For an indexed sequential data set with variable-length records, you can make three types of updates by using the basic access method. You can read a record and write it back with no change in its length, simply updating some part of the record. You do this with a READ KU, followed by a WRITE K, the same way you update fixed-length records.

Two other methods for updating variable-length records use the WRITE KN macro and lets you change the record length. In one method, a record read for update (by a READ KU) can be updated in a manner that will change the record length and be written back with its new length by a WRITE KN (key new). In the second method, you can replace a record with another record having the same key and possibly a different length using the WRITE KN macro. To replace a record, it is not necessary to have first read the record.

In either method, when changing the record length, you must place the new length in the DECBLGTH field of the DECB before issuing the WRITE KN macro. If you use a WRITE KN macro to update a variable-length record that has been marked for deletion, the first bit (no record found) of the exceptional condition code field (DECBEXC1) of the DECB is set on. If this condition is found, the record must be written using a WRITE KN with nothing specified in the DECBLGTH field.

Recommendation: Do not try to use the DECBLGTH field to determine the length of a record read because DECBLGTH is for use with writing records, not reading them.

If you are reading fixed-length records, the length of the record read is in DCBLRECL, and if you are reading variable-length records, the length is in the record descriptor word (RDW).