fstream::setbuf
streambuf* setbuf( char* pch**, int** nLength );
Attaches the specified reserve area to the stream’s filebuf object.
Return Value
If the file is open and a buffer has already been allocated, the function returns NULL; otherwise it returns a pointer to the filebuf cast as a streambuf. The reserve area will not be released by the destructor.
Parameters
pch
A pointer to a previously allocated reserve area of length nLength. A NULL value indicates an unbuffered stream.
nLength
The length (in bytes) of the reserve area. A length of 0 indicates an unbuffered stream.