From 6beeb1b708550be0d4a53b272283e17e5e35fe17 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sun, 7 Apr 2024 17:01:30 +0200 Subject: Adding upstream version 2.4.57. Signed-off-by: Daniel Baumann --- docs/manual/dso.html.en | 332 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 332 insertions(+) create mode 100644 docs/manual/dso.html.en (limited to 'docs/manual/dso.html.en') diff --git a/docs/manual/dso.html.en b/docs/manual/dso.html.en new file mode 100644 index 0000000..85ffc30 --- /dev/null +++ b/docs/manual/dso.html.en @@ -0,0 +1,332 @@ + + + + + +Dynamic Shared Object (DSO) Support - Apache HTTP Server Version 2.4 + + + + + + +
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Modules | Directives | FAQ | Glossary | Sitemap

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Apache HTTP Server Version 2.4

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Dynamic Shared Object (DSO) Support

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Available Languages:  en  | + fr  | + ja  | + ko  | + tr 

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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. + Modules will 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).

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Alternatively, the modules can be statically compiled into + the httpd binary when the server is built.

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

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Implementation

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Related ModulesRelated Directives
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The DSO support for loading individual Apache httpd modules is based + on a module named mod_so which must be statically + compiled into the Apache httpd core. It is the only module besides + core which cannot be put into a DSO + itself. Practically all other distributed Apache httpd modules will then + be placed into a DSO. After a module is compiled into a DSO named + mod_foo.so you can use mod_so's LoadModule directive in your + httpd.conf file to load this module at server startup + or restart.

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The DSO builds for individual modules can be disabled via + configure's --enable-mods-static + option as discussed in the install + documentation.

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To simplify this creation of DSO files for Apache httpd modules + (especially for third-party modules) a support program + named apxs (APache + eXtenSion) is available. It can be used to build DSO based + modules outside of the Apache httpd source tree. The idea is + simple: When installing Apache HTTP Server the configure's + make install procedure installs the Apache httpd 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 httpd module sources without the Apache httpd distribution + source tree and without having to fiddle with the + platform-dependent compiler and linker flags for DSO + support.

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Usage Summary

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To give you an overview of the DSO features of Apache HTTP Server 2.x, + here is a short and concise summary:

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  1. +

    Build and install a distributed Apache httpd module, say + mod_foo.c, into its own DSO + mod_foo.so:

    + +

    +$ ./configure --prefix=/path/to/install --enable-foo
    +$ make install +

    +
    2. + +
  2. +

    Configure Apache HTTP Server with all modules enabled. Only a basic + set will be loaded during server startup. You can change the set of loaded + modules by activating or deactivating the LoadModule directives in + httpd.conf.

    + +

    +$ ./configure --enable-mods-shared=all
    +$ make install +

    +
    3. + +
  3. +

    Some modules are only useful for developers and will not be build. + when using the module set all. To build all available modules + including developer modules use reallyall. In addition the + LoadModule directives for all + built modules can be activated via the configure option + --enable-load-all-modules.

    + +

    +$ ./configure --enable-mods-shared=reallyall --enable-load-all-modules
    +$ make install +

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    4. + +
  4. + Build and install a third-party Apache httpd module, say + mod_foo.c, into its own DSO + mod_foo.so outside of the Apache httpd + source tree using apxs: + +

    +$ cd /path/to/3rdparty
    +$ apxs -cia mod_foo.c +

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    5. +
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In all cases, once the shared module is compiled, you must + use a LoadModule + directive in httpd.conf to tell Apache httpd to activate + the module.

+ +

See the apxs documentation for more details.

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Background

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On modern Unix derivatives there exists a mechanism + 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.

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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().

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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.

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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.

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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.

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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.

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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.

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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.

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Advantages and Disadvantages

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The above DSO based features have the following + advantages:

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  • The server package is more flexible at run-time because + the server process can be assembled at run-time via + LoadModule + httpd.conf configuration directives instead of + configure options at build-time. For instance, + this way one is able to run different server instances + (standard & SSL version, minimalistic & dynamic + version [mod_perl, mod_php], etc.) with only one Apache httpd + installation.
    • + +
  • The server package can be easily extended with + third-party modules even after installation. This is + a great benefit for vendor package maintainers, who can create + an Apache httpd core package and additional packages containing + extensions like PHP, mod_perl, mod_security, etc.
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  • Easier Apache httpd module prototyping, because with the + DSO/apxs pair you can both work outside the + Apache httpd 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 HTTP Server.
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DSO has the following disadvantages:

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  • The server is approximately 20% slower at startup time + because of the symbol resolving overhead the Unix loader now + has to do.
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  • 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.
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  • 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 httpd core, from the C library + (libc) and all other dynamic or static libraries + used by the Apache httpd 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 + httpd core itself already contains a reference to it or + loading the code yourself via dlopen().
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Available Languages:  en  | + fr  | + ja  | + ko  | + tr 

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