About Handles and Objects - Win32 Programmer's Reference

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About Handles and Objects

Windows uses objects and handles to regulate access to system resources for two main reasons. First, the use of objects ensures that developers are not writing code specifically to low-level, internal structures. This enables Microsoft to add or change functionality of the operating system, as long as the original calling conventions are maintained. When subsequent versions of the operating system are released, applications will gain this new functionality with little or no additional development.

Second, the use of objects enables developers to take advantage of Win32 security. Each object has its own access-control list (ACL) that specifies the types of actions processes can perform on the object. The operating system examines an object's ACL each time an application attempts to create a handle to the object. For more information about security, see Security.

For most objects, the Win32 API provides functions that create the object, create an object handle, close the object handle, and destroy the object. These tasks may be combined or unnecessary, depending on the type of object and the situation. For example, an application could create an event object. Other applications could open the event and each would have a unique handle to the same event object. In this scenario, as the applications finish using the object, each closes its handle. When there are no open handles to the event object, the operating system removes the object from memory.

In contrast, an application could obtain the existing window-object handle. In this instance, when the window object is no longer needed, the application must remove the object from memory, which invalidates the window handle.

When a process terminates, the system automatically closes handles and deletes objects created by the process. However, when a thread terminates, the system usually does not close handles or delete objects. The only exceptions are window, hook, window position, and dynamic data exchange (DDE) conversation objects that are deleted when the creating thread terminates.

Handles and objects consume memory. Therefore, to preserve system performance, an application should close handles and delete objects as soon as they are no longer needed. Applications that do not do this can slow the operating system, due to excessive use of the paging file.

Windows provides three categories of objects: user, graphics device interface (GDI), and kernel, as shown in the following tables. The system uses user objects to support window management, GDI objects to support graphics, and kernel objects to support memory management, process execution, and interprocess communications (IPC). For information about creating and using a specific object, refer to the associated overview.

Windows User Objects

User object	Overview
Accelerator table	Keyboard Accelerators
Caret	Carets
Cursor	Cursors
Dynamic data exchange conversation	Dynamic Data Exchange Management Library
Desktop	Security
Hook	Hooks
Icon	Icons
Menu	Menus
Window	Windows
Window position	Windows
Window station	Windows

Windows GDI Objects

GDI object	Overview
Bitmap	Bitmaps
Brush	Brushes
Font	Fonts and Text
Palette	Colors
Pen	Pens
Extended pen	Pens
Region	Regions
Device context	Device Contexts
Memory device context	Device Contexts
Metafile	Metafiles
Metafile device context	Metafiles
Enhanced metafile	Metafiles
Enhanced-metafile device context	Metafiles

Windows Kernel Objects

Kernel object	Overview
Process	Processes and Threads
Thread	Processes and Threads
File	Files
File mapping	File Mapping
Event	Synchronization
Semaphore	Synchronization
Mutex	Synchronization
Pipe (named and anonymous)	Pipes
Mailslot	Mailslots
Communications device	Communications
Heap	Memory Management
Module	Dynamic-Link Libraries
Update resource	Resources
Find file	Files
Event log	Event Logging
Change notification	Files
Token	Security
Service object	Printing and Print Spooler
LZ file	Data Decompression Library

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