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97a9a944b5887e91042b019776c41d5dd74557aferikabele<?xml-stylesheet type="text/xsl" href="/style/manual.en.xsl"?>

a945f35eff8b6a88009ce73de6d4c862ce58de3cslive

a945f35eff8b6a88009ce73de6d4c862ce58de3cslive

b05ab3ff5ab54aa22610b13d56eaba6ddfc3db60nd

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd<manualpage metafile="modguide.xml.meta">

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd<parentdocument href="./">Developer</parentdocument>

3b3b7fc78d1f5bfc2769903375050048ff41ff26nd

b686b6a420bde7f78c416b90be11db94cb789979nd <title>Developing modules for the Apache HTTP Server 2.4</title>

06ba4a61654b3763ad65f52283832ebf058fdf1cslive

06ba4a61654b3763ad65f52283832ebf058fdf1cslive<summary>

06ba4a61654b3763ad65f52283832ebf058fdf1cslive<p>This document explains how you can develop modules for the Apache HTTP

06ba4a61654b3763ad65f52283832ebf058fdf1csliveServer 2.4</p>

06ba4a61654b3763ad65f52283832ebf058fdf1cslive</summary>

06ba4a61654b3763ad65f52283832ebf058fdf1cslive

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42nd<seealso><a href="request.html">Request Processing in Apache 2.4</a></seealso>

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2nd<seealso><a href="hooks.html">Apache 2.x Hook Functions</a></seealso>

5b10fd3977e6dfff19afe770e612e276962f7950nd

5b10fd3977e6dfff19afe770e612e276962f7950nd<section id="introduction"><title>Introduction</title>

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2nd<section id="what"><title>What we will be discussing in this document</title>

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2nd<p>

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2ndThis document will discuss how you can create modules for the Apache

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2ndHTTP Server 2.4, by exploring an example module called

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2nd<code>mod_example</code>. In the first part of this document, the purpose

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2ndof this module will be to calculate and print out various digest values for

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42ndexisting files on your web server, whenever we access the URL <code>

5b10fd3977e6dfff19afe770e612e276962f7950ndhttp://hostname/filename.sum</code>. For instance, if we want to know the

5b10fd3977e6dfff19afe770e612e276962f7950ndMD5 digest value of the file located at <code>

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42ndhttp://www.example.com/index.html</code>, we would visit <code>

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42ndhttp://www.example.com/index.html.sum</code>.

2aff288113d772cedca6add888eb643afffe9fb1nd</p>

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2nd

2aff288113d772cedca6add888eb643afffe9fb1nd<p>

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2ndIn the second part of this document, which deals with configuration

7fa75a06a4fee19e995c069ee00310455d1452e1pquernadirective and context awareness, we will be looking at a module that simply

7fa75a06a4fee19e995c069ee00310455d1452e1pquernawrites out its own configuration to the client.

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2nd</p>

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2nd</section>

2aff288113d772cedca6add888eb643afffe9fb1nd

2aff288113d772cedca6add888eb643afffe9fb1nd<section id="prerequisites"><title>Prerequisites</title>

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2nd<p>

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2ndFirst and foremost, you are expected to have a basic knowledge of how the C

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2ndprogramming language works. In most cases, we will try to be as pedagogical

2aff288113d772cedca6add888eb643afffe9fb1ndas possible and link to documents describing the functions used in the

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2ndexamples, but there are also many cases where it is necessary to either

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2ndjust assume that "it works" or do some digging yourself into what the hows

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2ndand whys of various function calls.

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2nd</p>

2aff288113d772cedca6add888eb643afffe9fb1nd<p>

2aff288113d772cedca6add888eb643afffe9fb1ndLastly, you will need to have a basic understanding of how modules are

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42ndloaded and configured in the Apache HTTP Server, as well as how to get the headers for

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42ndApache if you do not have them already, as these are needed for compiling

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42ndnew modules.

db99fa79ac42b9cc42b63386eb289aecb0f3cb9cnd</p>

7ec4d5cc4aa574e3191bc5a612e68fd8f25ab7earpluem</section>

7ec4d5cc4aa574e3191bc5a612e68fd8f25ab7earpluem<section id="compiling"><title>Compiling your module</title>

7ec4d5cc4aa574e3191bc5a612e68fd8f25ab7earpluem<p>

2aff288113d772cedca6add888eb643afffe9fb1ndTo compile the source code we are building in this document, we will be

2aff288113d772cedca6add888eb643afffe9fb1ndusing <a href="/programs/apxs.html">APXS</a>. Assuming your source file

2aff288113d772cedca6add888eb643afffe9fb1ndis called mod_example.c, compiling, installing and activating the module is

2aff288113d772cedca6add888eb643afffe9fb1ndas simple as:

