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Resources

Resources are a special kind of data type in PHP. The term resources doesn't really refer to any special kind of data, but to an abstraction method for maintaining any kind of information. Resources are kept in a special resource list within Zend. Each entry in the list has a correspondending type definition that denotes the kind of resource to which it refers. Zend then internally manages all references to this resource. Access to a resource is never possible directly - only via a provided API. As soon as all references to a specific resource are lost, a corresponding shutdown function is called.

For example, resources are used to store database links and file descriptors. The de facto standard implementation can be found in the MySQL module, but other modules such as the Oracle module also make use of resources.

Note: In fact, a resource can be a pointer to anything you need to handle in your functions (e.g. pointer to a structure) and the user only has to pass a single resource variable to your function.

To create a new resource you need to register a resource destruction handler for it. Since you can store any kind of data as a resource, Zend needs to know how to free this resource if its not longer needed. This works by registering your own resource destruction handler to Zend which in turn gets called by Zend whenever your resource can be freed (whether manually or automatically). Registering your resource handler within Zend returns you the resource type handle for that resource. This handle is needed whenever you want to access a resource of this type later and is most of time stored in a global static variable within your extension. There is no need to worry about thread safety here because you only register your resource handler once during module initialization.

The Zend function to register your resource handler is defined as:
ZEND_API int zend_register_list_destructors_ex(rsrc_dtor_func_t ld, rsrc_dtor_func_t pld, char *type_name, int module_number);

There are two different kinds of resource destruction handlers you can pass to this function: a handler for normal resources and a handler for persistent resources. Persistent resources are for example used for database connection. When registering a resource, either of these handlers must be given. For the other handler just pass NULL.

zend_register_list_destructors_ex() accepts the following parameters:

ldNormal resource destruction handler callback
pldPesistent resource destruction handler callback
type_nameA string specifying the name of your resource. It's always a good thing to specify an unique name within PHP for the resource type so when the user for example calls var_dump($resource); he also gets the name of the resource.
module_numberThe module_number is automatically available in your PHP_MINIT_FUNCTION function and therefore you just pass it over.

The return value is an unique integer ID for your resource type.

The resource destruction handler (either normal or persistent resources) has the following prototype:
void resource_destruction_handler(zend_rsrc_list_entry *rsrc TSRMLS_DC);
The passed rsrc is a pointer to the following structure:
typedef struct _zend_rsrc_list_entry {
     
    void *ptr;
    int type;
    int refcount;

 } zend_rsrc_list_entry;
The member void *ptr is the actual pointer to your resource.

Now we know how to start things, we define our own resource we want register within Zend. It is only a simple structure with two integer members:
typedef struct {
     
    int resource_link;
    int resource_type;

 } my_resource;
Our resource destruction handler is probably going to look something like this:
void my_destruction_handler(zend_rsrc_list_entry *rsrc TSRMLS_DC) {

    // You most likely cast the void pointer to your structure type

    my_resource *my_rsrc = (my_resource *) rsrc->ptr;

    // Now do whatever needs to be done with you resource. Closing
    // Files, Sockets, freeing additional memory, etc.
    // Also, don't forget to actually free the memory for your resource too!

    do_whatever_needs_to_be_done_with_the_resource(my_rsrc);
 }

Note: One important thing to mention: If your resource is a rather complex structure which also contains pointers to memory you allocated during runtime you have to free them before freeing the resource itself!

Now that we have defined

  1. what our resource is and

  2. our resource destruction handler

we can go on and do the rest of the steps:

  1. create a global variable within the extension holding the resource ID so it can be accessed from every function which needs it

  2. define the resource name

  3. write the resource destruction handler

  4. and finally register the handler

// Somewhere in your extension, define the variable for your registered resources.
    // If you wondered what 'le' stands for: it simply means 'list entry'.
    static int le_myresource;

    // It's nice to define your resource name somewhere
    #define le_myresource_name  "My type of resource"

    [...]

    // Now actually define our resource destruction handler
    void my_destruction_handler(zend_rsrc_list_entry *rsrc TSRMLS_DC) {

        my_resource *my_rsrc = (my_resource *) rsrc->ptr;
        do_whatever_needs_to_be_done_with_the_resource(my_rsrc);
    }

    [...]

    PHP_MINIT_FUNCTION(my_extension) {

        // Note that 'module_number' is already provided through the
        // PHP_MINIT_FUNCTION() function definition.

        le_myresource = zend_register_resource_destructors_ex(my_destruction_handler, NULL, le_myresource_name, module_number);

        // You can register additional resources, initialize
        // your global vars, constants, whatever.
     }

To actually register a new resource you use can either use the zend_register_resource() function or the ZEND_REGISTER_RESOURE() macro, both defined in zend_list.h . Although the arguments for both map 1:1 it's a good idea to always use macros to be upwards compatible:
int ZEND_REGISTER_RESOURCE(zval *rsrc_result, void *rsrc_pointer, int rsrc_type);

rsrc_resultThis is an already initialized zval * container.
rsrc_pointerYour resource pointer you want to store.
rsrc_typeThe type which you received when you registered the resource destruction handler. If you followed the naming scheme this would be le_myresource.

The return value is an unique integer identifier for that resource.

What is really going on when you register a new resource is it gets inserted in an internal list in Zend and the result is just stored in the given zval * container:
rsrc_id = zend_list_insert(rsrc_pointer, rsrc_type);
     
    if (rsrc_result) {
        rsrc_result->value.lval = rsrc_id;
        rsrc_result->type = IS_RESOURCE;
    }

     return rsrc_id;
The returned rsrc_id uniquly identifies the newly registered resource. You can use the macro RETURN_RESOURE to return it to the user:
RETURN_RESOURCE(rsrc_id)

Note: It is common practice that if you want to return the resource immidiately to the user you specify the return_value as the zval * container.

Zend now keeps track of all references to this resource. As soon as all references to the resource are lost, the destructor that you previously registered for this resource is called. The nice thing about this setup is that you don't have to worry about memory leakages introduced by allocations in your module - just register all memory allocations that your calling script will refer to as resources. As soon as the script decides it doesn't need them anymore, Zend will find out and tell you.

Now that the user got his resource, at some point he is passing it back to one of your functions. The value.lval inside the zval * container contains the key to your resource and thus can be used to fetch the resource with the following macro: ZEND_FETCH_RESOURCE:
ZEND_FETCH_RESOURCE(rsrc, rsrc_type, rsrc_id, default_rsrc_id, resource_type_name, resource_type)

rsrcThis is your pointer which will point to your previously registered resource.
rsrc_typeThis is the typecast argument for your pointer, e.g. myresource *.
rsrc_idThis is the address of the zval *container the user passed to your function, e.g. &z_resource if zval *z_resource is given.
default_rsrc_idThis integer specifies the default resource ID if no resource could be fetched or -1.
resource_type_nameThis is the name of the requested resource. It's a string and is used when the resource can't be found or is invalid to form a meaningful error message.
resource_typeThe resource_type you got back when registering the resource destruction handler. In our example this was le_myresource.

This macro has no return value. It is for the developers convenience and takes care of TSRMLS arguments passing and also does check if the resource could be fetched. It throws a warning message and returns the current PHP function with NULL if there was a problem retrieving the resource.

To force removal of a resource from the list, use the function zend_list_delete(). You can also force the reference count to increase if you know that you're creating another reference for a previously allocated value (for example, if you're automatically reusing a default database link). For this case, use the function zend_list_addref(). To search for previously allocated resource entries, use zend_list_find(). The complete API can be found in zend_list.h.

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