A member of a PDS can be updated in place. Only one user can update at a time. The system does not enforce this rule. If you violate this rule; you will corrupt the member or the directory. Two programs in different address spaces can violate this rule with DISP=SHR. Two programs in one address space can violate this rule with any value for DISP. When you update-in-place, you read records, process them, and write them back to their original positions without destroying the remaining records. The following rules apply:

• You must specify the UPDAT option in the OPEN macro to update the data set. To perform the update, you can use only the READ, WRITE, GET, PUTX, CHECK, NOTE, POINT, FIND, BLDL, and STOW macros.
• You cannot update concatenated data sets.
• You cannot delete any record or change its length; you cannot add new records.
• You do not need to issue a STOW macro unless you want to change the user data in the directory entry.
• You cannot use LBI.