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| <title>Apache 2.0 Hook Functions - Apache HTTP Server</title> |
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| <p class="apache">Apache HTTP Server Version 2.1</p> |
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| <a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="../">Version 2.1</a> > <a href="./">Developer Documentation</a></div><div id="page-content"><div id="preamble"><h1>Apache 2.0 Hook Functions</h1> |
| <div class="toplang"> |
| <p><span>Available Languages: </span><a href="../en/developer/hooks.html" title="English"> en </a></p> |
| </div> |
| |
| <div class="warning"><h3>Warning</h3> |
| <p>This document is still in development and may be partially out of |
| date.</p> |
| </div> |
| |
| <p>In general, a hook function is one that Apache will call at |
| some point during the processing of a request. Modules can |
| provide functions that are called, and specify when they get |
| called in comparison to other modules.</p> |
| </div> |
| <div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#create">Creating a hook function</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#hooking">Hooking the hook</a></li> |
| </ul></div> |
| <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="create" id="create">Creating a hook function</a></h2> |
| <p>In order to create a new hook, four things need to be |
| done:</p> |
| |
| <h3><a name="create-declare" id="create-declare">Declare the hook function</a></h3> |
| <p>Use the <code>AP_DECLARE_HOOK</code> macro, which needs to be given |
| the return type of the hook function, the name of the hook, and the |
| arguments. For example, if the hook returns an <code>int</code> and |
| takes a <code>request_rec *</code> and an <code>int</code> and is |
| called <code>do_something</code>, then declare it like this:</p> |
| <div class="example"><p><code> |
| AP_DECLARE_HOOK(int, do_something, (request_rec *r, int n)) |
| </code></p></div> |
| |
| <p>This should go in a header which modules will include if |
| they want to use the hook.</p> |
| |
| |
| <h3><a name="create-create" id="create-create">Create the hook structure</a></h3> |
| <p>Each source file that exports a hook has a private structure |
| which is used to record the module functions that use the hook. |
| This is declared as follows:</p> |
| |
| <div class="example"><p><code> |
| APR_HOOK_STRUCT(<br /> |
| <span class="indent"> |
| APR_HOOK_LINK(do_something)<br /> |
| ...<br /> |
| </span> |
| ) |
| </code></p></div> |
| |
| |
| <h3><a name="create-implement" id="create-implement">Implement the hook caller</a></h3> |
| <p>The source file that exports the hook has to implement a |
| function that will call the hook. There are currently three |
| possible ways to do this. In all cases, the calling function is |
| called <code>ap_run_<var>hookname</var>()</code>.</p> |
| |
| <h4>Void hooks</h4> |
| <p>If the return value of a hook is <code>void</code>, then all the |
| hooks are called, and the caller is implemented like this:</p> |
| |
| <div class="example"><p><code> |
| AP_IMPLEMENT_HOOK_VOID(do_something, (request_rec *r, int n), (r, n)) |
| </code></p></div> |
| |
| <p>The second and third arguments are the dummy argument |
| declaration and the dummy arguments as they will be used when |
| calling the hook. In other words, this macro expands to |
| something like this:</p> |
| |
| <div class="example"><p><code> |
| void ap_run_do_something(request_rec *r, int n)<br /> |
| {<br /> |
| <span class="indent"> |
| ...<br /> |
| do_something(r, n);<br /> |
| </span> |
| } |
| </code></p></div> |
| |
| |
| <h4>Hooks that return a value</h4> |
| <p>If the hook returns a value, then it can either be run until |
| the first hook that does something interesting, like so:</p> |
| |
| <div class="example"><p><code> |
| AP_IMPLEMENT_HOOK_RUN_FIRST(int, do_something, (request_rec *r, int n), (r, n), DECLINED) |
| </code></p></div> |
| |
| <p>The first hook that does <em>not</em> return <code>DECLINED</code> |
| stops the loop and its return value is returned from the hook |
| caller. Note that <code>DECLINED</code> is the tradition Apache |
| hook return meaning "I didn't do anything", but it can be |
| whatever suits you.</p> |
| |
| <p>Alternatively, all hooks can be run until an error occurs. |
| This boils down to permitting <em>two</em> return values, one of |
| which means "I did something, and it was OK" and the other |
| meaning "I did nothing". The first function that returns a |
