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<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
<p class="apache">Apache HTTP Server Version 2.4</p>
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<div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="../images/left.gif" /></a></div>
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<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="../">Version 2.4</a> &gt; <a href="./">Developer</a></div><div id="page-content"><div id="preamble"><h1>Developing modules for the Apache HTTP Server 2.4</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/developer/modguide.html" title="English">&nbsp;en&nbsp;</a></p>
</div>

<p>This document explains how you can develop modules for the Apache HTTP 
Server 2.4</p>
</div>
<div id="quickview"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#introduction">Introduction</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#basics">Defining a module</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#hooking">Getting started: Hooking into the server</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#handling">Building a handler</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#configuration">Adding configuration options</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#context">Context aware configurations</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#summary">Summing up</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#snippets">Some useful snippets of code</a></li>
</ul><h3>See also</h3><ul class="seealso"><li><a href="request.html">Request Processing in Apache 2.4</a></li><li><a href="hooks.html">Apache 2.x Hook Functions</a></li><li><a href="#comments_section">Comments</a></li></ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="introduction" id="introduction">Introduction</a></h2>
<h3><a name="what" id="what">What we will be discussing in this document</a></h3>
<p>
This document will discuss how you can create modules for the Apache 
HTTP Server 2.4, by exploring an example module called 
<code>mod_example</code>. In the first part of this document, the purpose 
of this module will be to calculate and print out various digest values for 
existing files on your web server, whenever we access the URL <code>
http://hostname/filename.sum</code>. For instance, if we want to know the 
MD5 digest value of the file located at <code>
http://www.example.com/index.html</code>, we would visit <code>
http://www.example.com/index.html.sum</code>. 
</p>

<p>
In the second part of this document, which deals with configuration 
directive and context awareness, we will be looking at a module that simply 
writes out its own configuration to the client.
</p>


<h3><a name="prerequisites" id="prerequisites">Prerequisites</a></h3>
<p>
First and foremost, you are expected to have a basic knowledge of how the C 
programming language works. In most cases, we will try to be as pedagogical 
as possible and link to documents describing the functions used in the 
examples, but there are also many cases where it is necessary to either 
just assume that "it works" or do some digging yourself into what the hows 
and whys of various function calls. 
</p>
<p>
Lastly, you will need to have a basic understanding of how modules are 
loaded and configured in the Apache HTTP Server, as well as how to get the headers for 
Apache if you do not have them already, as these are needed for compiling 
new modules.
</p>

<h3><a name="compiling" id="compiling">Compiling your module</a></h3>
<p>
To compile the source code we are building in this document, we will be 
using <a href="../programs/apxs.html">APXS</a>. Assuming your source file 
is called mod_example.c, compiling, installing and activating the module is 
as simple as: 
</p>
<div class="example"><pre>apxs -i -a -c mod_example.c</pre></div>


</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="basics" id="basics">Defining a module</a></h2>
<p>
<img src="../images/build_a_mod_3.png" alt="Module name tags" /><br />
Every module starts with the same declaration, or name tag if you will, 
that defines a module as <em>a separate entity within Apache</em>:</p>



<pre class="prettyprint lang-c">module AP_MODULE_DECLARE_DATA   example_module =
{ 
    STANDARD20_MODULE_STUFF,
    create_dir_conf, /* Per-directory configuration handler */
    merge_dir_conf,  /* Merge handler for per-directory configurations */
    create_svr_conf, /* Per-server configuration handler */
    merge_svr_conf,  /* Merge handler for per-server configurations */
    directives,      /* Any directives we may have for httpd */
    register_hooks   /* Our hook registering function */
};</pre>



<p>
This bit of code lets the server know that we have now registered a new module 
in the system, and that its name is <code>example_module</code>. The name 
of the module is used primarily for two things:<br />
</p>
<ul>
<li>Letting the server know how to load the module using the LoadModule</li>
<li>Setting up a namespace for the module to use in configurations</li>
</ul>
<p>
For now, we're only concerned with the first purpose of the module name, 
which comes into play when we need to load the module:
</p>
<pre class="prettyprint lang-config">LoadModule example_module modules/mod_example.so</pre>