2aff288113d772cedca6add888eb643afffe9fb1nd</p>

2aff288113d772cedca6add888eb643afffe9fb1nd<example><pre>

2aff288113d772cedca6add888eb643afffe9fb1ndapxs -i -a -c mod_example.c

635e08c1d8332adc365b1c20bbe3577d59ebcd78kess</pre></example>

2aff288113d772cedca6add888eb643afffe9fb1nd

2aff288113d772cedca6add888eb643afffe9fb1nd</section>

8951c7d73bfa2ae5a2c8fe5bd27f3e677be02564noirin</section>

909ce17e2bd0faef7b1c294f2307f009793fd493nd

2aff288113d772cedca6add888eb643afffe9fb1nd<section id="basics"><title>Defining a module</title>

aa0b2780958e9b1467c9d0153a05738e399811a5nd<p>

2aff288113d772cedca6add888eb643afffe9fb1nd<img src="/images/build_a_mod_3.png" alt="Module name tags"/><br/>

42af92a661a06b3cebc88d585aad75064a309d51ndEvery module starts with the same declaration, or name tag if you will,

42af92a661a06b3cebc88d585aad75064a309d51ndthat defines a module as <em>a separate entity within Apache</em>:</p>

8951c7d73bfa2ae5a2c8fe5bd27f3e677be02564noirin

6fe26506780e73be2a412d758af77fafdf03291and

aa0b2780958e9b1467c9d0153a05738e399811a5nd<highlight language="c">

2aff288113d772cedca6add888eb643afffe9fb1ndmodule AP_MODULE_DECLARE_DATA example_module =

2aff288113d772cedca6add888eb643afffe9fb1nd{

2aff288113d772cedca6add888eb643afffe9fb1nd STANDARD20_MODULE_STUFF,

64c02f1310b7747423957823ee09fb3608430f89nd create_dir_conf, /* Per-directory configuration handler */

aa0b2780958e9b1467c9d0153a05738e399811a5nd merge_dir_conf, /* Merge handler for per-directory configurations */

2aff288113d772cedca6add888eb643afffe9fb1nd create_svr_conf, /* Per-server configuration handler */

2aff288113d772cedca6add888eb643afffe9fb1nd merge_svr_conf, /* Merge handler for per-server configurations */

2aff288113d772cedca6add888eb643afffe9fb1nd directives, /* Any directives we may have for httpd */

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42nd register_hooks /* Our hook registering function */

2aff288113d772cedca6add888eb643afffe9fb1nd};

2aff288113d772cedca6add888eb643afffe9fb1nd</highlight>

2aff288113d772cedca6add888eb643afffe9fb1nd

2aff288113d772cedca6add888eb643afffe9fb1nd<p>

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42ndThis bit of code lets the server know that we have now registered a new module

2aff288113d772cedca6add888eb643afffe9fb1ndin the system, and that its name is <code>example_module</code>. The name

e67fa8d3f161e595dd448fc24a591ee17ae59131ndof the module is used primarily for two things:<br/>

05ede5110427cb9dc071cc671d5aaba5d3b88c79nd</p>

e8b603fa9ccf7b17b11b42df6d8916fd97c2331dnd<ul>

bf94bedcb62d7f0b926f4286069def5ee6b07b60nd<li>Letting the server know how to load the module using the LoadModule</li>

bf94bedcb62d7f0b926f4286069def5ee6b07b60nd<li>Setting up a namespace for the module to use in configurations</li>

2aff288113d772cedca6add888eb643afffe9fb1nd</ul>

2aff288113d772cedca6add888eb643afffe9fb1nd<p>

bbcbf978a3074512c627c797fedcb30eeab7b39dsliveFor now, we're only concerned with the first purpose of the module name,

ee5db395bc3723609919edfa96af387eea37c491jimwhich comes into play when we need to load the module:

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42nd</p>

2aff288113d772cedca6add888eb643afffe9fb1nd<highlight language="config">

e55e60efce8a3e2139132c1d6ad9f6f0d2976614ndLoadModule example_module "modules/mod_example.so"

e55e60efce8a3e2139132c1d6ad9f6f0d2976614nd</highlight>

2aff288113d772cedca6add888eb643afffe9fb1nd<p>

7f3fdc63be07f2ad39e01a44e68e9324998a5c03sliveIn essence, this tells the server to open up <code>mod_example.so</code> and look for a module

7f3fdc63be07f2ad39e01a44e68e9324998a5c03slivecalled <code>example_module</code>.

