Every installable driver exports a DriverProc function. This common entry-point function receives driver messages from the system that direct the driver to carry out actions or provide information. The system sends driver messages to the DriverProc function when an application or DLL opens or closes the driver or requests that a message be sent to the driver. The DriverProc function either processes the message or passes the message to the default message handler, the DefDriverProc function. In either case, DriverProc must return a value indicating whether the requested action was successful.