Manual Section... (3) - page: fflush NAMEfflush - flush a stream
DESCRIPTIONFor output streams, fflush() forces a write of all
user-space buffered data for the given output or update stream
via the stream's underlying write function. For input streams,
fflush() discards any buffered data that has been fetched from
the underlying file, but has not been by the application. The
open status of the stream is unaffected. If the stream argument
is NULL, fflush() flushes all open output streams. For a
nonlocking counterpart, see unlocked_stdio(3). RETURN VALUEUpon
successful completion 0 is returned. Otherwise, EOF is returned
and errno is set to indicate the error. - Stream is not an open
stream, or is not open for writing. The function fflush() may
also fail and set errno for any of the errors specified for
write(2). CONFORMING TOC89, C99, POSIX.1-2001, POSIX.1-2008.
NOTESNote that fflush() only flushes the user space buffers
provided by the C library. To ensure that the data is physically
stored on disk the kernel