Developing modules for the Apache HTTP Server 2.4

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- -

This document explains how you can develop modules for the Apache HTTP Server 2.4

Introduction
Defining a module
Getting started: Hooking into Apache
Building a handler
Adding configuration options
Context aware configurations
Summing up
Some useful snippets of code

Introduction

What we will be discussing in this document

-This document will discuss how you can easily create modules for the Apache HTTP Server 2.4 ("Apache"), -by exploring an example module called mod_example. In the first part of this document, the purpose of this -module will be to calculate and print out various digest values for existing files on your web server, whenever we -access the URL http://hostname/filename.sum. For instance, if we want to know the MD5 digest value of the file -located at http://www.example.com/index.html, we would visit http://www.example.com/index.html.sum. -

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

- -

Prerequisites

-First and foremost, you are expected to have a basic knowledge of how the C programming language works. In most cases, -we will try to be as pedagogical as possible and link to documents describing the functions used in the examples, -but there are also many cases where it is necessary to either just assume that "it works" or do some digging youself -into what the hows and whys of various function calls. -

-Lastly, you will need to have a basic understanding of how modules are loaded and configured in Apache, as well as -how to get the headers for Apache if you do not have them already, as these are needed for compiling new modules. -

- -

Compiling your module

-To compile the source code we are building in this document, we will be using APXS. -Assuming your source file is called mod_example.c, compiling, installing and activating the module is as simple as: -

-apxs -i -a -c mod_example.c
-

- -

Defining a module

Every module starts with the same declaration, or name tag if you will, that defines a module as a separate entity within Apache: - - - -

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

- - - -This bit of code lets Apache know that we have now registered a new module in the system, -and that its name is example_module. The name of the module is used primarilly -for two things:
-

Letting Apache know how to load the module using the LoadModule
Setting up a namespace for the module to use in configurations

-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:
-

LoadModule example_module modules/mod_example.so

-In essence, this tells Apache to open up mod_example.so and look for a module -called example_module. -

-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 -service. We'll return to all these elements later in this document. -

Getting started: Hooking into Apache

An introduction to hooks

-When handling requests in Apache, the first thing you will need to do is create a hook into -the request handling process. A hook is essentially a message telling Apache 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 Apache -how and when each module will come into place. Thus, Apache 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:
-
-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, Apache has directives for hinting to modules whether -their assistance is needed or not. Two of these are AddHandler and SetHandler. -Let's take a look at an example using AddHandler. -In our example case, we want every request ending with .sum to be served by mod_example, -so we'll add a configuration directive that tells Apache to do just that: -

-AddHandler example-handler .sum
-

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

- -

Hooking into httpd

-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:
- - - -

-module AP_MODULE_DECLARE_DATA   example_module =
-{
-    STANDARD20_MODULE_STUFF,
-    NULL,
-    NULL,
-    NULL,
-    NULL,
-    NULL,
-    register_hooks   /* Our hook registering function */
-};
-

- - - -This lets Apache know that we are not interesting in anything fancy, we just want to hook onto -the requests and possibly handle some of them. -

-The reference in our example declaration, register_hooks 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 Apache know, that we want to hook into its process -as one of the last modules: - - - -

-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);
-}
-

- - - -The example_handler reference is the function that will handle the request. We will -discuss how to create a handler in the next chapter. -

- -

Other useful hooks

-Hooking into the request handling phase is but one of many hooks that you can create. Some other ways of hooking are: -

ap_hook_child_init: Place a hook that executes when a child process is spawned (commonly used for initializing modules after Apache has forked)
ap_hook_pre_config: Place a hook that executes before any configuration data has been read (very early hook)
ap_hook_post_config: Place a hook that executes after configuration has been parsed, but before Apache has forked
ap_hook_translate_name: Place a hook that executes when a URI needs to be translated into a filename on the server (think mod_rewrite)