<p>
In essence, this tells the server to open up <code>mod_example.so</code> and look for a module 
called <code>example_module</code>.
</p>
<p>
Within this name tag of ours is also a bunch of references to how we would 
like to handle things: Which directives do we respond to in a configuration 
file or .htaccess, how do we operate within specific contexts, and what 
handlers are we interested in registering with the Apache HTTP service. We'll 
return to all these elements later in this document.
</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="hooking" id="hooking">Getting started: Hooking into the server</a></h2>
<h3><a name="hook_intro" id="hook_intro">An introduction to hooks</a></h3>
<p>
When handling requests in Apache HTTP Server 2.4, the first thing you will need to do is 
create a hook into the request handling process. A hook is essentially a 
message telling the server that you are willing to either serve or at least 
take a glance at certain requests given by clients. All handlers, whether 
it's mod_rewrite, mod_authn_*, mod_proxy and so on, are hooked into 
specific parts of the request process. As you are probably aware, modules 
serve different purposes; Some are authentication/authorization handlers, 
others are file or script handlers while some third modules rewrite URIs or 
proxies content. Furthermore, in the end, it is up to the user of the server 
how and when each module will come into place. Thus, the server itself does not 
presume to know which module is responsible for handling a specific 
request, and will ask each module whether they have an interest in a given 
request or not. It is then up to each module to either gently decline 
serving a request, accept serving it or flat out deny the request from 
being served, as authentication/authorization modules do: <br />
<img src="../images/build_a_mod_2.png" alt="Hook handling in httpd" /><br />
To make it a bit easier for handlers such as our mod_example to know 
whether the client is requesting content we should handle or not, the server 
has directives for hinting to modules whether their assistance is needed or 
not. Two of these are <code class="directive"><a href="../mod/mod_mime.html#addhandler">AddHandler</a></code> 
and <code class="directive"><a href="../mod/core.html#sethandler">SetHandler</a></code>. Let's take a look at 
an example using <code class="directive"><a href="../mod/mod_mime.html#addhandler">AddHandler</a></code>. In 
our example case, we want every request ending with .sum to be served by 
<code>mod_example</code>, so we'll add a configuration directive that tells 
the server to do just that:
</p>
<pre class="prettyprint lang-config">AddHandler example-handler .sum</pre>

<p>
What this tells the server is the following: <em>Whenever we receive a request 
for a URI ending in .sum, we are to let all modules know that we are 
looking for whoever goes by the name of "example-handler" </em>. 
Thus, when a request is being served that ends in .sum, the server will let all 
modules know, that this request should be served by "example-handler
". As you will see later, when we start building mod_example, we will 
check for this handler tag relayed by <code>AddHandler</code> and reply to 
the server based on the value of this tag.
</p>

<h3><a name="hook_declaration" id="hook_declaration">Hooking into httpd</a></h3>
<p>
To begin with, we only want to create a simple handler, that replies to the 
client browser when a specific URL is requested, so we won't bother setting 
up configuration handlers and directives just yet. Our initial module 
definition will look like this:</p>



<pre class="prettyprint lang-c">module AP_MODULE_DECLARE_DATA   example_module =
{
    STANDARD20_MODULE_STUFF,
    NULL,
    NULL,
    NULL,
    NULL,
    NULL,
    register_hooks   /* Our hook registering function */
};</pre>




<p>This lets the server know that we are not interested in anything fancy, we 
just want to hook onto the requests and possibly handle some of them. </p> 

<p> The reference in our example declaration, <code>register_hooks</code> 
is the name of a function we will create to manage how we hook onto the 
request process. In this example module, the function has just one purpose; 
To create a simple hook that gets called after all the rewrites, access 
control etc has been handled. Thus, we will let the server know, that we want 
to hook into its process as one of the last modules: 
</p>


<pre class="prettyprint lang-c">static void register_hooks(apr_pool_t *pool)
{
    /* Create a hook in the request handler, so we get called when a request arrives */
    ap_hook_handler(example_handler, NULL, NULL, APR_HOOK_LAST);
}</pre>



<p>
The <code>example_handler</code> reference is the function that will handle 
the request. We will discuss how to create a handler in the next chapter.
</p>

<h3><a name="hook_others" id="hook_others">Other useful hooks</a></h3>
<p>
Hooking into the request handling phase is but one of many hooks that you 
can create. Some other ways of hooking are:
</p>
<ul>
<li><code>ap_hook_child_init</code>: Place a hook that executes when a child process is spawned (commonly used for initializing modules after the server has forked)</li>
<li><code>ap_hook_pre_config</code>: Place a hook that executes before any configuration data has been read (very early hook)</li>
<li><code>ap_hook_post_config</code>: Place a hook that executes after configuration has been parsed, but before the server has forked</li>
<li><code>ap_hook_translate_name</code>: Place a hook that executes when a URI needs to be translated into a filename on the server (think <code>mod_rewrite</code>)</li>
<li><code>ap_hook_quick_handler</code>: Similar to <code>ap_hook_handler</code>, except it is run before any other request hooks (translation, auth, fixups etc)</li>
<li><code>ap_hook_log_transaction</code>: Place a hook that executes when the server is about to add a log entry of the current request</li>
</ul>


