System Messages and System-Message Broadcasts - Win32 Programmer's Reference

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System Messages and System-Message Broadcasts

Each system message consists of a message identifier and two 32-bit parameters, wParam and lParam. The message identifier is a unique value that specifies the message purpose. The parameters provide additional information that is message-specific, but the wParam parameter is often a notification value that further specifies the message purpose.

A system-message broadcast is simply the sending of a system message to components in the system. You broadcast a system message by using the BroadcastSystemMessage function and specifying the recipients of the message. Rather than specify individual recipients, you must specify one or more types of recipients. These types are applications, installable drivers, Windows-based network drivers, and system-level device drivers. BroadcastSystemMessage sends messages to all members of each type you specify.

Most applications do not broadcast system messages. Instead, they process system messages sent by other components. The operating system typically broadcasts system messages in response to changes that usually take place within system-level device drivers. The device driver or related component generates the system message and broadcasts it to applications and other components to notify them of the change. For example, the subsystem responsible for disk drives generates and broadcasts a system message whenever the device driver for the floppy disk drive detects a change of media such as when the user inserts a disk in the drive.

Applications receive system messages through the window procedure of their top-level windows. System messages are not sent to child windows. The action an application takes in response to a system message depends on the message. Some system messages, called query messages, require the application to respond by returning either TRUE or BROADCAST_QUERY_DENY to indicate whether the system should continue to broadcast the message to other recipients.

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