7f3fdc63be07f2ad39e01a44e68e9324998a5c03slive</p>

75585bd48fe0f30483dba4762e61edf39ea3e0f6nd<p>

75585bd48fe0f30483dba4762e61edf39ea3e0f6ndWithin this name tag of ours is also a bunch of references to how we would

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42ndlike to handle things: Which directives do we respond to in a configuration

03a4ff9ac4c9b8009249010e7c53bb86ff05915andfile or .htaccess, how do we operate within specific contexts, and what

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42ndhandlers are we interested in registering with the Apache HTTP service. We'll

3b58542e01ec69422f3086db5825a12fc77b726endreturn to all these elements later in this document.

a0d937b340692a3578f1d2f2535890c520c4bf0cnd</p>

7d15331eeb5429d7148d13d6fd914a641bf1c000pquerna</section>

2aff288113d772cedca6add888eb643afffe9fb1nd<section id="hooking"><title>Getting started: Hooking into the server</title>

2aff288113d772cedca6add888eb643afffe9fb1nd<section id="hook_intro"><title>An introduction to hooks</title>

2aff288113d772cedca6add888eb643afffe9fb1nd<p>

2aff288113d772cedca6add888eb643afffe9fb1ndWhen handling requests in Apache HTTP Server 2.4, the first thing you will need to do is

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42ndcreate a hook into the request handling process. A hook is essentially a

08842292d2f1550b40ae73e0dafc7641c5955c82ndmessage telling the server that you are willing to either serve or at least

2aff288113d772cedca6add888eb643afffe9fb1ndtake a glance at certain requests given by clients. All handlers, whether

d2b809e5d72658bff23819d8b77f20e4939af541ndit's mod_rewrite, mod_authn_*, mod_proxy and so on, are hooked into

2aff288113d772cedca6add888eb643afffe9fb1ndspecific parts of the request process. As you are probably aware, modules

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42ndserve different purposes; Some are authentication/authorization handlers,

2aff288113d772cedca6add888eb643afffe9fb1ndothers are file or script handlers while some third modules rewrite URIs or

2aff288113d772cedca6add888eb643afffe9fb1ndproxies content. Furthermore, in the end, it is up to the user of the server

2aff288113d772cedca6add888eb643afffe9fb1ndhow and when each module will come into place. Thus, the server itself does not

2aff288113d772cedca6add888eb643afffe9fb1ndpresume to know which module is responsible for handling a specific

2aff288113d772cedca6add888eb643afffe9fb1ndrequest, and will ask each module whether they have an interest in a given

2aff288113d772cedca6add888eb643afffe9fb1ndrequest or not. It is then up to each module to either gently decline

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42ndserving a request, accept serving it or flat out deny the request from

c023f60e35022146373e40249f0c8c8d623b6fcfndbeing served, as authentication/authorization modules do: <br/>

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42nd<img src="/images/build_a_mod_2.png" alt="Hook handling in httpd"/><br/>

a43bfa789f4e52dde53ae8e53fa0427b5c1cf977ndTo make it a bit easier for handlers such as our mod_example to know

a43bfa789f4e52dde53ae8e53fa0427b5c1cf977ndwhether the client is requesting content we should handle or not, the server

101bf3584c853027d9e51df6edfff5ff70c80238jimhas directives for hinting to modules whether their assistance is needed or

73ba54c33b4fcad0e13005e10ea8648c9fe4265bndnot. Two of these are <directive module="mod_mime">AddHandler</directive>

73ba54c33b4fcad0e13005e10ea8648c9fe4265bndand <directive module="core">SetHandler</directive>. Let's take a look at

73ba54c33b4fcad0e13005e10ea8648c9fe4265bndan example using <directive module="mod_mime">AddHandler</directive>. In

73ba54c33b4fcad0e13005e10ea8648c9fe4265bndour example case, we want every request ending with .sum to be served by

73ba54c33b4fcad0e13005e10ea8648c9fe4265bnd<code>mod_example</code>, so we'll add a configuration directive that tells

73ba54c33b4fcad0e13005e10ea8648c9fe4265bndthe server to do just that:

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42nd</p>

2aff288113d772cedca6add888eb643afffe9fb1nd<highlight language="config">

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42ndAddHandler example-handler ".sum"

2aff288113d772cedca6add888eb643afffe9fb1nd</highlight>

2aff288113d772cedca6add888eb643afffe9fb1nd<p>

2aff288113d772cedca6add888eb643afffe9fb1ndWhat this tells the server is the following: <em>Whenever we receive a request

2aff288113d772cedca6add888eb643afffe9fb1ndfor a URI ending in .sum, we are to let all modules know that we are

2aff288113d772cedca6add888eb643afffe9fb1ndlooking for whoever goes by the name of &quot;example-handler&quot; </em>.