- -

Building a handler

-A handler is essentially a function that receives a callback when a request to Apache 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 -Apache that it's not interested in the request or handle the request with the tools provided. -

A simple "Hello, world!" handler

-Let's start off by making a very simple request handler that does the following:
-

Check that this is a request that should be served by "example-handler"
Set the content type of our output to text/html
Write "Hello, world!" back to the client browser
Let Apache know that we took care of this request and everything went fine

-In C code, our example handler will now look like this:
- - - -

-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, it not, we simply return DECLINED,
-     * and Apache 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 Apache that we took care of this request and everything went fine.
-     * We do so by simply returning the value OK to Apache.
-     */
-    return OK;
-}
-

- - - -Now, we put all we have learned together and end up with a program that looks like -mod_example_1.c. The functions used in this example will be explained later in the section -"Some useful functions you should know". - -

The request_rec structure

The most essential part of any request is the request record. In a call to a handler function, this -is represented by the request_req* structure passed along with every call that is made. This -struct, typically just refered to as r in modules, contains all the information you need for -your module to fully process any HTTP request and respond accordingly.

Some key elements of the request_req structure are: -

r->handler (char*): Contains the name of the handler, Apache is currently asking to do the handling of this request
r->method (char*): Contains the HTTP method being used, f.x. GET or POST
r->filename (char*): Contains the translated filename the client is requesting
r->args (char*): Contains the query string of the request, if any
r->headers_in (apr_table_t*): Contains all the headers sent by the client
r->connection (conn_rec*): A record containing information about the current connection
r->useragent_ip (char*): The IP address of the client connecting to us
r->pool (apr_pool_t*): The memory pool of this request. We'll discuss this in the " -Memory management" chapter.

-A complete list of all the values contained with in the request_req structure can be found in -the httpd.h header -file or at [insert link here]. -

-Let's try out some of these variables in another example handler:
- - - -

-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, "<h2>Hello, %s!</h2>", r->useragent_ip);
-    
-    /* If we were reached through a GET or a POST request, be happy, else sad. */
-    if ( !strcmp(r->method, "POST") || !strcmp(r->method, "GET") ) {
-        ap_rputs("You used a GET or a POST method, that makes us happy!<br>", r);
-    }
-    else {
-        ap_rputs("You did not use POST or GET, that makes us sad :(<br>", r);
-    }
-
-    /* Lastly, if there was a query string, let's print that too! */
-    if (r->args) {
-        ap_rprintf(r, "Your query string was: %s", r->args);
-    }
-    return OK;
-}
-

- - - -

- - -

Return values

-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 DECLINED. If it is handling -a request, it should either return the generic value OK, or a specific HTTP status -code, for example: - - - -

-static int example_handler(request_rec *r)
-{
-    /* Return 404: Not found */
-    return HTTP_NOT_FOUND;
-}
-

- - - -Returning OK or a HTTP status code does not necessarilly mean that the request will end. Apache -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 DONE to let Apache know that it should cease all activity on this request and -carry on with the next, without informing other handlers. -
-General response codes: -

DECLINED: We are not handling this request
OK: We handled this request and it went well
DONE: We handled this request and Apache should just close this thread without further processing

-HTTP specific return codes (excerpt): -

HTTP_OK (200): Request was okay
HTTP_MOVED_PERMANENTLY (301): The resource has moved to a new URL
HTTP_UNAUTHORIZED (401): Client is not authorized to visit this page
HTTP_FORBIDDEN (403): Permission denied
HTTP_NOT_FOUND (404): File not found
HTTP_INTERNAL_SERVER_ERROR (500): Internal server error (self explanatory)

- - -

Some useful functions you should know

- -

- ap_rputs(const char *string, request_req *r):
- Sends a string of text to the client. This is a shorthand version of - ap_rwrite. - - - -
```
ap_rputs("Hello, world!", r);
```
- - - -
- - ap_rprintf:
- This function works just like printf, except it sends the result to the client. - - - -
```
ap_rprintf(r, "Hello, %s!", r->useragent_ip);
```
- - -
- - ap_set_content_type(request_req *r, const char *type):
- Sets the content type of the output you are sending. - - - -
```
ap_set_content_type(r, "text/plain"); /* force a raw text output */
```
- - -

- - -

Memory management

-Managing your resources in Apache 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?

