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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 DECLARE_HOOK macro, which needs to be given the name of the
 hook, the return type of the hook function 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>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>
 HOOK_STRUCT(
             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>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>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>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>
 </body>
 </html>
	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
	<html>
	<head>
	<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"
	>

	<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 DECLARE_HOOK macro, which needs to be given the name of the
	hook, the return type of the hook function 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>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>
	HOOK_STRUCT(
	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>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>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>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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