PHP Internals Book

The Resource type: zend_resource

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The Resource type: zend_resource

Even though PHP could really get rid of the “resource” type, because custom object storage allows to build a PHP representation of any abstract kind of data, that resource type still exists in PHP, and you may need to deal with it.

If you need to create resources, we really would like to push you not to, but instead use objects and their custom storage management. Objects is the PHP type that can embed anything of any type. However, for historical reasons, PHP still knows about that special type “Resource”, and still makes use of it in its heart or in some extensions. Let’s see that type together. Beware however, it is really cryptic and suffers from a long past history, so don’t be suprised about its design especially when reading the source code about it

What is the “Resource” type?

Easy enough you know about it. We are talking about this here:

$fp = fopen('/proc/cpuinfo', 'r');
var_dump($fp); /* resource(2) of type (stream) */

Internally, a resource is bound to the zend_resource structure type:

struct _zend_resource {
        zend_refcounted_h gc;
        int               handle;
        int               type;
        void             *ptr;

We find the traditionnal zend_refcounted_h header, meaning that resources are reference countable. If you feel lost with reference counting and memory tracking, you may refer to the ./zvals/memory_management chapter.

The handle is an integer that is used internally by the engine to locate the resource into an internal resource table. It is used as the key for such a table.

The type is used to regroup resources of the same type together. This is about the way resources get destroyed and how they are fetched back from their handle.

Finally, the ptr field in zend_resource is your abstract data. Remember resources are about storing an abstract data that cannot fit in any data type PHP can represent natively.

Resource types and resource destruction

Resources must register a destructor. When users use resources in PHP userland, they usually don’t bother cleaning those when they don’t make use of them anymore. For example, it is not uncommon to see an fopen() call, and not see the fclose() call. Using the C language, this would be at best a bad idea, at most a disaster. But using a high level language like PHP, you ease things.

You, as an internal developer, must be prepared to the fact that the user would create a lot of resources you’ll allow him to use, without properly cleaning them and releasing memory/OS resource. You hence must register a destructor that will be called anytime the engine is about to destroy a resource of that type.

Destructors are grouped by types, so are resources themselves. You won’t apply the destructor for a resource of type ‘database’ than for a resource of type ‘file’.

There also exists two kinds of resources, here again differenciated about their lifetime.

  • Classical resources, the most used ones, do not persist across several requests, their destructor is called at request shutdown
  • Persistent resources will persist across several requests and will only get destroyed when the PHP process dies.

Playing with resources

The resources related API can be found in zend/zend_list.c. You may find some inconsistencies into it, like talking about “lists” for “resources”.

Creating resources

To create a resource, one must first register a destructor for it and associate it to a resource type name using zend_register_list_destructors_ex(). That call will return an integer that represents the type of resource you register. You must remember that integer because you will need it later-on to fetch back your resource from the user.

After that, you can register a new resource using zend_register_resource(). That one will return you a zend_resource. Let’s see together a simple use-case example:

#include <stdio.h>

int res_num;
FILE *fp;
zend_resource *my_res;
zval my_val;

static void my_res_dtor(zend_resource *rsrc)
    fclose((FILE *)rsrc->ptr);

/* module_number should be your PHP extension number here */
res_num = zend_register_list_destructors_ex(my_res_dtor, NULL, "my_res", module_number);
fp      = fopen('/proc/cpuinfo', "r");
my_res  = zend_register_resource((void *)fp, res_num);

ZVAL_RES(&my_val, my_res);

What we do in the code above, is that we open a file using libc’s fopen(), and store the returned pointer into a resource. Before that, we registered a destructor which will use libc’s fclose() on the pointer. Then, we register the resource against the engine, and we pass the resource into a zval container that could get returned to userland.


Zvals chapter can be found here.

What must be remembered is resource type. Here, we register a resource of type “my_res”. This is the type name. The engine does not really care about type name, but type identifier, the integer returned by zend_register_list_destructors_ex(). You should remember it somewhere, like we do in the res_num variable.

Fetching back resources

Now that we registered a resource and put it in a zval for an example, we should learn how to fetch back that resource from the userland. Remember, the resource is stored into the zval. Into the resource is stored the resource type number (on the type field). Thus, to be given back our resource from the user, we must extract the zend_resource from the zval, and call zend_fetch_resource() to get back our FILE * pointer:

/* ... later on ... */

zval *user_zval = /* fetch zval from userland, assume type IS_RESOURCE */

ZEND_ASSERT(Z_TYPE_P(user_zval) == IS_RESOURCE); /* just a check to be sure */

fp = (FILE *)zend_fetch_resource(Z_RESVAL_P(user_zval), "my_res", res_num);

Like we said : get back a zval from the user (of type IS_RESOURCE), and fetch the resource pointer back from it by calling zend_fetch_resource().

That function will check if the type of the resource is of the type you pass as third parameter (res_num here). If yes, it extracts back the void * resource pointer you need and we are done. If not, then it throws a warning like “supplied resource is not a valid {type name} resource”. This could happen if for example you expect a resource of type “my_res”, and you are given a zval with a resource of type “gzip”, like one returned by gzopen() PHP function.

Resource types are just a way for the engine to mix different kind of resources (of type “file”, “gzip” or even “mysql connection”) into the same resource table. Resource types have names, so that those can be used in error messages or in debug statement (like a var_dump($my_resource)), and they also are represented as an integer used internaly to fetch back the resource pointer from it, and to register a destructor with the resource type.

Reference counting resources

Like many other types, zend_resource is reference counted. We can see its zend_refcounted_h header. Here is the API to play with reference counting, if you need it (you shouldn’t really need it on an average):

  • zend_list_delete(zend_resource *res) decrements refcount and destroys resource if drops to zero
  • zend_list_free(zend_resource *res) checks if refcount is zero, and destroys the resource if true.
  • zend_list_close(zend_resource *res) calls the resource destructor whatever the conditions

Persistent resources

Persistent resources don’t get destroyed at the end of the request. The classical use-case for that are persistent database connections. Those are connections that are recycled from request to request (with all the bullshit that will bring).

Traditionnaly, you should not be using persistent resources, as one request will be different from the other. Reusing the same resource should really be thought about deeply before going this way.

To register a persistent resource, use a persistent destructor instead of a classical one. This is done in the call to zend_register_list_destructors_ex(), which API is like:

zend_register_list_destructors_ex(rsrc_dtor_func_t destructor, rsrc_dtor_func_t persistent_destructor,
                                  const char *type_name, int module_number);

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