fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kess<?xml version='1.0' encoding='UTF-8' ?>

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kess<!DOCTYPE manualpage SYSTEM "/style/manualpage.dtd">

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kess<?xml-stylesheet type="text/xsl" href="/style/manual.en.xsl"?>

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kess

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd

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

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

3b3b7fc78d1f5bfc2769903375050048ff41ff26nd

3b3b7fc78d1f5bfc2769903375050048ff41ff26nd <title>Developing modules for the Apache HTTP Server 2.4</title>

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowen

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowen<summary>

10ca15b877e1d7f36b5d4ccd0a38e984cd635da4rbowen<p>This document explains how you can develop modules for the Apache HTTP

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kessServer 2.4</p>

0e13d2a04f8c7cb647556e890bb69d5789a31ce1nd</summary>

0e13d2a04f8c7cb647556e890bb69d5789a31ce1nd

2bd55c0d474884fe40c4a081efcbd82bc1c913a8rbowen<seealso><a href="request.html">Request Processing in Apache 2.4</a></seealso>

2bd55c0d474884fe40c4a081efcbd82bc1c913a8rbowen<seealso><a href="hooks.html">Apache 2.x Hook Functions</a></seealso>

2bd55c0d474884fe40c4a081efcbd82bc1c913a8rbowen

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowen<section id="introduction"><title>Introduction</title>

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowen<section id="what"><title>What we will be discussing in this document</title>

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowen<p>

10ca15b877e1d7f36b5d4ccd0a38e984cd635da4rbowenThis document will discuss how you can easily create modules for the Apache

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowenHTTP Server 2.4, by exploring an example module called

10ca15b877e1d7f36b5d4ccd0a38e984cd635da4rbowen<code>mod_example</code>. In the first part of this document, the purpose

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowenof this module will be to calculate and print out various digest values for

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowenexisting files on your web server, whenever we access the URL <code>

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowenhttp://hostname/filename.sum</code>. For instance, if we want to know the

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowenMD5 digest value of the file located at <code>

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowenhttp://www.example.com/index.html</code>, we would visit <code>

5a58787efeb02a1c3f06569d019ad81fd2efa06endhttp://www.example.com/index.html.sum</code>.

5a58787efeb02a1c3f06569d019ad81fd2efa06end</p>

5a58787efeb02a1c3f06569d019ad81fd2efa06end

be809bbd44119f26f27a701f6912d21f334d85ffrbowen<p>

ae89c529be7bde57cc051ec8f85da6fddd502926erikabeleIn the second part of this document, which deals with configuration

5a58787efeb02a1c3f06569d019ad81fd2efa06enddirective and context awareness, we will be looking at a module that simply

5a58787efeb02a1c3f06569d019ad81fd2efa06endwrite out its own configuration to the client.

5a58787efeb02a1c3f06569d019ad81fd2efa06end</p>

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowen</section>

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowen

2bd55c0d474884fe40c4a081efcbd82bc1c913a8rbowen<section id="prerequisites"><title>Prerequisites</title>

2bd55c0d474884fe40c4a081efcbd82bc1c913a8rbowen<p>

10ca15b877e1d7f36b5d4ccd0a38e984cd635da4rbowenFirst and foremost, you are expected to have a basic knowledge of how the C

10ca15b877e1d7f36b5d4ccd0a38e984cd635da4rbowenprogramming language works. In most cases, we will try to be as pedagogical

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kessas possible and link to documents describing the functions used in the

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowenexamples, but there are also many cases where it is necessary to either

48160cfe05c7614376dc7b1f57f5493eb73f06dbrbowenjust assume that "it works" or do some digging yourself into what the hows

48160cfe05c7614376dc7b1f57f5493eb73f06dbrbowenand whys of various function calls.

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowen</p>

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowen<p>

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kessLastly, you will need to have a basic understanding of how modules are

10ca15b877e1d7f36b5d4ccd0a38e984cd635da4rbowenloaded and configured in the Apache HTTP Server, as well as how to get the headers for

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowenApache if you do not have them already, as these are needed for compiling

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowennew modules.

5a58787efeb02a1c3f06569d019ad81fd2efa06end</p>

5a58787efeb02a1c3f06569d019ad81fd2efa06end</section>

5a58787efeb02a1c3f06569d019ad81fd2efa06end<section id="compiling"><title>Compiling your module</title>

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowen<p>

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowenTo compile the source code we are building in this document, we will be

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kessusing <a href="/programs/apxs.html">APXS</a>. Assuming your source file

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kessis called mod_example.c, compiling, installing and activating the module is

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kessas simple as:

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kess</p>

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kess<example><pre>

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowenapxs -i -a -c mod_example.c

2eaf662cbc81e823e8d9aeb8d54e69e63032493erbowen</pre></example>

48160cfe05c7614376dc7b1f57f5493eb73f06dbrbowen

b55e805b1e1f5258452901038edc7097056eba7eerikabele</section>

48160cfe05c7614376dc7b1f57f5493eb73f06dbrbowen</section>

48160cfe05c7614376dc7b1f57f5493eb73f06dbrbowen

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kess<section id="basics"><title>Defining a module</title>

1855f4a7304fe643dd91b222128facaa185753e8erikabele<p>

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kess<img src="/images/build_a_mod_3.png" alt="Module name tags"/><br/>

