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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