Verifying block count in label when reading

To verify that all records on the input data set on the current volume have been read, the system compares the block count shown in the first trailer label (EOV1 or EOF1) against the block count that was accumulated in the DCB. For reading backward, the system compares the 0 block count shown in the HDR1 label against the block count in the DCB.

If the block count in the label does not equal the block count in the DCB, the EOV routine gives control to the block count unequal exit in the user's DCB exit list. This entry in the exit list is identified by the code hex 0B. The EOV routine passes the following information to the exit routine:

Register: Contents
0: The block count shown in the label
1: The address of the DCB

After your exit routine analyzes the discrepancy (and possibly prints a message), your exit routine must return to the EOV routine with one of the following return codes in register 15:

ReturnCode: Meaning
0: Abnormally terminate with completion code of X'237'
4: Continue processing.

For more information, see z/OS DFSMS Using Data Sets and Verifying block count.

If the appropriate user exit entry is not provided in the DCB exit list, or a return code of 0 is received, then a block count discrepancy causes the system to call the optional DCB ABEND exit and issue ABEND 237. When the FEOV macro is executed, the block count is not verified. For more information, see z/OS DFSMS Using Data Sets.

If the data set was created with an EXCP DCB with no device-dependent section, then the block count is written as zero and is not verified. The data set can be created that way only if the label validation installation exit allows it. This is a requirement of the Version 3 standard.