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kessEvery module starts with the same declaration, or name tag if you will,

48160cfe05c7614376dc7b1f57f5493eb73f06dbrbowenthat defines a module as <em>a separate entity within Apache</em>:</p>

48160cfe05c7614376dc7b1f57f5493eb73f06dbrbowen

fefb8b844b6286bfc41bb2e0c4cc003b8e7d4ff2kess

48160cfe05c7614376dc7b1f57f5493eb73f06dbrbowen<p class="source">

5a58787efeb02a1c3f06569d019ad81fd2efa06endmodule AP_MODULE_DECLARE_DATA example_module <code class="sh_misc">=</code>

3b3b7fc78d1f5bfc2769903375050048ff41ff26nd<code class="sh_misc">{</code>

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd STANDARD20_MODULE_STUFF<code class="sh_misc">,</code>

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd create_dir_conf<code class="sh_misc">,</code> <code class="sh_comment">/* Per-directory configuration handler */</code>

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd merge_dir_conf<code class="sh_misc">,</code> <code class="sh_comment">/* Merge handler for per-directory configurations */</code>

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd create_svr_conf<code class="sh_misc">,</code> <code class="sh_comment">/* Per-server configuration handler */</code>

3b3b7fc78d1f5bfc2769903375050048ff41ff26nd merge_svr_conf<code class="sh_misc">,</code> <code class="sh_comment">/* Merge handler for per-server configurations */</code>

35714556a25fceb7c9bf9c4e01791b2e2a4c27c3nd directives<code class="sh_misc">,</code> <code class="sh_comment">/* Any directives we may have for httpd */</code>

5a58787efeb02a1c3f06569d019ad81fd2efa06end register_hooks <code class="sh_comment">/* Our hook registering function */</code>

5a58787efeb02a1c3f06569d019ad81fd2efa06end<code class="sh_misc">}</code><code class="sh_misc">;</code>

</p>

<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>

<example><pre><a href="/mod/mod_so.html#LoadModule">LoadModule</a> example_module modules/mod_example.so</pre></example>

<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>

</section>

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

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

<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 <directive module="mod_mime">AddHandler</directive>

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

an example using <directive module="mod_mime">AddHandler</directive>. 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>

<example><pre>

AddHandler example-handler .sum

</pre></example>

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

&quot;. 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>

</section>

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

<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>

<p class="source">

module AP_MODULE_DECLARE_DATA example_module =

{

STANDARD20_MODULE_STUFF,

<code class="sh_constant">NULL</code>,

<code class="sh_constant">NULL</code>,

<code class="sh_constant">NULL</code>,

<code class="sh_constant">NULL</code>,

<code class="sh_constant">NULL</code>,

<code class="sh_function">register_hooks</code> <code class="sh_comment"><code class="sh_comment">/* Our hook registering function */</code></code>

};

</p>

<p>This lets the server know that we are not interesting 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>

<p class="source">

<code class="sh_declare">static</code> <code class="sh_declare">void</code> register_hooks<code class="sh_misc">(</code>apr_pool_t <code class="sh_misc">*</code>pool<code class="sh_misc">)</code>

<code class="sh_misc">{</code>

<code class="sh_comment">/* Create a hook in the request handler, so we get called when a request arrives */</code>

<code class="sh_function">ap_hook_handler</code><code class="sh_misc">(</code>example_handler<code class="sh_misc">,</code> <code class="sh_constant">NULL</code><code class="sh_misc">,</code> <code class="sh_constant">NULL</code><code class="sh_misc">,</code> APR_HOOK_LAST<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code>

</p>

<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>

</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>

<p class="source">

<code class="sh_declare">static</code> <code class="sh_declare">int</code> example_handler<code class="sh_misc">(</code>request_rec <code class="sh_misc">*</code>r<code class="sh_misc">)</code>

<code class="sh_misc">{</code>

<code class="sh_comment">/* First off, we need to check if this is a call for the "example-handler" handler.</code>

<code class="sh_comment">&#xa0;&#xa0;&#xa0;&#xa0;&#xa0;* If it is, we accept it and do our things, if not, we simply return DECLINED,</code>

<code class="sh_comment">&#xa0;&#xa0;&#xa0;&#xa0;&#xa0;* and the server will try somewhere else.</code>

<code class="sh_comment">&#xa0;&#xa0;&#xa0;&#xa0;&#xa0;*/</code>

<code class="sh_declare">if</code> <code class="sh_misc">(</code><code class="sh_misc">!</code>r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">handler</code> <code class="sh_misc">|</code><code class="sh_misc">|</code> <code class="sh_function">strcmp</code><code class="sh_misc">(</code>r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">handler</code><code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">example-handler</code><code class="sh_character">"</code><code class="sh_misc">)</code><code class="sh_misc">)</code> <code class="sh_declare">return</code> <code class="sh_misc">(</code>DECLINED<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* Now that we are handling this request, we'll write out "Hello, world!" to the client.</code>

<code class="sh_comment">&#xa0;&#xa0;&#xa0;&#xa0;&#xa0;* To do so, we must first set the appropriate content type, followed by our output.</code>

<code class="sh_comment">&#xa0;&#xa0;&#xa0;&#xa0;&#xa0;*/</code>

<a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__PROTO.html#gaa2f8412c400197338ec509f4a45e4579">ap_set_content_type</a><code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">text/html</code><code class="sh_character">"</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__PROTO.html#ga5e91eb6ca777c9a427b2e82bf1eeb81d">ap_rprintf</a><code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">Hello, world!</code><code class="sh_character">"</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* Lastly, we must tell the server that we took care of this request and everything went fine.</code>

<code class="sh_comment">&#xa0;&#xa0;&#xa0;&#xa0;&#xa0;* We do so by simply returning the value OK to the server.</code>

<code class="sh_comment">&#xa0;&#xa0;&#xa0;&#xa0;&#xa0;*/</code>

<code class="sh_declare">return</code> OK<code class="sh_misc">;</code>

<code class="sh_misc">}</code>

</p>

<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_req* </code> structure passed along with every call that is made.

