The size argument may be given as zero to obtain deferred optimal-size buffer allocation as usual. If it is not zero, then except for unbuffered files, the buf argument should point to a buffer at least size bytes long; this buffer will be used instead of the current buffer. If buf is not NULL, it is the callers responsibility to free(3) this buffer after closing the stream. (If the size argument is not zero but buf is NULL, a buffer of the given size will be allocated immediately, and released on close. This is an extension to ANSI C; portable code should use a size of 0 with any NULL buffer.)
The setvbuf function may be used at any time, but may have peculiar side effects (such as discarding input or flushing output) if the stream is active. Portable applications should call it only once on any given stream, and before any I/O is performed.
The other three calls are, in effect, simply aliases for calls to setvbuf. Except for the lack of a return value, the setbuf function is exactly equivalent to the call
The setbuffer function is the same, except that the size of the buffer is up to the caller, rather than being determined by the default BUFSIZ. The setlinebuf function is exactly equivalent to the call: