Overview

OSObject is the concrete root class of the Libkern and I/O Kit C++ class hierarchy.

Discussion

OSObject defines the minimal functionality required of Libkern and I/O Kit C++ classes: tie-in to the run-time type information facility, the dynamic allocation/initialization paradigm, and reference counting. While kernel extensions are free to use their own C++ classes internally, any interaction they have with Libkern or the I/O Kit will require classes ultimately derived from OSObject.

Run-Time Type Information

OSObject is derived from the abstract root class OSMetaClassBase, which declares (and defines many of) the primitives on which the run-time type information facility is based. A parallel inheritance hierarchy of metaclass objects provides run-time introspection, including access to class names, inheritance, and safe type-casting. See OSMetaClass for more information.

Dynamic Allocation/Initialization

The kernel-resident C++ runtime does not support exceptions, so Libkern classes cannot use standard C++ object constructors and destructors, which use exceptions to report errors. To support error-handling during instance creation, then, OSObject separates object allocation from initialization. You can create a new OSObject-derived instance with the new operator, but this does nothing more than allocate memory and initialize the reference count to 1. Following this, you must call a designated initialization function and check its bool return value. If the initialization fails, you must immediately call release on the instance and handle the failure in whatever way is appropriate. Many Libkern and I/O Kit classes define static instance-creation functions (beginning with the word "with") to make construction a one-step process for clients.

Reference Counting

OSObject provides reference counting services using the retain, release(), release(int freeWhen) and free functions. The public interface to the reference counting is retain, and release; release(int freeWhen) is provided for objects that have internal retain cycles.

In general, a subclass is expected to only override free. It may also choose to override release(int freeWhen) if the object has a circular retain count, as noted above.

Use Restrictions

With very few exceptions in the I/O Kit, all Libkern-based C++ classes, functions, and macros are unsafe to use in a primary interrupt context. Consult the I/O Kit documentation related to primary interrupts for more information.

Concurrency Protection

The basic features of OSObject are thread-safe. Most Libkern subclasses are not, and require locking or other protection if instances are shared between threads. I/O Kit driver objects are either designed for use within thread-safe contexts or designed to inherently be thread-safe. Always check the individual class documentation to see what steps are necessary for concurrent use of instances.

Functions

free

Deallocates/releases resources held by the object.

getRetainCount

Returns the reference count of the object.

init

Initializes a newly-allocated object.

operator delete

Frees the memory of the object itself.

operator new

Allocates memory for an instance of the class.

release()

Releases a reference to the object, freeing it immediately if the reference count drops to zero.

release(int)

Releases a reference to an object, freeing it immediately if the reference count drops below the specified threshold.

retain

Retains a reference to the object.

serialize

Overridden by subclasses to archive the receiver into the provided OSSerialize object.

taggedRelease(const void *)

Releases a tagged reference to an object, freeing it immediately if the reference count drops to zero.

taggedRelease(const void *, const int)

Releases a tagged reference to an object, freeing it immediately if the reference count drops below the specified threshold.

taggedRetain

Retains a reference to the object with an optional tag used for reference-tracking.

Deallocates/releases resources held by the object.

protected

virtual void free(); 

Discussion

Classes derived from OSObject should override this function to deallocate or release all dynamic resources held by the instance, then call the superclass's implementation.

Caution:

1. You can not assume that you have completed initialization before free is called, so be very careful in your implementation.

2. OSObject's implementation performs the C++ delete of the instance, so be sure that you call the superclass implementation last in your implementation.

3. free must not fail; all resources must be deallocated or released on completion.

Returns the reference count of the object.

public

virtual int getRetainCount() const; 

Return Value

The reference count of the object.

Initializes a newly-allocated object.

protected

virtual bool init(); 

Return Value

true on success, false on failure.

Discussion

Classes derived from OSObject must override the primary init method of their parent. In general most implementations call super::init() before doing local initialisation. If the superclass call fails then return false immediately. If the subclass encounters a failure then it should return false.

Frees the memory of the object itself.

protected

static void operator delete(
    void *mem,
    size_t size); 

Parameters

Discussion

Never use delete on objects derived from OSObject; use release instead.

Allocates memory for an instance of the class.

public

static void * operator new(
    size_t size); 

Parameters

Return Value

A pointer to block of memory if available, NULL otherwise.

Releases a reference to the object, freeing it immediately if the reference count drops to zero.

public

virtual void release() const; 

Discussion

This function decrements the reference count of the receiver by 1. If the reference count drops to zero, the object is immediately freed using free.

Releases a reference to an object, freeing it immediately if the reference count drops below the specified threshold.

protected

virtual void release(
    int freeWhen) const; 

Parameters

Discussion

If the receiver has freeWhen or fewer references after its reference count is decremented, it is immediately freed.

This version of release can be used to break certain retain cycles in object graphs. In general, however, it should be avoided.

Retains a reference to the object.

public

virtual void retain() const; 

Discussion

This function increments the reference count of the receiver by 1. If you need to maintain a reference to an object outside the context in which you received it, you should always retain it immediately.

Overridden by subclasses to archive the receiver into the provided OSSerialize object.

public

virtual bool serialize(
    OSSerialize *serializer) const; 

Parameters

Return Value

true if serialization succeeds, false if not.

Discussion

OSObject's implementation writes a string indicating that the class of the object receiving the function call is not serializable. Subclasses that can meaningfully encode themselves in I/O Kit-style property list XML can override this function to do so. See OSSerialize for more information.

Releases a tagged reference to an object, freeing it immediately if the reference count drops to zero.

public

virtual void taggedRelease(
    const void *tag = 0) const; 

Parameters

Discussion

Kernel extensions should not use this function. It is for use by OSCollection and subclasses to track inclusion in collections.

Releases a tagged reference to an object, freeing it immediately if the reference count drops below the specified threshold.

protected

virtual void taggedRelease(
    const void *tag,
    const int freeWhen) const; 

Parameters

Discussion

Kernel extensions should not use this function. It is for use by OSCollection and subclasses to track inclusion in collections.

If the receiver has freeWhen or fewer references after its reference count is decremented, it is immediately freed.

This version of release can be used to break certain retain cycles in object graphs. In general, however, it should be avoided.

Retains a reference to the object with an optional tag used for reference-tracking.

public

virtual void taggedRetain(
    const void *tag = 0) const; 

Parameters

Discussion

Kernel extensions should not use this function. It is for use by OSCollection and subclasses to track inclusion in collections.

If you need to maintain a reference to an object outside the context in which you received it, you should always retain it immediately.

Last Updated: 2010-07-29