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200 lines
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200 lines
6.5 KiB
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<meta name="generator" content="HTML Tidy, see www.w3.org" />
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<title>Apache 2.0 Hook Functions</title>
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<!--#include virtual="header.html" -->
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<h1 align="center">Apache Hook Functions</h1>
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<p>In general, a hook function is one that Apache will call at
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some point during the processing of a request. Modules can
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provide functions that are called, and specify when they get
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called in comparison to other modules.</p>
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<h2>Creating a hook function</h2>
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<p>In order to create a new hook, four things need to be
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done:</p>
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<h3>Declare the hook function</h3>
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<p>Use the AP_DECLARE_HOOK macro, which needs to be given the
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return type of the hook function, the name of the hook, and the
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arguments. For example, if the hook returns an <tt>int</tt> and
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takes a <tt>request_rec *</tt> and an <tt>int</tt> and is
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called "do_something", then declare it like this:</p>
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<tt>AP_DECLARE_HOOK(int,do_something,(request_rec *r,int
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n))</tt>
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<p>This should go in a header which modules will include if
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they want to use the hook.</p>
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<h3>Create the hook structure</h3>
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<p>Each source file that exports a hook has a private structure
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which is used to record the module functions that use the hook.
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This is declared as follows:</p>
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<pre>
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APR_HOOK_STRUCT(
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APR_HOOK_LINK(do_something)
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...
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)
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</pre>
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<h3>Implement the hook caller</h3>
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<p>The source file that exports the hook has to implement a
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function that will call the hook. There are currently three
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possible ways to do this. In all cases, the calling function is
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called <tt>ap_run_<i>hookname</i>()</tt>.</p>
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<h4>Void hooks</h4>
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<p>If the return value of a hook is <tt>void</tt>, then all the
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hooks are called, and the caller is implemented like this:</p>
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<tt>AP_IMPLEMENT_HOOK_VOID(do_something,(request_rec *r,int
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n),(r,n))</tt>
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<p>The second and third arguments are the dummy argument
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declaration and the dummy arguments as they will be used when
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calling the hook. In other words, this macro expands to
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something like this:</p>
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<pre>
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void ap_run_do_something(request_rec *r,int n)
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{
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...
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do_something(r,n);
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}
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</pre>
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<h4>Hooks that return a value</h4>
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<p>If the hook returns a value, then it can either be run until
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the first hook that does something interesting, like so:</p>
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<tt>AP_IMPLEMENT_HOOK_RUN_FIRST(int,do_something,(request_rec
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*r,int n),(r,n),DECLINED)</tt>
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<p>The first hook that <i>doesn't</i> return <tt>DECLINED</tt>
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stops the loop and its return value is returned from the hook
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caller. Note that <tt>DECLINED</tt> is the tradition Apache
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hook return meaning "I didn't do anything", but it can be
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whatever suits you.</p>
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<p>Alternatively, all hooks can be run until an error occurs.
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This boils down to permitting <i>two</i> return values, one of
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which means "I did something, and it was OK" and the other
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meaning "I did nothing". The first function that returns a
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value other than one of those two stops the loop, and its
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return is the return value. Declare these like so:</p>
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<tt>AP_IMPLEMENT_HOOK_RUN_ALL(int,do_something,(request_rec
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*r,int n),(r,n),OK,DECLINED)</tt>
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<p>Again, <tt>OK</tt> and <tt>DECLINED</tt> are the traditional
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values. You can use what you want.</p>
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<h3>Call the hook callers</h3>
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<p>At appropriate moments in the code, call the hook caller,
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like so:</p>
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<pre>
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int n,ret;
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request_rec *r;
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ret=ap_run_do_something(r,n);
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</pre>
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<h2>Hooking the hook</h2>
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<p>A module that wants a hook to be called needs to do two
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things.</p>
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<h3>Implement the hook function</h3>
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<p>Include the appropriate header, and define a static function
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of the correct type:</p>
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<pre>
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static int my_something_doer(request_rec *r,int n)
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{
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...
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return OK;
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}
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</pre>
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<h3>Add a hook registering function</h3>
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<p>During initialisation, Apache will call each modules hook
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registering function, which is included in the module
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structure:</p>
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<pre>
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static void my_register_hooks()
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{
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ap_hook_do_something(my_something_doer,NULL,NULL,HOOK_MIDDLE);
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}
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mode MODULE_VAR_EXPORT my_module =
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{
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...
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my_register_hooks /* register hooks */
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};
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</pre>
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<h3>Controlling hook calling order</h3>
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<p>In the example above, we didn't use the three arguments in
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the hook registration function that control calling order.
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There are two mechanisms for doing this. The first, rather
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crude, method, allows us to specify roughly where the hook is
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run relative to other modules. The final argument control this.
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There are three possible values:</p>
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<pre>
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HOOK_FIRST
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HOOK_MIDDLE
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HOOK_LAST
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</pre>
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<p>All modules using any particular value may be run in any
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order relative to each other, but, of course, all modules using
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<tt>HOOK_FIRST</tt> will be run before <tt>HOOK_MIDDLE</tt>
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which are before <tt>HOOK_LAST</tt>. Modules that don't care
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when they are run should use <tt>HOOK_MIDDLE</tt>. <i>(I spaced
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these out so people could do stuff like <tt>HOOK_FIRST-2</tt>
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to get in slightly earlier, but is this wise? - Ben)</i></p>
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<p>Note that there are two more values,
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<tt>HOOK_REALLY_FIRST</tt> and <tt>HOOK_REALLY_LAST</tt>. These
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should only be used by the hook exporter.</p>
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<p>The other method allows finer control. When a module knows
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that it must be run before (or after) some other modules, it
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can specify them by name. The second (third) argument is a
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NULL-terminated array of strings consisting of the names of
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modules that must be run before (after) the current module. For
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example, suppose we want "mod_xyz.c" and "mod_abc.c" to run
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before we do, then we'd hook as follows:</p>
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<pre>
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static void register_hooks()
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{
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static const char * const aszPre[]={ "mod_xyz.c", "mod_abc.c", NULL };
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ap_hook_do_something(my_something_doer,aszPre,NULL,HOOK_MIDDLE);
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}
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</pre>
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<p>Note that the sort used to achieve this is stable, so
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ordering set by <tt>HOOK_<i>ORDER</i></tt> is preserved, as far
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as is possible.</p>
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<i>Ben Laurie, 15th August 1999</i>
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