</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="handling" id="handling">Building a handler</a></h2>
<p>
A handler is essentially a function that receives a callback when a request 
to the server is made. It is passed a record of the current request (how it was 
made, which headers and requests were passed along, who's giving the 
request and so on), and is put in charge of either telling the server that it's 
not interested in the request or handle the request with the tools provided.
</p>
<h3><a name="simple_handler" id="simple_handler">A simple "Hello, world!" 
handler</a></h3> 
<p>Let's start off by making a very simple request handler 
that does the following:
</p>
<ol>
<li>Check that this is a request that should be served by "example-handler"</li>
<li>Set the content type of our output to <code>text/html</code></li>
<li>Write "Hello, world!" back to the client browser</li>
<li>Let the server know that we took care of this request and everything went fine</li>
</ol>
<p>
In C code, our example handler will now look like this:
</p>


<pre class="prettyprint lang-c">static int example_handler(request_rec *r)
{
    /* First off, we need to check if this is a call for the "example-handler" handler.
     * If it is, we accept it and do our things, if not, we simply return DECLINED,
     * and the server will try somewhere else.
     */
    if (!r-&gt;handler || strcmp(r-&gt;handler, "example-handler")) return (DECLINED);
    
    /* Now that we are handling this request, we'll write out "Hello, world!" to the client.
     * To do so, we must first set the appropriate content type, followed by our output.
     */
    ap_set_content_type(r, "text/html");
    ap_rprintf(r, "Hello, world!");
    
    /* Lastly, we must tell the server that we took care of this request and everything went fine.
     * We do so by simply returning the value OK to the server.
     */
    return OK;
}</pre>



<p>
Now, we put all we have learned together and end up with a program that 
looks like 
<a href="http://people.apache.org/~humbedooh/mods/examples/mod_example_1.c">mod_example_1.c</a>
. The functions used in this example will be explained later in the section 
<a href="#functions">"Some useful functions you should know"</a>. 
</p>
 
<h3><a name="request_rec" id="request_rec">The request_rec structure</a></h3> 
<p>The most essential part of any request is the <em>request record
</em>. In a call to a handler function, this is represented by the <code>
request_rec* </code> structure passed along with every call that is made. 
This struct, typically just referred to as <code>r</code> in modules, 
contains all the information you need for your module to fully process any 
HTTP request and respond accordingly.</p> <p>Some key elements of the <code>
request_rec </code> structure are:
</p>
<ul>
<li><code>r-&gt;handler (char*):</code> Contains the name of the handler the server is currently asking to do the handling of this request</li>
<li><code>r-&gt;method (char*):</code> Contains the HTTP method being used, f.x. GET or POST</li>
<li><code>r-&gt;filename (char*):</code> Contains the translated filename the client is requesting</li>
<li><code>r-&gt;args (char*):</code> Contains the query string of the request, if any</li>
<li><code>r-&gt;headers_in (apr_table_t*):</code> Contains all the headers sent by the client</li>
<li><code>r-&gt;connection (conn_rec*):</code> A record containing information about the current connection</li>
<li><code>r-&gt;user (char*):</code> If the URI requires authentication, this is set to the username provided</li>
<li><code>r-&gt;useragent_ip (char*):</code> The IP address of the client connecting to us</li>
<li><code>r-&gt;pool (apr_pool_t*)</code>: The memory pool of this request. We'll discuss this in the 
"<a href="#memory">Memory management</a>" chapter.</li>
</ul>
<p>
A complete list of all the values contained within the <code>request_rec</code> structure can be found in 
the <a href="http://svn.apache.org/repos/asf/httpd/httpd/trunk/include/httpd.h"><code>httpd.h</code></a> header 
file or at <a href="http://ci.apache.org/projects/httpd/trunk/doxygen/structrequest__rec.html">http://ci.apache.org/projects/httpd/trunk/doxygen/structrequest__rec.html</a>.
</p>


<p>
Let's try out some of these variables in another example handler:<br />
</p>


<pre class="prettyprint lang-c">static int example_handler(request_rec *r)
{
    /* Set the appropriate content type */
    ap_set_content_type(r, "text/html");

    /* Print out the IP address of the client connecting to us: */
    ap_rprintf(r, "&lt;h2&gt;Hello, %s!&lt;/h2&gt;", r-&gt;useragent_ip);
    
    /* If we were reached through a GET or a POST request, be happy, else sad. */
    if ( !strcmp(r-&gt;method, "POST") || !strcmp(r-&gt;method, "GET") ) {
        ap_rputs("You used a GET or a POST method, that makes us happy!&lt;br/&gt;", r);
    }
    else {
        ap_rputs("You did not use POST or GET, that makes us sad :(&lt;br/&gt;", r);
    }

    /* Lastly, if there was a query string, let's print that too! */
    if (r-&gt;args) {
        ap_rprintf(r, "Your query string was: %s", r-&gt;args);
    }
    return OK;
}</pre>





<h3><a name="return_value" id="return_value">Return values</a></h3>
<p>
Apache relies on return values from handlers to signify whether a request 
was handled or not, and if so, whether the request went well or not. If a 
module is not interested in handling a specific request, it should always 
return the value <code>DECLINED</code>. If it is handling a request, it 
should either return the generic value <code>OK</code>, or a specific HTTP 
status code, for example:
</p>


