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title>Hook Functions in the Apache HTTP Server
2.x - Apache HTTP Server</
title>
1370N/A<
body id="manual-page"><
div id="page-header">
1370N/A<
p class="apache">Apache HTTP Server Version 2.5</
p>
486N/A <
div class="warning"><
h3>Warning</
h3>
1370N/A <
p>This document is still in development and may be partially out of
1370N/A <
p>In general, a hook function is one that the Apache HTTP Server
486N/A will call at some point during the processing of a request.
486N/A Modules can provide functions that are called, and specify when
486N/A they get called in comparison to other modules.</
p>
486N/A<
div id="quickview"><
ul id="toc"><
li><
img alt="" src="/images/down.gif" /> <
a href="#create">Creating a hook function</
a></
li>
486N/A</
ul><
ul class="seealso"><
li><
a href="#comments_section">Comments</
a></
li></
ul></
div>
486N/A<
div class="top"><
a href="#page-header"><
img alt="top" src="/images/up.gif" /></
a></
div>
486N/A<
h2><
a name="create" id="create">Creating a hook function</
a></
h2>
486N/A <
p>In order to create a new hook, four things need to be
486N/A <
h3><
a name="create-declare" id="create-declare">Declare the hook function</
a></
h3>
486N/A <
p>Use the <
code>AP_DECLARE_HOOK</
code> macro, which needs to be given
486N/A the return type of the hook function, the name of the hook, and the
486N/A arguments. For example, if the hook returns an <
code>int</
code> and
486N/A takes a <
code>request_rec *</
code> and an <
code>int</
code> and is
486N/A called <
code>do_something</
code>, then declare it like this:</
p>
486N/A <
pre class="prettyprint lang-c">
1068N/A AP_DECLARE_HOOK(int, do_something, (request_rec *r, int n))
486N/A <
p>This should go in a header which modules will include if
1068N/A they want to use the hook.</
p>
1370N/A <
h3><
a name="create-create" id="create-create">Create the hook structure</
a></
h3>
486N/A <
p>Each source file that exports a hook has a private structure
486N/A which is used to record the module functions that use the hook.
486N/A This is declared as follows:</
p>
486N/A <
pre class="prettyprint lang-c">
486N/A APR_HOOK_LINK(do_something)
486N/A <
h3><
a name="create-implement" id="create-implement">Implement the hook caller</
a></
h3>
486N/A <
p>The source file that exports the hook has to implement a
486N/A function that will call the hook. There are currently three
486N/A possible ways to do this. In all cases, the calling function is
486N/A called <
code>ap_run_<
var>hookname</
var>()</
code>.</
p>
1370N/A <
p>If the return value of a hook is <
code>void</
code>, then all the
486N/A hooks are called, and the caller is implemented like this:</
p>
1276N/A <
pre class="prettyprint lang-c">
486N/A AP_IMPLEMENT_HOOK_VOID(do_something, (request_rec *r, int n), (r, n))
486N/A <
p>The second and third arguments are the dummy argument
1370N/A declaration and the dummy arguments as they will be used when
486N/A calling the hook. In other words, this macro expands to
486N/A something like this:</
p>
486N/A <
pre class="prettyprint lang-c">
486N/A void ap_run_do_something(request_rec *r, int n)
486N/A <
h4>Hooks that return a value</
h4>
1370N/A <
p>If the hook returns a value, then it can either be run until
486N/A the first hook that does something interesting, like so:</
p>
486N/A <
pre class="prettyprint lang-c">
486N/A AP_IMPLEMENT_HOOK_RUN_FIRST(int, do_something, (request_rec *r, int n), (r, n), DECLINED)
1276N/A <
p>The first hook that does <
em>not</
em> return <
code>DECLINED</
code>
486N/A stops the loop and its return value is returned from the hook
486N/A caller. Note that <
code>DECLINED</
code> is the traditional
486N/A hook return value meaning "I didn't do anything", but it can be
486N/A <
p>Alternatively, all hooks can be run until an error occurs.
486N/A This boils down to permitting <
em>two</
em> return values, one of
486N/A which means "I did something, and it was OK" and the other
486N/A meaning "I did nothing". The first function that returns a
486N/A value other than one of those two stops the loop, and its
486N/A return is the return value. Declare these like so:</
p>
486N/A <
pre class="prettyprint lang-c">
486N/A AP_IMPLEMENT_HOOK_RUN_ALL(int, do_something, (request_rec *r, int n), (r, n), OK, DECLINED)
<
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,
<
pre class="prettyprint lang-c">
ret=ap_run_do_something(r, n);
</
div><
div class="top"><
a href="#page-header"><
img alt="top" src="/images/up.gif" /></
a></
div>
<
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
<
h3><
a name="hooking-implement" id="hooking-implement">Implement the hook function</
a></
h3>
<
p>Include the appropriate header, and define a static function
<
pre class="prettyprint lang-c">
static int my_something_doer(request_rec *r, int n)<
br />
<
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
<
pre class="prettyprint lang-c">
static void my_register_hooks()
ap_hook_do_something(my_something_doer, NULL, NULL, APR_HOOK_MIDDLE);
mode MODULE_VAR_EXPORT my_module =
my_register_hooks /* register hooks */
<
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
before we do, then we'd hook as follows:</
p>
<
pre class="prettyprint lang-c">
static void register_hooks()
ap_hook_do_something(my_something_doer, aszPre, NULL, APR_HOOK_MIDDLE);
<
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
<
p><
span>Available Languages: </
span><
a href="/en/developer/hooks.html" title="English"> en </
a></
p>
</
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h2><
a id="comments_section" name="comments_section">Comments</
a></
h2><
div class="warning"><
strong>Notice:</
strong><
br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered
invalid/
off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <
a href="http://httpd.apache.org/lists.html">mailing lists</
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