docs/manual/developer/hooks.xml

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<manualpage metafile="hooks.xml.meta">
<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This document is still in development and may be partially out of
      date.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="corehooks"><title>Core Hooks</title>
    <p>The httpd's core modules offer a predefined list of hooks
    used during the standard <a href="./request.html">request processing</a>
    phase. Creating a new hook will expose a function that 
    implements it (see sections below) but it is essential to understand that you will not 
    extend the httpd's core hooks. Their presence and order in the request processing is in fact 
    a consequence of how they are called in <code>server/request.c</code> 
    (check <a href="./modguide.html#hooking">this section</a> 
    for an overview). The core hooks are listed in the 
    <a href="https://ci.apache.org/projects/httpd/trunk/doxygen/group__hooks.html">doxygen documentation</a>.</p>

    <p>Reading <a href="./modguide.html">guide for developing modules</a> and 
    <a href="./request.html">request processing</a> before proceeding is 
    highly recommended.
    </p> 
</section>

<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)
{
    ...
    return OK;
}
      </highlight>
    </section>

    <section id="hooking-add"><title>Add a hook registering function</title>
      <p>During initialization, 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 of 
      all the functions registered within the hook.
      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>