For input streams associated with seekable files (e.g., disk files, but not pipes or terminals), fflush() discards any buffered data that has been fetched from the underlying file, but has not been consumed 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).
The function fflush() may also fail and set errno for any of the errors specified for write(2).