2aff288113d772cedca6add888eb643afffe9fb1ndThus, when a request is being served that ends in .sum, the server will let all

2aff288113d772cedca6add888eb643afffe9fb1ndmodules know, that this request should be served by &quot;example-handler&quot;.

2aff288113d772cedca6add888eb643afffe9fb1ndAs you will see later, when we start building mod_example, we will

2aff288113d772cedca6add888eb643afffe9fb1ndcheck for this handler tag relayed by <code>AddHandler</code> and reply to

2aff288113d772cedca6add888eb643afffe9fb1ndthe server based on the value of this tag.

2aff288113d772cedca6add888eb643afffe9fb1nd</p>

2aff288113d772cedca6add888eb643afffe9fb1nd</section>

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42nd<section id="hook_declaration"><title>Hooking into httpd</title>

2aff288113d772cedca6add888eb643afffe9fb1nd<p>

2aff288113d772cedca6add888eb643afffe9fb1ndTo begin with, we only want to create a simple handler, that replies to the

2aff288113d772cedca6add888eb643afffe9fb1ndclient browser when a specific URL is requested, so we won't bother setting

2aff288113d772cedca6add888eb643afffe9fb1ndup configuration handlers and directives just yet. Our initial module

2aff288113d772cedca6add888eb643afffe9fb1nddefinition will look like this:</p>

03c25fb6f628ac81f2ecb637d1e7502dcee783f3nd

03c25fb6f628ac81f2ecb637d1e7502dcee783f3nd

a63f0ab647ad2ab72efc9bea7a66e24e9ebc5cc2nd<highlight language="c">

3b3b7fc78d1f5bfc2769903375050048ff41ff26ndmodule AP_MODULE_DECLARE_DATA example_module =

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd{

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd STANDARD20_MODULE_STUFF,

b05ab3ff5ab54aa22610b13d56eaba6ddfc3db60nd NULL,

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd NULL,

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd NULL,

3b3b7fc78d1f5bfc2769903375050048ff41ff26nd NULL,

8951c7d73bfa2ae5a2c8fe5bd27f3e677be02564noirin NULL,

b686b6a420bde7f78c416b90be11db94cb789979nd register_hooks /* Our hook registering function */

80d3dc69b0e6ad772135f6a78af3d16bd6cccc42nd};

</highlight>

<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>

<highlight language="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);

}

</highlight>

<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>

</section>

<section id="hook_others"><title>Other useful hooks</title>

<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>

</section>

</section>

<section id="handling"><title>Building a handler</title>

<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>

<section id="simple_handler"><title>A simple &quot;Hello, world!&quot;

handler</title>

<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 &quot;example-handler&quot;</li>

<li>Set the content type of our output to <code>text/html</code></li>

<li>Write &quot;Hello, world!&quot; 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>

<highlight language="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->handler || strcmp(r->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;

}

</highlight>

<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">&quot;Some useful functions you should know&quot;</a>.

</p>

</section>

<section id="request_rec"><title>The request_rec structure</title>

<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

&quot;<a href="#memory">Memory management</a>&quot; 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>

<highlight language="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;

}

</highlight>

</section>

<section id="return_value"><title>Return values</title>

<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>

<highlight language="c">

static int example_handler(request_rec *r)

{

/* Return 404: Not found */

return HTTP_NOT_FOUND;

}

</highlight>

<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>

</section>

<section id="functions"><title>Some useful functions you should know</title>

<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>.

<highlight language="c">ap_rputs("Hello, world!", r);</highlight>

</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.

<highlight language="c">ap_rprintf(r, "Hello, %s!", r->useragent_ip);</highlight>

</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.

<highlight language="c">ap_set_content_type(r, "text/plain"); /* force a raw text output */</highlight>

</li>

</ul>

</section>

<section id="memory"><title>Memory management</title>

<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>

<highlight language="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->pool, sizeof(int)*10);

/* Create a copy of the 'original' variable that we can edit. */

copy = apr_pstrdup(r->pool, original);

return OK;

}

</highlight>

<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>

<highlight language="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);

}

</highlight>

<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>

</section>

<section id="parsing"><title>Parsing request data</title>

<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>

<highlight language="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);

</highlight>

<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>

<highlight language="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";

</highlight>

<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>

</section>

<section id="advanced_handler"><title>Making an advanced handler</title>

<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>

<highlight language="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;

}

</highlight>

<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>

</section>

</section>

<section id="configuration"><title>Adding configuration options</title>

<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>

<section id="config_intro"><title>An introduction to configuration

directives</title>

<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>

<highlight language="config">

RewriteEngine On

RewriteCond "%{REQUEST_URI}" "^/foo/bar"

RewriteRule "^/foo/bar/(.*)$" "/foobar?page=$1"

</highlight>

<p>

Each of these configuration directives are handled by a separate function,

that parses the parameters given and sets up a configuration accordingly.

