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<parentdocument href="./">Developer Documentation</parentdocument>

<title>Hook Functions in the Apache HTTP Server 2.x</title>

<summary>

<note type="warning"><title>Warning</title>

<p>This document is still in development and may be partially out of

date.</p>

</note>

<p>In general, a hook function is one that the Apache HTTP Server

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>

</summary>

<section id="create"><title>Creating a hook function</title>

<p>In order to create a new hook, four things need to be

done:</p>

<section id="create-declare"><title>Declare the hook function</title>

<p>Use the <code>AP_DECLARE_HOOK</code> 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 <code>int</code> and

takes a <code>request_rec *</code> and an <code>int</code> and is

called <code>do_something</code>, then declare it like this:</p>

<highlight language="c">

AP_DECLARE_HOOK(int, do_something, (request_rec *r, int n))

</highlight>

<p>This should go in a header which modules will include if

they want to use the hook.</p>

</section>

<section id="create-create"><title>Create the hook structure</title>

<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>

<highlight language="c">

APR_HOOK_STRUCT(

APR_HOOK_LINK(do_something)

...

)

</highlight>

</section>

<section id="create-implement"><title>Implement the hook caller</title>

<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 <code>ap_run_<var>hookname</var>()</code>.</p>

<section><title>Void hooks</title>

<p>If the return value of a hook is <code>void</code>, then all the

hooks are called, and the caller is implemented like this:</p>

<highlight language="c">

AP_IMPLEMENT_HOOK_VOID(do_something, (request_rec *r, int n), (r, n))

</highlight>

<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>

<highlight language="c">

void ap_run_do_something(request_rec *r, int n)

{

...

do_something(r, n);

}

</highlight>

</section>

<section><title>Hooks that return a value</title>

<p>If the hook returns a value, then it can either be run until

the first hook that does something interesting, like so:</p>

<highlight language="c">

AP_IMPLEMENT_HOOK_RUN_FIRST(int, do_something, (request_rec *r, int n), (r, n), DECLINED)

</highlight>

<p>The first hook that does <em>not</em> return <code>DECLINED</code>

stops the loop and its return value is returned from the hook

caller. Note that <code>DECLINED</code> is the traditional

hook return value 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 <em>two</em> 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>

<highlight language="c">

AP_IMPLEMENT_HOOK_RUN_ALL(int, do_something, (request_rec *r, int n), (r, n), OK, DECLINED)

</highlight>

<p>Again, <code>OK</code> and <code>DECLINED</code> are the traditional

values. You can use what you want.</p>

</section>

</section>

<section id="create-call"><title>Call the hook callers</title>

<p>At appropriate moments in the code, call the hook caller,

like so:</p>

<highlight language="c">

int n, ret;

request_rec *r;

ret=ap_run_do_something(r, n);

</highlight>

</section>

</section>

<section id="hooking"><title>Hooking the hook</title>

<p>A module that wants a hook to be called needs to do two

things.</p>

<section id="hooking-implement"><title>Implement the hook function</title>

<p>Include the appropriate header, and define a static function

of the correct type:</p>

<highlight language="c">

static int my_something_doer(request_rec *r, int n)<br />

{

...

return OK;

}

</highlight>

</section>

<section id="hooking-add"><title>Add a hook registering function</title>

<p>During initialisation, the server will call each modules hook

registering function, which is included in the module

structure:</p>

<highlight language="c">

static void my_register_hooks()

{

ap_hook_do_something(my_something_doer, NULL, NULL, APR_HOOK_MIDDLE);

}

mode MODULE_VAR_EXPORT my_module =

{

...

my_register_hooks /* register hooks */

};

</highlight>

</section>

<section id="hooking-order"><title>Controlling hook calling order</title>

<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: <code>APR_HOOK_FIRST</code>,

<code>APR_HOOK_MIDDLE</code> and <code>APR_HOOK_LAST</code>.</p>

<p>All modules using any particular value may be run in any

order relative to each other, but, of course, all modules using

<code>APR_HOOK_FIRST</code> will be run before <code>APR_HOOK_MIDDLE</code>

which are before <code>APR_HOOK_LAST</code>. Modules that don't care

when they are run should use <code>APR_HOOK_MIDDLE</code>. <em>These

values are spaced out, so that positions like <code>APR_HOOK_FIRST-2</code>

are possible to hook slightly earlier than other functions.</em></p>

<p>Note that there are two more values,

<code>APR_HOOK_REALLY_FIRST</code> and <code>APR_HOOK_REALLY_LAST</code>. 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>

<highlight language="c">

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, APR_HOOK_MIDDLE);

}

</highlight>

<p>Note that the sort used to achieve this is stable, so

ordering set by <code>APR_HOOK_<var>ORDER</var></code> is preserved, as far

as is possible.</p>

</section>

</section>

</manualpage>