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authorAndré Malo <nd@apache.org>2003-04-05 02:58:03 +0000
committerAndré Malo <nd@apache.org>2003-04-05 02:58:03 +0000
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- <head>
- <meta name="generator" content="HTML Tidy, see www.w3.org" />
-
- <title>Apache 2.0 Hook Functions</title>
- </head>
- <!-- Background white, links blue (unvisited), navy (visited), red (active) -->
-
- <body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
- vlink="#000080" alink="#FF0000">
- <!--#include virtual="header.html" -->
-
- <h1 align="center">Apache Hook Functions</h1>
-
- <p>In general, a hook function is one that Apache 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>
-
- <h2>Creating a hook function</h2>
-
- <p>In order to create a new hook, four things need to be
- done:</p>
-
- <h3>Declare the hook function</h3>
-
- <p>Use the AP_DECLARE_HOOK 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 <tt>int</tt> and
- takes a <tt>request_rec *</tt> and an <tt>int</tt> and is
- called "do_something", then declare it like this:</p>
- <tt>AP_DECLARE_HOOK(int,do_something,(request_rec *r,int
- n))</tt>
-
- <p>This should go in a header which modules will include if
- they want to use the hook.</p>
-
- <h3>Create the hook structure</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>
-APR_HOOK_STRUCT(
- APR_HOOK_LINK(do_something)
- ...
- )
-</pre>
-
- <h3>Implement the hook caller</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 <tt>ap_run_<i>hookname</i>()</tt>.</p>
-
- <h4>Void hooks</h4>
-
- <p>If the return value of a hook is <tt>void</tt>, then all the
- hooks are called, and the caller is implemented like this:</p>
- <tt>AP_IMPLEMENT_HOOK_VOID(do_something,(request_rec *r,int
- n),(r,n))</tt>
-
- <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>
-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>
- <tt>AP_IMPLEMENT_HOOK_RUN_FIRST(int,do_something,(request_rec
- *r,int n),(r,n),DECLINED)</tt>
-
- <p>The first hook that <i>doesn't</i> return <tt>DECLINED</tt>
- stops the loop and its return value is returned from the hook
- caller. Note that <tt>DECLINED</tt> is the tradition Apache
- hook return 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 <i>two</i> 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>
- <tt>AP_IMPLEMENT_HOOK_RUN_ALL(int,do_something,(request_rec
- *r,int n),(r,n),OK,DECLINED)</tt>
-
- <p>Again, <tt>OK</tt> and <tt>DECLINED</tt> are the traditional
- values. You can use what you want.</p>
-
- <h3>Call the hook callers</h3>
-
- <p>At appropriate moments in the code, call the hook caller,
- like so:</p>
-<pre>
- int n,ret;
- request_rec *r;
-
- ret=ap_run_do_something(r,n);
-</pre>
-
- <h2>Hooking the hook</h2>
-
- <p>A module that wants a hook to be called needs to do two
- things.</p>
-
- <h3>Implement the hook function</h3>
-
- <p>Include the appropriate header, and define a static function
- of the correct type:</p>
-<pre>
-static int my_something_doer(request_rec *r,int n)
-{
- ...
- return OK;
-}
-</pre>
-
- <h3>Add a hook registering function</h3>
-
- <p>During initialisation, Apache will call each modules hook
- registering function, which is included in the module
- structure:</p>
-<pre>
-static void my_register_hooks()
-{
- ap_hook_do_something(my_something_doer,NULL,NULL,HOOK_MIDDLE);
-}
-
-mode MODULE_VAR_EXPORT my_module =
-{
- ...
- my_register_hooks /* register hooks */
-};
-</pre>
-
- <h3>Controlling hook calling order</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:</p>
-<pre>
-HOOK_FIRST
-HOOK_MIDDLE
-HOOK_LAST
-</pre>
-
- <p>All modules using any particular value may be run in any
- order relative to each other, but, of course, all modules using
- <tt>HOOK_FIRST</tt> will be run before <tt>HOOK_MIDDLE</tt>
- which are before <tt>HOOK_LAST</tt>. Modules that don't care
- when they are run should use <tt>HOOK_MIDDLE</tt>. <i>(I spaced
- these out so people could do stuff like <tt>HOOK_FIRST-2</tt>
- to get in slightly earlier, but is this wise? - Ben)</i></p>
-
- <p>Note that there are two more values,
- <tt>HOOK_REALLY_FIRST</tt> and <tt>HOOK_REALLY_LAST</tt>. 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>
-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,HOOK_MIDDLE);
-}
-</pre>
-
- <p>Note that the sort used to achieve this is stable, so
- ordering set by <tt>HOOK_<i>ORDER</i></tt> is preserved, as far
- as is possible.</p>
- <i>Ben Laurie, 15th August 1999</i>
- <!--#include virtual="footer.html" -->
- </body>
-</html>
-
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