This struct, typically just refered 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_req </code> structure are:

</p>

<ul>

<li><code><code class="sh_subobject">r-&gt;handler</code> (char*)</code>: Contains the name of the handler the server is currently asking to do the handling of this request</li>

<li><code><code class="sh_subobject">r-&gt;method</code> (char*)</code>: Contains the HTTP method being used, f.x. GET or POST</li>

<li><code><code class="sh_subobject">r-&gt;filename</code> (char*)</code>: Contains the translated filename the client is requesting</li>

<li><code><code class="sh_subobject">r-&gt;args</code> (char*)</code>: Contains the query string of the request, if any</li>

<li><code><code class="sh_subobject">r-&gt;headers_in</code> (apr_table_t*)</code>: Contains all the headers sent by the client</li>

<li><code><code class="sh_subobject">r-&gt;connection</code> (conn_rec*)</code>: A record containing information about the current connection</li>

<li><code><code class="sh_subobject">r-&gt;useragent_ip</code> (char*)</code>: The IP address of the client connecting to us</li>

<li><code><code class="sh_subobject">r-&gt;pool</code> (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 with in the <code>request_req</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>

<p class="source">

<code class="sh_declare">static</code> <code class="sh_declare">int</code> example_handler<code class="sh_misc">(</code>request_rec <code class="sh_misc">*</code>r<code class="sh_misc">)</code>

<code class="sh_misc">{</code>

<code class="sh_comment">/* Set the appropriate content type */</code>

<a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__PROTO.html#gaa2f8412c400197338ec509f4a45e4579">ap_set_content_type</a><code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">text/html</code><code class="sh_character">"</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* Print out the IP address of the client connecting to us: */</code>

<a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__PROTO.html#ga5e91eb6ca777c9a427b2e82bf1eeb81d">ap_rprintf</a><code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">&lt;h2>Hello, </code><code class="sh_reference">%s</code><code class="sh_value">!&lt;/h2></code><code class="sh_character">"</code><code class="sh_misc">,</code> r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">useragent_ip</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* If we were reached through a GET or a POST request, be happy, else sad. */</code>

<code class="sh_declare">if</code> <code class="sh_misc">(</code> <code class="sh_misc">!</code><code class="sh_function">strcmp</code><code class="sh_misc">(</code>r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">method</code><code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">POST</code><code class="sh_character">"</code><code class="sh_misc">)</code> <code class="sh_misc">|</code><code class="sh_misc">|</code> <code class="sh_misc">!</code><code class="sh_function">strcmp</code><code class="sh_misc">(</code>r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">method</code><code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">GET</code><code class="sh_character">"</code><code class="sh_misc">)</code> <code class="sh_misc">)</code> <code class="sh_misc">{</code>

ap_rputs<code class="sh_misc">(</code><code class="sh_character">"</code><code class="sh_value">You used a GET or a POST method, that makes us happy!&lt;br></code><code class="sh_character">"</code><code class="sh_misc">,</code> r<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code>

<code class="sh_declare">else</code> <code class="sh_misc">{</code>

ap_rputs<code class="sh_misc">(</code><code class="sh_character">"</code><code class="sh_value">You did not use POST or GET, that makes us sad :(&lt;br></code><code class="sh_character">"</code><code class="sh_misc">,</code> r<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code>

<code class="sh_comment">/* Lastly, if there was a query string, let's print that too! */</code>

<code class="sh_declare">if</code> <code class="sh_misc">(</code>r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">args</code><code class="sh_misc">)</code> <code class="sh_misc">{</code>

<a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__PROTO.html#ga5e91eb6ca777c9a427b2e82bf1eeb81d">ap_rprintf</a><code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">Your query string was: </code><code class="sh_reference">%s</code><code class="sh_character">"</code><code class="sh_misc">,</code> r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">args</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code>

<code class="sh_declare">return</code> OK<code class="sh_misc">;</code>

<code class="sh_misc">}</code>

</p>

</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>

<p class="source">

<code class="sh_declare">static</code> <code class="sh_declare">int</code> example_handler<code class="sh_misc">(</code>request_rec <code class="sh_misc">*</code>r<code class="sh_misc">)</code>

<code class="sh_misc">{</code>

<code class="sh_comment">/* Return 404: Not found */</code>

<code class="sh_declare">return</code> HTTP_NOT_FOUND<code class="sh_misc">;</code>

<code class="sh_misc">}</code>

</p>

<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_req *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>.

