Windows Messages

Windows passes input to a window procedure in the form of messages. Messages are generated by Windows and by applications. Windows generates a message at each input event for example, when the user types, moves the mouse, or clicks a control such as a scroll bar. Windows also generates messages in response to changes in the system brought about by an application, such as when an application changes the pool of system font resources or resizes one of its windows. An application can generate messages to direct its own windows to perform tasks or to communicate with windows in other applications.

Windows sends a message to a window procedure with a set of four parameters: a window handle, a message identifier, and two 32-bit values called message parameters. The window handle identifies the window for which the message is intended. Windows uses it to determine which window procedure to send the message to.

A message identifier is a named constant that identifies the purpose of a message. When a window procedure receives a message, it uses a message identifier to determine how to process the message. For example, the message identifier WM_PAINT tells the window procedure that the window's client area has changed and must be repainted.

Message parameters specify data or the location of data used by a window procedure when processing a message. The meaning and value of the message parameters depend on the message. A message parameter can contain an integer, packed bit flags, a pointer to a structure containing additional data, and so on. When a message does not use message parameters, they are typically set to NULL. A window procedure must check the message identifier to determine how to interpret the message parameters.

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