- -

-In our module, we will primarilly be allocating memory for each request, so it's appropriate to -use the r->pool reference when creating new objects. A few of the functions for -allocating memory within a pool are: -

void* apr_palloc( -apr_pool_t *p, apr_size_t size): Allocates size number of bytes in the pool for you
void* apr_pcalloc( -apr_pool_t *p, apr_size_t size): Allocates size number of bytes in the pool for you and sets all bytes to 0
char* apr_pstrdup( -apr_pool_t *p, const char *s): Creates a duplicate of the string s. This is useful for copying constant values so you can edit them

-Let's put these functions into an example handler:
- - - -

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

- - -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 register_hooks function to sort it out: - - -

-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);
-}
-

- - -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 -Apache for allocating memory on a per-process based level. - -

- - -

Parsing request data

-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 -valueA=yes&valueB=no&valueC=maybe. 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 digest, -and if set to md5, we'll produce an MD5 digest, otherwise we'll produce a SHA1 digest. -

-Since the introduction of Apache 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: - - - -

-apr_table_t *GET;
-apr_array_header_t *POST;
-
-ap_args_to_table(r, &GET);
-ap_parse_form_data(r, NULL, &POST, -1, 8192);
-

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

-/* 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";
-
-

- - - -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 -this example in the last chapter of this document. -

- - -

Making an advanced handler

-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:
- - - -

-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->handler || strcmp(r->handler, "example-handler")) return (DECLINED);
-    
-    /* Figure out which file is being requested by removing the .sum from it */
-    filename = apr_pstrdup(r->pool, r->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(&finfo, filename, APR_FINFO_MIN, r->pool);
-    if (rc == APR_SUCCESS) {
-        exists =
-        (
-            (finfo.filetype != APR_NOFILE)
-        &&  !(finfo.filetype & 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, &GET);
-    ap_parse_form_data(r, NULL, &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, "<h2>Information on %s:</h2>", filename);
-    ap_rprintf(r, "<b>Size:</b> %u bytes<br/>", 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(&file, filename, APR_READ, APR_OS_DEFAULT, r->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(&md5);
-            readBytes = 256;
-            while ( apr_file_read(file, buffer, &readBytes) == APR_SUCCESS ) {
-                apr_md5_update(&md5, buffer, readBytes);
-            }
-            apr_md5_final(digest.chr, &md5);
-            
-            /* Print out the MD5 digest */
-            ap_rputs("<b>MD5: </b><code>", r);
-            for (n = 0; n < APR_MD5_DIGESTSIZE/4; n++) {
-                ap_rprintf(r, "%08x", digest.num[n]);
-            }
-            ap_rputs("</code>", r);
-            /* Print a link to the SHA1 version */
-            ap_rputs("<br/><a href='?digest=sha1'>View the SHA1 hash instead</a>", 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(&sha1);
-            readBytes = 256;
-            while ( apr_file_read(file, buffer, &readBytes) == APR_SUCCESS ) {
-                apr_sha1_update(&sha1, buffer, readBytes);
-            }
-            apr_sha1_final(digest.chr, &sha1);
-            
-            /* Print out the SHA1 digest */
-            ap_rputs("<b>SHA1: </b><code>", r);
-            for (n = 0; n < APR_SHA1_DIGESTSIZE/4; n++) {
-                ap_rprintf(r, "%08x", digest.num[n]);
-            }
-            ap_rputs("</code>", r);
-            
-            /* Print a link to the MD5 version */
-            ap_rputs("<br/><a href='?digest=md5'>View the MD5 hash instead</a>", r);
-        }
-        apr_file_close(file);
-        
-    }
-    
-    
-    
-    /* Let Apache know that we responded to this request. */
-    return OK;
-}
-

