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224 lines
8.4 KiB
XML
224 lines
8.4 KiB
XML
<?xml version="1.0" encoding="UTF-8" ?>
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<!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
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<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
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<!-- $LastChangedRevision$ -->
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<!--
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Licensed to the Apache Software Foundation (ASF) under one or more
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contributor license agreements. See the NOTICE file distributed with
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this work for additional information regarding copyright ownership.
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The ASF licenses this file to You under the Apache License, Version 2.0
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(the "License"); you may not use this file except in compliance with
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the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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<manualpage metafile="hooks.xml.meta">
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<parentdocument href="./">Developer Documentation</parentdocument>
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<title>Hook Functions in the Apache HTTP Server 2.x</title>
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<summary>
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<note type="warning"><title>Warning</title>
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<p>This document is still in development and may be partially out of
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date.</p>
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</note>
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<p>In general, a hook function is one that the Apache HTTP Server
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will call at some point during the processing of a request.
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Modules can provide functions that are called, and specify when
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they get called in comparison to other modules.</p>
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</summary>
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<section id="create"><title>Creating a hook function</title>
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<p>In order to create a new hook, four things need to be
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done:</p>
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<section id="create-declare"><title>Declare the hook function</title>
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<p>Use the <code>AP_DECLARE_HOOK</code> macro, which needs to be given
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the return type of the hook function, the name of the hook, and the
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arguments. For example, if the hook returns an <code>int</code> and
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takes a <code>request_rec *</code> and an <code>int</code> and is
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called <code>do_something</code>, then declare it like this:</p>
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<highlight language="c">
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AP_DECLARE_HOOK(int, do_something, (request_rec *r, int n))
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</highlight>
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<p>This should go in a header which modules will include if
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they want to use the hook.</p>
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</section>
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<section id="create-create"><title>Create the hook structure</title>
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<p>Each source file that exports a hook has a private structure
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which is used to record the module functions that use the hook.
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This is declared as follows:</p>
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<highlight language="c">
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APR_HOOK_STRUCT(
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APR_HOOK_LINK(do_something)
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...
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)
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</highlight>
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</section>
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<section id="create-implement"><title>Implement the hook caller</title>
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<p>The source file that exports the hook has to implement a
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function that will call the hook. There are currently three
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possible ways to do this. In all cases, the calling function is
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called <code>ap_run_<var>hookname</var>()</code>.</p>
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<section><title>Void hooks</title>
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<p>If the return value of a hook is <code>void</code>, then all the
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hooks are called, and the caller is implemented like this:</p>
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<highlight language="c">
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AP_IMPLEMENT_HOOK_VOID(do_something, (request_rec *r, int n), (r, n))
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</highlight>
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<p>The second and third arguments are the dummy argument
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declaration and the dummy arguments as they will be used when
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calling the hook. In other words, this macro expands to
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something like this:</p>
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<highlight language="c">
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void ap_run_do_something(request_rec *r, int n)
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{
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...
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do_something(r, n);
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}
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</highlight>
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</section>
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<section><title>Hooks that return a value</title>
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<p>If the hook returns a value, then it can either be run until
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the first hook that does something interesting, like so:</p>
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<highlight language="c">
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AP_IMPLEMENT_HOOK_RUN_FIRST(int, do_something, (request_rec *r, int n), (r, n), DECLINED)
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</highlight>
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<p>The first hook that does <em>not</em> return <code>DECLINED</code>
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stops the loop and its return value is returned from the hook
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caller. Note that <code>DECLINED</code> is the traditional
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hook return value meaning "I didn't do anything", but it can be
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whatever suits you.</p>
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<p>Alternatively, all hooks can be run until an error occurs.
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This boils down to permitting <em>two</em> return values, one of
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which means "I did something, and it was OK" and the other
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meaning "I did nothing". The first function that returns a
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value other than one of those two stops the loop, and its
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return is the return value. Declare these like so:</p>
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<highlight language="c">
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AP_IMPLEMENT_HOOK_RUN_ALL(int, do_something, (request_rec *r, int n), (r, n), OK, DECLINED)
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</highlight>
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<p>Again, <code>OK</code> and <code>DECLINED</code> are the traditional
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values. You can use what you want.</p>
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</section>
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</section>
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<section id="create-call"><title>Call the hook callers</title>
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<p>At appropriate moments in the code, call the hook caller,
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like so:</p>
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<highlight language="c">
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int n, ret;
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request_rec *r;
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ret=ap_run_do_something(r, n);
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</highlight>
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</section>
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</section>
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<section id="hooking"><title>Hooking the hook</title>
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<p>A module that wants a hook to be called needs to do two
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things.</p>
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<section id="hooking-implement"><title>Implement the hook function</title>
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<p>Include the appropriate header, and define a static function
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of the correct type:</p>
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<highlight language="c">
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static int my_something_doer(request_rec *r, int n)<br />
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{
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...
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return OK;
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}
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</highlight>
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</section>
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<section id="hooking-add"><title>Add a hook registering function</title>
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<p>During initialisation, the server will call each modules hook
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registering function, which is included in the module
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structure:</p>
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<highlight language="c">
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static void my_register_hooks()
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{
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ap_hook_do_something(my_something_doer, NULL, NULL, APR_HOOK_MIDDLE);
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}
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mode MODULE_VAR_EXPORT my_module =
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{
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...
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my_register_hooks /* register hooks */
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};
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</highlight>
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</section>
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<section id="hooking-order"><title>Controlling hook calling order</title>
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<p>In the example above, we didn't use the three arguments in
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the hook registration function that control calling order.
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There are two mechanisms for doing this. The first, rather
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crude, method, allows us to specify roughly where the hook is
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run relative to other modules. The final argument control this.
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There are three possible values: <code>APR_HOOK_FIRST</code>,
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<code>APR_HOOK_MIDDLE</code> and <code>APR_HOOK_LAST</code>.</p>
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<p>All modules using any particular value may be run in any
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order relative to each other, but, of course, all modules using
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<code>APR_HOOK_FIRST</code> will be run before <code>APR_HOOK_MIDDLE</code>
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which are before <code>APR_HOOK_LAST</code>. Modules that don't care
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when they are run should use <code>APR_HOOK_MIDDLE</code>. <em>These
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values are spaced out, so that positions like <code>APR_HOOK_FIRST-2</code>
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are possible to hook slightly earlier than other functions.</em></p>
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<p>Note that there are two more values,
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<code>APR_HOOK_REALLY_FIRST</code> and <code>APR_HOOK_REALLY_LAST</code>. These
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should only be used by the hook exporter.</p>
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<p>The other method allows finer control. When a module knows
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that it must be run before (or after) some other modules, it
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can specify them by name. The second (third) argument is a
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NULL-terminated array of strings consisting of the names of
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modules that must be run before (after) the current module. For
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example, suppose we want "mod_xyz.c" and "mod_abc.c" to run
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before we do, then we'd hook as follows:</p>
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<highlight language="c">
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static void register_hooks()
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{
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static const char * const aszPre[] = { "mod_xyz.c", "mod_abc.c", NULL };
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ap_hook_do_something(my_something_doer, aszPre, NULL, APR_HOOK_MIDDLE);
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}
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</highlight>
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<p>Note that the sort used to achieve this is stable, so
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ordering set by <code>APR_HOOK_<var>ORDER</var></code> is preserved, as far
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as is possible.</p>
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</section>
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</section>
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</manualpage>
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