Hook Functions in the Apache HTTP Server 2.x

This document is still in development and may be partially out of date.

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.

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

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 int

and takes a request_rec *

, then declare it like this:

APDECLAREHOOK(int, dosomething, (requestrec *r, int n))

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

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:

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 aprun

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 aprundosomething(requestrec *r, int n) { ... do_something(r, n); }

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

APIMPLEMENTHOOKRUNFIRST(int, dosomething, (requestrec *r, int n), (r, n), DECLINED)

stops the loop and its return value is returned from the hook caller. Note that DECLINED

is the traditional hook return value 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:

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

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

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

static void myregisterhooks() { aphookdosomething(mysomethingdoer, NULL, NULL, APRHOOKMIDDLE); } mode MODULEVAREXPORT mymodule = { ... myregisterhooks / register hooks / };

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: APRHOOKFIRST

All modules using any particular value may be run in any order relative to each other, but, of course, all modules using APRHOOKFIRST

which are before APRHOOKLAST

. These values are spaced out, so that positions like APRHOOKFIRST-2 are possible to hook slightly earlier than other functions.

. 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 "modxyz.c" and "modabc.c" to run before we do, then we'd hook as follows:

static void registerhooks() { static const char * const aszPre[] = { "modxyz.c", "modabc.c", NULL }; aphookdosomething(mysomethingdoer, aszPre, NULL, APRHOOKMIDDLE); }

Note that the sort used to achieve this is stable, so ordering set by APRHOOK

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Apache HTTP Server Version 2.4