| value other than one of those two stops the loop, and its |
| return is the return value. Declare these like so:</p> |
| |
| <div class="example"><p><code> |
| AP_IMPLEMENT_HOOK_RUN_ALL(int, do_something, (request_rec *r, int n), (r, n), OK, DECLINED) |
| </code></p></div> |
| |
| <p>Again, <code>OK</code> and <code>DECLINED</code> are the traditional |
| values. You can use what you want.</p> |
| |
| |
| |
| <h3><a name="create-call" id="create-call">Call the hook callers</a></h3> |
| <p>At appropriate moments in the code, call the hook caller, |
| like so:</p> |
| |
| <div class="example"><p><code> |
| int n, ret;<br /> |
| request_rec *r;<br /> |
| <br /> |
| ret=ap_run_do_something(r, n); |
| </code></p></div> |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="hooking" id="hooking">Hooking the hook</a></h2> |
| <p>A module that wants a hook to be called needs to do two |
| things.</p> |
| |
| <h3><a name="hooking-implement" id="hooking-implement">Implement the hook function</a></h3> |
| <p>Include the appropriate header, and define a static function |
| of the correct type:</p> |
| |
| <div class="example"><p><code> |
| static int my_something_doer(request_rec *r, int n)<br /> |
| {<br /> |
| <span class="indent"> |
| ...<br /> |
| return OK;<br /> |
| </span> |
| } |
| </code></p></div> |
| |
| |
| <h3><a name="hooking-add" id="hooking-add">Add a hook registering function</a></h3> |
| <p>During initialisation, Apache will call each modules hook |
| registering function, which is included in the module |
| structure:</p> |
| |
| <div class="example"><p><code> |
| static void my_register_hooks()<br /> |
| {<br /> |
| <span class="indent"> |
| ap_hook_do_something(my_something_doer, NULL, NULL, APR_HOOK_MIDDLE);<br /> |
| </span> |
| }<br /> |
| <br /> |
| mode MODULE_VAR_EXPORT my_module =<br /> |
| {<br /> |
| <span class="indent"> |
| ...<br /> |
| my_register_hooks /* register hooks */<br /> |
| </span> |
| }; |
| </code></p></div> |
| |
| |
| <h3><a name="hooking-order" id="hooking-order">Controlling hook calling order</a></h3> |
| <p>In the example above, we didn't use the three arguments in |
| the hook registration function that control calling order. |
| There are two mechanisms for doing this. The first, rather |
| crude, method, allows us to specify roughly where the hook is |
| run relative to other modules. The final argument control this. |
| There are three possible values: <code>APR_HOOK_FIRST</code>, |
| <code>APR_HOOK_MIDDLE</code> and <code>APR_HOOK_LAST</code>.</p> |
| |
| <p>All modules using any particular value may be run in any |
| order relative to each other, but, of course, all modules using |
| <code>APR_HOOK_FIRST</code> will be run before <code>APR_HOOK_MIDDLE</code> |
| which are before <code>APR_HOOK_LAST</code>. Modules that don't care |
| when they are run should use <code>APR_HOOK_MIDDLE</code>. <em>(I spaced |
| these out so people could do stuff like <code>APR_HOOK_FIRST-2</code> |
| to get in slightly earlier, but is this wise? - Ben)</em></p> |
| |
| <p>Note that there are two more values, |
| <code>APR_HOOK_REALLY_FIRST</code> and <code>APR_HOOK_REALLY_LAST</code>. These |
| should only be used by the hook exporter.</p> |
| |
| <p>The other method allows finer control. When a module knows |
| that it must be run before (or after) some other modules, it |
| can specify them by name. The second (third) argument is a |
| NULL-terminated array of strings consisting of the names of |
| modules that must be run before (after) the current module. For |
| example, suppose we want "mod_xyz.c" and "mod_abc.c" to run |
| before we do, then we'd hook as follows:</p> |
| |
| <div class="example"><p><code> |
| static void register_hooks()<br /> |
| {<br /> |
| <span class="indent"> |
| static const char * const aszPre[] = { "mod_xyz.c", "mod_abc.c", NULL };<br /> |
| <br /> |
| ap_hook_do_something(my_something_doer, aszPre, NULL, APR_HOOK_MIDDLE);<br /> |
| </span> |
| } |
| </code></p></div> |
| |
| <p>Note that the sort used to achieve this is stable, so |
| ordering set by <code>APR_HOOK_<var>ORDER</var></code> is preserved, as far |
| as is possible.</p> |
| |
| <p class="cite"><cite>Ben Laurie</cite>, 15th August 1999</p> |
| |
| </div></div> |
| <div class="bottomlang"> |
| <p><span>Available Languages: </span><a href="../en/developer/hooks.html" title="English"> en </a></p> |
| </div><div id="footer"> |
| <p class="apache">Copyright 1999-2004 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p> |
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