- - - -This version in its entirity can be found here: mod_example_2.c. - - -

Adding configuration options

-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 Apache works with configuration, and what happens when you start writing advanced configurations -for your modules. -

An introduction to configuration directives

-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 mod_rewrite works: -

-RewriteEngine On
-RewriteCond %{REQUEST_URI} ^/foo/bar
-RewriteRule ^/foo/bar/(.*)$ /foobar?page=$1
-

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

Making an example configuration

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

-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;
-

- - - -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 register_hooks function for initializing the -configuration values to their defaults: - - - -

-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->handler || strcmp(r->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 */
-};
-

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

-<Location /example>
-    SetHandler example-handler
-</Location>
-

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

Registering directives with Apache

-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 Apache -know that we want this to be possible. To do so, we must first change our name tag -to include a reference to the configuration directives we want to register with Apache: - - - -

-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 */
-};
-

- - - -This will tell Apache that we are now accepting directives from the configuration files, and that the -structure called example_directives 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: - - - -

-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 }
-};
-

- - - -

-As you can see, each directive needs at least 5 parameters set: -

AP_INIT_TAKE1: This is a macro that tells Apache that this directive takes one and only one argument. -If we required two arguments, we could use the macro AP_INIT_TAKE2 and so on (refer to httpd_conf.h -for more macros).
exampleEnabled: 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.
example_set_enabled: 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.
RSRC_CONF: This tells Apache where the directive is permissable. We'll go into details on this value in the -later chapters, but for now, RSRC_CONF means that Apache will only accept these directives in a server context.
"Enable or disable....": This is simply a brief description of what the directive does.

-(The "missing" parameter in our definition, which is usually set to NULL, 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.) -

- -

The directive handler function

-Now that we've told Apache to expect some directives for our module, it's time to make a few functions for handling these. What -Apache 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 exampleAction -directive to accept two arguments, its C function also has an additional parameter defined:
- - - -

-/* Handler for the "exambleEnabled" 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;
-}
-

- - - -

- -

Putting it all together

-Now that we have our directives set up, and handlers configured for them, we can assemble our module -into one big file: - - - -

-/* mod_example_config_simple.c: */
-#include <stdio.h>
-#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 "exambleEnabled" 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->handler || strcmp(r->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 */
-};
-

- - - -

-In our httpd.conf file, we can now change the hard-coded configuration by adding a few lines: -

-ExampleEnabled On
-ExamplePath "/usr/bin/foo"
-ExampleAction file allow
-

-And thus we apply the configuration, visit /example on our web site, and we see the configuration has -adapted to what we wrote in our configuration file. -

- - - -

Context aware configurations

Introduction to context aware configurations

-In Apache, different URLs, virtual hosts, directories etc can have very different meanings -to the user of Apache, and thus different contexts within which modules must operate. For example, -let's assume you have this configuration set up for mod_rewrite: -

-<Directory "/var/www">
-    RewriteCond %{HTTP_HOST} ^example.com$
-    RewriteRule (.*) http://www.example.com/$1
-</Directory>
-<Directory "/var/www/sub">
-    RewriteRule ^foobar$ index.php?foobar=true
-</Directory>
-

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

Inside /var/www, all requests for http://example.com must go to http://www.example.com
Inside /var/www/sub, all requests for foobar must go to index.php?foobar=true

-If mod_rewrite (or Apache 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 Apache, it -does not need to know itself, which of the directives are valid in this context, since Apache takes care of this.