</p>

</section>

<section id="config_simple"><title>Making an example configuration</title>

<p>To begin with, we'll create a basic configuration in C-space:</p>

<highlight language="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;

</highlight>

<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>

<highlight language="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 */

};

</highlight>

<p>

So far so good. To access our new handler, we could add the following to

our configuration:

</p>

<highlight language="config">

&lt;Location "/example"&gt;

SetHandler example-handler

&lt;/Location&gt;

</highlight>

<p>

When we visit, we'll see our current configuration being spit out by our

module.

</p>

</section>

<section id="register_directive"><title>Registering directives with the server</title>

<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>

<highlight language="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 */

};

</highlight>

<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>

<highlight language="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 }

};

</highlight>

<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>

</section>

<section id="directive_handler"><title>The directive handler function</title>

<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>

<highlight language="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;

}

</highlight>

</section>

<section id="directive_complete"><title>Putting it all together</title>

<p>

Now that we have our directives set up, and handlers configured for them,

we can assemble our module into one big file:

</p>

<highlight language="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 }

};

/*

==============================================================================

Our module handler:

==============================================================================

*/

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;

}

/*

==============================================================================

The hook registration function (also initializes the default config values):

==============================================================================

*/

static void register_hooks(apr_pool_t *pool)

{

config.enabled = 1;

config.path = "/foo/bar";

config.typeOfAction = 3;

ap_hook_handler(example_handler, NULL, NULL, APR_HOOK_LAST);

}

/*

==============================================================================

Our module name tag:

==============================================================================

*/

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 */

};

</highlight>

<p>

In our httpd.conf file, we can now change the hard-coded configuration by

adding a few lines:

</p>

<highlight language="config">

ExampleEnabled On

ExamplePath "/usr/bin/foo"

ExampleAction file allow

</highlight>

<p>

And thus we apply the configuration, visit <code>/example</code> on our

web site, and we see the configuration has adapted to what we wrote in our

configuration file.

</p>

</section>

</section>

<section id="context"><title>Context aware configurations</title>

<section id="context_intro"><title>Introduction to context aware configurations</title>

<p>

In Apache HTTP Server 2.4, different URLs, virtual hosts, directories etc can have very

different meanings to the user of the server, and thus different contexts

within which modules must operate. For example, let's assume you have this

configuration set up for mod_rewrite:

</p>

<highlight language="config">

&lt;Directory "/var/www"&gt;

RewriteCond "%{HTTP_HOST}" "^example.com$"

RewriteRule "(.*)" "http://www.example.com/$1"

&lt;/Directory&gt;

&lt;Directory "/var/www/sub"&gt;

RewriteRule "^foobar$" "index.php?foobar=true"

&lt;/Directory&gt;

</highlight>

<p>

In this example, you will have set up two different contexts for

mod_rewrite:</p>

<ol>

<li>Inside <code>/var/www</code>, all requests for <code>http://example.com</code> must go to <code>http://www.example.com</code></li>

<li>Inside <code>/var/www/sub</code>, all requests for <code>foobar</code> must go to <code>index.php?foobar=true</code></li>

</ol>

<p>

If mod_rewrite (or the entire server for that matter) wasn't context aware, then

these rewrite rules would just apply to every and any request made,

regardless of where and how they were made, but since the module can pull

the context specific configuration straight from the server, it does not need

to know itself, which of the directives are valid in this context, since

the server takes care of this.</p>

<p>

So how does a module get the specific configuration for the server,

directory or location in question? It does so by making one simple call:

</p>

<highlight language="c">

example_config *config = (example_config*) <a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__CONFIG.html#ga1093a5908a384eacc929b028c79f2a02">ap_get_module_config</a>(r-&gt;per_dir_config, &amp;example_module);

</highlight>

<p>

That's it! Of course, a whole lot goes on behind the scenes, which we will

discuss in this chapter, starting with how the server came to know what our

configuration looks like, and how it came to be set up as it is in the

specific context.

