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0N/A<title>Apache 2.0 Hook Functions - Apache HTTP Server</title>
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0N/A<p class="apache">Apache HTTP Server Version 2.3</p>
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0N/A<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="../">Version 2.3</a> &gt; <a href="./">Developer Documentation</a></div><div id="page-content"><div id="preamble"><h1>Apache 2.0 Hook Functions</h1>
1237N/A<div class="toplang">
1190N/A<p><span>Available Languages: </span><a href="/en/developer/hooks.html" title="English">&nbsp;en&nbsp;</a></p>
1185N/A</div>
0N/A
0N/A <div class="warning"><h3>Warning</h3>
0N/A <p>This document is still in development and may be partially out of
388N/A date.</p>
1185N/A </div>
388N/A
388N/A <p>In general, a hook function is one that Apache will call at
388N/A some point during the processing of a request. Modules can
388N/A provide functions that are called, and specify when they get
388N/A called in comparison to other modules.</p>
388N/A</div>
388N/A<div id="quickview"><ul id="toc"><li><img alt="" src="/images/down.gif" /> <a href="#create">Creating a hook function</a></li>
508N/A<li><img alt="" src="/images/down.gif" /> <a href="#hooking">Hooking the hook</a></li>
1327N/A</ul></div>
1185N/A<div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div>
388N/A<div class="section">
388N/A<h2><a name="create" id="create">Creating a hook function</a></h2>
1185N/A <p>In order to create a new hook, four things need to be
1436N/A done:</p>
819N/A
350N/A <h3><a name="create-declare" id="create-declare">Declare the hook function</a></h3>
1185N/A <p>Use the <code>AP_DECLARE_HOOK</code> macro, which needs to be given
0N/A the return type of the hook function, the name of the hook, and the
1385N/A arguments. For example, if the hook returns an <code>int</code> and
615N/A takes a <code>request_rec *</code> and an <code>int</code> and is
1195N/A called <code>do_something</code>, then declare it like this:</p>
1185N/A <div class="example"><p><code>
1185N/A AP_DECLARE_HOOK(int, do_something, (request_rec *r, int n))
388N/A </code></p></div>
0N/A
1111N/A <p>This should go in a header which modules will include if
1185N/A they want to use the hook.</p>
1111N/A
456N/A
1327N/A <h3><a name="create-create" id="create-create">Create the hook structure</a></h3>
456N/A <p>Each source file that exports a hook has a private structure
456N/A which is used to record the module functions that use the hook.
456N/A This is declared as follows:</p>
1185N/A
1111N/A <div class="example"><p><code>
1185N/A APR_HOOK_STRUCT(<br />
1185N/A <span class="indent">
1185N/A APR_HOOK_LINK(do_something)<br />
1185N/A ...<br />
1185N/A </span>
1185N/A )
1185N/A </code></p></div>
1111N/A
1111N/A
1237N/A <h3><a name="create-implement" id="create-implement">Implement the hook caller</a></h3>
1318N/A <p>The source file that exports the hook has to implement a
1190N/A function that will call the hook. There are currently three
615N/A possible ways to do this. In all cases, the calling function is
1190N/A called <code>ap_run_<var>hookname</var>()</code>.</p>
1185N/A
1185N/A <h4>Void hooks</h4>
1185N/A <p>If the return value of a hook is <code>void</code>, then all the
819N/A hooks are called, and the caller is implemented like this:</p>
0N/A
1185N/A <div class="example"><p><code>
0N/A AP_IMPLEMENT_HOOK_VOID(do_something, (request_rec *r, int n), (r, n))
1185N/A </code></p></div>
0N/A
0N/A <p>The second and third arguments are the dummy argument
0N/A declaration and the dummy arguments as they will be used when
0N/A calling the hook. In other words, this macro expands to
0N/A something like this:</p>
1185N/A
1185N/A <div class="example"><p><code>
1190N/A void ap_run_do_something(request_rec *r, int n)<br />
1461N/A {<br />
1385N/A <span class="indent">
1185N/A ...<br />
1185N/A do_something(r, n);<br />
1185N/A </span>
1185N/A }
1385N/A </code></p></div>
1185N/A
1385N/A
1385N/A <h4>Hooks that return a value</h4>
1185N/A <p>If the hook returns a value, then it can either be run until
1385N/A the first hook that does something interesting, like so:</p>
1185N/A
1185N/A <div class="example"><p><code>
1185N/A AP_IMPLEMENT_HOOK_RUN_FIRST(int, do_something, (request_rec *r, int n), (r, n), DECLINED)
1185N/A </code></p></div>
1385N/A
1385N/A <p>The first hook that does <em>not</em> return <code>DECLINED</code>
1327N/A stops the loop and its return value is returned from the hook
1185N/A caller. Note that <code>DECLINED</code> is the tradition Apache
1195N/A hook return meaning "I didn't do anything", but it can be
1185N/A whatever suits you.</p>
1185N/A
1185N/A <p>Alternatively, all hooks can be run until an error occurs.
