Dynamic Shared Object (DSO) support

+ + Dynamic Shared Object (DSO) Support + +

The Apache HTTP Server is a modular program where the + administrator can choose the functionality to include in the + server by selecting a set of modules. The modules can be + statically compiled into the httpd binary when the + server is built. Alternatively, modules can be compiled as + Dynamic Shared Objects (DSOs) that exist separately from the + main httpd binary file. DSO modules may be + compiled at the time the server is built, or they may be + compiled and added at a later time using the Apache Extension + Tool (apxs).

+ +

This document describes how to use DSO modules as well as + the theory behind their use.

+ + +

Implementation + + + +mod_so + + +LoadModule + + + +

The DSO support for loading individual Apache modules is based + on a module named mod_so which must be statically + compiled into the Apache core. It is the only module besides + core which cannot be put into a DSO + itself. Practically all other distributed Apache modules can then + be placed into a DSO by individually enabling the DSO build for + them via configure's + --enable-module=shared option as disucussed + in the install documentation. 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 modules) a new support program + named apxs (APache + eXtenSion) 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-dependent 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-dependent compiler and linker flags for DSO + support.

+ +

Usage Summary + +

To give you an overview of the DSO features of Apache 2.0, + here is a short and concise summary:

+ +

+ Build and install a distributed Apache module, say + mod_foo.c, into its own DSO + mod_foo.so: + + +$ ./configure --prefix=/path/to/install --enable-foo=shared
+$ make install + +
+ Build and install a third-party Apache module, say + mod_foo.c, into its own DSO + mod_foo.so: + + +$ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c --enable-foo=shared
+$ make install + +
+ Configure Apache for later installation of shared + modules: + + +$ ./configure --enable-so
+$ make install + +
+ 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 using apxs: + + +$ cd /path/to/3rdparty
+$ apxs -c mod_foo.c
+$ apxs -i -a -n foo mod_foo.so + +

+ +

In all cases, once the shared module is compiled, you must + use a LoadModule + directive in httpd.conf to tell Apache to activate + the module.

+ +

Background + +

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 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 + reside in a system directory (usually /usr/lib) + and the link to the executable program is established at + build-time by specifying -lfoo to the linker + command. This hard-codes library references into the executable + program file so that at start-time the Unix loader is able to + locate libfoo.so in /usr/lib, in + paths hard-coded via linker-options like -R or in + paths configured via the environment variable + LD_LIBRARY_PATH. It then 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 reusable 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 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 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.

+ +

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 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.

+ +

Advantages and Disadvantages + +

The above DSO based features have the following + advantages:

+ +

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 + configure options 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 pair you can both work outside the + Apache source tree and only need an apxs -i + command followed by an apachectl restart to + bring a new version of your currently developed module into + the running Apache server.

+ +

DSO has the following disadvantages:

+ +

The DSO mechanism cannot be used on every platform + because not all operating systems support dynamic loading of + code into the address space of a program.
The server is approximately 20% slower at startup time + because of the symbol resolving overhead the Unix loader now + has to do.
The server is approximately 5% slower at execution time + under some platforms because position independent code (PIC) + sometimes needs complicated assembler tricks for relative + addressing which are not necessarily as fast as absolute + addressing.
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 independent code. + The only chances 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().

+ +

Dynamic Shared Object (DSO) Support

Apache HTTP Server Version 2.0

Dynamic Shared Object (DSO) Support

Implementation

Implementation

Usage Summary

Usage Summary

Background

Background

Advantages and - Disadvantages

Advantages and Disadvantages

Apache HTTP Server Version 2.0