</p>

</section>

<section id="context_base"><title>Our basic configuration setup</title>

<p>In this chapter, we will be working with a slightly modified version of

our previous context structure. We will set a <code>context</code>

variable that we can use to track which context configuration is being

used by the server in various places:

</p>

<highlight language="c">

typedef struct {

char context[256];

char path[256];

int typeOfAction;

int enabled;

} example_config;

</highlight>

<p>Our handler for requests will also be modified, yet still very simple:</p>

<highlight language="c">

static int example_handler(request_rec *r)

{

if(!r->handler || strcmp(r->handler, "example-handler")) return(DECLINED);

example_config *config = (example_config*) ap_get_module_config(r->per_dir_config, &amp;example_module);

ap_set_content_type(r, "text/plain");

ap_rprintf("Enabled: %u\n", config->enabled);

ap_rprintf("Path: %s\n", config->path);

ap_rprintf("TypeOfAction: %x\n", config->typeOfAction);

ap_rprintf("Context: %s\n", config->context);

return OK;

}

</highlight>

</section>

<section id="context_which"><title>Choosing a context</title>

<p>

Before we can start making our module context aware, we must first define,

which contexts we will accept. As we saw in the previous chapter, defining

a directive required five elements be set:</p>

<highlight language="c">

AP_INIT_TAKE1("exampleEnabled", example_set_enabled, NULL, RSRC_CONF, "Enable or disable mod_example"),

</highlight>

<p>The <code>RSRC_CONF</code> definition told the server that we would only allow

this directive in a global server context, but since we are now trying out

a context aware version of our module, we should set this to something

more lenient, namely the value <code>ACCESS_CONF</code>, which lets us use

the directive inside &lt;Directory&gt; and &lt;Location&gt; blocks. For more

control over the placement of your directives, you can combine the following

restrictions together to form a specific rule:

</p>

<ul>

<li><code>RSRC_CONF</code>: Allow in .conf files (not .htaccess) outside &lt;Directory&gt; or &lt;Location&gt;</li>

<li><code>ACCESS_CONF</code>: Allow in .conf files (not .htaccess) inside &lt;Directory&gt; or &lt;Location&gt;</li>

<li><code>OR_OPTIONS</code>: Allow in .conf files and .htaccess when <code>AllowOverride Options</code> is set</li>

<li><code>OR_FILEINFO</code>: Allow in .conf files and .htaccess when <code>AllowOverride FileInfo</code> is set</li>

<li><code>OR_AUTHCFG</code>: Allow in .conf files and .htaccess when <code>AllowOverride AuthConfig</code> is set</li>

<li><code>OR_INDEXES</code>: Allow in .conf files and .htaccess when <code>AllowOverride Indexes</code> is set</li>

<li><code>OR_ALL</code>: Allow anywhere in .conf files and .htaccess</li>

</ul>

</section>

<section id="context_pool"><title>Using the server to allocate configuration slots</title>

<p> A much smarter way to manage your configurations is by letting the server

help you create them. To do so, we must first start off by changing our

<em>name tag</em> to let the server know, that it should assist us in creating

and managing our configurations. Since we have chosen the per-directory

(or per-location) context for our module configurations, we'll add a

per-directory creator and merger function reference in our tag:</p>

<highlight language="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 */

NULL, /* Per-server configuration handler */

NULL, /* Merge handler for per-server configurations */

directives, /* Any directives we may have for httpd */

register_hooks /* Our hook registering function */

};

</highlight>

</section>

<section id="context_new"><title>Creating new context configurations</title>

<p>

Now that we have told the server to help us create and manage configurations,

our first step is to make a function for creating new, blank

configurations. We do so by creating the function we just referenced in

our name tag as the Per-directory configuration handler:</p>

<highlight language="c">

void *create_dir_conf(apr_pool_t *pool, char *context) {

context = context ? context : "(undefined context)";

example_config *cfg = apr_pcalloc(pool, sizeof(example_config));

if(cfg) {

/* Set some default values */

strcpy(cfg->context, context);

cfg->enabled = 0;

cfg->path = "/foo/bar";

cfg->typeOfAction = 0x11;

}

return cfg;

}

</highlight>

</section>

<section id="context_merge"><title>Merging configurations</title>

<p>

Our next step in creating a context aware configuration is merging

configurations. This part of the process particularly applies to scenarios

where you have a parent configuration and a child, such as the following:

</p>

<highlight language="config">

&lt;Directory "/var/www"&gt;

ExampleEnabled On

ExamplePath "/foo/bar"

ExampleAction file allow

&lt;/Directory&gt;

&lt;Directory "/var/www/subdir"&gt;

ExampleAction file deny

&lt;/Directory&gt;

</highlight>

<p>

In this example, it is natural to assume that the directory <code>

/var/www/subdir</code> should inherit the values set for the <code>/var/www

</code> directory, as we did not specify an <code>ExampleEnabled</code> nor

an <code>ExamplePath</code> for this directory. The server does not presume to

know if this is true, but cleverly does the following:

</p>

<ol>

<li>Creates a new configuration for <code>/var/www</code></li>

<li>Sets the configuration values according to the directives given for <code>/var/www</code></li>

<li>Creates a new configuration for <code>/var/www/subdir</code></li>

<li>Sets the configuration values according to the directives given for <code>/var/www/subdir</code></li>

