C runtime buffer

      The  three  types  of  buffering  available  are  unbuffered,  block  buffered,  and  line buffered.  When an output stream is
       unbuffered, information appears on the destination file or terminal as soon as written; when it is block buffered many charac‐
       ters  are saved up and written as a block; when it is line buffered characters are saved up until a newline is output or input
       is read from any stream attached to a terminal device (typically stdin).  The function fflush(3) may  be  used  to  force  the
       block  out  early.   (See  fclose(3).)  Normally all files are block buffered.  When the first I/O operation occurs on a file,
       malloc(3) is called, and a buffer is obtained.  If a stream refers to  a  terminal  (as  stdout  normally  does)  it  is  line
       buffered.  The standard error stream stderr is always unbuffered by default.