<p class="source">ap_rputs<code class="sh_misc">(</code><code class="sh_character">"</code><code class="sh_value">Hello, world!</code><code class="sh_character">"</code><code class="sh_misc">,</code> r<code class="sh_misc">)</code><code class="sh_misc">;</code></p>

</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.

<p class="source"><code class="sh_declare"><a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__PROTO.html#ga5e91eb6ca777c9a427b2e82bf1eeb81d">ap_rprintf</a></code><code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">Hello, </code><code class="sh_reference">%s</code><code class="sh_value">!</code><code class="sh_character">"</code><code class="sh_misc">,</code> r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">useragent_ip</code><code class="sh_misc">)</code><code class="sh_misc">;</code></p>

</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_req *r, const char *type)</code>: <br/>

Sets the content type of the output you are sending.

<p class="source"><code class="sh_declare"><a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__PROTO.html#gaa2f8412c400197338ec509f4a45e4579">ap_set_content_type</a></code><code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">text/plain</code><code class="sh_character">"</code><code class="sh_misc">)</code><code class="sh_misc">;</code> <code class="sh_comment">/* force a raw text output */</code></p>

</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 class="sh_subobject">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>

<p class="source">

<code class="sh_declare">static</code> <code class="sh_declare">int</code> example_handler<code class="sh_misc">(</code>request_rec <code class="sh_misc">*</code>r<code class="sh_misc">)</code>

<code class="sh_misc">{</code>

<code class="sh_declare">const</code> <code class="sh_declare">char</code><code class="sh_misc">*</code> original <code class="sh_misc">=</code> <code class="sh_character">"</code><code class="sh_value">You can't edit this!</code><code class="sh_character">"</code><code class="sh_misc">;</code>

<code class="sh_declare">char</code><code class="sh_misc">*</code> copy<code class="sh_misc">;</code>

<code class="sh_declare">int</code><code class="sh_misc">*</code> integers<code class="sh_misc">;</code>

<code class="sh_comment">/* Allocate space for 10 integer values and set them all to zero. */</code>

integers <code class="sh_misc">=</code> apr_pcalloc<code class="sh_misc">(</code>r<code class="sh_misc">-</code><code class="sh_misc">></code>pool<code class="sh_misc">,</code> <code class="sh_declare">sizeof</code><code class="sh_misc">(</code><code class="sh_declare">int</code><code class="sh_misc">)</code><code class="sh_misc">*</code><code class="sh_reference">10</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* Create a copy of the 'original' variable that we can edit. */</code>

copy <code class="sh_misc">=</code> apr_pstrdup<code class="sh_misc">(</code>r<code class="sh_misc">-</code><code class="sh_misc">></code>pool<code class="sh_misc">,</code> original<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_declare">return</code> OK<code class="sh_misc">;</code>

<code class="sh_misc">}</code>

</p>

<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>

<p class="source">

<code class="sh_declare">static</code> <code class="sh_declare">void</code> register_hooks<code class="sh_misc">(</code>apr_pool_t <code class="sh_misc">*</code>pool<code class="sh_misc">)</code>

<code class="sh_misc">{</code>

<code class="sh_comment">/* Call a function that initializes some stuff */</code>

<code class="sh_function">example_init_function</code><code class="sh_misc">(</code>pool<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* Create a hook in the request handler, so we get called when a request arrives */</code>

<code class="sh_function">ap_hook_handler</code><code class="sh_misc">(</code>example_handler<code class="sh_misc">,</code> <code class="sh_constant">NULL</code><code class="sh_misc">,</code> <code class="sh_constant">NULL</code><code class="sh_misc">,</code> APR_HOOK_LAST<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code>

</p>

<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>

<p class="source">

<a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__apr__tables.html#gad7ea82d6608a4a633fc3775694ab71e4">apr_table_t</a> *GET;

<a href="http://ci.apache.org/projects/httpd/trunk/doxygen/structapr__array__header__t.html">apr_array_header_t</a> *POST;

<a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__SCRIPT.html#gaed25877b529623a4d8f99f819ba1b7bd">ap_args_to_table</a>(r, &amp;GET);

<a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__DAEMON.html#ga9d426b6382b49754d4f87c55f65af202">ap_parse_form_data</a>(r, <code class="sh_constant">NULL</code>, &amp;POST, -1, 8192);

</p>

<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>

<p class="source">

<code class="sh_comment">/* Get the "digest" key from the query string, if any. */</code>

<code class="sh_declare">const</code> <code class="sh_declare">char</code><code class="sh_misc"> *</code>digestType <code class="sh_misc">=</code> <a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__apr__tables.html#ga4db13e3915c6b9a3142b175d4c15d915">apr_table_get</a><code class="sh_misc">(</code>GET<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">digest</code><code class="sh_character">"</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* If no key was returned, we will set a default value instead. */</code>

<code class="sh_declare">if</code> <code class="sh_misc">(</code><code class="sh_misc">!</code>digestType<code class="sh_misc">)</code> digestType <code class="sh_misc">=</code> <code class="sh_character">"</code><code class="sh_value">sha1</code><code class="sh_character">"</code><code class="sh_misc">;</code>

</p>

<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>

<p class="source">

<code class="sh_declare">static</code> <code class="sh_declare">int</code> example_handler<code class="sh_misc">(</code>request_rec <code class="sh_misc">*</code>r<code class="sh_misc">)</code>

