man2/fsync.2

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  26 .\" Modified 21 Aug 1994 by Michael Chastain <mec@shell.portal.com>:
  27 .\"   Removed note about old libc (pre-4.5.26) translating to 'sync'.
  28 .\" Modified 15 Apr 1995 by Michael Chastain <mec@shell.portal.com>:
  29 .\"   Added `see also' section.
  30 .\" Modified 13 Apr 1996 by Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
  31 .\"   Added remarks about fdatasync.
  32 .\" Modified 31 Jan 1997 by Eric S. Raymond <esr@thyrsus.com>
  33 .\" Modified 18 Apr 2001 by Andi Kleen
  34 .\"   Fix description to describe what it really does; add a few caveats.
  35 .\" 2006-04-28, mtk, substantial rewrite of various parts.
  36 .\" 2012-02-27 Various changes by Christoph Hellwig <hch@lst.de>
  37 .\"
  38 .TH FSYNC 2 2012-02-27 "Linux" "Linux Programmer's Manual"
  39 .SH NAME
  40 fsync, fdatasync \- synchronize a file's in-core state with storage device
  41 .SH SYNOPSIS
  42 .B #include <unistd.h>
  43 .sp
  44 .BI "int fsync(int " fd );
  45 .sp
  46 .BI "int fdatasync(int " fd );
  47 .sp
  48 .in -4n
  49 Feature Test Macro Requirements for glibc (see
  50 .BR feature_test_macros (7)):
  51 .in
  52 .sp
  53 .BR fsync ():
  54 _BSD_SOURCE || _XOPEN_SOURCE
  55 .br
  56          || /* since glibc 2.8: */ _POSIX_C_SOURCE\ >=\ 200112L
  57 .\" _POSIX_C_SOURCE\ >=\ 200112L only since glibc 2.8
  58 .br
  59 .BR fdatasync ():
  60 _POSIX_C_SOURCE\ >=\ 199309L || _XOPEN_SOURCE\ >=\ 500
  61 .SH DESCRIPTION
  62 .BR fsync ()
  63 transfers ("flushes") all modified in-core data of
  64 (i.e., modified buffer cache pages for) the
  65 file referred to by the file descriptor
  66 .I fd
  67 to the disk device (or other permanent storage device) so that all
  68 changed information can be retrieved even after the system crashed or
  69 was rebooted.
  70 This includes writing through or flushing a disk cache if present.
  71 The call blocks until the device reports that the transfer has completed.
  72 It also flushes metadata information associated with the file (see
  73 .BR stat (2)).
  74
  75
  76 Calling
  77 .BR fsync ()
  78 does not necessarily ensure
  79 that the entry in the directory containing the file has also reached disk.
  80 For that an explicit
  81 .BR fsync ()
  82 on a file descriptor for the directory is also needed.
  83
  84 .BR fdatasync ()
  85 is similar to
  86 .BR fsync (),
  87 but does not flush modified metadata unless that metadata
  88 is needed in order to allow a subsequent data retrieval to be
  89 correctly handled.
  90 For example, changes to
  91 .I st_atime
  92 or
  93 .I st_mtime
  94 (respectively, time of last access and
  95 time of last modification; see
  96 .BR stat (2))
  97 do not require flushing because they are not necessary for
  98 a subsequent data read to be handled correctly.
  99 On the other hand, a change to the file size
 100 .RI ( st_size ,
 101 as made by say
 102 .BR ftruncate (2)),
 103 would require a metadata flush.
 104
 105 The aim of
 106 .BR fdatasync ()
 107 is to reduce disk activity for applications that do not
 108 require all metadata to be synchronized with the disk.
 109 .SH "RETURN VALUE"
 110 On success, these system calls return zero.
 111 On error, \-1 is returned, and
 112 .I errno
 113 is set appropriately.
 114 .SH ERRORS
 115 .TP
 116 .B EBADF
 117 .I fd
 118 is not a valid open file descriptor.
 119 .TP
 120 .B EIO
 121 An error occurred during synchronization.
 122 .TP
 123 .BR EROFS ", " EINVAL
 124 .I fd
 125 is bound to a special file which does not support synchronization.
 126 .SH "CONFORMING TO"
 127 4.3BSD, POSIX.1-2001.
 128 .SH AVAILABILITY
 129 On POSIX systems on which
 130 .BR fdatasync ()
 131 is available,
 132 .B _POSIX_SYNCHRONIZED_IO
 133 is defined in
 134 .I <unistd.h>
 135 to a value greater than 0.
 136 (See also
 137 .BR sysconf (3).)
 138 .\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.
 139 .\" -1: unavailable, 0: ask using sysconf().
 140 .\" glibc defines them to 1.
 141 .SH NOTES
 142 In Linux 2.2 and earlier,
 143 .BR fdatasync ()
 144 is equivalent to
 145 .BR fsync (),
 146 and so has no performance advantage.
 147
 148 The
 149 .BR fsync ()
 150 implementations in older kernels and lesser used filesystems
 151 does not know how to flush disk caches.
 152 In these cases disk caches need to be disabled using
 153 .BR hdparm (8)
 154 or
 155 .BR sdparm (8)
 156 to guarantee safe operation.
 157 .SH "SEE ALSO"
 158 .BR bdflush (2),
 159 .BR open (2),
 160 .BR sync (2),
 161 .BR sync_file_range (2),
 162 .BR hdparm (8),
 163 .BR mount (8),
 164 .BR sync (8),
 165 .BR update (8)