The function fflush forces a write of all buffered data for the given output or update stream via the streams underlying write function. The open status of the stream is unaffected.
If the stream argument is NULL, fflush flushes all open output streams.
The function fpurge erases any input or output buffered in the given stream. For output streams this discards any unwritten output. For input streams this discards any input read from the underlying object but not yet obtained via getc(3); this includes any text pushed back via ungetc(3).
Upon successful completion 0 is returned. Otherwise, EOF is returned and the global variable errno is set to indicate the error.