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8b7e19de6d547ab1ad4256316fbf0d2497f724f8humbedooh<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>
f086b4b402fa9a2fefc7dda85de2a3cc1cd0a654rjung<p><span>Available Languages: </span><a href="/en/developer/hooks.html" title="English"> en </a></p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>This document is still in development and may be partially out of
8b7e19de6d547ab1ad4256316fbf0d2497f724f8humbedooh <p>In general, a hook function is one that the Apache HTTP Server
8b7e19de6d547ab1ad4256316fbf0d2497f724f8humbedooh will call at some point during the processing of a request.
8b7e19de6d547ab1ad4256316fbf0d2497f724f8humbedooh Modules can provide functions that are called, and specify when
8b7e19de6d547ab1ad4256316fbf0d2497f724f8humbedooh they get called in comparison to other modules.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd<div id="quickview"><ul id="toc"><li><img alt="" src="/images/down.gif" /> <a href="#create">Creating a hook function</a></li>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd<li><img alt="" src="/images/down.gif" /> <a href="#hooking">Hooking the hook</a></li>
30471a4650391f57975f60bbb6e4a90be7b284bfhumbedooh</ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
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1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd<h2><a name="create" id="create">Creating a hook function</a></h2>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>In order to create a new hook, four things need to be
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <h3><a name="create-declare" id="create-declare">Declare the hook function</a></h3>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>Use the <code>AP_DECLARE_HOOK</code> macro, which needs to be given
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd the return type of the hook function, the name of the hook, and the
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd arguments. For example, if the hook returns an <code>int</code> and
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd takes a <code>request_rec *</code> and an <code>int</code> and is
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd called <code>do_something</code>, then declare it like this:</p>
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh <pre class="prettyprint lang-c">AP_DECLARE_HOOK(int, do_something, (request_rec *r, int n))</pre>
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh <p>This should go in a header which modules will include if
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd they want to use the hook.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <h3><a name="create-create" id="create-create">Create the hook structure</a></h3>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>Each source file that exports a hook has a private structure
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd which is used to record the module functions that use the hook.
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd This is declared as follows:</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd APR_HOOK_LINK(do_something)
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh <h3><a name="create-implement" id="create-implement">Implement the hook caller</a></h3>
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh <p>The source file that exports the hook has to implement a
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd function that will call the hook. There are currently three
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd possible ways to do this. In all cases, the calling function is
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>If the return value of a hook is <code>void</code>, then all the
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd hooks are called, and the caller is implemented like this:</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <pre class="prettyprint lang-c">AP_IMPLEMENT_HOOK_VOID(do_something, (request_rec *r, int n), (r, n))</pre>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>The second and third arguments are the dummy argument
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh declaration and the dummy arguments as they will be used when
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd calling the hook. In other words, this macro expands to
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh something like this:</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <pre class="prettyprint lang-c">void ap_run_do_something(request_rec *r, int n)
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd do_something(r, n);
fd5a091eb07540ab440f8ac3a277caa590be46c1humbedooh <p>If the hook returns a value, then it can either be run until
fd5a091eb07540ab440f8ac3a277caa590be46c1humbedooh the first hook that does something interesting, like so:</p>
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh <pre class="prettyprint lang-c">AP_IMPLEMENT_HOOK_RUN_FIRST(int, do_something, (request_rec *r, int n), (r, n), DECLINED)</pre>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>The first hook that does <em>not</em> return <code>DECLINED</code>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd stops the loop and its return value is returned from the hook
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd caller. Note that <code>DECLINED</code> is the traditional
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd hook return value meaning "I didn't do anything", but it can be
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd whatever suits you.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>Alternatively, all hooks can be run until an error occurs.
