1 .\" Hey Emacs! This file is -*- nroff -*- source.
3 .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) and
4 .\" Copyright 2006 Michael Kerrisk
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7 .\" manual provided the copyright notice and this permission notice are
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12 .\" entire resulting derived work is distributed under the terms of a
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15 .\" Since the Linux kernel and libraries are constantly changing, this
16 .\" manual page may be incorrect or out-of-date. The author(s) assume no
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20 .\" which is licensed free of charge, as they might when working
23 .\" Formatted or processed versions of this manual, if unaccompanied by
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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.
37 .TH FSYNC 2 2006-04-28 "Linux 1.3.85" "Linux Programmer's Manual"
39 fsync, fdatasync \- synchronize a file's in-core state with storage device
41 .B #include <unistd.h>
43 .BI "int fsync(int " fd );
45 .BI "int fdatasync(int " fd );
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
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
60 does not necessarily ensure
61 that the entry in the directory containing the file has also reached disk.
64 on a file descriptor for the directory is also needed.
69 but does not flush modified metadata unless that metadata
70 is needed in order to allow a subsequent data retrieval to be
72 For example, changes to
76 (respectively, time of last access and
77 time of last modification; see
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
85 would require a metadata flush.
89 is to reduce disk activity for applications that do not
90 require all metadata to be synchronised with the disk.
92 On success, zero is returned. On error, \-1 is returned, and
99 is not a valid file descriptor open for writing.
102 An error occurred during synchronization.
104 .BR EROFS ", " EINVAL
106 is bound to a special file which does not support synchronization.
108 If the underlying hard disk has write caching enabled, then
109 the data may not really be on permanent storage when
116 .\" for how to disable that cache for IDE disks.
118 When an ext2 file system is mounted with the
120 option, directory entries are also implicitly synced by
123 On kernels before 2.4,
125 on big files can be inefficient.
126 An alternative might be to use the