<pre class="prettyprint lang-c">static int example_handler(request_rec *r)
{
    /* Return 404: Not found */
    return HTTP_NOT_FOUND;
}</pre>



<p>
Returning <code>OK</code> or a HTTP status code does not necessarily mean 
that the request will end. The server may still have other handlers that are 
interested in this request, for instance the logging modules which, upon a 
successful request, will write down a summary of what was requested and how 
it went. To do a full stop and prevent any further processing after your 
module is done, you can return the value <code>DONE</code> to let the server 
know that it should cease all activity on this request and carry on with 
the next, without informing other handlers.
<br />
<strong>General response codes:</strong>
</p>
<ul>
<li><code>DECLINED</code>: We are not handling this request</li>
<li><code>OK</code>: We handled this request and it went well</li>
<li><code>DONE</code>: We handled this request and the server should just close this thread without further processing</li>
</ul>
<p>
<strong>HTTP specific return codes (excerpt):</strong>
</p>
<ul>
<li><code>HTTP_OK (200)</code>: Request was okay</li>
<li><code>HTTP_MOVED_PERMANENTLY (301)</code>: The resource has moved to a new URL</li>
<li><code>HTTP_UNAUTHORIZED (401)</code>: Client is not authorized to visit this page</li>
<li><code>HTTP_FORBIDDEN (403)</code>: Permission denied</li>
<li><code>HTTP_NOT_FOUND (404)</code>: File not found</li>
<li><code>HTTP_INTERNAL_SERVER_ERROR (500)</code>: Internal server error (self explanatory)</li>
</ul>


<h3><a name="functions" id="functions">Some useful functions you should know</a></h3>

<ul>
<li>
    <code>ap_rputs(const char *string, request_rec *r)</code>: <br />
    Sends a string of text to the client. This is a shorthand version of <a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__PROTO.html#gac827cd0537d2b6213a7c06d7c26cc36e">
    ap_rwrite</a>.
    


<pre class="prettyprint lang-c">ap_rputs("Hello, world!", r);</pre>




</li>
<li>
    <code>
    <a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__PROTO.html#ga5e91eb6ca777c9a427b2e82bf1eeb81d">ap_rprintf</a></code>: <br />
    This function works just like <code>printf</code>, except it sends the result to the client. 
    


<pre class="prettyprint lang-c">ap_rprintf(r, "Hello, %s!", r-&gt;useragent_ip);</pre>



</li>
<li>
    <code>
    <a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__PROTO.html#gaa2f8412c400197338ec509f4a45e4579">ap_set_content_type</a>(request_rec *r, const char *type)</code>: <br />
    Sets the content type of the output you are sending.
    


<pre class="prettyprint lang-c">ap_set_content_type(r, "text/plain"); /* force a raw text output */</pre>



</li>


</ul>


<h3><a name="memory" id="memory">Memory management</a></h3>
<p>
Managing your resources in Apache HTTP Server 2.4 is quite easy, thanks to the memory pool 
system. In essence, each server, connection and request have their own 
memory pool that gets cleaned up when its scope ends, e.g. when a request 
is done or when a server process shuts down. All your module needs to do is 
latch onto this memory pool, and you won't have to worry about having to 
clean up after yourself - pretty neat, huh?
</p>

<p>
In our module, we will primarily be allocating memory for each request, so 
it's appropriate to use the <code>r-&gt;pool</code> 
reference when creating new objects. A few of the functions for allocating 
memory within a pool are:
</p>
<ul>
<li><code>void* <a href="http://apr.apache.org/docs/apr/1.4/group__apr__pools.html#ga85f1e193c31d109affda72f9a92c6915">apr_palloc</a>(
apr_pool_t *p, apr_size_t size)</code>: Allocates <code>size</code> number of bytes in the pool for you</li>
<li><code>void* <a href="http://apr.apache.org/docs/apr/1.4/group__apr__pools.html#gaf61c098ad258069d64cdf8c0a9369f9e">apr_pcalloc</a>(
apr_pool_t *p, apr_size_t size)</code>: Allocates <code>size</code> number of bytes in the pool for you and sets all bytes to 0</li>
<li><code>char* <a href="http://apr.apache.org/docs/apr/1.4/group__apr__strings.html#gabc79e99ff19abbd7cfd18308c5f85d47">apr_pstrdup</a>(
apr_pool_t *p, const char *s)</code>: Creates a duplicate of the string <code>s</code>. This is useful for copying constant values so you can edit them</li>
<li><code>char* <a href="http://apr.apache.org/docs/apr/1.4/group__apr__strings.html#ga3eca76b8d293c5c3f8021e45eda813d8">apr_psprintf</a>(
apr_pool_t *p, const char *fmt, ...)</code>: Similar to <code>sprintf</code>, except the server supplies you with an appropriately allocated target variable</li>
</ul>

