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 .\"
  37 .TH FSYNC 2 2006-04-28 "Linux 1.3.85" "Linux Programmer's Manual"
  38 .SH NAME
  39 fsync, fdatasync \- synchronize a file's in-core state with storage device
  40 .SH SYNOPSIS
  41 .B #include <unistd.h>
  42 .sp
  43 .BI "int fsync(int " fd );
  44 .sp
  45 .BI "int fdatasync(int " fd );
  46 .SH DESCRIPTION
  47 .BR fsync ()
  48 transfers ("flushes") all modified in-core data of
  49 (i.e., modified buffer cache pages for) the
  50 file referred to by the file descriptor
  51 .I fd
  52 to the disk device (or other permanent storage device)
  53 where that file resides.
  54 The call blocks until the device reports that the transfer has completed.
  55 It also flushes  metadata information associated with the file (see
  56 .BR stat (2)).
  57
  58 Calling
  59 .BR fsync ()
  60 does not necessarily ensure
  61 that the entry in the directory containing the file has also reached disk.
  62 For that an explicit
  63 .BR fsync ()
  64 on a file descriptor for the directory is also needed.
  65
  66 .BR fdatasync ()
  67 is similar to
  68 .BR fsync (),
  69 but does not flush modified metadata unless that metadata
  70 is needed in order to allow a subsequent data retrieval to be
  71 correctly handled.
  72 For example, changes to
  73 .I st_atime
  74 or
  75 .I st_mtime
  76 (respectively, time of last access and
  77 time of last modification; see
  78 .BR stat (2))
  79 do not not require flushing because they are not necessary for
  80 a subsequent data read to be handled correctly.
  81 On the other hand, a change to the file size
  82 .RI ( st_size ,
  83 as made by say
  84 .BR ftruncate (2)),
  85 would require a metadata flush.
  86
  87 The aim of
  88 .BR fdatasync (2)
  89 is to reduce disk activity for applications that do not
  90 require all metadata to be synchronised with the disk.
  91 .SH "RETURN VALUE"
  92 On success, zero is returned.  On error, \-1 is returned, and
  93 .I errno
  94 is set appropriately.
  95 .SH ERRORS
  96 .TP
  97 .B EBADF
  98 .I fd
  99 is not a valid file descriptor open for writing.
 100 .TP
 101 .B EIO
 102 An error occurred during synchronization.
 103 .TP
 104 .BR EROFS ", " EINVAL
 105 .I fd
 106 is bound to a special file which does not support synchronization.
 107 .SH NOTES
 108 If the underlying hard disk has write caching enabled, then
 109 the data may not really be on permanent storage when
 110 .BR fsync ()
 111 /
 112 .BR fdatasync ()
 113 return.
 114 .\" See
 115 .\" .BR hdparm (8)
 116 .\" for how to disable that cache for IDE disks.
 117 .LP
 118 When an ext2 file system is mounted with the
 119 .I sync
 120 option, directory entries are also implicitly synced by
 121 .BR fsync ().
 122 .LP
 123 On kernels before 2.4,
 124 .BR fsync ()
 125 on big files can be inefficient.
 126 An alternative might be to use the
 127 .I O_SYNC
 128 flag to
 129 .BR open (2).
 130 .SH "CONFORMING TO"
 131 4.3BSD, POSIX.1-2001
 132 .SH "SEE ALSO"
 133 .BR bdflush (2),
 134 .BR open (2),
 135 .BR sync (2),
 136 .BR sync_file_range (2),
 137 .BR hdparm (8),
 138 .BR mount (8),
 139 .BR sync (8),
 140 .BR update (8)