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FreeBSD 11.0 - man page for fflush (freebsd section 3)

FFLUSH(3)						   BSD Library Functions Manual 						 FFLUSH(3)

NAME
fflush, fpurge -- flush a stream
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdio.h> int fflush(FILE *stream); int fpurge(FILE *stream);
DESCRIPTION
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).
RETURN VALUES
Upon successful completion 0 is returned. Otherwise, EOF is returned and the global variable errno is set to indicate the error.
ERRORS
[EBADF] The stream argument 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).
SEE ALSO
write(2), fclose(3), fopen(3), setbuf(3)
STANDARDS
The fflush() function conforms to ISO/IEC 9899:1990 (``ISO C90'').
BSD
June 4, 1993 BSD