- -

-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: - - - -

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

-That's it! Of course, a whole lot goes on behind the scenes, which we will discuss in this chapter, starting with how -Apache came to know what our configuration looks like, and how it came to be set up as it is in the specific context. -

- - -

Our basic configuration setup

In this chapter, we will be working with a slightly modified version of our previous -context structure. We will set a context variable that we can use to track -which context configuration is being used by Apache in various places: - - - -

-typedef struct {
-    char        context[256];
-    char        path[256];
-    int         typeOfAction;
-    int         enabled;
-} example_config;
-

- - - -

- -

Our handler for requests will also be modified, yet still very simple: - - - -

-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, &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;
-}
-

- - - -

Choosing a context

-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: - - - -

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

- - - -The RSRC_CONF definition told Apache 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 ACCESS_CONF, which lets us use the directive inside <Directory> and <Location> blocks. -

- - -

Using Apache to allocate configuration slots

A much smarter way to manage your configurations is by letting Apache help you create them. -To do so, we must first start off by chancing our name tag to let Apache 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: - - -

-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 */
-};
-

- - - -

- - - - - -

Creating new context configurations

-Now that we have told Apache 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: - -

-void* example_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, x);
-        cfg->enabled = 0;
-        cfg->path = "/foo/bar";
-        cfg->typeOfAction = 0x11;
-    }
-    return dir;
-}
-

- - - -

- - -

Merging configurations

-Our next step in creating a context aware configuration is merging configurations. This part of the process -particularly apply to scenarios where you have a parent configuration and a child, such as the following: -

-<Directory "/var/www">
-    ExampleEnable On
-    ExamplePath /foo/bar
-    ExampleAction file allow
-</Directory>
-<Directory "/var/www/subdir">
-    ExampleAction file deny
-</Directory>
-

-In this example, it is natural to assume that the directory /var/www/subdir should inherit the -value set for the /var/www directory, as we did not specify a ExampleEnable nor an -ExamplePath for this directory. Apache does not presume to know if this is true, but cleverly -does the following: -

Creates a new configuration for /var/www
Sets the configuration values according to the directives given for /var/www
Creates a new configuration for /var/www/subdir
Sets the configuration values according to the directives given for /var/www/subdir
Proposes a merge of the two configurations into a new configuration for /var/www/subdir

-This proposal is handled by the merge_dir_conf 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: - - - -

-void* merge_dir_conf(apr_pool_t* pool, void* BASE, void* ADD) {
-    example_config* base = BASE ;
-    example_config* add = ADD ;
-    example_config* conf = create_dir_conf(pool, "Merged configuration");
-    
-    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 ;
-}
-

- - -

Trying out our new context aware configurations

-Now, let's try putting it all together to create a new module that it context aware. First off, we'll -create a configuration that lets us test how the module works: -

-<Location "/a">
-    SetHandler example-handler
-    ExampleEnabled on
-    ExamplePath "/foo/bar"
-    ExampleAction file allow
-</Location>
-
-<Location "/a/b">
-    ExampleAction file deny
-    ExampleEnabled off
-</Location>
-
-<Location "/a/b/c">
-    ExampleAction db deny
-    ExamplePath "/foo/bar/baz"
-    ExampleEnabled on
-</Location>
-

-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: -

- - -

-/*$6
- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
- * mod_example_config.c
- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
- */
-
-
-#include <stdio.h>
-#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->handler || strcmp(r->handler, "example-handler")) return(DECLINED);
-
-    /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-    example_config    *config = (example_config *) ap_get_module_config(r->per_dir_config, &example_module);
-    /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-
-    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);
-    ap_rprintf(r, "Context: %s\n", config->context);
-    return OK;
-}
-
-/*
- =======================================================================================================================
-    Handler for the "exambleEnabled" 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->enabled = 1;
-        else
-            conf->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->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->typeOfAction = 0x01;
-            else
-                conf->typeOfAction = 0x02;
-            if(!strcasecmp(arg2, "deny"))
-                conf->typeOfAction += 0x10;
-            else
-                conf->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->context, context);
-            cfg->enabled = 0;
-            memset(cfg->path, 0, 256);
-            cfg->typeOfAction = 0x00;
-        }
-    }
-
-    return cfg;
-}
-
-/*
- =======================================================================================================================
-    Merging function for configurations
- =======================================================================================================================
- */
-void *merge_dir_conf(apr_pool_t *pool, void *BASE, void *ADD)
-{
-    /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-    example_config    *base = BASE;
-    example_config    *add = ADD;
-    example_config    *conf = create_dir_conf(pool, "Merged configuration");
-    /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-
-    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;
-}
-

