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| <manualpage metafile="dso.xml.meta"> |
| |
| <title>Dynamic Shared Object (DSO) Support</title> |
| |
| <summary> |
| <p>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 <program>httpd</program> |
| 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 (<program>apxs</program>).</p> |
| <p>Alternatively, the modules can be statically compiled into |
| the <program>httpd</program> binary when the server is built.</p> |
| |
| <p>This document describes how to use DSO modules as well as |
| the theory behind their use.</p> |
| </summary> |
| |
| |
| <section id="implementation"><title>Implementation</title> |
| |
| <related> |
| <modulelist> |
| <module>mod_so</module> |
| </modulelist> |
| <directivelist> |
| <directive module="mod_so">LoadModule</directive> |
| </directivelist> |
| </related> |
| |
| <p>The DSO support for loading individual Apache httpd modules is based |
| on a module named <module>mod_so</module> which must be statically |
| compiled into the Apache httpd core. It is the only module besides |
| <module>core</module> 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 |
| <code>mod_foo.so</code> you can use <module>mod_so</module>'s <directive |
| module="mod_so">LoadModule</directive> directive in your |
| <code>httpd.conf</code> file to load this module at server startup |
| or restart.</p> |
| <p>The DSO builds for individual modules can be disabled via |
| <program>configure</program>'s <code>--enable-mods-static</code> |
| option as discussed in the <a href="install.html">install |
| documentation</a>.</p> |
| |
| <p>To simplify this creation of DSO files for Apache httpd modules |
| (especially for third-party modules) a support program |
| named <program>apxs</program> (<dfn>APache |
| eXtenSion</dfn>) is available. It can be used to build DSO based |
| modules <em>outside of</em> the Apache httpd source tree. The idea is |
| simple: When installing Apache HTTP Server the <program>configure</program>'s |
| <code>make install</code> procedure installs the Apache httpd C |
| header files and puts the platform-dependent compiler and |
| linker flags for building DSO files into the <program>apxs</program> |
| program. This way the user can use <program>apxs</program> 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.</p> |
| </section> |
| |
| <section id="usage"><title>Usage Summary</title> |
| |
| <p>To give you an overview of the DSO features of Apache HTTP Server 2.x, |
| here is a short and concise summary:</p> |
| |
| <ol> |
| <li> |
| <p>Build and install a <em>distributed</em> Apache httpd module, say |
| <code>mod_foo.c</code>, into its own DSO |
| <code>mod_foo.so</code>:</p> |
| |
| <example> |
| $ ./configure --prefix=/path/to/install --enable-foo<br /> |
| $ make install |
| </example> |
| </li> |
| |
| <li> |
| <p>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 <directive |
| module="mod_so">LoadModule</directive> directives in |
| <code>httpd.conf</code>.</p> |
| |
| <example> |
| $ ./configure --enable-mods-shared=all<br /> |
| $ make install |
| </example> |
| </li> |
| |
| <li> |
| <p>Some modules are only useful for developers and will not be build. |
| when using the module set <em>all</em>. To build all available modules |
| including developer modules use <em>reallyall</em>. In addition the |
| <directive module="mod_so">LoadModule</directive> directives for all |
| built modules can be activated via the configure option |
| <code>--enable-load-all-modules</code>.</p> |
| |
| <example> |
| $ ./configure --enable-mods-shared=reallyall --enable-load-all-modules<br /> |
| $ make install |
| </example> |
| </li> |
| |
| <li> |
| Build and install a <em>third-party</em> Apache httpd module, say |
| <code>mod_foo.c</code>, into its own DSO |
| <code>mod_foo.so</code> <em>outside of</em> the Apache httpd |
| source tree using <program>apxs</program>: |
| |
| <example> |
| $ cd /path/to/3rdparty<br /> |
| $ apxs -cia mod_foo.c |
| </example> |
| </li> |
| </ol> |
| |
| <p>In all cases, once the shared module is compiled, you must |
| use a <directive module="mod_so">LoadModule</directive> |
| directive in <code>httpd.conf</code> to tell Apache httpd to activate |
| the module.</p> |
| |
| <p>See the <a href="programs/apxs.html">apxs documentation</a> for more details.</p> |
| </section> |
| |
| <section id="background"><title>Background</title> |
| |
| <p>On modern Unix derivatives there exists a mechanism |
| 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> |
| |
| <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> |
| |
| <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 hard-codes 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> |
| |
| <p>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 |
| <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> |
| |
| <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> |
| |
| <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 |
| <em>etc.</em> 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> |
| |
| <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.</p> |
| |
| <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.</p> |
| |
| </section> |
| |
| <section id="advantages"><title>Advantages and Disadvantages</title> |
| |
| <p>The above DSO based features have the following |
| advantages:</p> |
| |
| <ul> |
| <li>The server package is more flexible at run-time because |
| the server process can be assembled at run-time via |
| <directive module="mod_so">LoadModule</directive> |
| <code>httpd.conf</code> configuration directives instead of |
| <program>configure</program> 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], <em>etc.</em>) with only one Apache httpd |
| installation.</li> |
| |
| <li>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, <em>etc.</em></li> |
| |
| <li>Easier Apache httpd module prototyping, because with the |
| DSO/<program>apxs</program> pair you can both work outside the |
| Apache httpd 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 HTTP Server.</li> |
| </ul> |
| |
| <p>DSO has the following disadvantages:</p> |
| |
| <ul> |
| <li>The server is approximately 20% slower at startup time |
| because of the symbol resolving overhead the Unix loader now |
| has to do.</li> |
| |
| <li>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.</li> |
| |
| <li>Because DSO modules cannot be linked against other |
| DSO-based libraries (<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 httpd core, from the C library |
| (<code>libc</code>) and all other dynamic or static libraries |
| used by the Apache httpd core, or from static library archives |
| (<code>libfoo.a</code>) 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 <code>dlopen()</code>.</li> |
| </ul> |
| |
| </section> |
| |
| </manualpage> |