#include <stdio.h>void setbuf(FILE *stream, char *buf);
The setbuf( ) function specifies the buffer that stream will use or, if called with buf set to null, turns off buffering. If a programmer-defined buffer is to be specified, it must be BUFSIZ characters long. BUFSIZ is defined in <stdio.h>.
In C99, stream and buf are qualified by restrict.
Related functions are fopen( ), fclose( ), and setvbuf( ).