Full buffering is the default. You can specify line buffering, but z/OS® XL C/C++ treats line buffering as full buffering for VSAM data sets. Unbuffered I/O is not supported under VSAM; if you specify it, your setvbuf() call fails.
To find out how to optimize VSAM performance by controlling the number of VSAM buffers used for your data set, refer to z/OS DFSMS Access Method Services Commands.