<p>Let's put these functions into an example handler:</p>



<pre class="prettyprint lang-c">static int example_handler(request_rec *r)
{
    const char *original = "You can't edit this!";
    char *copy;
    int *integers;
    
    /* Allocate space for 10 integer values and set them all to zero. */
    integers = apr_pcalloc(r-&gt;pool, sizeof(int)*10); 
    
    /* Create a copy of the 'original' variable that we can edit. */
    copy = apr_pstrdup(r-&gt;pool, original);
    return OK;
}</pre>



<p>
This is all well and good for our module, which won't need any 
pre-initialized variables or structures. However, if we wanted to 
initialize something early on, before the requests come rolling in, we 
could simply add a call to a function in our <code>register_hooks</code> 
function to sort it out:
</p>


<pre class="prettyprint lang-c">static void register_hooks(apr_pool_t *pool)
{
    /* Call a function that initializes some stuff */
    example_init_function(pool);
    /* Create a hook in the request handler, so we get called when a request arrives */
    ap_hook_handler(example_handler, NULL, NULL, APR_HOOK_LAST);
}</pre>



<p>
In this pre-request initialization function we would not be using the 
same pool as we did when allocating resources for request-based functions. 
Instead, we would use the pool given to us by the server for allocating memory 
on a per-process based level.
</p>


<h3><a name="parsing" id="parsing">Parsing request data</a></h3>
<p>
In our example module, we would like to add a feature, that checks which 
type of digest, MD5 or SHA1 the client would like to see. This could be 
solved by adding a query string to the request. A query string is typically 
comprised of several keys and values put together in a string, for instance 
<code>valueA=yes&amp;valueB=no&amp;valueC=maybe</code>. It is up to the 
module itself to parse these and get the data it requires. In our example, 
we'll be looking for a key called <code>digest</code>, and if set to <code>
md5</code>, we'll produce an MD5 digest, otherwise we'll produce a SHA1 
digest.
</p>
<p>
Since the introduction of Apache HTTP Server 2.4, parsing request data from GET and 
POST requests have never been easier. All we require to parse both GET and 
POST data is four simple lines:
</p> 



<pre class="prettyprint lang-c">
<a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__apr__tables.html#gad7ea82d6608a4a633fc3775694ab71e4">apr_table_t</a> *GET; <em>
</em><a href="http://ci.apache.org/projects/httpd/trunk/doxygen/structapr__array__header__t.html">apr_array_header_t</a>*POST; 
<em>
</em>
<a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__SCRIPT.html#gaed25877b529623a4d8f99f819ba1b7bd">
ap_args_to_table</a>(r, &amp;GET); <em>
</em><a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__DAEMON.html#ga9d426b6382b49754d4f87c55f65af202">
ap_parse_form_data</a>(r, NULL, &amp;POST, -1, 8192);</pre>



<p>
In our specific example module, we're looking for the <code>digest</code> 
value from the query string, which now resides inside a table called <code>
GET</code>. To extract this value, we need only perform a simple operation:
</p>



<pre class="prettyprint lang-c">/* Get the "digest" key from the query string, if any. */
const char *digestType = apr_table_get(GET, "digest");

/* If no key was returned, we will set a default value instead. */
if (!digestType) digestType = "sha1";</pre>



<p>
The structures used for the POST and GET data are not exactly the same, so 
if we were to fetch a value from POST data instead of the query string, we 
would have to resort to a few more lines, as outlined in <a href="#get_post">this example</a> in the last chapter of this document.
</p>


<h3><a name="advanced_handler" id="advanced_handler">Making an advanced handler</a></h3>
<p>
Now that we have learned how to parse form data and manage our resources, 
we can move on to creating an advanced version of our module, that spits 
out the MD5 or SHA1 digest of files:
</p>



<pre class="prettyprint lang-c">static int example_handler(request_rec *r)
{
    int rc, exists;
    apr_finfo_t finfo;
    apr_file_t *file;
    char *filename;
    char buffer[256];
    apr_size_t readBytes;
    int n;
    apr_table_t *GET;
    apr_array_header_t *POST;
    const char *digestType;
    
    
    /* Check that the "example-handler" handler is being called. */
    if (!r-&gt;handler || strcmp(r-&gt;handler, "example-handler")) return (DECLINED);
    
    /* Figure out which file is being requested by removing the .sum from it */
    filename = apr_pstrdup(r-&gt;pool, r-&gt;filename);
    filename[strlen(filename)-4] = 0; /* Cut off the last 4 characters. */
    
