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'\" t
.\" Copyright (c) 1990, 1991 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Chris Torek and the American National Standards Committee X3,
.\" on Information Processing Systems.
.\"
.\" SPDX-License-Identifier: BSD-4-Clause-UC
.\"
.\" @(#)fflush.3 5.4 (Berkeley) 6/29/91
.\"
.\" Converted for Linux, Mon Nov 29 15:22:01 1993, faith@cs.unc.edu
.\"
.\" Modified 2000-07-22 by Nicolás Lichtmaier <nick@debian.org>
.\" Modified 2001-10-16 by John Levon <moz@compsoc.man.ac.uk>
.\"
.TH fflush 3 2023-10-31 "Linux man-pages 6.7"
.SH NAME
fflush \- flush a stream
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.P
.BI "int fflush(FILE *_Nullable " stream );
.fi
.SH DESCRIPTION
For output streams,
.BR fflush ()
forces a write of all user-space buffered data for the given output or update
.I stream
via the stream's underlying write function.
.P
For input streams associated with seekable files
(e.g., disk files, but not pipes or terminals),
.BR fflush ()
discards any buffered data that has been fetched from the underlying file,
but has not been consumed by the application.
.P
The open status of the stream is unaffected.
.P
If the
.I stream
argument is NULL,
.BR fflush ()
flushes
.I all
open output streams.
.\" mtk: POSIX specifies that only output streams are flushed for this case.
.\" Also verified for glibc by experiment.
.P
For a nonlocking counterpart, see
.BR unlocked_stdio (3).
.SH RETURN VALUE
Upon successful completion 0 is returned.
Otherwise,
.B EOF
is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I stream
is not an open stream, or is not open for writing.
.P
The function
.BR fflush ()
may also fail and set
.I errno
for any of the errors specified for
.BR write (2).
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface Attribute Value
T{
.na
.nh
.BR fflush ()
T} Thread safety MT-Safe
.TE
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
C89, POSIX.1-2001, POSIX.1-2008.
.P
POSIX.1-2001 did not specify the behavior for flushing of input streams,
but the behavior is specified in POSIX.1-2008.
.SH NOTES
Note that
.BR fflush ()
flushes only the user-space buffers provided by the C library.
To ensure that the data is physically stored on disk
the kernel buffers must be flushed too, for example, with
.BR sync (2)
or
.BR fsync (2).
.SH SEE ALSO
.BR fsync (2),
.BR sync (2),
.BR write (2),
.BR fclose (3),
.BR fileno (3),
.BR fopen (3),
.BR fpurge (3),
.BR setbuf (3),
.BR unlocked_stdio (3)
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