The parameters passed to a user label processing routine are addresses of: the 80-byte label buffer, the DCB being processed, the status information if an unrecoverable input/output error occurs, and the totaling area.

The 80-byte label buffer contains an image of the user label when an input label is being processed. When an output label is being processed, the buffer contains no significant information at entry to your label processing routine. When the utility program has been requested to generate labels, your label processing routine must construct a label in the label buffer.

If standard user labels (SUL) are specified on the DD statement for a data set, but the data set has no user labels, the system still takes the specified exits to the appropriate user routine. In such a case, the user input label processing routine is entered with the buffer address parameter set to zero.

The format and content of the DCB are explained in z/OS DFSMS Macro Instructions for Data Sets.

If an unrecoverable input/output error occurs while reading or writing a user label, the appropriate label processing routine is entered with bit 0 of flag 2 in the status information address parameter set on. The three low order bytes of this parameter contain the address of standard status information as supplied for SYNAD routines. (The SYNAD routine is not entered.)