<li><strong>Proposes a merge</strong> of the two configurations into a new configuration for <code>/var/www/subdir</code></li>

</ol>

<p>

This proposal is handled by the <code>merge_dir_conf</code> function we

referenced in our name tag. The purpose of this function is to assess the

two configurations and decide how they are to be merged:</p>

<highlight language="c">

void *merge_dir_conf(apr_pool_t *pool, void *BASE, void *ADD) {

example_config *base = (example_config *) BASE ; /* This is what was set in the parent context */

example_config *add = (example_config *) ADD ; /* This is what is set in the new context */

example_config *conf = (example_config *) create_dir_conf(pool, "Merged configuration"); /* This will be the merged configuration */

/* Merge configurations */

conf->enabled = ( add->enabled == 0 ) ? base->enabled : add->enabled ;

conf->typeOfAction = add->typeOfAction ? add->typeOfAction : base->typeOfAction;

strcpy(conf->path, strlen(add->path) ? add->path : base->path);

return conf ;

}

</highlight>

</section>

<section id="context_example"><title>Trying out our new context aware configurations</title>

<p>

Now, let's try putting it all together to create a new module that is

context aware. First off, we'll create a configuration that lets us test

how the module works:

</p>

<highlight language="config">

&lt;Location "/a"&gt;

SetHandler example-handler

ExampleEnabled on

ExamplePath "/foo/bar"

ExampleAction file allow

&lt;/Location&gt;

&lt;Location "/a/b"&gt;

ExampleAction file deny

ExampleEnabled off

&lt;/Location&gt;

&lt;Location "/a/b/c"&gt;

ExampleAction db deny

ExamplePath "/foo/bar/baz"

ExampleEnabled on

&lt;/Location&gt;

</highlight>

<p>

Then we'll assemble our module code. Note, that since we are now using our

name tag as reference when fetching configurations in our handler, I have

added some prototypes to keep the compiler happy:

</p>

<highlight language="c">

/*$6

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

* mod_example_config.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"

/*$1

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Configuration structure

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

*/

typedef struct

{

char context[256];

char path[256];

int typeOfAction;

int enabled;

} example_config;

/*$1

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Prototypes

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

*/

static int example_handler(request_rec *r);

const char *example_set_enabled(cmd_parms *cmd, void *cfg, const char *arg);

const char *example_set_path(cmd_parms *cmd, void *cfg, const char *arg);

const char *example_set_action(cmd_parms *cmd, void *cfg, const char *arg1, const char *arg2);

void *create_dir_conf(apr_pool_t *pool, char *context);

void *merge_dir_conf(apr_pool_t *pool, void *BASE, void *ADD);

static void register_hooks(apr_pool_t *pool);

/*$1

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Configuration directives

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

*/

static const command_rec directives[] =

{

AP_INIT_TAKE1("exampleEnabled", example_set_enabled, NULL, ACCESS_CONF, "Enable or disable mod_example"),

AP_INIT_TAKE1("examplePath", example_set_path, NULL, ACCESS_CONF, "The path to whatever"),

AP_INIT_TAKE2("exampleAction", example_set_action, NULL, ACCESS_CONF, "Special action value!"),

{ NULL }

};

/*$1

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Our name tag

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

*/

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 */

NULL, /* Per-server configuration handler */

NULL, /* Merge handler for per-server configurations */

directives, /* Any directives we may have for httpd */

register_hooks /* Our hook registering function */

};

/*

=======================================================================================================================

Hook registration function

=======================================================================================================================

*/

static void register_hooks(apr_pool_t *pool)

{

ap_hook_handler(example_handler, NULL, NULL, APR_HOOK_LAST);

}

/*

=======================================================================================================================

Our example web service handler

=======================================================================================================================

*/

static int example_handler(request_rec *r)

{

if(!r-&gt;handler || strcmp(r-&gt;handler, "example-handler")) return(DECLINED);

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

example_config *config = (example_config *) ap_get_module_config(r-&gt;per_dir_config, &amp;example_module);

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

ap_set_content_type(r, "text/plain");

ap_rprintf(r, "Enabled: %u\n", config-&gt;enabled);

ap_rprintf(r, "Path: %s\n", config-&gt;path);

ap_rprintf(r, "TypeOfAction: %x\n", config-&gt;typeOfAction);

ap_rprintf(r, "Context: %s\n", config-&gt;context);

return OK;

}

/*

=======================================================================================================================

Handler for the "exampleEnabled" directive

=======================================================================================================================

*/

const char *example_set_enabled(cmd_parms *cmd, void *cfg, const char *arg)

{

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

example_config *conf = (example_config *) cfg;

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

if(conf)

{

if(!strcasecmp(arg, "on"))

conf-&gt;enabled = 1;

else

conf-&gt;enabled = 0;