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh This boils down to permitting <em>two</em> return values, one of
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh which means "I did something, and it was OK" and the other
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd meaning "I did nothing". The first function that returns a
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd value other than one of those two stops the loop, and its
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd return is the return value. Declare these like so:</p>
8b7e19de6d547ab1ad4256316fbf0d2497f724f8humbedooh <pre class="prettyprint lang-c">AP_IMPLEMENT_HOOK_RUN_ALL(int, do_something, (request_rec *r, int n), (r, n), OK, DECLINED)</pre>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>Again, <code>OK</code> and <code>DECLINED</code> are the traditional
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd values. You can use what you want.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <h3><a name="create-call" id="create-call">Call the hook callers</a></h3>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>At appropriate moments in the code, call the hook caller,
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh like so:</p>
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedoohrequest_rec *r;
1eba1be63201689b2d3e651d3a35f0bc2057e2c0ndret=ap_run_do_something(r, n);</pre>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd<h2><a name="hooking" id="hooking">Hooking the hook</a></h2>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>A module that wants a hook to be called needs to do two
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd things.</p>
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh <h3><a name="hooking-implement" id="hooking-implement">Implement the hook function</a></h3>
e41ae559e8408edb1c949c1237034f277d423eccminfrin <p>Include the appropriate header, and define a static function
e41ae559e8408edb1c949c1237034f277d423eccminfrin of the correct type:</p>
e41ae559e8408edb1c949c1237034f277d423eccminfrin <pre class="prettyprint lang-c">static int my_something_doer(request_rec *r, int n)<br />
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd return OK;
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <h3><a name="hooking-add" id="hooking-add">Add a hook registering function</a></h3>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>During initialisation, the server will call each modules hook
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd registering function, which is included in the module
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd structure:</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <pre class="prettyprint lang-c">static void my_register_hooks()
e41ae559e8408edb1c949c1237034f277d423eccminfrin ap_hook_do_something(my_something_doer, NULL, NULL, APR_HOOK_MIDDLE);
e41ae559e8408edb1c949c1237034f277d423eccminfrinmode MODULE_VAR_EXPORT my_module =
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh my_register_hooks /* register hooks */
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <h3><a name="hooking-order" id="hooking-order">Controlling hook calling order</a></h3>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>In the example above, we didn't use the three arguments in
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd the hook registration function that control calling order.
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh There are two mechanisms for doing this. The first, rather
e41ae559e8408edb1c949c1237034f277d423eccminfrin crude, method, allows us to specify roughly where the hook is
e41ae559e8408edb1c949c1237034f277d423eccminfrin run relative to other modules. The final argument control this.
e41ae559e8408edb1c949c1237034f277d423eccminfrin There are three possible values: <code>APR_HOOK_FIRST</code>,
e41ae559e8408edb1c949c1237034f277d423eccminfrin <code>APR_HOOK_MIDDLE</code> and <code>APR_HOOK_LAST</code>.</p>
e41ae559e8408edb1c949c1237034f277d423eccminfrin <p>All modules using any particular value may be run in any
e41ae559e8408edb1c949c1237034f277d423eccminfrin order relative to each other, but, of course, all modules using
e41ae559e8408edb1c949c1237034f277d423eccminfrin <code>APR_HOOK_FIRST</code> will be run before <code>APR_HOOK_MIDDLE</code>
e41ae559e8408edb1c949c1237034f277d423eccminfrin which are before <code>APR_HOOK_LAST</code>. Modules that don't care
e41ae559e8408edb1c949c1237034f277d423eccminfrin when they are run should use <code>APR_HOOK_MIDDLE</code>. <em>These
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh values are spaced out, so that positions like <code>APR_HOOK_FIRST-2</code>
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh are possible to hook slightly earlier than other functions.</em></p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>Note that there are two more values,
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <code>APR_HOOK_REALLY_FIRST</code> and <code>APR_HOOK_REALLY_LAST</code>. These
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd should only be used by the hook exporter.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>The other method allows finer control. When a module knows
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd that it must be run before (or after) some other modules, it
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd can specify them by name. The second (third) argument is a
1c14060ebaf0bab91387c4be294e0d3fddf2068cyoshiki NULL-terminated array of strings consisting of the names of
1c14060ebaf0bab91387c4be294e0d3fddf2068cyoshiki modules that must be run before (after) the current module. For
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd example, suppose we want "mod_xyz.c" and "mod_abc.c" to run
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd before we do, then we'd hook as follows:</p>
1c14060ebaf0bab91387c4be294e0d3fddf2068cyoshiki <pre class="prettyprint lang-c">static void register_hooks()
8b7e19de6d547ab1ad4256316fbf0d2497f724f8humbedooh static const char * const aszPre[] = { "mod_xyz.c", "mod_abc.c", NULL };
8b7e19de6d547ab1ad4256316fbf0d2497f724f8humbedooh ap_hook_do_something(my_something_doer, aszPre, NULL, APR_HOOK_MIDDLE);
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>Note that the sort used to achieve this is stable, so
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd ordering set by <code>APR_HOOK_<var>ORDER</var></code> is preserved, as far
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd as is possible.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd<p><span>Available Languages: </span><a href="/en/developer/hooks.html" title="English"> en </a></p>
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