man3/fflush.3

   1 '\" t
   2 .\" Copyright (c) 1990, 1991 The Regents of the University of California.
   3 .\" All rights reserved.
   4 .\"
   5 .\" This code is derived from software contributed to Berkeley by
   6 .\" Chris Torek and the American National Standards Committee X3,
   7 .\" on Information Processing Systems.
   8 .\"
   9 .\" SPDX-License-Identifier: BSD-4-Clause-UC
  10 .\"
  11 .\"     @(#)fflush.3    5.4 (Berkeley) 6/29/91
  12 .\"
  13 .\" Converted for Linux, Mon Nov 29 15:22:01 1993, faith@cs.unc.edu
  14 .\"
  15 .\" Modified 2000-07-22 by Nicolás Lichtmaier <nick@debian.org>
  16 .\" Modified 2001-10-16 by John Levon <moz@compsoc.man.ac.uk>
  17 .\"
  18 .TH fflush 3 (date) "Linux man-pages (unreleased)"
  19 .SH NAME
  20 fflush \- flush a stream
  21 .SH LIBRARY
  22 Standard C library
  23 .RI ( libc ", " \-lc )
  24 .SH SYNOPSIS
  25 .nf
  26 .B #include <stdio.h>
  27 .PP
  28 .BI "int fflush(FILE *_Nullable " stream );
  29 .fi
  30 .SH DESCRIPTION
  31 For output streams,
  32 .BR fflush ()
  33 forces a write of all user-space buffered data for the given output or update
  34 .I stream
  35 via the stream's underlying write function.
  36 .PP
  37 For input streams associated with seekable files
  38 (e.g., disk files, but not pipes or terminals),
  39 .BR fflush ()
  40 discards any buffered data that has been fetched from the underlying file,
  41 but has not been consumed by the application.
  42 .PP
  43 The open status of the stream is unaffected.
  44 .PP
  45 If the
  46 .I stream
  47 argument is NULL,
  48 .BR fflush ()
  49 flushes
  50 .I all
  51 open output streams.
  52 .\" mtk: POSIX specifies that only output streams are flushed for this case.
  53 .\" Also verified for glibc by experiment.
  54 .PP
  55 For a nonlocking counterpart, see
  56 .BR unlocked_stdio (3).
  57 .SH RETURN VALUE
  58 Upon successful completion 0 is returned.
  59 Otherwise,
  60 .B EOF
  61 is returned and
  62 .I errno
  63 is set to indicate the error.
  64 .SH ERRORS
  65 .TP
  66 .B EBADF
  67 .I stream
  68 is not an open stream, or is not open for writing.
  69 .PP
  70 The function
  71 .BR fflush ()
  72 may also fail and set
  73 .I errno
  74 for any of the errors specified for
  75 .BR write (2).
  76 .SH ATTRIBUTES
  77 For an explanation of the terms used in this section, see
  78 .BR attributes (7).
  79 .TS
  80 allbox;
  81 lbx lb lb
  82 l l l.
  83 Interface       Attribute       Value
  84 T{
  85 .na
  86 .nh
  87 .BR fflush ()
  88 T}      Thread safety   MT-Safe
  89 .TE
  90 .sp 1
  91 .SH STANDARDS
  92 C11, POSIX.1-2008.
  93 .SH HISTORY
  94 C89, POSIX.1-2001, POSIX.1-2008.
  95 .PP
  96 POSIX.1-2001 did not specify the behavior for flushing of input streams,
  97 but the behavior is specified in POSIX.1-2008.
  98 .SH NOTES
  99 Note that
 100 .BR fflush ()
 101 flushes only the user-space buffers provided by the C library.
 102 To ensure that the data is physically stored on disk
 103 the kernel buffers must be flushed too, for example, with
 104 .BR sync (2)
 105 or
 106 .BR fsync (2).
 107 .SH SEE ALSO
 108 .BR fsync (2),
 109 .BR sync (2),
 110 .BR write (2),
 111 .BR fclose (3),
 112 .BR fileno (3),
 113 .BR fopen (3),
 114 .BR fpurge (3),
 115 .BR setbuf (3),
 116 .BR unlocked_stdio (3)