Apache Hook Functions

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.

Creating a hook function

In order to create a new hook, four things need to be done:

Declare the hook function

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 int and takes a request_rec * and an int and is called "do_something", then declare it like this:

This should go in a header which modules will include if they want to use the hook.

Create the hook structure

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:

HOOK_STRUCT(
            HOOK_LINK(do_something)
            ...
           )

Implement the hook caller

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 ap_run_hookname().

Void hooks

If the return value of a hook is void, then all the hooks are called, and the caller is implemented like this:

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:

void ap_run_do_something(request_rec *r,int n)
{
    ...
    do_something(r,n);
}

Hooks that return a value

If the hook returns a value, then it can either be run until the first hook that does something interesting, like so:

The first hook that doesn't return DECLINED stops the loop and its return value is returned from the hook caller. Note that DECLINED is the tradition Apache hook return meaning "I didn't do anything", but it can be whatever suits you.

Alternatively, all hooks can be run until an error occurs. This boils down to permitting two 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:

Again, OK and DECLINED are the traditional values. You can use what you want.

Call the hook callers

At appropriate moments in the code, call the hook caller, like so:

    int n,ret;
    request_rec *r;

    ret=ap_run_do_something(r,n);

Hooking the hook

A module that wants a hook to be called needs to do two things.

Implement the hook function

Include the appropriate header, and define a static function of the correct type:

static int my_something_doer(request_rec *r,int n)
{
    ...
    return OK;
}

Add a hook registering function

During initialisation, Apache will call each modules hook registering function, which is included in the module structure:

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 */
};

Controlling hook calling order

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:

HOOK_FIRST
HOOK_MIDDLE
HOOK_LAST

All modules using any particular value may be run in any order relative to each other, but, of course, all modules using HOOK_FIRST will be run before HOOK_MIDDLE which are before HOOK_LAST. Modules that don't care when they are run should use HOOK_MIDDLE. (I spaced these out so people could do stuff like HOOK_FIRST-2 to get in slightly earlier, but is this wise? - Ben)

Note that there are two more values, HOOK_REALLY_FIRST and HOOK_REALLY_LAST. These should only be used by the hook exporter.

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:

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);
}

Note that the sort used to achieve this is stable, so ordering set by HOOK_ORDER is preserved, as far as is possible.