If DCBLRECL has been set up and DCBRECFM specifies either F, V, or D you can write a routine as part of the IFG0EX0B exit to have the system determine the block size for the data set. Do this by setting the user's block size field to zero before the exit returns to its caller. If you are requesting a system determined block size, the DCBBLKSI field in the DCB and the DCBEBLKSI field in the DCBE, if present, should be set to zero. A sample is shown in Example of the IFG0EX0B Exit Routine.

The user's block size value is in the 2-byte DCBBLKSI in the DCB if both of the following are true:

• Bit DCBMRECP is off, meaning it is not an EXCP DCB.
• Any of DCBH0, DCBH1, DCBESLBI and DCBEULBI is off. The user is not using LBI, large block interface.

The user's block size value is in the 4-byte DCBEBLKSI in the DCBE if all of the following are true:

• The DCB is not for EXCP, meaning that bit DCBMRECP is off or it is on and the 5-word device interface section of the DCB is valid, meaning bit DCBMR5WD is on.
• Bits DCBH0 and DCBH1 are on, meaning that DCBDCBE points to the DCBE.
• Bit DCBESLBI is on, meaning that the system supports LBI for this DCB.
• Bit DCBEULBI is on, meaning that the user requests LBI.

For an EXCP DCB that is not using the DCBE BLKSIZE field, your exit can use the BLKSIZE field in the JFCB but the application program might not use it.

If a system-determined block size is used, open turns on the reblockable indicator in the format-1 DSCB (bit 2 of the DS1SMSFG field). When the data set is opened for OUTPUT or OUTIN and the merging of the block size fields occurs, open checks the indicator. If the block size is a system-determined block size, and the logical record length or record format have changed from those specified in the data set label, open determines the block size again.