Understanding IFG0EX0B Execution Environment

Table 1 shows when IFG0EX0B receives control, and the processing that occurs before and after it is called.

Table 1. IFG0EX0B's Execution Environment
Step	Action
1	Begin open processing For DASD data sets: Mount the volume Read the format -1, -2, and -3 DSCBs Merge information from the format 1 DSCB to the JFCB For tape data sets: Mount the volume Verify the header labels Merge information from the header labels to the JFCB For all data sets: Merge information from the JFCB to the DCB Take the DCB open user exit or the JFCBE user exit, if either is specified in the DCB's exit list. Perform RACF or password verification processing.
2	Call IFG0EX0B in the following cases: When the OPEN macro (with or without the TYPE=J parameter) processes a DCB, including when two or more DCBs are opened in parallel through a single invocation. When a program is reading a sequential concatenation of data sets with unlike attributes. IFG0EX0B receives control as the program begins reading each data set. Data sets are considered unlike only if the program has turned on the DCBOFPPC flag in the DCBOFLGS field. For example, data sets with different record format are considered unlike.
3	Complete open processing: Calculated the block size if the block size in the DCB or DCBE is zero. Merge information from the DCB to the JFCB (not all fields are merged). Merge information from the JFCB to the format-1 DSCB for DASD data sets (not all fields are merged). Write header labels for output tape data sets. Perform access-method-dependent processing (obtain buffers, DEB, and other control blocks). Update the system copy of the JFCB.