When processing multiple subtasks sharing a single control block, concurrent GET and PUT requests are allowed. A control interval is protected for write operations using an exclusive control facility provided in VSAM record management. Other PUT requests to the same control interval are not allowed and a logical error is returned to the user issuing the request macro. Depending on the selected buffer option, nonshared (NSR) or shared (LSR/GSR) resources, GET requests to the same control interval as that being updated can or cannot be allowed. Figure 1 illustrates the exclusive control facility.

When a subtask issues OPEN to an ACB that will share a control block structure that can have been previously used, issue the POINT macro to obtain the position for the data set. In this case, it should not be assumed that positioning is at the beginning of the data set.