Persistence of DCB and DCBE Fields

Between the completion of OPEN and the completion of CLOSE with like concatenation, the system can change certain DCB or DCBE fields that represent data attributes. Your program and the system do not change the following attribute fields:

RECFM
LRECL for format-F for BSAM and for QSAM XLRI
BLKSIZE for BSAM (your program can change this)
KEYLEN
NCP or BUFNO

With like concatenation the system can change the following when switching to another data set:

BLKSIZE and BUFL for QSAM
Field DCBDEVT in the DCB (device type)
TRTCH (tape recording technique)
DEN (tape density)

With or without concatenation the system sets LRECL in the DCB for each QSAM GET macro when reading format-V, format-D, or format-U records, except with XLRI. GET issues an ABEND if it encounters a record that is longer than LRECL was at the completion of OPEN.

If your program indicates like concatenation (by taking no special action about DCBOFPPC) and one of the like concatenation rules is broken, the results are unpredictable. A typical result is an I/O error, resulting in an ABEND, or entry to the SYNAD routine. The program might even appear to run correctly.