man3/fflush.3

   1 .\" Copyright (c) 1990, 1991 The Regents of the University of California.
   2 .\" All rights reserved.
   3 .\"
   4 .\" This code is derived from software contributed to Berkeley by
   5 .\" Chris Torek and the American National Standards Committee X3,
   6 .\" on Information Processing Systems.
   7 .\"
   8 .\" SPDX-License-Identifier: BSD-4-Clause-UC
   9 .\"
  10 .\"     @(#)fflush.3    5.4 (Berkeley) 6/29/91
  11 .\"
  12 .\" Converted for Linux, Mon Nov 29 15:22:01 1993, faith@cs.unc.edu
  13 .\"
  14 .\" Modified 2000-07-22 by Nicolás Lichtmaier <nick@debian.org>
  15 .\" Modified 2001-10-16 by John Levon <moz@compsoc.man.ac.uk>
  16 .\"
  17 .TH FFLUSH 3  2021-08-27 "Linux man-pages (unreleased)"
  18 .SH NAME
  19 fflush \- flush a stream
  20 .SH LIBRARY
  21 Standard C library
  22 .RI ( libc ", " \-lc )
  23 .SH SYNOPSIS
  24 .nf
  25 .B #include <stdio.h>
  26 .PP
  27 .BI "int fflush(FILE *" stream );
  28 .fi
  29 .SH DESCRIPTION
  30 For output streams,
  31 .BR fflush ()
  32 forces a write of all user-space buffered data for the given output or update
  33 .I stream
  34 via the stream's underlying write function.
  35 .PP
  36 For input streams associated with seekable files
  37 (e.g., disk files, but not pipes or terminals),
  38 .BR fflush ()
  39 discards any buffered data that has been fetched from the underlying file,
  40 but has not been consumed by the application.
  41 .PP
  42 The open status of the stream is unaffected.
  43 .PP
  44 If the
  45 .I stream
  46 argument is NULL,
  47 .BR fflush ()
  48 flushes
  49 .I all
  50 open output streams.
  51 .\" mtk: POSIX specifies that only output streams are flushed for this case.
  52 .\" Also verified for glibc by experiment.
  53 .PP
  54 For a nonlocking counterpart, see
  55 .BR unlocked_stdio (3).
  56 .SH RETURN VALUE
  57 Upon successful completion 0 is returned.
  58 Otherwise,
  59 .B EOF
  60 is returned and
  61 .I errno
  62 is set to indicate the error.
  63 .SH ERRORS
  64 .TP
  65 .B EBADF
  66 .I stream
  67 is not an open stream, or is not open for writing.
  68 .PP
  69 The function
  70 .BR fflush ()
  71 may also fail and set
  72 .I errno
  73 for any of the errors specified for
  74 .BR write (2).
  75 .SH ATTRIBUTES
  76 For an explanation of the terms used in this section, see
  77 .BR attributes (7).
  78 .ad l
  79 .nh
  80 .TS
  81 allbox;
  82 lbx lb lb
  83 l l l.
  84 Interface       Attribute       Value
  85 T{
  86 .BR fflush ()
  87 T}      Thread safety   MT-Safe
  88 .TE
  89 .hy
  90 .ad
  91 .sp 1
  92 .SH STANDARDS
  93 C89, C99, POSIX.1-2001, POSIX.1-2008.
  94 .PP
  95 POSIX.1-2001 did not specify the behavior for flushing of input streams,
  96 but the behavior is specified in POSIX.1-2008.
  97 .SH NOTES
  98 Note that
  99 .BR fflush ()
 100 flushes only the user-space buffers provided by the C library.
 101 To ensure that the data is physically stored on disk
 102 the kernel buffers must be flushed too, for example, with
 103 .BR sync (2)
 104 or
 105 .BR fsync (2).
 106 .SH SEE ALSO
 107 .BR fsync (2),
 108 .BR sync (2),
 109 .BR write (2),
 110 .BR fclose (3),
 111 .BR fileno (3),
 112 .BR fopen (3),
 113 .BR fpurge (3),
 114 .BR setbuf (3),
 115 .BR unlocked_stdio (3)