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c82fca6d3f5608b946f18d37e8710b1d71e3478dnd<parentdocument href="./">Developer Documentation</parentdocument>
c98b488eb0a76ff64674644913f4bbd1739ed03ahumbedooh<title>Hook Functions in the Apache HTTP Server 2.x</title>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>This document is still in development and may be partially out of
a99c5d4cc3cab6a62b04d52000dbc22ce1fa2d94coar <p>In general, a hook function is one that the Apache HTTP Server
a99c5d4cc3cab6a62b04d52000dbc22ce1fa2d94coar will call at some point during the processing of a request.
a99c5d4cc3cab6a62b04d52000dbc22ce1fa2d94coar Modules can provide functions that are called, and specify when
d6d9a8670a4620266fc9aa4958d8e42668a6bb68humbedooh they get called in comparison to other modules.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd<section id="create"><title>Creating a hook function</title>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>In order to create a new hook, four things need to be
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <section id="create-declare"><title>Declare the hook function</title>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>Use the <code>AP_DECLARE_HOOK</code> macro, which needs to be given
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd the return type of the hook function, the name of the hook, and the
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd arguments. For example, if the hook returns an <code>int</code> and
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd takes a <code>request_rec *</code> and an <code>int</code> and is
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd called <code>do_something</code>, then declare it like this:</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd AP_DECLARE_HOOK(int, do_something, (request_rec *r, int n))
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh </highlight>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>This should go in a header which modules will include if
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd they want to use the hook.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd </section>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <section id="create-create"><title>Create the hook structure</title>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>Each source file that exports a hook has a private structure
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd which is used to record the module functions that use the hook.
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd This is declared as follows:</p>
94e836655ae06d0a14ec9aaaf6455bcaf2872d1dhumbedoohAPR_HOOK_STRUCT(
94e836655ae06d0a14ec9aaaf6455bcaf2872d1dhumbedooh APR_HOOK_LINK(do_something)
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh </highlight>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd </section>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <section id="create-implement"><title>Implement the hook caller</title>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>The source file that exports the hook has to implement a
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd function that will call the hook. There are currently three
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd possible ways to do this. In all cases, the calling function is
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>If the return value of a hook is <code>void</code>, then all the
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd hooks are called, and the caller is implemented like this:</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd AP_IMPLEMENT_HOOK_VOID(do_something, (request_rec *r, int n), (r, n))
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh </highlight>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>The second and third arguments are the dummy argument
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd declaration and the dummy arguments as they will be used when
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd calling the hook. In other words, this macro expands to
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd something like this:</p>
94e836655ae06d0a14ec9aaaf6455bcaf2872d1dhumbedoohvoid ap_run_do_something(request_rec *r, int n)
bb10b4c35ab42a1950a733a36e4fb2dd1168b236jailletc do_something(r, n);
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh </highlight>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd </section>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>If the hook returns a value, then it can either be run until
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd the first hook that does something interesting, like so:</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd AP_IMPLEMENT_HOOK_RUN_FIRST(int, do_something, (request_rec *r, int n), (r, n), DECLINED)
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh </highlight>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>The first hook that does <em>not</em> return <code>DECLINED</code>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd stops the loop and its return value is returned from the hook
a99c5d4cc3cab6a62b04d52000dbc22ce1fa2d94coar caller. Note that <code>DECLINED</code> is the traditional
d6d9a8670a4620266fc9aa4958d8e42668a6bb68humbedooh hook return value meaning "I didn't do anything", but it can be
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd whatever suits you.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>Alternatively, all hooks can be run until an error occurs.
