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1.1 apache-2.0/htdocs/manual/developer/hooks.html
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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>
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1.1 apache-2.0/htdocs/manual/developer/modules.html
Index: modules.html
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<TITLE>Converting Modules from Apache 1.3 to Apache 2.0</TITLE>
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<H1 ALIGN="CENTER">From Apache 1.3 to Apache 2.0<br>Modules</H1>
<p>
This is a first attempt at writing the lessons I learned when trying to
convert the mod_mmap_static module to Apache 2.0. It's by no means definitive
and probably won't even be correct in some ways, but it's a start.
</p>
<hr>
<h2>The easier changes...
</h2>
<h3>Cleanup Routines</h3>
<p>
These now need to be of type ap_status_t and return a value of that type.
Normally the return value will be APR_SUCCESS unless there is some need to
signal an error in the cleanup. Be aware that even though you signal an error
not all code yet checks and acts upon the error.
</p>
<h3>Initialisation Routines</h3>
<p>
These should now be renamed to better signify where they sit in the overall
process. So the name gets a small change from mmap_init to mmap_post_config.
The arguments passed have undergone a radical change and now look like
</p>
<ul style="list-style:none">
<li>ap_context_t *p,</li>
<li>ap_context_t *plog,</li>
<li>ap_context_t *ptemp,</li>
<li>server_rec *s</li>
</ul>
<p>
Throughout Apache the old pools have been replced by the ap_context_t, though
their use remains remarkably similar.
</p>
<h3>Data Types</h3>
<p>
A lot of the data types have been moved into the APR. This means that some
have had a name change, such as the one shown above. The following is a brief
list of some of the changes that you are likely to have to make.
<ul style="list-style:none">
<li>pool becomes ap_context_t</li>
<li>table becomes ap_table_t</li>
</ul>
<hr>
<h2>
The <em>messier</em> changes...
</h2>
<h3>Register Hooks</h3>
<p>
The new architecture uses a series of hooks to provide for calling your
functions. These you'll need to add to your module by way of a new function,
static void register_hooks(void). The function is really reasonably
straightforward once you understand what needs to be done. Each function that
needs calling at some stage in the processing of a request needs to be
registered, handlers do not. There are a number of phases where functions can
be added, and for each you can specify with a high degree of control the
relative order that the function will be called in.
</p>
<p>
This is the code that was added to mod_mmap_static
</p>
<pre>
static void register_hooks(void)
{
static const char * const aszPre[]={ "http_core.c",NULL };
ap_hook_post_config(mmap_post_config,NULL,NULL,HOOK_MIDDLE);
ap_hook_translate_name(mmap_static_xlat,aszPre,NULL,HOOK_LAST);
};
</pre>
<p>
This registers 2 functions that need to be called, one in the post_config
stage (virtually every module will need this one) and one for the
translate_name phase. note that while there are different function names the
format of each is identical. So what is the format?
</p>
<p><strong>
ap_hook_[phase_name](function_name, predecessors, successors, position);
</strong></p>
<p>
There are 3 hook positions defined...
</p>
<ul style="list-style:none">
<li>HOOK_FIRST</li>
<li>HOOK_MIDDLE</li>
<li>HOOK_LAST</li>
</ul>
<p>
To define the position you use the position and then modify it with the
predecessors and successors. each of the modifiers can be a list of functions
that should be called, either before the function is run (predecessors) or
after the function has run (successors).
</p>
<p>
In the mod_mmap_static case I didn't care about the post_config stage, but
the mmap_static_xlat MUST be called after the core module had done it's name
translation, hence the use of the aszPre to define a modifier to the position
HOOK_LAST.
</p>
<h3>Module Definition</h3>
<p>
There are now a lot fewer stages to worry about when creating your module
definition. The old defintion looked like
</p>
<pre>
module MODULE_VAR_EXPORT [module_name]_module =
{
STANDARD_MODULE_STUFF,
/* initializer */
/* dir config creater */
/* dir merger --- default is to override */
/* server config */
/* merge server config */
/* command handlers */
/* handlers */
/* filename translation */
/* check_user_id */
/* check auth */
/* check access */
/* type_checker */
/* fixups */
/* logger */
/* header parser */
/* child_init */
/* child_exit */
/* post read-request */
};
</pre>
<p>
The new structure is a great deal simpler...
</p>
<pre>
module MODULE_VAR_EXPORT [module_name]_module =
{
STANDARD20_MODULE_STUFF,
/* create per-directory config structures */
/* merge per-directory config structures */
/* create per-server config structures */
/* merge per-server config structures */
/* command handlers */
/* handlers */
/* register hooks */
};
</pre>
<p>
Some of these read directly across, some don't. I'll try to summarise what
should be done below.
</p>
<p>
The stages that read directly across :
</p>
<ul style="list-style:none">
<li>
/* dir config creater */ ==> /* create per-directory config structures */
</li>
<li>
/* server config */ ==> /* create per-server config structures */
</li>
<li>
/* dir merger */ ==> /* merge per-directory config structures */
</li>
<li>
/* merge server config */ ==> /* merge per-server config structures */
</li>
<li>
/* command table */ ==> /* command ap_table_t */
</li>
<li>
/* handlers */ ==> /* handlers */
</li>
</ul>
<P>
The remainder of the old functions should be registered as hooks. There are
the following hook stages defined so far...
</P>
<ul style="list-style:none">
<li>
ap_hook_post_config <em>(this is where the old _init routines get
registered)</em>
</li>
<li>
ap_hook_http_method
</li>
<li>
ap_hook_open_logs
</li>
<li>
ap_hook_auth_checker
</li>
<li>
ap_hook_default_port
</li>
<li>
ap_hook_access_checker
</li>
<li>
ap_hook_process_connection
</li>
<li>
ap_hook_child_init_hook
</li>
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