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15 <p class="apache">Apache HTTP Server Version 2.0</p>
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19 <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>
21 <p><span>Available Languages: </span><a href="../en/developer/hooks.html" title="English"> en </a></p>
24 <div class="warning"><h3>Warning</h3>
25 <p>This document is still in development and may be partially out of
29 <p>In general, a hook function is one that Apache will call at
30 some point during the processing of a request. Modules can
31 provide functions that are called, and specify when they get
32 called in comparison to other modules.</p>
34 <div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#create">Creating a hook function</a></li>
35 <li><img alt="" src="../images/down.gif" /> <a href="#hooking">Hooking the hook</a></li>
37 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
39 <h2><a name="create" id="create">Creating a hook function</a></h2>
40 <p>In order to create a new hook, four things need to be
43 <h3><a name="create-declare" id="create-declare">Declare the hook function</a></h3>
44 <p>Use the <code>AP_DECLARE_HOOK</code> macro, which needs to be given
45 the return type of the hook function, the name of the hook, and the
46 arguments. For example, if the hook returns an <code>int</code> and
47 takes a <code>request_rec *</code> and an <code>int</code> and is
48 called <code>do_something</code>, then declare it like this:</p>
49 <div class="example"><p><code>
50 AP_DECLARE_HOOK(int, do_something, (request_rec *r, int n))
53 <p>This should go in a header which modules will include if
54 they want to use the hook.</p>
57 <h3><a name="create-create" id="create-create">Create the hook structure</a></h3>
58 <p>Each source file that exports a hook has a private structure
59 which is used to record the module functions that use the hook.
60 This is declared as follows:</p>
62 <div class="example"><p><code>
63 APR_HOOK_STRUCT(<br />
65 APR_HOOK_LINK(do_something)<br />
72 <h3><a name="create-implement" id="create-implement">Implement the hook caller</a></h3>
73 <p>The source file that exports the hook has to implement a
74 function that will call the hook. There are currently three
75 possible ways to do this. In all cases, the calling function is
76 called <code>ap_run_<var>hookname</var>()</code>.</p>
79 <p>If the return value of a hook is <code>void</code>, then all the
80 hooks are called, and the caller is implemented like this:</p>
82 <div class="example"><p><code>
83 AP_IMPLEMENT_HOOK_VOID(do_something, (request_rec *r, int n), (r, n))
86 <p>The second and third arguments are the dummy argument
87 declaration and the dummy arguments as they will be used when
88 calling the hook. In other words, this macro expands to
89 something like this:</p>
91 <div class="example"><p><code>
92 void ap_run_do_something(request_rec *r, int n)<br />
96 do_something(r, n);<br />
102 <h4>Hooks that return a value</h4>
103 <p>If the hook returns a value, then it can either be run until
104 the first hook that does something interesting, like so:</p>
106 <div class="example"><p><code>
107 AP_IMPLEMENT_HOOK_RUN_FIRST(int, do_something, (request_rec *r, int n), (r, n), DECLINED)
110 <p>The first hook that does <em>not</em> return <code>DECLINED</code>
111 stops the loop and its return value is returned from the hook
112 caller. Note that <code>DECLINED</code> is the tradition Apache
113 hook return meaning "I didn't do anything", but it can be
114 whatever suits you.</p>
116 <p>Alternatively, all hooks can be run until an error occurs.
117 This boils down to permitting <em>two</em> return values, one of
118 which means "I did something, and it was OK" and the other
119 meaning "I did nothing". The first function that returns a
120 value other than one of those two stops the loop, and its
121 return is the return value. Declare these like so:</p>
123 <div class="example"><p><code>
124 AP_IMPLEMENT_HOOK_RUN_ALL(int, do_something, (request_rec *r, int n), (r, n), OK, DECLINED)
127 <p>Again, <code>OK</code> and <code>DECLINED</code> are the traditional
128 values. You can use what you want.</p>
132 <h3><a name="create-call" id="create-call">Call the hook callers</a></h3>
133 <p>At appropriate moments in the code, call the hook caller,
136 <div class="example"><p><code>
138 request_rec *r;<br />
140 ret=ap_run_do_something(r, n);
143 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
144 <div class="section">
145 <h2><a name="hooking" id="hooking">Hooking the hook</a></h2>
146 <p>A module that wants a hook to be called needs to do two
149 <h3><a name="hooking-implement" id="hooking-implement">Implement the hook function</a></h3>
150 <p>Include the appropriate header, and define a static function
151 of the correct type:</p>
153 <div class="example"><p><code>
154 static int my_something_doer(request_rec *r, int n)<br />
156 <span class="indent">
164 <h3><a name="hooking-add" id="hooking-add">Add a hook registering function</a></h3>
165 <p>During initialisation, Apache will call each modules hook
166 registering function, which is included in the module
169 <div class="example"><p><code>
170 static void my_register_hooks()<br />
172 <span class="indent">
173 ap_hook_do_something(my_something_doer, NULL, NULL, HOOK_MIDDLE);<br />
177 mode MODULE_VAR_EXPORT my_module =<br />
179 <span class="indent">
181 my_register_hooks /* register hooks */<br />
187 <h3><a name="hooking-order" id="hooking-order">Controlling hook calling order</a></h3>
188 <p>In the example above, we didn't use the three arguments in
189 the hook registration function that control calling order.
190 There are two mechanisms for doing this. The first, rather
191 crude, method, allows us to specify roughly where the hook is
192 run relative to other modules. The final argument control this.
193 There are three possible values: <code>HOOK_FIRST</code>,
194 <code>HOOK_MIDDLE</code> and <code>HOOK_LAST</code>.</p>
196 <p>All modules using any particular value may be run in any
197 order relative to each other, but, of course, all modules using
198 <code>HOOK_FIRST</code> will be run before <code>HOOK_MIDDLE</code>
199 which are before <code>HOOK_LAST</code>. Modules that don't care
200 when they are run should use <code>HOOK_MIDDLE</code>. <em>(I spaced
201 these out so people could do stuff like <code>HOOK_FIRST-2</code>
202 to get in slightly earlier, but is this wise? - Ben)</em></p>
204 <p>Note that there are two more values,
205 <code>HOOK_REALLY_FIRST</code> and <code>HOOK_REALLY_LAST</code>. These
206 should only be used by the hook exporter.</p>
208 <p>The other method allows finer control. When a module knows
209 that it must be run before (or after) some other modules, it
210 can specify them by name. The second (third) argument is a
211 NULL-terminated array of strings consisting of the names of
212 modules that must be run before (after) the current module. For
213 example, suppose we want "mod_xyz.c" and "mod_abc.c" to run
214 before we do, then we'd hook as follows:</p>
216 <div class="example"><p><code>
217 static void register_hooks()<br />
219 <span class="indent">
220 static const char * const aszPre[] = { "mod_xyz.c", "mod_abc.c", NULL };<br />
222 ap_hook_do_something(my_something_doer, aszPre, NULL, HOOK_MIDDLE);<br />
227 <p>Note that the sort used to achieve this is stable, so
228 ordering set by <code>HOOK_<var>ORDER</var></code> is preserved, as far
231 <p class="cite"><cite>Ben Laurie</cite>, 15th August 1999</p>
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235 <p><span>Available Languages: </span><a href="../en/developer/hooks.html" title="English"> en </a></p>
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237 <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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