The following list defines many of the terms used in discussions about the Windows API and especially on this site. Please refer to this glossary whenever you are confused about the meaning of a term. A link to this glossary appears whenever one of the defined terms appears in the Windows API Guide.

An integer used to identify a string or integer stored in an atom table. Atoms can be stored in either an application's private atom table or the public atom table defined by the operating system. Back
A type of bitmap which is used to fill an area. Brushes are usually 8x8 pixel images, forming either a solid color or a pattern. Back
Callback Function
A function defined by the application which the Windows API will call. Callback functions are usually used whenever the program must process certain information itself at some time in the midst of the execution of an API function. In Visual Basic, all callback function must be declared Public and be defined in a module. Back
The flashing indicator which appears inside of text boxes. The caret identifies where the current position in the text box is. When text is typed, it (usually) appears inserted at the caret's position. Back
Client Area
The area of a window which contains the "actual" contents of the window. The client area does not include the title bar, the menu bar (if any), any toolbars or status bars, or the window's border. Coordinates measured within a window are usually taken relative to the client area. Back
Common Dialog
One of the dialog boxes built into the Windows API designed to allow the user to select something. The common dialog boxes include the Open File, Save File, Choose Color, Choose Font, Print, Print Setup, Find Text, and Find & Replace Text boxes. Back
Current Point
A point on a device also known as the last point referenced. Almost all drawing function ending in -To usually use the current point as the implied point to begin drawing from; they also set the current point to the end point of whatever was drawn. Other drawing functions completely ignore the current point. Back
An object which performs input or output operations (or both). Devices are used to communicate with the user and include such objects as the keyboard, mouse, and printers. Many windows are also considered to be devices. Devices are usually refered to using a device context. Back
Device Context
An object which both holds information about a device and provides a way to use the device. Device contexts refer to an internal data structure in Windows not accessible to applications; programs can only reference device context via a handle. Back
A portion of memory, usually a variable, which has a length of four bytes. The term dword is given to anything which is four bytes in length. Literally, a dword is a "double word." Back
File Pointer
A value identifying the current position in an open file. Any synchronous (not overlapped) read or write operations on the file begin at the byte position identified by the file pointer. Back
A four-byte integer used to identify a wide variety of objects. Handles refer to an internal data structure not accessible to Windows applications which contain information about an object. Back
Hook Function
A special type of callback function. Hook functions are used to process messages associated with an object such as a window or a dialog box. In general, hook functions are called to respond to some sort of message. Back
Information Context
A special type of device context. Unlike device contexts, an information context can only be used to read properties, settings, or other information about device. It cannot be used to actually do anything with the device. Back
Item Identifier
Identifies some object, usually a folder, used chiefly by the Windows shell. Programs rarely access these directly, but instead refer to them via a pointer. API functions almost always create these as necessary. Back
A collection of information specifying the type of language used to present certain kinds of information to the user. A locale consists of things such as the language, the date format, the currency format, the time format, etc. None of these settings actually affect the information itself, but how that information is displayed according to the user's preferences. Locales are typically referenced in the API using a numeric identifier. Back
When used to describe a graphics object (such as a font or brush), this means that the object described is an "ideal" object. Information about the logical object is used by the system to create a physical object which may not exactly match the logical one. However, only physical objects can actually be used. Back
A type of command sent to an object. Physically, a message is identified by a 32-bit integer. Most messages have two parameters associated with them. Most objects continually monitor for newly received messages and act on them. Back
A synchronization object used for interprocess communication. A mutex can be owned by at most one thread at any one time, or it can be owned by none. A mutex is in a signaled state when it is unowned by a thread, and in a nonsignaled state when it is owned. Back
An object which is used to draw lines and curves. Pens define the style, size, and shape of a line or curve. Pens are usually refered to using a handle. Back
When used to describe a graphics object (such as a font or brush), this means that the object described is an actual object. Physical objects can be used to actually draw on devices. Back
A four-byte integer used to identify a physical location in memory (a memory address). Often when using the API in Visual Basic, an implicit pointer to a variable can be used by passing it ByRef (except for Strings, which are always ByVal) as a parameter. Pointers can also be stored in Long-type variables. Back
An object which identifies one or more areas of an area. Each component of a region can be of any shape, including (but not limited to) ellipses, rectangles, and other polygons. Regions can be combined in a variety of ways to create more complicated regions. Back
A synchronization object used for interprocess communication. A semaphore maintains a count between zero and a maximum value. A semaphore is in a signaled state when its count is positive, and in a nonsignaled state when its count is zero. Back
A path of execution in a program. Each application has at least one thread executing it, although it is possible to have more than one thread in the same instance of a program. Back
Virtual Folder
A folder which is treated like a folder by the Windows shell but does not correspond to a physical directory on any disk. For example, My Computer is a virtual folder which contains all the root drives, the Control Panel virtual folder, and other items. Virtual folders are rarely used by any non-shell functions. Back
Window Class
Defines attributes common to a group of windows. Windows which are created as a member of a certain window class inherit many properties of that class. The window class largely determines the look of its associated windows. Back
Window Property
An additional piece of information attached to a window. Window properties are referenced by a string identifier and hold a handle to the related piece of information. Window properties can be added, read, and removed from a window and do not by default have any bearing on the basic operations of a window. Back
A portion of memory, usually a variable, which has a length of two bytes. The term word is given to anything which is two bytes in length. Back
An internal list identifying the overlapping of windows. Basically, the Z-order determines which windows lie on top of other windows. The top of the Z-order is the window which is on top of all other windows. Back

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Last Modified: August 26, 2000
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