Defined in header
int fflush( std::FILE* stream );
For output streams (and for update streams on which the last operation was output), writes any unwritten data from the
stream's buffer to the associated output device.
For input streams (and for update streams on which the last operation was input), the behavior is undefined.
stream is a null pointer, all open output streams are flushed, including the ones manipulated within library packages or otherwise not directly accessible to the program.
|the file stream to write out
 Return value
Returns zero on success. Otherwise EOF is returned and the error indicator of the file stream is set.
POSIX extends the specification of
fflush by defining its effects on an input stream, as long as that stream represents a file or another seekable device: in that case the POSIX file pointer is repositioned to match the C stream pointer (which effectively undoes any read buffering) and the effects of any std::ungetc or std::ungetwc that weren't yet read back from the stream are discarded.
Microsoft also extends the specification of
fflush by defining its effects on an input stream: in Visual Studio 2013 and prior, it discarded the input buffer, in Visual Studio 2015 and newer, it has no effect, buffers are retained.
 See also
| opens a file
| closes a file
C documentation for fflush