man3/fflush.3

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  37 .\"
  38 .\"     @(#)fflush.3    5.4 (Berkeley) 6/29/91
  39 .\"
  40 .\" Converted for Linux, Mon Nov 29 15:22:01 1993, faith@cs.unc.edu
  41 .\"
  42 .\" Modified 2000-07-22 by Nicolás Lichtmaier <nick@debian.org>
  43 .\" Modified 2001-10-16 by John Levon <moz@compsoc.man.ac.uk>
  44 .\"
  45 .TH FFLUSH 3  2017-09-15 "GNU" "Linux Programmer's Manual"
  46 .SH NAME
  47 fflush \- flush a stream
  48 .SH SYNOPSIS
  49 .B #include <stdio.h>
  50 .PP
  51 .BI "int fflush(FILE *" stream );
  52 .SH DESCRIPTION
  53 For output streams,
  54 .BR fflush ()
  55 forces a write of all user-space buffered data for the given output or update
  56 .I stream
  57 via the stream's underlying write function.
  58 .PP
  59 For input streams associated with seekable files
  60 (e.g., disk files, but not pipes or terminals),
  61 .BR fflush ()
  62 discards any buffered data that has been fetched from the underlying file,
  63 but has not been consumed by the application.
  64 .PP
  65 The open status of the stream is unaffected.
  66 .PP
  67 If the
  68 .I stream
  69 argument is NULL,
  70 .BR fflush ()
  71 flushes
  72 .I all
  73 open output streams.
  74 .\" mtk: POSIX specifies that only output streams are flushed for this case.
  75 .\" Also verified for glibc by experiment.
  76 .PP
  77 For a nonlocking counterpart, see
  78 .BR unlocked_stdio (3).
  79 .SH RETURN VALUE
  80 Upon successful completion 0 is returned.
  81 Otherwise,
  82 .B EOF
  83 is returned and
  84 .I errno
  85 is set to indicate the error.
  86 .SH ERRORS
  87 .TP
  88 .B EBADF
  89 .I stream
  90 is not an open stream, or is not open for writing.
  91 .PP
  92 The function
  93 .BR fflush ()
  94 may also fail and set
  95 .I errno
  96 for any of the errors specified for
  97 .BR write (2).
  98 .SH ATTRIBUTES
  99 For an explanation of the terms used in this section, see
 100 .BR attributes (7).
 101 .TS
 102 allbox;
 103 lb lb lb
 104 l l l.
 105 Interface       Attribute       Value
 106 T{
 107 .BR fflush ()
 108 T}      Thread safety   MT-Safe
 109 .TE
 110 .SH CONFORMING TO
 111 C89, C99, POSIX.1-2001, POSIX.1-2008.
 112 .PP
 113 POSIX.1-2001 did not specify the behavior for flushing of input streams,
 114 but the behavior is specified in POSIX.1-2008.
 115 .SH NOTES
 116 Note that
 117 .BR fflush ()
 118 flushes only the user-space buffers provided by the C library.
 119 To ensure that the data is physically stored on disk
 120 the kernel buffers must be flushed too, for example, with
 121 .BR sync (2)
 122 or
 123 .BR fsync (2).
 124 .SH SEE ALSO
 125 .BR fsync (2),
 126 .BR sync (2),
 127 .BR write (2),
 128 .BR fclose (3),
 129 .BR fileno (3),
 130 .BR fopen (3),
 131 .BR setbuf (3),
 132 .BR unlocked_stdio (3)