<code class="sh_misc">{</code>

<code class="sh_declare">int</code> rc<code class="sh_misc">,</code> exists<code class="sh_misc">;</code>

apr_finfo_t finfo<code class="sh_misc">;</code>

apr_file_t<code class="sh_misc"> *</code>file<code class="sh_misc">;</code>

<code class="sh_declare">char</code> <code class="sh_misc">*</code>filename<code class="sh_misc">;</code>

<code class="sh_declare">char</code> buffer<code class="sh_misc">[</code><code class="sh_reference">256</code><code class="sh_misc">]</code><code class="sh_misc">;</code>

apr_size_t readBytes<code class="sh_misc">;</code>

<code class="sh_declare">int</code> n<code class="sh_misc">;</code>

apr_table_t<code class="sh_misc"> *</code>GET<code class="sh_misc">;</code>

apr_array_header_t<code class="sh_misc"> *</code>POST<code class="sh_misc">;</code>

<code class="sh_declare">const</code> <code class="sh_declare">char</code><code class="sh_misc"> *</code>digestType<code class="sh_misc">;</code>

<code class="sh_comment">/* Check that the "example-handler" handler is being called. */</code>

<code class="sh_declare">if</code> <code class="sh_misc">(</code><code class="sh_misc">!</code>r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">handler</code> <code class="sh_misc">|</code><code class="sh_misc">|</code> <code class="sh_function">strcmp</code><code class="sh_misc">(</code>r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">handler</code><code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">example-handler</code><code class="sh_character">"</code><code class="sh_misc">)</code><code class="sh_misc">)</code> <code class="sh_declare">return</code> <code class="sh_misc">(</code>DECLINED<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* Figure out which file is being requested by removing the .sum from it */</code>

filename <code class="sh_misc">=</code> apr_pstrdup<code class="sh_misc">(</code>r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">pool</code><code class="sh_misc">,</code> r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">filename</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

filename<code class="sh_misc">[</code><code class="sh_function">strlen</code><code class="sh_misc">(</code>filename<code class="sh_misc">)</code><code class="sh_misc">-</code><code class="sh_reference">4</code><code class="sh_misc">]</code> <code class="sh_misc">=</code> <code class="sh_reference">0</code><code class="sh_misc">;</code> <code class="sh_comment">/* Cut off the last 4 characters. */</code>

<code class="sh_comment">/* Figure out if the file we request a sum on exists and isn't a directory */</code>

rc <code class="sh_misc">=</code> apr_stat<code class="sh_misc">(</code><code class="sh_misc">&amp;</code>finfo<code class="sh_misc">,</code> filename<code class="sh_misc">,</code> APR_FINFO_MIN<code class="sh_misc">,</code> r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">pool</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_declare">if</code> <code class="sh_misc">(</code>rc <code class="sh_misc">=</code><code class="sh_misc">=</code> APR_SUCCESS<code class="sh_misc">)</code> <code class="sh_misc">{</code>

exists <code class="sh_misc">=</code>

<code class="sh_misc">(</code>

<code class="sh_misc">(</code>finfo<code class="sh_misc">.</code>filetype <code class="sh_misc">!</code><code class="sh_misc">=</code> APR_NOFILE<code class="sh_misc">)</code>

<code class="sh_misc">&amp;</code><code class="sh_misc">&amp;</code> <code class="sh_misc">!</code><code class="sh_misc">(</code>finfo<code class="sh_misc">.</code>filetype <code class="sh_misc">&amp;</code> APR_DIR<code class="sh_misc">)</code>

<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_declare">if</code> <code class="sh_misc">(</code><code class="sh_misc">!</code>exists<code class="sh_misc">)</code> <code class="sh_declare">return</code> HTTP_NOT_FOUND<code class="sh_misc">;</code> <code class="sh_comment">/* Return a 404 if not found. */</code>

<code class="sh_misc">}</code>

<code class="sh_comment">/* If apr_stat failed, we're probably not allowed to check this file. */</code>

<code class="sh_declare">else</code> <code class="sh_declare">return</code> HTTP_FORBIDDEN<code class="sh_misc">;</code>

<code class="sh_comment">/* Parse the GET and, optionally, the POST data sent to us */</code>

ap_args_to_table<code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_misc">&amp;</code>GET<code class="sh_misc">)</code><code class="sh_misc">;</code>

ap_parse_form_data<code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_constant">NULL</code><code class="sh_misc">,</code> <code class="sh_misc">&amp;</code>POST<code class="sh_misc">,</code> <code class="sh_misc">-</code><code class="sh_reference">1</code><code class="sh_misc">,</code> <code class="sh_reference">8192</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* Set the appropriate content type */</code>

ap_set_content_type<code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">text/html</code><code class="sh_character">"</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* Print a title and some general information */</code>

ap_rprintf<code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">&lt;h2>Information on </code><code class="sh_reference">%s</code><code class="sh_value">:&lt;/h2></code><code class="sh_character">"</code><code class="sh_misc">,</code> filename<code class="sh_misc">)</code><code class="sh_misc">;</code>

ap_rprintf<code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">&lt;b>Size:&lt;/b> </code><code class="sh_reference">%u</code><code class="sh_value"> bytes&lt;br/></code><code class="sh_character">"</code><code class="sh_misc">,</code> finfo<code class="sh_misc">.</code>size<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* Get the digest type the client wants to see */</code>