    /* Figure out if the file we request a sum on exists and isn't a directory */
    rc = apr_stat(&amp;finfo, filename, APR_FINFO_MIN, r-&gt;pool);
    if (rc == APR_SUCCESS) {
        exists =
        (
            (finfo.filetype != APR_NOFILE)
        &amp;&amp;  !(finfo.filetype &amp; APR_DIR)
        );
        if (!exists) return HTTP_NOT_FOUND; /* Return a 404 if not found. */
    }
    /* If apr_stat failed, we're probably not allowed to check this file. */
    else return HTTP_FORBIDDEN;
    
    /* Parse the GET and, optionally, the POST data sent to us */
    
    ap_args_to_table(r, &amp;GET);
    ap_parse_form_data(r, NULL, &amp;POST, -1, 8192);
    
    /* Set the appropriate content type */
    ap_set_content_type(r, "text/html");
    
    /* Print a title and some general information */
    ap_rprintf(r, "&lt;h2&gt;Information on %s:&lt;/h2&gt;", filename);
    ap_rprintf(r, "&lt;b&gt;Size:&lt;/b&gt; %u bytes&lt;br/&gt;", finfo.size);
    
    /* Get the digest type the client wants to see */
    digestType = apr_table_get(GET, "digest");
    if (!digestType) digestType = "MD5";
    
    
    rc = apr_file_open(&amp;file, filename, APR_READ, APR_OS_DEFAULT, r-&gt;pool);
    if (rc == APR_SUCCESS) {
        
        /* Are we trying to calculate the MD5 or the SHA1 digest? */
        if (!strcasecmp(digestType, "md5")) {
            /* Calculate the MD5 sum of the file */
            union {
                char      chr[16];
                uint32_t  num[4];
            } digest;
            apr_md5_ctx_t md5;
            apr_md5_init(&amp;md5);
            readBytes = 256;
            while ( apr_file_read(file, buffer, &amp;readBytes) == APR_SUCCESS ) {
                apr_md5_update(&amp;md5, buffer, readBytes);
            }
            apr_md5_final(digest.chr, &amp;md5);
            
            /* Print out the MD5 digest */
            ap_rputs("&lt;b&gt;MD5: &lt;/b&gt;&lt;code&gt;", r);
            for (n = 0; n &lt; APR_MD5_DIGESTSIZE/4; n++) {
                ap_rprintf(r, "%08x", digest.num[n]);
            }
            ap_rputs("&lt;/code&gt;", r);
            /* Print a link to the SHA1 version */
            ap_rputs("&lt;br/&gt;&lt;a href='?digest=sha1'&gt;View the SHA1 hash instead&lt;/a&gt;", r);
        }
        else {
            /* Calculate the SHA1 sum of the file */
            union {
                char      chr[20];
                uint32_t  num[5];
            } digest;
            apr_sha1_ctx_t sha1;
            apr_sha1_init(&amp;sha1);
            readBytes = 256;
            while ( apr_file_read(file, buffer, &amp;readBytes) == APR_SUCCESS ) {
                apr_sha1_update(&amp;sha1, buffer, readBytes);
            }
            apr_sha1_final(digest.chr, &amp;sha1);
            
            /* Print out the SHA1 digest */
            ap_rputs("&lt;b&gt;SHA1: &lt;/b&gt;&lt;code&gt;", r);
            for (n = 0; n &lt; APR_SHA1_DIGESTSIZE/4; n++) {
                ap_rprintf(r, "%08x", digest.num[n]);
            }
            ap_rputs("&lt;/code&gt;", r);
            
            /* Print a link to the MD5 version */
            ap_rputs("&lt;br/&gt;&lt;a href='?digest=md5'&gt;View the MD5 hash instead&lt;/a&gt;", r);
        }
        apr_file_close(file);
        
    }    
    /* Let the server know that we responded to this request. */
    return OK;
}</pre>



<p>
This version in its entirety can be found here: 
<a href="http://people.apache.org/~humbedooh/mods/examples/mod_example_2.c">mod_example_2.c</a>.
</p>


</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="configuration" id="configuration">Adding configuration options</a></h2>
<p>
In this next segment of this document, we will turn our eyes away from the 
digest module and create a new example module, whose only function is to 
write out its own configuration. The purpose of this is to examine how 
the server works with configuration, and what happens when you start writing 
advanced configurations 
for your modules.
</p>
<h3><a name="config_intro" id="config_intro">An introduction to configuration 
directives</a></h3>
<p>
If you are reading this, then you probably already know 
what a configuration directive is. Simply put, a directive is a way of 
telling an individual module (or a set of modules) how to behave, such as 
these directives control how <code>mod_rewrite</code> works:
</p>
<pre class="prettyprint lang-config">RewriteEngine On
RewriteCond "%{REQUEST_URI}" "^/foo/bar"
RewriteRule "^/foo/bar/(.*)$" "/foobar?page=$1"</pre>

