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FFLUSH(3) Library Functions Manual FFLUSH(3)

fflush, fpurge
flush a stream

#include <stdio.h>
int
fflush(FILE *stream);
int
fpurge(FILE *stream);

The function fflush() forces a write of all buffered data for the given output or update stream via the stream's 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.

 
 
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stream is not an open stream or, in the case of fflush(), not a stream open for writing.
The function fflush() may also fail and set errno for any of the errors specified for the routine write(2).

write(2), fclose(3), fopen(3), setvbuf(3)

The fflush() function conforms to ANSI X3.159-1989 (“ANSI C89”).

The fflush() function first appeared in Version 7 AT&T UNIX.
November 4, 2015 OpenBSD-current