After closing the data set, you should issue a FREEPOOL macro to release the virtual storage used for the buffer pool unless you specified RMODE31=BUFF on the DCBE macro with QSAM or it is BSAM and BUFNO was not supplied from any source. If you plan to process other data sets, use FREEPOOL to regain the buffer pool storage space. If you expect to reopen a data set using the same DCB, use FREEPOOL unless the buffer pool created the first time the data set was opened will meet your needs when you reopen the data set. FREEPOOL is discussed in more detail in Constructing a Buffer Pool.

After the data set has been closed, the DCB can be used for another data set. If you do not close the data set before a task completes, the operating system tries to close it automatically. If the DCB is not available to the system at that time, the operating system abnormally ends the task, and data results can be unpredictable. The operating system, however, cannot automatically close any DCBs in dynamic storage (outside your program) or after the normal end of a program that was brought into virtual storage by the loader. Therefore, reentrant or loaded programs must include CLOSE macros for all open data sets.