To control mixer operations, clients call the user-mode driver's mxdMessage function. The user-mode driver should first expect the client to send a MXDM_OPEN message to open a driver instance. Then the driver can expect to receive MXDM_GETLINEINFO, MXDM_GETLINECONTROLS, MXDM_GETCONTROLDETAILS, and MXDM_SETCONTROLDETAILS messages from the opened the instance, until the client sends MXDM_CLOSE.