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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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| <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> |
| <p class="apache">Apache HTTP Server Version 2.5</p> |
| <img alt="" src="../images/feather.png" /></div> |
| <div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div> |
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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> |
| <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="#corehooks">Core Hooks</a></li> |
| <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><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</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="corehooks" id="corehooks">Core Hooks</a></h2> |
| <p>The httpd's core modules offer a predefinined list of hooks |
| used during the standard <a href="./request.html">request processing</a> |
| phase. Creating a new hook will expose a function that |
| implements it (see sections below) but it is essential to undestand that you will not |
| extend the httpd's core hooks. Their presence and order in the request processing is in fact |
| a consequence of how they are called in <code>server/request.c</code> |
| (check <a href="./modguide.html#hooking">this section</a> |
| for an overview). The core hooks are listed in the |
| <a href="https://ci.apache.org/projects/httpd/trunk/doxygen/group__hooks.html">doxygen documentation</a>.</p> |
| |
| <p>Reading <a href="./modguide.html">guide for developing modules</a> and |
| <a href="./request.html">request processing</a> before proceeding is |
| highly recomended. |
| </p> |
| </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> |
| <pre class="prettyprint lang-c">AP_DECLARE_HOOK(int, do_something, (request_rec *r, int n))</pre> |
| |
| |
| <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> |
| |
| <pre class="prettyprint lang-c">APR_HOOK_STRUCT( |
| APR_HOOK_LINK(do_something) |
| ... |
| )</pre> |
| |
| |
| |
| <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> |
| |
| <pre class="prettyprint lang-c">AP_IMPLEMENT_HOOK_VOID(do_something, (request_rec *r, int n), (r, n))</pre> |
| |
| |
| <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> |
| |
| <pre class="prettyprint lang-c">void ap_run_do_something(request_rec *r, int n) |
| { |
| ... |
| do_something(r, n); |
| }</pre> |
| |
| |
| |
| <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> |
| |
| <pre class="prettyprint lang-c">AP_IMPLEMENT_HOOK_RUN_FIRST(int, do_something, (request_rec *r, int n), (r, n), DECLINED)</pre> |
| |
| |
| <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> |
| |
| <pre class="prettyprint lang-c">AP_IMPLEMENT_HOOK_RUN_ALL(int, do_something, (request_rec *r, int n), (r, n), OK, DECLINED)</pre> |
| |
| |
| <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> |
| |
| <pre class="prettyprint lang-c">int n, ret; |
| request_rec *r; |
| |
| ret=ap_run_do_something(r, n);</pre> |
| |
| |
| </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> |
| |
| <pre class="prettyprint lang-c">static int my_something_doer(request_rec *r, int n) |
| { |
| ... |
| return OK; |
| }</pre> |
| |
| |
| |
| <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> |
| |
| <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 */ |
| };</pre> |
| |
| |
| |
| <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 of |
| all the functions registered within the hook. |
| 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> |
| |
| <pre class="prettyprint lang-c">static void register_hooks() |
| { |
| static const char * const aszPre[] = { "mod_xyz.c", "mod_abc.c", NULL }; |
| |
| ap_hook_do_something(my_something_doer, aszPre, NULL, APR_HOOK_MIDDLE); |
| }</pre> |
| |
| |
| <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 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 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</a>.</div> |
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