- - - - - - -

Summing up

-We have now looked at how to create simple modules for Apache 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 mailing lists -or check out the rest of our documentation for further tips. -

Some useful snippets of code

- -

Retrieve a variable from POST form data

- - - -

-const char *read_post_value(const char *key) 
-{
-    /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-    const apr_array_header_t    *fields;
-    int                         i;
-    apr_table_entry_t           *e = 0;
-    /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-    e = (apr_table_entry_t *) fields->elts;
-    for(i = 0; i < fields->nelts; i++) {
-        if(!strcmp(e[i].key, key)) return e[i].val;
-    }
-    return 0;
-}
-static int example_handler(request_req *r) 
-{
-    /*~~~~~~~~~~~~~~~~~~~~~~*/
-    apr_array_header_t *POST;
-    const char         *value;
-    /*~~~~~~~~~~~~~~~~~~~~~~*/
-    ap_parse_form_data(r, NULL, &POST, -1, 8192);
-    
-    value = read_post_value(POST, "valueA");
-    if (!value) value = "(undefined)";
-    ap_rprintf(r, "The value of valueA is: %s", value);
-    return OK;
-}
-

- - - - - -

Printing out every HTTP header received

- - - -

-static int example_handler(request_req *r) 
-{
-    /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-    const apr_array_header_t    *fields;
-    int                         i;
-    apr_table_entry_t           *e = 0;
-    /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-
-    fields = apr_table_elts(r->headers_in);
-    e = (apr_table_entry_t *) fields->elts;
-    for(i = 0; i < fields->nelts; i++) {
-        ap_rprintf(r, "<b>%s</b>: %s<br/>", e[i].key, e[i].val);
-    }
-    return OK;
-}
-

- - - - - -

Reading the request body into memory

- - - -

-static int util_read(request_rec *r, const char **rbuf, apr_off_t *size)
-{
-    /*~~~~~~~~*/
-    int rc = OK;
-    /*~~~~~~~~*/
-
-    if((rc = ap_setup_client_block(r, REQUEST_CHUNKED_ERROR))) {
-        return(rc);
-    }
-
-    if(ap_should_client_block(r)) {
-
-        /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-        char         argsbuffer[HUGE_STRING_LEN];
-        apr_off_t    rsize, len_read, rpos = 0;
-        apr_off_t length = r->remaining;
-        /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-
-        *rbuf = (const char *) apr_pcalloc(r->pool, (apr_size_t) (length + 1));
-        *size = length;
-        while((len_read = ap_get_client_block(r, argsbuffer, sizeof(argsbuffer))) > 0) {
-            if((rpos + len_read) > length) {
-                rsize = length - rpos;
-            }
-            else {
-                rsize = len_read;
-            }
-
-            memcpy((char *) *rbuf + rpos, argsbuffer, (size_t) rsize);
-            rpos += rsize;
-        }
-    }
-    return(rc);
-}
-
-static int example_handler(request_req* r) 
-{
-    /*~~~~~~~~~~~~~~~~*/
-    apr_off_t   size;
-    const char  *buffer;
-    /*~~~~~~~~~~~~~~~~*/
-
-    if(util_read(r, &data, &size) == OK) {
-        ap_rprintf("We read a request body that was %u bytes long", size);
-    }
-    return OK;
-}
-

- - - - - -

Context aware configurations -

Introduction to context aware configurations -

-<Directory "/var/www">
-    RewriteCond %{HTTP_HOST} ^example.com$
-    RewriteRule (.*) http://www.example.com/$1
-</Directory>
-<Directory "/var/www/sub">
-    RewriteRule ^foobar$ index.php?foobar=true
-</Directory>
-

