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     <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>
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	<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>
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