<p>
Each of these configuration directives are handled by a separate function, 
that parses the parameters given and sets up a configuration accordingly.
</p>

<h3><a name="config_simple" id="config_simple">Making an example configuration</a></h3>
<p>To begin with, we'll create a basic configuration in C-space:</p>



<pre class="prettyprint lang-c">typedef struct {
    int         enabled;      /* Enable or disable our module */
    const char *path;         /* Some path to...something */
    int         typeOfAction; /* 1 means action A, 2 means action B and so on */
} example_config;</pre>



<p>
Now, let's put this into perspective by creating a very small module that 
just prints out a hard-coded configuration. You'll notice that we use the 
<code>register_hooks</code> function for initializing the configuration 
values to their defaults:
</p>


<pre class="prettyprint lang-c">typedef struct {
    int         enabled;      /* Enable or disable our module */
    const char *path;         /* Some path to...something */
    int         typeOfAction; /* 1 means action A, 2 means action B and so on */
} example_config;

static example_config config;

static int example_handler(request_rec *r)
{
    if (!r-&gt;handler || strcmp(r-&gt;handler, "example-handler")) return(DECLINED);
    ap_set_content_type(r, "text/plain");
    ap_rprintf(r, "Enabled: %u\n", config.enabled);
    ap_rprintf(r, "Path: %s\n", config.path);
    ap_rprintf(r, "TypeOfAction: %x\n", config.typeOfAction);
    return OK;
}

static void register_hooks(apr_pool_t *pool) 
{
    config.enabled = 1;
    config.path = "/foo/bar";
    config.typeOfAction = 0x00;
    ap_hook_handler(example_handler, NULL, NULL, APR_HOOK_LAST);
}

/* Define our module as an entity and assign a function for registering hooks  */

module AP_MODULE_DECLARE_DATA   example_module =
{
    STANDARD20_MODULE_STUFF,
    NULL,            /* Per-directory configuration handler */
    NULL,            /* Merge handler for per-directory configurations */
    NULL,            /* Per-server configuration handler */
    NULL,            /* Merge handler for per-server configurations */
    NULL,            /* Any directives we may have for httpd */
    register_hooks   /* Our hook registering function */
};</pre>



<p>
So far so good. To access our new handler, we could add the following to 
our configuration:
</p>
<pre class="prettyprint lang-config">&lt;Location "/example"&gt;
    SetHandler example-handler
&lt;/Location&gt;</pre>

<p>
When we visit, we'll see our current configuration being spit out by our 
module. 
</p>


<h3><a name="register_directive" id="register_directive">Registering directives with the server</a></h3>
<p>
What if we want to change our configuration, not by hard-coding new values 
into the module, but by using either the httpd.conf file or possibly a 
.htaccess file? It's time to let the server know that we want this to be 
possible. To do so, we must first change our <em>name tag</em> to include a 
reference to the configuration directives we want to register with the server:
</p>


<pre class="prettyprint lang-c">module AP_MODULE_DECLARE_DATA   example_module =
{
    STANDARD20_MODULE_STUFF,
    NULL,               /* Per-directory configuration handler */
    NULL,               /* Merge handler for per-directory configurations */
    NULL,               /* Per-server configuration handler */
    NULL,               /* Merge handler for per-server configurations */
    example_directives, /* Any directives we may have for httpd */
    register_hooks      /* Our hook registering function */
};</pre>



<p>
This will tell the server that we are now accepting directives from the 
configuration files, and that the structure called <code>example_directives
</code> holds information on what our directives are and how they work. 
Since we have three different variables in our module configuration, we 
will add a structure with three directives and a NULL at the end:
</p>


<pre class="prettyprint lang-c">static const command_rec        example_directives[] =
{
    AP_INIT_TAKE1("exampleEnabled", example_set_enabled, NULL, RSRC_CONF, "Enable or disable mod_example"),
    AP_INIT_TAKE1("examplePath", example_set_path, NULL, RSRC_CONF, "The path to whatever"),
    AP_INIT_TAKE2("exampleAction", example_set_action, NULL, RSRC_CONF, "Special action value!"),
    { NULL }
};</pre>