460N/A This boils down to permitting <em>two</em> return values, one of
1185N/A which means "I did something, and it was OK" and the other
1185N/A meaning "I did nothing". The first function that returns a
1185N/A value other than one of those two stops the loop, and its
1185N/A return is the return value. Declare these like so:</p>
1185N/A
1185N/A <div class="example"><p><code>
1185N/A AP_IMPLEMENT_HOOK_RUN_ALL(int, do_something, (request_rec *r, int n), (r, n), OK, DECLINED)
1185N/A </code></p></div>
1185N/A
1185N/A <p>Again, <code>OK</code> and <code>DECLINED</code> are the traditional
1185N/A values. You can use what you want.</p>
1190N/A
1185N/A
1185N/A
1190N/A <h3><a name="create-call" id="create-call">Call the hook callers</a></h3>
1185N/A <p>At appropriate moments in the code, call the hook caller,
1185N/A like so:</p>
1185N/A
1185N/A <div class="example"><p><code>
1190N/A int n, ret;<br />
1190N/A request_rec *r;<br />
1190N/A <br />
1185N/A ret=ap_run_do_something(r, n);
1190N/A </code></p></div>
1185N/A
1185N/A</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div>
1185N/A<div class="section">
1469N/A<h2><a name="hooking" id="hooking">Hooking the hook</a></h2>
1185N/A <p>A module that wants a hook to be called needs to do two
1185N/A things.</p>
1185N/A
1185N/A <h3><a name="hooking-implement" id="hooking-implement">Implement the hook function</a></h3>
1185N/A <p>Include the appropriate header, and define a static function
1185N/A of the correct type:</p>
158N/A
1237N/A <div class="example"><p><code>
1237N/A static int my_something_doer(request_rec *r, int n)<br />
1185N/A {<br />
1185N/A <span class="indent">
1390N/A ...<br />
1185N/A return OK;<br />
1469N/A </span>
1185N/A }
1469N/A </code></p></div>
1185N/A
1185N/A
1390N/A <h3><a name="hooking-add" id="hooking-add">Add a hook registering function</a></h3>
1469N/A <p>During initialisation, Apache will call each modules hook
1390N/A registering function, which is included in the module
1185N/A structure:</p>
1185N/A
1185N/A <div class="example"><p><code>
0N/A static void my_register_hooks()<br />
1469N/A {<br />
1390N/A <span class="indent">
1185N/A ap_hook_do_something(my_something_doer, NULL, NULL, APR_HOOK_MIDDLE);<br />
1185N/A </span>
1185N/A }<br />
1185N/A <br />
1185N/A mode MODULE_VAR_EXPORT my_module =<br />
1185N/A {<br />
1185N/A <span class="indent">
1185N/A ...<br />
1185N/A my_register_hooks /* register hooks */<br />
1111N/A </span>
1390N/A };
1185N/A </code></p></div>
1185N/A
1185N/A
1469N/A <h3><a name="hooking-order" id="hooking-order">Controlling hook calling order</a></h3>
1390N/A <p>In the example above, we didn't use the three arguments in
1185N/A the hook registration function that control calling order.
1185N/A There are two mechanisms for doing this. The first, rather
350N/A crude, method, allows us to specify roughly where the hook is
350N/A run relative to other modules. The final argument control this.
350N/A There are three possible values: <code>APR_HOOK_FIRST</code>,
928N/A <code>APR_HOOK_MIDDLE</code> and <code>APR_HOOK_LAST</code>.</p>
350N/A
1185N/A <p>All modules using any particular value may be run in any
1385N/A order relative to each other, but, of course, all modules using
1185N/A <code>APR_HOOK_FIRST</code> will be run before <code>APR_HOOK_MIDDLE</code>
1185N/A which are before <code>APR_HOOK_LAST</code>. Modules that don't care
1185N/A when they are run should use <code>APR_HOOK_MIDDLE</code>. <em>(I spaced
1185N/A these out so people could do stuff like <code>APR_HOOK_FIRST-2</code>
1185N/A to get in slightly earlier, but is this wise? - Ben)</em></p>
1185N/A
1185N/A <p>Note that there are two more values,
1185N/A <code>APR_HOOK_REALLY_FIRST</code> and <code>APR_HOOK_REALLY_LAST</code>. These
1461N/A should only be used by the hook exporter.</p>
1190N/A
1185N/A <p>The other method allows finer control. When a module knows
1190N/A that it must be run before (or after) some other modules, it
1190N/A can specify them by name. The second (third) argument is a
1185N/A NULL-terminated array of strings consisting of the names of
0N/A modules that must be run before (after) the current module. For
0N/A example, suppose we want "mod_xyz.c" and "mod_abc.c" to run
1185N/A before we do, then we'd hook as follows:</p>
1190N/A
1185N/A <div class="example"><p><code>
0N/A static void register_hooks()<br />
0N/A {<br />
0N/A <span class="indent">
0N/A static const char * const aszPre[] = { "mod_xyz.c", "mod_abc.c", NULL };<br />
0N/A <br />
0N/A 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>
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