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

Inside /var/www, all requests for http://example.com must go to http://www.example.com
Inside /var/www/sub, all requests for foobar must go to index.php?foobar=true

- -

-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: - - - -

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

- -

Our basic configuration setup -

-typedef struct {
-    char        context[256];
-    char        path[256];
-    int         typeOfAction;
-    int         enabled;
-} example_config;
-

- - - -

- -

Our handler for requests will also be modified, yet still very simple: - - - -

-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, &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;
-}
-

- - - -

- -

Choosing a context -

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

- -

Using Apache to allocate configuration slots -

-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 */
-};
-

- - - -

- -

Creating new context configurations -

-void* example_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, x);
-        cfg->enabled = 0;
-        cfg->path = "/foo/bar";
-        cfg->typeOfAction = 0x11;
-    }
-    return dir;
-}
-

- - - -

- -

Merging configurations -

-<Directory "/var/www">
-    ExampleEnable On
-    ExamplePath /foo/bar
-    ExampleAction file allow
-</Directory>
-<Directory "/var/www/subdir">
-    ExampleAction file deny
-</Directory>
-

Creates a new configuration for /var/www
Sets the configuration values according to the directives given for /var/www
Creates a new configuration for /var/www/subdir
Sets the configuration values according to the directives given for /var/www/subdir
Proposes a merge of the two configurations into a new configuration for /var/www/subdir

-void* merge_dir_conf(apr_pool_t* pool, void* BASE, void* ADD) {
-    example_config* base = BASE ;
-    example_config* add = ADD ;
-    example_config* conf = create_dir_conf(pool, "Merged configuration");
-    
-    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 ;
-}
-

- - -

- -

Trying out our new context aware configurations -

-Now, let's try putting it all together to create a new module that it context aware. First off, we'll -create a configuration that lets us test how the module works: -

-<Location "/a">
-    SetHandler example-handler
-    ExampleEnabled on
-    ExamplePath "/foo/bar"
-    ExampleAction file allow
-</Location>
-
-<Location "/a/b">
-    ExampleAction file deny
-    ExampleEnabled off
-</Location>
-
-<Location "/a/b/c">
-    ExampleAction db deny
-    ExamplePath "/foo/bar/baz"
-    ExampleEnabled on
-</Location>
-

- - -

-/*$6
- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
- * mod_example_config.c
- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
- */
-
-
-#include <stdio.h>
-#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->handler || strcmp(r->handler, "example-handler")) return(DECLINED);
-
-    /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-    example_config    *config = (example_config *) ap_get_module_config(r->per_dir_config, &example_module);
-    /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-
-    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);
-    ap_rprintf(r, "Context: %s\n", config->context);
-    return OK;
-}
-
-/*
- =======================================================================================================================
-    Handler for the "exambleEnabled" 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->enabled = 1;
-        else
-            conf->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->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->typeOfAction = 0x01;
-            else
-                conf->typeOfAction = 0x02;
-            if(!strcasecmp(arg2, "deny"))
-                conf->typeOfAction += 0x10;
-            else
-                conf->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->context, context);
-            cfg->enabled = 0;
-            memset(cfg->path, 0, 256);
-            cfg->typeOfAction = 0x00;
-        }
-    }
-
-    return cfg;
-}
-
-/*
- =======================================================================================================================
-    Merging function for configurations
- =======================================================================================================================
- */
-void *merge_dir_conf(apr_pool_t *pool, void *BASE, void *ADD)
-{
-    /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-    example_config    *base = BASE;
-    example_config    *add = ADD;
-    example_config    *conf = create_dir_conf(pool, "Merged configuration");
-    /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
-
-    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;
-}
-

- - - -

- - -

Developing modules for the Apache HTTP Server 2.4

See also