Any driver wishing to receive messages using the USB messaging system must provide and register at least one message handler. This is a routine which will be executed by the USB system to deliver messages to the application. It should be noted that the routine is executed in the context of the USB system and a crash in this routine will most likely render the whole USB system unusable. Hence, this routine should be robust and not block execution for an extended period of time.

The routine is called with a pointer to a message and the private word that it was registered with. The handler will be called once for each message to be delivered. The handler may either return 0 to allow the message to propagate to another application or a value which will stop further processing of the message and return the value to the caller of USB_MsgSend (SWI&55398).

Registering a handler is done by calling the USB_MsgOp (SWI&55399) with the USB_MSGOP_REGISTER reason code, the address of the message handler and a value to be passed to the handler on each call. This may also be achieved by using the usbmsg_register_handler C API call.

De-registering a handler is done by calling the USB_MsgOp (SWI&55399) with the USB_MSGOP_DEREGISTER reason code and the values of message handler and private word passed to the register call. This may also be achieved by using the usbmsg_deregister_handler C API call.

Setting a filter based on message class is done by calling USB_MsgOp (SWI&55399) with the USB_MSGOP_FILTER reason code, the values of message handler and private word used to register the handler, the message class and a flag to indicate either the setting or clearing of the filter. This may also be achieved using the usbmsg_filter C API call.

A special case filter can be applied which prevents the reception of all messages. This is useful for either temporary suspension of message reception or filtering of all but one or two message classes. This is achieved by calling USB_MsgOp (SWI&55399) with the USB_MSGOP_FILTERALL reason code, the values of message handler and private word used to register the handler. This may also be achieved by using the usbmsg_filterall C API call.