ben 99/08/15 06:50:27
Added: mpm/htdocs hooks.html
Log:
First-cut docco for hook functions.
Revision Changes Path
1.1 apache-2.0/mpm/htdocs/hooks.html
Index: hooks.html
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<H1>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>
