You can create your own system messages and use them to coordinate activities between applications and other components in the system. This is especially useful if you have created your own installable drivers or system-level device drivers. Your custom system messages can carry information to and from your driver and the applications that use the driver.
You broadcast custom system messages using the BroadcastSystemMessage function. (System-level device drivers use a related, system-level function.) The function sends the messages to the recipients in this order: system-level device drivers, network drivers, installable drivers, and applications. This means that system-level device drivers, if chosen as recipients, always get the first opportunity to respond to a system message. Within a given recipient type, no driver is guaranteed to receive a given message before any other driver. This means that a system message intended for a specific driver must have a globally-unique message identifier so that no other driver unintentionally processes it.
Query messages are a useful way to poll recipients for permission to carry out a given action. You can generate your own query messages by setting the BSF_QUERY value in the dwFlags parameter when calling BroadcastSystemMessage. Each recipient of the query message must return TRUE for the function to send the message to the next recipient. If any recipient returns BROADCAST_QUERY_DENY, the broadcast ends immediately and the function returns 0.
You can create installable drivers that broadcast and process system messages. An installable driver is a dynamic-link library (DLL) that exports a DriverProc function. The driver receives system messages through its DriverProc function and can broadcast messages using BroadcastSystemMessage. Installable drivers are typically used to support multimedia devices, such as sound boards, but can be used for other devices and purposes too.
Network drivers are dynamic-link libraries that provide the underlying support for applications that use the network functions to connect to and browse network resources. System-level device drivers are -system-specific executable components that provide direct access to and management of the hardware devices of the computer. The details regarding how these components process system messages is beyond the scope of this overview.