digestType <code class="sh_misc">=</code> apr_table_get<code class="sh_misc">(</code>GET<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">digest</code><code class="sh_character">"</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_declare">if</code> <code class="sh_misc">(</code><code class="sh_misc">!</code>digestType<code class="sh_misc">)</code> digestType <code class="sh_misc">=</code> <code class="sh_character">"</code><code class="sh_value">MD5</code><code class="sh_character">"</code><code class="sh_misc">;</code>

rc <code class="sh_misc">=</code> apr_file_open<code class="sh_misc">(</code><code class="sh_misc">&amp;</code>file<code class="sh_misc">,</code> filename<code class="sh_misc">,</code> APR_READ<code class="sh_misc">,</code> APR_OS_DEFAULT<code class="sh_misc">,</code> r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">pool</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_declare">if</code> <code class="sh_misc">(</code>rc <code class="sh_misc">=</code><code class="sh_misc">=</code> APR_SUCCESS<code class="sh_misc">)</code> <code class="sh_misc">{</code>

<code class="sh_comment">/* Are we trying to calculate the MD5 or the SHA1 digest? */</code>

<code class="sh_declare">if</code> <code class="sh_misc">(</code><code class="sh_misc">!</code>strcasecmp<code class="sh_misc">(</code>digestType<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">md5</code><code class="sh_character">"</code><code class="sh_misc">)</code><code class="sh_misc">)</code> <code class="sh_misc">{</code>

<code class="sh_comment">/* Calculate the MD5 sum of the file */</code>

<code class="sh_declare">union</code> <code class="sh_misc">{</code>

<code class="sh_declare">char</code> chr<code class="sh_misc">[</code><code class="sh_reference">16</code><code class="sh_misc">]</code><code class="sh_misc">;</code>

uint32_t num<code class="sh_misc">[</code><code class="sh_reference">4</code><code class="sh_misc">]</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code> digest<code class="sh_misc">;</code>

apr_md5_ctx_t md5<code class="sh_misc">;</code>

apr_md5_init<code class="sh_misc">(</code><code class="sh_misc">&amp;</code>md5<code class="sh_misc">)</code><code class="sh_misc">;</code>

readBytes <code class="sh_misc">=</code> <code class="sh_reference">256</code><code class="sh_misc">;</code>

<code class="sh_declare">while</code> <code class="sh_misc">(</code> apr_file_read<code class="sh_misc">(</code>file<code class="sh_misc">,</code> buffer<code class="sh_misc">,</code> <code class="sh_misc">&amp;</code>readBytes<code class="sh_misc">)</code> <code class="sh_misc">=</code><code class="sh_misc">=</code> APR_SUCCESS <code class="sh_misc">)</code> <code class="sh_misc">{</code>

apr_md5_update<code class="sh_misc">(</code><code class="sh_misc">&amp;</code>md5<code class="sh_misc">,</code> buffer<code class="sh_misc">,</code> readBytes<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code>

apr_md5_final<code class="sh_misc">(</code>digest<code class="sh_misc">.</code>chr<code class="sh_misc">,</code> <code class="sh_misc">&amp;</code>md5<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* Print out the MD5 digest */</code>

ap_rputs<code class="sh_misc">(</code><code class="sh_character">"</code><code class="sh_value">&lt;b>MD5: &lt;/b>&lt;code></code><code class="sh_character">"</code><code class="sh_misc">,</code> r<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_declare">for</code> <code class="sh_misc">(</code>n <code class="sh_misc">=</code> <code class="sh_reference">0</code><code class="sh_misc">;</code> n <code class="sh_misc">&lt;</code> APR_MD5_DIGESTSIZE<code class="sh_misc">/</code><code class="sh_reference">4</code><code class="sh_misc">;</code> n<code class="sh_misc">+</code><code class="sh_misc">+</code><code class="sh_misc">)</code> <code class="sh_misc">{</code>

ap_rprintf<code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_reference">%08x</code><code class="sh_character">"</code><code class="sh_misc">,</code> digest<code class="sh_misc">.</code>num<code class="sh_misc">[</code>n<code class="sh_misc">]</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code>

ap_rputs<code class="sh_misc">(</code><code class="sh_character">"</code><code class="sh_value">&lt;/code></code><code class="sh_character">"</code><code class="sh_misc">,</code> r<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* Print a link to the SHA1 version */</code>

ap_rputs<code class="sh_misc">(</code><code class="sh_character">"</code><code class="sh_value">&lt;br/>&lt;a href='?digest=sha1'>View the SHA1 hash instead&lt;/a></code><code class="sh_character">"</code><code class="sh_misc">,</code> r<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code>

<code class="sh_declare">else</code> <code class="sh_misc">{</code>

<code class="sh_comment">/* Calculate the SHA1 sum of the file */</code>

<code class="sh_declare">union</code> <code class="sh_misc">{</code>

<code class="sh_declare">char</code> chr<code class="sh_misc">[</code><code class="sh_reference">20</code><code class="sh_misc">]</code><code class="sh_misc">;</code>

uint32_t num<code class="sh_misc">[</code><code class="sh_reference">5</code><code class="sh_misc">]</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code> digest<code class="sh_misc">;</code>

apr_sha1_ctx_t sha1<code class="sh_misc">;</code>

apr_sha1_init<code class="sh_misc">(</code><code class="sh_misc">&amp;</code>sha1<code class="sh_misc">)</code><code class="sh_misc">;</code>

readBytes <code class="sh_misc">=</code> <code class="sh_reference">256</code><code class="sh_misc">;</code>

