1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
|
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!--
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
This file is generated from xml source: DO NOT EDIT
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
-->
<title>Hook Functions in the Apache HTTP Server 2.x - Apache HTTP Server</title>
<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
<link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" />
<link href="../images/favicon.ico" rel="shortcut icon" /></head>
<body id="manual-page"><div id="page-header">
<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
<p class="apache">Apache HTTP Server Version 2.5</p>
<img alt="" src="../images/feather.gif" /></div>
<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
<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 2.5</a> > <a href="./">Developer Documentation</a></div><div id="page-content"><div id="preamble"><h1>Hook Functions in the Apache HTTP Server 2.x</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 the Apache HTTP Server
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 traditional
hook return value 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, the server 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>These
values are spaced out, so that positions like <code>APR_HOOK_FIRST-2</code>
are possible to hook slightly earlier than other functions.</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>
</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 2012 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>
<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div>
</body></html>
|
