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23 <manualpage metafile="hooks.xml.meta">
24 <parentdocument href="./">Developer Documentation</parentdocument>
26 <title>Hook Functions in the Apache HTTP Server 2.x</title>
29 <note type="warning"><title>Warning</title>
30 <p>This document is still in development and may be partially out of
34 <p>In general, a hook function is one that the Apache HTTP Server
35 will call at some point during the processing of a request.
36 Modules can provide functions that are called, and specify when
37 they get called in comparison to other modules.</p>
40 <section id="create"><title>Creating a hook function</title>
41 <p>In order to create a new hook, four things need to be
44 <section id="create-declare"><title>Declare the hook function</title>
45 <p>Use the <code>AP_DECLARE_HOOK</code> macro, which needs to be given
46 the return type of the hook function, the name of the hook, and the
47 arguments. For example, if the hook returns an <code>int</code> and
48 takes a <code>request_rec *</code> and an <code>int</code> and is
49 called <code>do_something</code>, then declare it like this:</p>
50 <highlight language="c">
51 AP_DECLARE_HOOK(int, do_something, (request_rec *r, int n))
54 <p>This should go in a header which modules will include if
55 they want to use the hook.</p>
58 <section id="create-create"><title>Create the hook structure</title>
59 <p>Each source file that exports a hook has a private structure
60 which is used to record the module functions that use the hook.
61 This is declared as follows:</p>
63 <highlight language="c">
65 APR_HOOK_LINK(do_something)
71 <section id="create-implement"><title>Implement the hook caller</title>
72 <p>The source file that exports the hook has to implement a
73 function that will call the hook. There are currently three
74 possible ways to do this. In all cases, the calling function is
75 called <code>ap_run_<var>hookname</var>()</code>.</p>
77 <section><title>Void hooks</title>
78 <p>If the return value of a hook is <code>void</code>, then all the
79 hooks are called, and the caller is implemented like this:</p>
81 <highlight language="c">
82 AP_IMPLEMENT_HOOK_VOID(do_something, (request_rec *r, int n), (r, n))
85 <p>The second and third arguments are the dummy argument
86 declaration and the dummy arguments as they will be used when
87 calling the hook. In other words, this macro expands to
88 something like this:</p>
90 <highlight language="c">
91 void ap_run_do_something(request_rec *r, int n)
99 <section><title>Hooks that return a value</title>
100 <p>If the hook returns a value, then it can either be run until
101 the first hook that does something interesting, like so:</p>
103 <highlight language="c">
104 AP_IMPLEMENT_HOOK_RUN_FIRST(int, do_something, (request_rec *r, int n), (r, n), DECLINED)
107 <p>The first hook that does <em>not</em> return <code>DECLINED</code>
108 stops the loop and its return value is returned from the hook
109 caller. Note that <code>DECLINED</code> is the traditional
110 hook return value meaning "I didn't do anything", but it can be
111 whatever suits you.</p>
113 <p>Alternatively, all hooks can be run until an error occurs.
114 This boils down to permitting <em>two</em> return values, one of
115 which means "I did something, and it was OK" and the other
116 meaning "I did nothing". The first function that returns a
117 value other than one of those two stops the loop, and its
118 return is the return value. Declare these like so:</p>
120 <highlight language="c">
121 AP_IMPLEMENT_HOOK_RUN_ALL(int, do_something, (request_rec *r, int n), (r, n), OK, DECLINED)
124 <p>Again, <code>OK</code> and <code>DECLINED</code> are the traditional
125 values. You can use what you want.</p>
129 <section id="create-call"><title>Call the hook callers</title>
130 <p>At appropriate moments in the code, call the hook caller,
133 <highlight language="c">
137 ret=ap_run_do_something(r, n);
142 <section id="hooking"><title>Hooking the hook</title>
143 <p>A module that wants a hook to be called needs to do two
146 <section id="hooking-implement"><title>Implement the hook function</title>
147 <p>Include the appropriate header, and define a static function
148 of the correct type:</p>
150 <highlight language="c">
151 static int my_something_doer(request_rec *r, int n)<br />
159 <section id="hooking-add"><title>Add a hook registering function</title>
160 <p>During initialisation, the server will call each modules hook
161 registering function, which is included in the module
164 <highlight language="c">
165 static void my_register_hooks()
167 ap_hook_do_something(my_something_doer, NULL, NULL, APR_HOOK_MIDDLE);
170 mode MODULE_VAR_EXPORT my_module =
173 my_register_hooks /* register hooks */
178 <section id="hooking-order"><title>Controlling hook calling order</title>
179 <p>In the example above, we didn't use the three arguments in
180 the hook registration function that control calling order.
181 There are two mechanisms for doing this. The first, rather
182 crude, method, allows us to specify roughly where the hook is
183 run relative to other modules. The final argument control this.
184 There are three possible values: <code>APR_HOOK_FIRST</code>,
185 <code>APR_HOOK_MIDDLE</code> and <code>APR_HOOK_LAST</code>.</p>
187 <p>All modules using any particular value may be run in any
188 order relative to each other, but, of course, all modules using
189 <code>APR_HOOK_FIRST</code> will be run before <code>APR_HOOK_MIDDLE</code>
190 which are before <code>APR_HOOK_LAST</code>. Modules that don't care
191 when they are run should use <code>APR_HOOK_MIDDLE</code>. <em>These
192 values are spaced out, so that positions like <code>APR_HOOK_FIRST-2</code>
193 are possible to hook slightly earlier than other functions.</em></p>
195 <p>Note that there are two more values,
196 <code>APR_HOOK_REALLY_FIRST</code> and <code>APR_HOOK_REALLY_LAST</code>. These
197 should only be used by the hook exporter.</p>
199 <p>The other method allows finer control. When a module knows
200 that it must be run before (or after) some other modules, it
201 can specify them by name. The second (third) argument is a
202 NULL-terminated array of strings consisting of the names of
203 modules that must be run before (after) the current module. For
204 example, suppose we want "mod_xyz.c" and "mod_abc.c" to run
205 before we do, then we'd hook as follows:</p>
207 <highlight language="c">
208 static void register_hooks()
210 static const char * const aszPre[] = { "mod_xyz.c", "mod_abc.c", NULL };
212 ap_hook_do_something(my_something_doer, aszPre, NULL, APR_HOOK_MIDDLE);
216 <p>Note that the sort used to achieve this is stable, so
217 ordering set by <code>APR_HOOK_<var>ORDER</var></code> is preserved, as far