<code class="sh_declare">while</code> <code class="sh_misc">(</code> apr_file_read<code class="sh_misc">(</code>file<code class="sh_misc">,</code> buffer<code class="sh_misc">,</code> <code class="sh_misc">&amp;</code>readBytes<code class="sh_misc">)</code> <code class="sh_misc">=</code><code class="sh_misc">=</code> APR_SUCCESS <code class="sh_misc">)</code> <code class="sh_misc">{</code>

apr_sha1_update<code class="sh_misc">(</code><code class="sh_misc">&amp;</code>sha1<code class="sh_misc">,</code> buffer<code class="sh_misc">,</code> readBytes<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code>

apr_sha1_final<code class="sh_misc">(</code>digest<code class="sh_misc">.</code>chr<code class="sh_misc">,</code> <code class="sh_misc">&amp;</code>sha1<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* Print out the SHA1 digest */</code>

ap_rputs<code class="sh_misc">(</code><code class="sh_character">"</code><code class="sh_value">&lt;b>SHA1: &lt;/b>&lt;code></code><code class="sh_character">"</code><code class="sh_misc">,</code> r<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_declare">for</code> <code class="sh_misc">(</code>n <code class="sh_misc">=</code> <code class="sh_reference">0</code><code class="sh_misc">;</code> n <code class="sh_misc">&lt;</code> APR_SHA1_DIGESTSIZE<code class="sh_misc">/</code><code class="sh_reference">4</code><code class="sh_misc">;</code> n<code class="sh_misc">+</code><code class="sh_misc">+</code><code class="sh_misc">)</code> <code class="sh_misc">{</code>

ap_rprintf<code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_reference">%08x</code><code class="sh_character">"</code><code class="sh_misc">,</code> digest<code class="sh_misc">.</code>num<code class="sh_misc">[</code>n<code class="sh_misc">]</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code>

ap_rputs<code class="sh_misc">(</code><code class="sh_character">"</code><code class="sh_value">&lt;/code></code><code class="sh_character">"</code><code class="sh_misc">,</code> r<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_comment">/* Print a link to the MD5 version */</code>

ap_rputs<code class="sh_misc">(</code><code class="sh_character">"</code><code class="sh_value">&lt;br/>&lt;a href='?digest=md5'>View the MD5 hash instead&lt;/a></code><code class="sh_character">"</code><code class="sh_misc">,</code> r<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code>

apr_file_close<code class="sh_misc">(</code>file<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code>

<code class="sh_comment">/* Let the server know that we responded to this request. */</code>

<code class="sh_declare">return</code> OK<code class="sh_misc">;</code>

<code class="sh_misc">}</code>

</p>

<p>

This version in its entirity 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>

<example><pre>

RewriteEngine On

RewriteCond %{REQUEST_URI} ^/foo/bar

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

</pre></example>

<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>

<p class="source">

<code class="sh_declare">typedef</code> <code class="sh_declare">struct</code> <code class="sh_misc">{</code>

<code class="sh_declare">int</code> enabled<code class="sh_misc">;</code> <code class="sh_comment">/* Enable or disable our module */</code>

<code class="sh_declare">const</code> <code class="sh_declare">char</code> <code class="sh_misc">*</code>path<code class="sh_misc">;</code> <code class="sh_comment">/* Some path to...something */</code>

<code class="sh_declare">int</code> typeOfAction<code class="sh_misc">;</code> <code class="sh_comment">/* 1 means action A, 2 means action B and so on */</code>

<code class="sh_misc">}</code> example_config<code class="sh_misc">;</code>

</p>

<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>

<p class="source">

<code class="sh_declare">typedef</code> <code class="sh_declare">struct</code> <code class="sh_misc">{</code>

<code class="sh_declare">int</code> enabled<code class="sh_misc">;</code> <code class="sh_comment">/* Enable or disable our module */</code>

<code class="sh_declare">const</code> <code class="sh_declare">char</code> <code class="sh_misc">*</code>path<code class="sh_misc">;</code> <code class="sh_comment">/* Some path to...something */</code>

<code class="sh_declare">int</code> typeOfAction<code class="sh_misc">;</code> <code class="sh_comment">/* 1 means action A, 2 means action B and so on */</code>

<code class="sh_misc">}</code> example_config<code class="sh_misc">;</code>

<code class="sh_declare">static</code> example_config config<code class="sh_misc">;</code>

<code class="sh_declare">static</code> <code class="sh_declare">int</code> example_handler<code class="sh_misc">(</code>request_rec <code class="sh_misc">*</code>r<code class="sh_misc">)</code>

