If the application does not request that the CSM storage be returned to its buffer return exit routine, the application must not access the CSM storage after the send macroinstruction has been accepted. VTAM® returns the storage to the buffer pool after the send process is complete.
If the application requests that the CSM storage be returned (by using the FREERTN parameter on the GET_BUFFER request), the application must not access the CSM storage until the exit is given control. The scheduling and subsequent execution of this exit is asynchronous to the completion of the APPCCMD macroinstruction. The storage may be marked as eligible to be pagefreed by CSM upon return to the application's buffer return exit routine. Eligible to be pagefreed is a status maintained by CSM. The actual system state of a buffer with this status can be either fixed or pageable. The application is responsible for using CSM services to ensure the buffer is fixed if subsequent use of the storage requires that the buffer be fixed.
Prior to normal application termination, the application should wait for all buffers to be returned to the exit. For abnormal termination, CSM ensures that all CSM storage is returned to CSM.
Refer to z/OS Communications Server: CSM Guide for a discussion of the buffer return exit routine and the CSM method of managing real storage.