Opening an input data set

Before OPEN verifies the volume, OPEN will abnormally terminate the program if the system-wide RACF tape protection option is specified and the DD statement specifies PROTECT=YES without previously opening the DD statement for output processing.

If you specify ISO/ANSI labels, the first record on the tape must be at least 80 bytes and must contain the ASCII identifier VOL1 in the first 4 bytes. The volume label can be Version 1, Version 3, or Version 4. If the record is over 80 bytes long, the excess characters are ignored.

The system also checks for label type conflicts (the type of label requested differs from the type of label read at load point from the mounted volume), version compatibility conflicts (the label is not Version 1 or Version 3 or Version 4), and density conflicts (the density specified by the user for the data set is not compatible with the density of the VOL1 label). In the case of a label type conflict or version compatibility conflict, the volume is rejected. In the case of a density conflict, the density of the VOL1 label is used for all further processing of the data set.

If system security checking should be bypassed, the validation suppression exit is entered. Otherwise, if the system-wide RACF tape protection option has been specified, RACF access authorization to the volume is checked for READ authority. If a tape with Version 3 or Version 4 labels is not RACF protected and the VOL1 accessibility field contains a valid Version 3 or Version 4 character, the volume access exit is entered. For more details of RACF and accessibility checking, see Protecting data.

For a tape with Version 3 or Version 4 labels, (after access to the volume is authorized, the request is checked for possible symmetry violations). Because INOUT can result in asymmetrical labels, specifying this option will cause the label validation exit to be entered (unless it is Version 4 or validation has been suppressed). In addition, unless validation has been suppressed, the VOL1 label is checked for conditions not allowed by Version 3 or Version 4 standards. If an invalid condition is found, the label validation exit is entered.