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<p class="apache">Apache HTTP Server Version 2.0</p>
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<div id="path">
<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.0</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This document is still in development and may be partially out of
date.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, 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>HOOK_FIRST</code>,
<code>HOOK_MIDDLE</code> and <code>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>HOOK_FIRST</code> will be run before <code>HOOK_MIDDLE</code>
which are before <code>HOOK_LAST</code>. Modules that don't care
when they are run should use <code>HOOK_MIDDLE</code>. <em>(I spaced
these out so people could do stuff like <code>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>HOOK_REALLY_FIRST</code> and <code>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, HOOK_MIDDLE);<br />
</span>
}
</code></p></div>
<p>Note that the sort used to achieve this is stable, so
ordering set by <code>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>
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<p class="apache">Copyright 2009 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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