<code class="sh_misc">{</code>

<code class="sh_declare">if</code> <code class="sh_misc">(</code><code class="sh_misc">!</code>r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">handler</code> <code class="sh_misc">|</code><code class="sh_misc">|</code> <code class="sh_function">strcmp</code><code class="sh_misc">(</code>r<code class="sh_misc">-</code><code class="sh_misc">></code><code class="sh_subobject">handler</code><code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">example-handler</code><code class="sh_character">"</code><code class="sh_misc">)</code><code class="sh_misc">)</code> <code class="sh_declare">return</code><code class="sh_misc">(</code>DECLINED<code class="sh_misc">)</code><code class="sh_misc">;</code>

ap_set_content_type<code class="sh_misc">(</code>r<code class="sh_misc">,</code> <code class="sh_character">"</code><code class="sh_value">text/plain</code><code class="sh_character">"</code><code class="sh_misc">)</code><code class="sh_misc">;</code>

ap_rprintf<code class="sh_misc">(r, </code><code class="sh_character">"</code><code class="sh_value">Enabled: </code><code class="sh_reference">%u</code><code class="sh_reference">\n</code><code class="sh_character">"</code><code class="sh_misc">,</code> config<code class="sh_misc">.</code>enabled<code class="sh_misc">)</code><code class="sh_misc">;</code>

ap_rprintf<code class="sh_misc">(r, </code><code class="sh_character">"</code><code class="sh_value">Path: </code><code class="sh_reference">%s</code><code class="sh_reference">\n</code><code class="sh_character">"</code><code class="sh_misc">,</code> config<code class="sh_misc">.</code>path<code class="sh_misc">)</code><code class="sh_misc">;</code>

ap_rprintf<code class="sh_misc">(r, </code><code class="sh_character">"</code><code class="sh_value">TypeOfAction: </code><code class="sh_reference">%x</code><code class="sh_reference">\n</code><code class="sh_character">"</code><code class="sh_misc">,</code> config<code class="sh_misc">.</code>typeOfAction<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_declare">return</code> OK<code class="sh_misc">;</code>

<code class="sh_misc">}</code>

<code class="sh_declare">static</code> <code class="sh_declare">void</code> register_hooks<code class="sh_misc">(</code>apr_pool_t <code class="sh_misc">*</code>pool<code class="sh_misc">)</code>

<code class="sh_misc">{</code>

config<code class="sh_misc">.</code>enabled <code class="sh_misc">=</code> <code class="sh_reference">1</code><code class="sh_misc">;</code>

config<code class="sh_misc">.</code>path <code class="sh_misc">=</code> <code class="sh_character">"</code><code class="sh_value">/foo/bar</code><code class="sh_character">"</code><code class="sh_misc">;</code>

config<code class="sh_misc">.</code>typeOfAction <code class="sh_misc">=</code> <code class="sh_reference">0x00</code><code class="sh_misc">;</code>

ap_hook_handler<code class="sh_misc">(</code>example_handler<code class="sh_misc">,</code> <code class="sh_constant">NULL</code><code class="sh_misc">,</code> <code class="sh_constant">NULL</code><code class="sh_misc">,</code> APR_HOOK_LAST<code class="sh_misc">)</code><code class="sh_misc">;</code>

<code class="sh_misc">}</code>

<code class="sh_comment">/* Define our module as an entity and assign a function for registering hooks */</code>

module AP_MODULE_DECLARE_DATA example_module <code class="sh_misc">=</code>

<code class="sh_misc">{</code>

STANDARD20_MODULE_STUFF<code class="sh_misc">,</code>

<code class="sh_constant">NULL</code><code class="sh_misc">,</code> <code class="sh_comment">/* Per-directory configuration handler */</code>

<code class="sh_constant">NULL</code><code class="sh_misc">,</code> <code class="sh_comment">/* Merge handler for per-directory configurations */</code>

<code class="sh_constant">NULL</code><code class="sh_misc">,</code> <code class="sh_comment">/* Per-server configuration handler */</code>

<code class="sh_constant">NULL</code><code class="sh_misc">,</code> <code class="sh_comment">/* Merge handler for per-server configurations */</code>

<code class="sh_constant">NULL</code><code class="sh_misc">,</code> <code class="sh_comment">/* Any directives we may have for httpd */</code>

register_hooks <code class="sh_comment">/* Our hook registering function */</code>

<code class="sh_misc">}</code><code class="sh_misc">;</code>

</p>

<p>

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

our configuration:

</p>

<example><pre>

&lt;Location /example&gt;

SetHandler example-handler

&lt;/Location&gt;

</pre></example>

<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>

<p class="source">

module AP_MODULE_DECLARE_DATA example_module =

{

STANDARD20_MODULE_STUFF,

<code class="sh_constant">NULL</code>, <code class="sh_comment">/* Per-directory configuration handler */</code>

<code class="sh_constant">NULL</code>, <code class="sh_comment">/* Merge handler for per-directory configurations */</code>

<code class="sh_constant">NULL</code>, <code class="sh_comment">/* Per-server configuration handler */</code>

<code class="sh_constant">NULL</code>, <code class="sh_comment">/* Merge handler for per-server configurations */</code>

example_directives, <code class="sh_comment">/* Any directives we may have for httpd */</code>

register_hooks <code class="sh_comment">/* Our hook registering function */</code>

};

</p>

<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>

<p class="source">

static const command_rec example_directives[] =

{

<code class="sh_declare"><a href="http://ci.apache.org/projects/httpd/trunk/doxygen/group__APACHE__CORE__CONFIG.html#ga07c7d22ae17805e61204463326cf9c34">AP_INIT_TAKE1</a></code>("exampleEnabled", example_set_enabled, <code class="sh_constant">NULL</code>, RSRC_CONF, "Enable or disable mod_example"),