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| <h1 align="center">Dynamic Shared Object (DSO) Support</h1> |
| |
| <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. The modules can be |
| statically compiled into the <code>httpd</code> binary when the |
| server is built. Alternatively, modules can be compiled as |
| Dynamic Shared Objects (DSOs) that exist separately from the |
| main <code>httpd</code> 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 (<a href="programs/apxs.html">apxs</a>).</p> |
| |
| <p>This document describes how to use DSO modules as well as |
| the theory behind their use.</p> |
| |
| <ul> |
| <li><a href="#implementation">Implementation</a></li> |
| |
| <li><a href="#usage">Usage Summary</a></li> |
| |
| <li><a href="#background">Background</a></li> |
| |
| <li><a href="#advantages">Advantages and |
| Disadvantages</a></li> |
| </ul> |
| <hr /> |
| |
| <table border="1"> |
| <tr> |
| <td valign="top"><strong>Related Modules</strong><br /> |
| <br /> |
| <a href="mod/mod_so.html">mod_so</a><br /> |
| </td> |
| |
| <td valign="top"><strong>Related Directives</strong><br /> |
| <br /> |
| <a href="mod/mod_so.html#loadmodule">LoadModule</a><br /> |
| </td> |
| </tr> |
| </table> |
| |
| <h2><a id="implementation" |
| name="implementation">Implementation</a></h2> |
| |
| <p>The DSO support for loading individual Apache modules is |
| based on a module named <a |
| href="mod/mod_so.html"><code>mod_so.c</code></a> which must be |
| statically compiled into the Apache core. It is the only module |
| besides <code>core.c</code> 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 <code>configure</code>'s |
| <code>--enable-<i>module</i>=shared</code> option as disucussed |
| in the <a href="install.html">install documentation</a>. After |
| a module is compiled into a DSO named <code>mod_foo.so</code> |
| you can use <a href="mod/mod_so.html"><code>mod_so</code></a>'s |
| <a |
| href="mod/mod_so.html#loadmodule"><code>LoadModule</code></a> |
| command in your <code>httpd.conf</code> file to load this |
| module at server startup or restart.</p> |
| |
| <p>To simplify this creation of DSO files for Apache modules |
| (especially for third-party modules) a new support program |
| named <a href="programs/apxs.html">apxs</a> (<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-dependent 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-dependent compiler and linker flags for DSO |
| support.</p> |
| |
| <h2><a id="usage" name="usage">Usage Summary</a></h2> |
| |
| <p>To give you an overview of the DSO features of Apache 2.0, |
| here is a short and concise summary:</p> |
| |
| <ol> |
| <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>: |
| |
| <table bgcolor="#f0f0f0" cellpadding="10"> |
| <tr> |
| <td> |
| <pre> |
| $ ./configure --prefix=/path/to/install |
| --enable-foo=shared |
| $ make install |
| </pre> |
| </td> |
| </tr> |
| </table> |
| </li> |
| |
| <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>: |
| |
| <table bgcolor="#f0f0f0" cellpadding="10"> |
| <tr> |
| <td> |
| <pre> |
| $ ./configure --add-module=module_type:/path/to/3rdparty/mod_foo.c |
| --enable-foo=shared |
| $ make install |
| </pre> |
| </td> |
| </tr> |
| </table> |
| </li> |
| |
| <li> |
| Configure Apache for <em>later installation</em> of shared |
| modules: |
| |
| <table bgcolor="#f0f0f0" cellpadding="10"> |
| <tr> |
| <td> |
| <pre> |
| $ ./configure --enable-so |
| $ make install |
| </pre> |
| </td> |
| </tr> |
| </table> |
| </li> |
| |
| <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 using <a href="programs/apxs.html">apxs</a>: |
| |
| <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> |
| </li> |
| </ol> |
| |
| <p>In all cases, once the shared module is compiled, you must |
| use a <a |
| href="mod/mod_so.html#loadmodule"><code>LoadModule</code></a> |
| directive in <code>httpd.conf</code> to tell Apache to activate |
| the module.</p> |
| |
| <h2><a id="background" name="background">Background</a></h2> |
| |
| <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> |
| |
| <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. On the other hand using shared objects for extending |
| a program is not used by a lot of programs.</p> |
| |
| <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, <em>etc.</em> 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> |
| |
| <h2><a id="advantages" name="advantages">Advantages and |
| Disadvantages</a></h2> |
| |
| <p>The above DSO based features have the following |
| advantages:</p> |
| |
| <ul> |
| <li>The server package is more flexible at run-time because |
| the actual server process can be assembled at run-time via <a |
| href="mod/mod_so.html#loadmodule"><code>LoadModule</code></a> |
| <code>httpd.conf</code> configuration commands instead of |
| <code>configure</code> 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], <em>etc.</em>) with only one Apache |
| installation.</li> |
| |
| <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, |
| <em>etc.</em></li> |
| |
| <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.</li> |
| </ul> |
| |
| <p>DSO has the following disadvantages:</p> |
| |
| <ul> |
| <li>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.</li> |
| |
| <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 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 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 <code>dlopen()</code>.</li> |
| </ul> |
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