ctx1

Optional pointer to a caller-supplied context or other data type. This pointer will be passed as the ctx1 argument to the caller-supplied routines process_message and process_done. A value of NULL may be passed for this argument.

process_message

The address of a caller-supplied routine to process each message. 

process_done

Optional address of a caller-supplied clean up routine. A NULL value may be passed for this argument. 

item_code

An optional list of item codes. See the description section that follow for a list of item codes. The list must be terminated with an integer argument with value 0.