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd This boils down to permitting <em>two</em> return values, one of
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd which means "I did something, and it was OK" and the other
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd meaning "I did nothing". The first function that returns a
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd value other than one of those two stops the loop, and its
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd return is the return value. Declare these like so:</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd AP_IMPLEMENT_HOOK_RUN_ALL(int, do_something, (request_rec *r, int n), (r, n), OK, DECLINED)
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh </highlight>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>Again, <code>OK</code> and <code>DECLINED</code> are the traditional
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd values. You can use what you want.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd </section>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd </section>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <section id="create-call"><title>Call the hook callers</title>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>At appropriate moments in the code, call the hook caller,
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd like so:</p>
94e836655ae06d0a14ec9aaaf6455bcaf2872d1dhumbedoohrequest_rec *r;
94e836655ae06d0a14ec9aaaf6455bcaf2872d1dhumbedoohret=ap_run_do_something(r, n);
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh </highlight>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd </section>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>A module that wants a hook to be called needs to do two
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd things.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <section id="hooking-implement"><title>Implement the hook function</title>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>Include the appropriate header, and define a static function
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd of the correct type:</p>
94e836655ae06d0a14ec9aaaf6455bcaf2872d1dhumbedoohstatic int my_something_doer(request_rec *r, int n)<br />
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh </highlight>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd </section>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <section id="hooking-add"><title>Add a hook registering function</title>
d6d9a8670a4620266fc9aa4958d8e42668a6bb68humbedooh <p>During initialisation, the server will call each modules hook
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd registering function, which is included in the module
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd structure:</p>
94e836655ae06d0a14ec9aaaf6455bcaf2872d1dhumbedoohstatic void my_register_hooks()
94e836655ae06d0a14ec9aaaf6455bcaf2872d1dhumbedooh ap_hook_do_something(my_something_doer, NULL, NULL, APR_HOOK_MIDDLE);
94e836655ae06d0a14ec9aaaf6455bcaf2872d1dhumbedoohmode MODULE_VAR_EXPORT my_module =
94e836655ae06d0a14ec9aaaf6455bcaf2872d1dhumbedooh my_register_hooks /* register hooks */
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh </highlight>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd </section>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <section id="hooking-order"><title>Controlling hook calling order</title>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>In the example above, we didn't use the three arguments in
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd the hook registration function that control calling order.
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd There are two mechanisms for doing this. The first, rather
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd crude, method, allows us to specify roughly where the hook is
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd run relative to other modules. The final argument control this.
1c14060ebaf0bab91387c4be294e0d3fddf2068cyoshiki There are three possible values: <code>APR_HOOK_FIRST</code>,
1c14060ebaf0bab91387c4be294e0d3fddf2068cyoshiki <code>APR_HOOK_MIDDLE</code> and <code>APR_HOOK_LAST</code>.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>All modules using any particular value may be run in any
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd order relative to each other, but, of course, all modules using
1c14060ebaf0bab91387c4be294e0d3fddf2068cyoshiki <code>APR_HOOK_FIRST</code> will be run before <code>APR_HOOK_MIDDLE</code>
1c14060ebaf0bab91387c4be294e0d3fddf2068cyoshiki which are before <code>APR_HOOK_LAST</code>. Modules that don't care
a99c5d4cc3cab6a62b04d52000dbc22ce1fa2d94coar when they are run should use <code>APR_HOOK_MIDDLE</code>. <em>These
d6d9a8670a4620266fc9aa4958d8e42668a6bb68humbedooh values are spaced out, so that positions like <code>APR_HOOK_FIRST-2</code>
d6d9a8670a4620266fc9aa4958d8e42668a6bb68humbedooh are possible to hook slightly earlier than other functions.</em></p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>Note that there are two more values,
1c14060ebaf0bab91387c4be294e0d3fddf2068cyoshiki <code>APR_HOOK_REALLY_FIRST</code> and <code>APR_HOOK_REALLY_LAST</code>. These
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd should only be used by the hook exporter.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>The other method allows finer control. When a module knows
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd that it must be run before (or after) some other modules, it
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd can specify them by name. The second (third) argument is a
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd NULL-terminated array of strings consisting of the names of
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd modules that must be run before (after) the current module. For
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd example, suppose we want "mod_xyz.c" and "mod_abc.c" to run
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd before we do, then we'd hook as follows:</p>
94e836655ae06d0a14ec9aaaf6455bcaf2872d1dhumbedoohstatic void register_hooks()
94e836655ae06d0a14ec9aaaf6455bcaf2872d1dhumbedooh static const char * const aszPre[] = { "mod_xyz.c", "mod_abc.c", NULL };
94e836655ae06d0a14ec9aaaf6455bcaf2872d1dhumbedooh ap_hook_do_something(my_something_doer, aszPre, NULL, APR_HOOK_MIDDLE);
1050464f9f91f75e7a1c5c3daf3fb7b8aa74592ahumbedooh </highlight>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd <p>Note that the sort used to achieve this is stable, so
1c14060ebaf0bab91387c4be294e0d3fddf2068cyoshiki ordering set by <code>APR_HOOK_<var>ORDER</var></code> is preserved, as far
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd as is possible.</p>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd </section>
1eba1be63201689b2d3e651d3a35f0bc2057e2c0nd</manualpage>