}

return NULL;

}

/*

=======================================================================================================================

Handler for the "examplePath" directive

=======================================================================================================================

*/

const char *example_set_path(cmd_parms *cmd, void *cfg, const char *arg)

{

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

example_config *conf = (example_config *) cfg;

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

if(conf)

{

strcpy(conf-&gt;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)

{

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

example_config *conf = (example_config *) cfg;

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

if(conf)

{

{

if(!strcasecmp(arg1, "file"))

conf-&gt;typeOfAction = 0x01;

else

conf-&gt;typeOfAction = 0x02;

if(!strcasecmp(arg2, "deny"))

conf-&gt;typeOfAction += 0x10;

else

conf-&gt;typeOfAction += 0x20;

}

}

return NULL;

}

/*

=======================================================================================================================

Function for creating new configurations for per-directory contexts

=======================================================================================================================

*/

void *create_dir_conf(apr_pool_t *pool, char *context)

{

context = context ? context : "Newly created configuration";

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

example_config *cfg = apr_pcalloc(pool, sizeof(example_config));

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

if(cfg)

{

{

/* Set some default values */

strcpy(cfg-&gt;context, context);

cfg-&gt;enabled = 0;

memset(cfg-&gt;path, 0, 256);

cfg-&gt;typeOfAction = 0x00;

}

}

return cfg;

}

/*

=======================================================================================================================

Merging function for configurations

=======================================================================================================================

*/

void *merge_dir_conf(apr_pool_t *pool, void *BASE, void *ADD)

{

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

example_config *base = (example_config *) BASE;

example_config *add = (example_config *) ADD;

example_config *conf = (example_config *) create_dir_conf(pool, "Merged configuration");

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

conf-&gt;enabled = (add-&gt;enabled == 0) ? base-&gt;enabled : add-&gt;enabled;

conf-&gt;typeOfAction = add-&gt;typeOfAction ? add-&gt;typeOfAction : base-&gt;typeOfAction;

strcpy(conf-&gt;path, strlen(add-&gt;path) ? add-&gt;path : base-&gt;path);

return conf;

}

</highlight>

</section>

</section>

<section id="summary"><title>Summing up</title>

<p>

We have now looked at how to create simple modules for Apache HTTP Server 2.4 and

configuring them. What you do next is entirely up to you, but it is my

hope that something valuable has come out of reading this documentation.

If you have questions on how to further develop modules, you are welcome

to join our <a href="http://httpd.apache.org/lists.html">mailing lists</a>

or check out the rest of our documentation for further tips.

</p>

</section>

<section id="snippets"><title>Some useful snippets of code</title>

<section id="get_post"><title>Retrieve variables from POST form data</title>

<highlight language="c">

typedef struct {

const char *key;

const char *value;

} keyValuePair;

keyValuePair *readPost(request_rec *r) {

apr_array_header_t *pairs = NULL;

apr_off_t len;

apr_size_t size;

int res;

int i = 0;

char *buffer;

keyValuePair *kvp;

res = ap_parse_form_data(r, NULL, &amp;pairs, -1, HUGE_STRING_LEN);

if (res != OK || !pairs) return NULL; /* Return NULL if we failed or if there are is no POST data */

kvp = apr_pcalloc(r->pool, sizeof(keyValuePair) * (pairs->nelts + 1));

while (pairs &amp;&amp; !apr_is_empty_array(pairs)) {

ap_form_pair_t *pair = (ap_form_pair_t *) apr_array_pop(pairs);

apr_brigade_length(pair->value, 1, &amp;len);

size = (apr_size_t) len;

buffer = apr_palloc(r->pool, size + 1);

apr_brigade_flatten(pair->value, buffer, &amp;size);

buffer[len] = 0;

kvp[i].key = apr_pstrdup(r->pool, pair->name);

kvp[i].value = buffer;

i++;

}

return kvp;

}

static int example_handler(request_rec *r)

{

/*~~~~~~~~~~~~~~~~~~~~~~*/

keyValuePair *formData;

/*~~~~~~~~~~~~~~~~~~~~~~*/

formData = readPost(r);

if (formData) {

int i;

for (i = 0; &amp;formData[i]; i++) {

if (formData[i].key &amp;&amp; formData[i].value) {

ap_rprintf(r, "%s = %s\n", formData[i].key, formData[i].value);

} else if (formData[i].key) {

ap_rprintf(r, "%s\n", formData[i].key);

} else if (formData[i].value) {

ap_rprintf(r, "= %s\n", formData[i].value);

} else {

break;

}

}

}

return OK;

}

</highlight>

</section>

<section id="headers_out"><title>Printing out every HTTP header received</title>