration:
+ <&lt "Rule SHARED_CORE=default"
+ >> "Rule SHARED_CORE=yes"
+ <&lt "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>
+</TD></TR></TABLE>
+</UL>

-<LI>Build and install a distributed Apache module, say mod_foo.c,
- into its own DSO mod_foo.so:
+<LI>Build and install a <EM>distributed</EM> Apache module, say
+<CODE>mod_foo.c</CODE>, into its own DSO <CODE>mod_foo.so</CODE>:

+<P>
+<UL>
+<LI>Build and install via <CODE>configure</CODE> (preferred):
+<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
<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"
+$ ./configure --prefix=/path/to/install
+ --enable-shared=foo
+$ make install
</PRE>
+</TD></TR></TABLE>

-<LI>Build and install a third-party Apache module, say mod_foo.c,
- into its own DSO mod_foo.so
-
+<LI>Build and install manually:
+<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
<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"
+- Edit src/Configuration:
+ <&lt "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>
+</TD></TR></TABLE>
+</UL>

-<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:
+<LI>Build and install a <EM>third-party</EM> Apache module, say
+<CODE>mod_foo.c</CODE>, into its own DSO <CODE>mod_foo.so</CODE>

+<P>
+<UL>
+<LI>Build and install via <CODE>configure</CODE> (preferred):
+<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
<PRE>
- o Build and install via APXS:
- $ cd /path/to/3rdparty
- $ apxs -c mod_foo.c
- $ apxs -i -a -n foo mod_foo.so
+$ ./configure --add-module=/path/to/3rdparty/mod_foo.c
+ --enable-shared=foo
+$ make install
</PRE>
+</TD></TR></TABLE>

+<LI>Build and install manually:
+<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
+<PRE>
+$ 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>
+</TD></TR></TABLE>
</UL>

+<P>
+<LI>Build and install a <EM>third-party</EM> Apache module, say
+<CODE>mod_foo.c</CODE>, into its own DSO <CODE>mod_foo.so</CODE> <EM>outside
+of</EM> the Apache source tree:
+
+<P>
+<UL>
+<LI>Build and install via <CODE>apxs</CODE>:
+<TABLE BGCOLOR="#f0f0f0" CELLPADDING=10><TR><TD>
+<PRE>
+$ cd /path/to/3rdparty
+$ apxs -c mod_foo.c
+$ apxs -i -a -n foo mod_foo.so
+</PRE>
+</TD></TR></TABLE>
+</UL>
+
+</OL>
+
<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
+ process can be assembled at run-time via <CODE>LoadModule</CODE>
+ <CODE>httpd.conf</CODE> configuration commands instead of
+ <CODE>Configuration</CODE> <CODE>AddModule</CODE> 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.
-
+<P>
<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>
+<LI> Easier Apache module prototyping because with the DSO/<CODE>apxs</CODE>
+ pair you can both work outside the Apache source tree and only need an
+ <CODE>apxs -i</CODE> command followed by an <CODE>apachectl
+ restart</CODE> 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.
-
+ operating systems support dynamic loading of code into the address space
+ of a program.
+<P>
<LI> The server is approximately 20% slower at startup time because of the
symbol resolving overhead the Unix loader now has to do.
-
+<P>
<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.
-
+<P>
<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().
-
+ (<CODE>ld -lfoo</CODE>) 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 (<CODE>libc</CODE>)
+ and all other dynamic or static libraries used by the Apache core, or
+ from static library archives (<CODE>libfoo.a</CODE>) 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 <CODE>dlopen()</CODE>.
+<P>
<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.
+ chance here is to use the <CODE>SHARED_CORE</CODE> feature because this
+ way the global symbols are forced to be exported. As a consequence the
+ Apache <CODE>src/Configure</CODE> script automatically enforces
+ <CODE>SHARED_CORE</CODE> on these platforms when DSO features are used in
+ the <CODE>Configuration</CODE> file or on the configure command line.
</UL>

-<PRE>
- Ralf S. Engelschall
- rse@engelschall.com
- www.engelschall.com
-</PRE>
-
-

+</BLOCKQUOTE>
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