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<title>Hook Functions in the Apache HTTP Server 2.x - Apache HTTP Server Version 2.5</title>
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<body id="manual-page"><div id="page-header">
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<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/quickreference.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
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<p class="apache">Apache HTTP Server Version 2.5</p>
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<img alt="" src="../images/feather.png" /></div>
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<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
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<div id="path">
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<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>
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<div class="toplang">
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<p><span>Available Languages: </span><a href="../en/developer/hooks.html" title="English"> en </a></p>
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</div>
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<div class="warning"><h3>Warning</h3>
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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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</div>
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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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</div>
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<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#corehooks">Core Hooks</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#create">Creating a hook function</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#hooking">Hooking the hook</a></li>
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</ul><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
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<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="corehooks" id="corehooks">Core Hooks</a> <a title="Permanent link" href="#corehooks" class="permalink">¶</a></h2>
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<p>The httpd's core modules offer a predefinined list of hooks
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used during the standard <a href="./request.html">request processing</a>
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phase. Creating a new hook will expose a function that
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implements it (see sections below) but it is essential to understand that you will not
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extend the httpd's core hooks. Their presence and order in the request processing is in fact
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a consequence of how they are called in <code>server/request.c</code>
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(check <a href="./modguide.html#hooking">this section</a>
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for an overview). The core hooks are listed in the
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<a href="https://ci.apache.org/projects/httpd/trunk/doxygen/group__hooks.html">doxygen documentation</a>.</p>
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<p>Reading <a href="./modguide.html">guide for developing modules</a> and
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<a href="./request.html">request processing</a> before proceeding is
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highly recommended.
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</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="create" id="create">Creating a hook function</a> <a title="Permanent link" href="#create" class="permalink">¶</a></h2>
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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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<h3><a name="create-declare" id="create-declare">Declare the hook function</a></h3>
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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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<pre class="prettyprint lang-c">AP_DECLARE_HOOK(int, do_something, (request_rec *r, int n))</pre>
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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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<h3><a name="create-create" id="create-create">Create the hook structure</a></h3>
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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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<pre class="prettyprint lang-c">APR_HOOK_STRUCT(
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APR_HOOK_LINK(do_something)
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...
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)</pre>
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<h3><a name="create-implement" id="create-implement">Implement the hook caller</a></h3>
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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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<h4>Void hooks</h4>
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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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<pre class="prettyprint lang-c">AP_IMPLEMENT_HOOK_VOID(do_something, (request_rec *r, int n), (r, n))</pre>
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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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<pre class="prettyprint lang-c">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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}</pre>
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<h4>Hooks that return a value</h4>
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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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<pre class="prettyprint lang-c">AP_IMPLEMENT_HOOK_RUN_FIRST(int, do_something, (request_rec *r, int n), (r, n), DECLINED)</pre>
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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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<pre class="prettyprint lang-c">AP_IMPLEMENT_HOOK_RUN_ALL(int, do_something, (request_rec *r, int n), (r, n), OK, DECLINED)</pre>
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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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<h3><a name="create-call" id="create-call">Call the hook callers</a></h3>
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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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<pre class="prettyprint lang-c">int n, ret;
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request_rec *r;
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ret=ap_run_do_something(r, n);</pre>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="hooking" id="hooking">Hooking the hook</a> <a title="Permanent link" href="#hooking" class="permalink">¶</a></h2>
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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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<h3><a name="hooking-implement" id="hooking-implement">Implement the hook function</a></h3>
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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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<pre class="prettyprint lang-c">static int my_something_doer(request_rec *r, int n)
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{
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...
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return OK;
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}</pre>
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<h3><a name="hooking-add" id="hooking-add">Add a hook registering function</a></h3>
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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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<pre class="prettyprint lang-c">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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};</pre>
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<h3><a name="hooking-order" id="hooking-order">Controlling hook calling order</a></h3>
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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 of
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all the functions registered within the hook.
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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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<pre class="prettyprint lang-c">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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}</pre>
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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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</div></div>
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<div class="bottomlang">
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<p><span>Available Languages: </span><a href="../en/developer/hooks.html" title="English"> en </a></p>
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</div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><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 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 Libera.chat, or sent to our <a href="https://httpd.apache.org/lists.html">mailing lists</a>.</div>
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