<p>
<img src="../images/build_a_mod_4.png" alt="Directives structure" /><br />
As you can see, each directive needs at least 5 parameters set:
</p>
<ol>
<li><code><a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__CONFIG.html#ga07c7d22ae17805e61204463326cf9c34">AP_INIT_TAKE1</a></code>: This is a macro that tells the server that this directive takes one and only one argument. 
If we required two arguments, we could use the macro <code><a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__CONFIG.html#gafaec43534fcf200f37d9fecbf9247c21">AP_INIT_TAKE2</a></code> and so on (refer to httpd_conf.h 
for more macros).</li>
<li><code>exampleEnabled</code>: This is the name of our directive. More precisely, it is what the user must put in his/her 
configuration in order to invoke a configuration change in our module.</li>
<li><code>example_set_enabled</code>: This is a reference to a C function that parses the directive and sets the configuration 
accordingly. We will discuss how to make this in the following paragraph.</li>
<li><code>RSRC_CONF</code>: This tells the server where the directive is permitted. We'll go into details on this value in the 
later chapters, but for now, <code>RSRC_CONF</code> means that the server will only accept these directives in a server context.</li>
<li><code>"Enable or disable...."</code>: This is simply a brief description of what the directive does.</li>
</ol>
<p>
(<em>The "missing" parameter in our definition, which is usually set to 
<code>NULL</code>, is an optional function that can be run after the 
initial function to parse the arguments have been run. This is usually 
omitted, as the function for verifying arguments might as well be used to 
set them.</em>)
</p>

<h3><a name="directive_handler" id="directive_handler">The directive handler function</a></h3>
<p>
Now that we have told the server to expect some directives for our module, it's 
time to make a few functions for handling these. What the server reads in the 
configuration file(s) is text, and so naturally, what it passes along to 
our directive handler is one or more strings, that we ourselves need to 
recognize and act upon. You'll notice, that since we set our <code>
exampleAction</code> directive to accept two arguments, its C function also 
has an additional parameter defined:</p> 


<pre class="prettyprint lang-c">/* Handler for the "exampleEnabled" directive */
const char *example_set_enabled(cmd_parms *cmd, void *cfg, const char *arg)
{
    if(!strcasecmp(arg, "on")) config.enabled = 1;
    else config.enabled = 0;
    return NULL;
}

/* Handler for the "examplePath" directive */
const char *example_set_path(cmd_parms *cmd, void *cfg, const char *arg)
{
    config.path = arg;
    return NULL;
}

/* Handler for the "exampleAction" directive */
/* Let's pretend this one takes one argument (file or db), and a second (deny or allow), */
/* and we store it in a bit-wise manner. */
const char *example_set_action(cmd_parms *cmd, void *cfg, const char *arg1, const char *arg2)
{
    if(!strcasecmp(arg1, "file")) config.typeOfAction = 0x01;
    else config.typeOfAction = 0x02;
    
    if(!strcasecmp(arg2, "deny")) config.typeOfAction += 0x10;
    else config.typeOfAction += 0x20;
    return NULL;
}</pre>





<h3><a name="directive_complete" id="directive_complete">Putting it all together</a></h3>
<p>
Now that we have our directives set up, and handlers configured for them, 
we can assemble our module into one big file:
</p>


<pre class="prettyprint lang-c">/* mod_example_config_simple.c: */
#include &lt;stdio.h&gt;
#include "apr_hash.h"
#include "ap_config.h"
#include "ap_provider.h"
#include "httpd.h"
#include "http_core.h"
#include "http_config.h"
#include "http_log.h"
#include "http_protocol.h"
#include "http_request.h"

/*
 ==============================================================================
 Our configuration prototype and declaration:
 ==============================================================================
 */
typedef struct {
    int         enabled;      /* Enable or disable our module */
    const char *path;         /* Some path to...something */
    int         typeOfAction; /* 1 means action A, 2 means action B and so on */
} example_config;

static example_config config;

/*
 ==============================================================================
 Our directive handlers:
 ==============================================================================
 */
/* Handler for the "exampleEnabled" directive */
const char *example_set_enabled(cmd_parms *cmd, void *cfg, const char *arg)
{
    if(!strcasecmp(arg, "on")) config.enabled = 1;
    else config.enabled = 0;
    return NULL;
}

/* Handler for the "examplePath" directive */
const char *example_set_path(cmd_parms *cmd, void *cfg, const char *arg)
{
    config.path = arg;
    return NULL;
}

/* Handler for the "exampleAction" directive */
/* Let's pretend this one takes one argument (file or db), and a second (deny or allow), */
/* and we store it in a bit-wise manner. */
const char *example_set_action(cmd_parms *cmd, void *cfg, const char *arg1, const char *arg2)
{
    if(!strcasecmp(arg1, "file")) config.typeOfAction = 0x01;
    else config.typeOfAction = 0x02;
    
    if(!strcasecmp(arg2, "deny")) config.typeOfAction += 0x10;
    else config.typeOfAction += 0x20;
    return NULL;
}

/*
 ==============================================================================
 The directive structure for our name tag:
 ==============================================================================
 */
static const command_rec        example_directives[] =
{
    AP_INIT_TAKE1("exampleEnabled", example_set_enabled, NULL, RSRC_CONF, "Enable or disable mod_example"),
    AP_INIT_TAKE1("examplePath", example_set_path, NULL, RSRC_CONF, "The path to whatever"),
    AP_INIT_TAKE2("exampleAction", example_set_action, NULL, RSRC_CONF, "Special action value!"),
    { NULL }
};