-.\" Hey Emacs! This file is -*- nroff -*- source.
-.\"
-.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
+.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) and
+.\" and Copyright 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
+.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
-.\"
+.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
-.\"
+.\"
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.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
.\"
.\" Modified 21 Aug 1994 by Michael Chastain <mec@shell.portal.com>:
.\" Removed note about old libc (pre-4.5.26) translating to 'sync'.
.\" Modified 31 Jan 1997 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 18 Apr 2001 by Andi Kleen
.\" Fix description to describe what it really does; add a few caveats.
+.\" 2006-04-28, mtk, substantial rewrite of various parts.
+.\" 2012-02-27 Various changes by Christoph Hellwig <hch@lst.de>
.\"
-.TH FSYNC 2 2001-04-18 "Linux 1.3.85" "Linux Programmer's Manual"
+.TH FSYNC 2 2016-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
-fsync, fdatasync \- synchronize a file's complete in-core state with that on disk
+fsync, fdatasync \- synchronize a file's in-core state with storage device
.SH SYNOPSIS
.B #include <unistd.h>
.sp
.BI "int fsync(int " fd );
.sp
.BI "int fdatasync(int " fd );
+.sp
+.in -4n
+Feature Test Macro Requirements for glibc (see
+.BR feature_test_macros (7)):
+.in
+.sp
+.BR fsync ():
+ Glibc 2.16 and later:
+ No feature test macros need be defined
+ Glibc up to and including 2.15:
+ _BSD_SOURCE || _XOPEN_SOURCE
+ || /* since glibc 2.8: */ _POSIX_C_SOURCE\ >=\ 200112L
+.br
+.BR fdatasync ():
+ _POSIX_C_SOURCE\ >=\ 199309L || _XOPEN_SOURCE\ >=\ 500
.SH DESCRIPTION
-.B fsync
-copies all in-core parts of a file to disk, and waits until the
-device reports that all parts are on stable storage.
-It also updates metadata stat information. It does not necessarily ensure
+.BR fsync ()
+transfers ("flushes") all modified in-core data of
+(i.e., modified buffer cache pages for) the
+file referred to by the file descriptor
+.I fd
+to the disk device (or other permanent storage device) so that all
+changed information can be retrieved even after the system crashed or
+was rebooted.
+This includes writing through or flushing a disk cache if present.
+The call blocks until the device reports that the transfer has completed.
+It also flushes metadata information associated with the file (see
+.BR stat (2)).
+
+Calling
+.BR fsync ()
+does not necessarily ensure
that the entry in the directory containing the file has also reached disk.
For that an explicit
-.B fsync
-on the file descriptor of the directory is also needed.
+.BR fsync ()
+on a file descriptor for the directory is also needed.
-.B fdatasync
-does the same as
-.B fsync
-but only flushes user data, not the meta data like the mtime or atime.
+.BR fdatasync ()
+is similar to
+.BR fsync (),
+but does not flush modified metadata unless that metadata
+is needed in order to allow a subsequent data retrieval to be
+correctly handled.
+For example, changes to
+.I st_atime
+or
+.I st_mtime
+(respectively, time of last access and
+time of last modification; see
+.BR stat (2))
+do not require flushing because they are not necessary for
+a subsequent data read to be handled correctly.
+On the other hand, a change to the file size
+.RI ( st_size ,
+as made by say
+.BR ftruncate (2)),
+would require a metadata flush.
-.SH "RETURN VALUE"
-On success, zero is returned. On error, \-1 is returned, and
+The aim of
+.BR fdatasync ()
+is to reduce disk activity for applications that do not
+require all metadata to be synchronized with the disk.
+.SH RETURN VALUE
+On success, these system calls return zero.
+On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EBADF
.I fd
-is not a valid file descriptor open for writing.
+is not a valid open file descriptor.
.TP
.B EIO
An error occurred during synchronization.
.BR EROFS ", " EINVAL
.I fd
is bound to a special file which does not support synchronization.
+.SH CONFORMING TO
+POSIX.1-2001, POSIX.1-2008, 4.3BSD.
+.SH AVAILABILITY
+On POSIX systems on which
+.BR fdatasync ()
+is available,
+.B _POSIX_SYNCHRONIZED_IO
+is defined in
+.I <unistd.h>
+to a value greater than 0.
+(See also
+.BR sysconf (3).)
+.\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.
+.\" -1: unavailable, 0: ask using sysconf().
+.\" glibc defines them to 1.
.SH NOTES
-In case the hard disk has write cache enabled,
-the data may not really be on permanent storage when
-.BR fsync / fdatasync
-return.
-.\" See
-.\" .BR hdparm (8)
-.\" for how to disable that cache for IDE disks.
-.LP
-When an ext2 file system is mounted with the
-.I sync
-option, directory entries are also implicitly synced by
-.BR fsync .
-.LP
-On kernels before 2.4,
-.B fsync
-on big files can be inefficient.
-An alternative might be to use the
-.I O_SYNC
-flag to
-.BR open (2).
-.SH "CONFORMING TO"
-POSIX.1b (formerly POSIX.4)
-.SH "SEE ALSO"
+On some UNIX systems (but not Linux),
+.I fd
+must be a
+.I writable
+file descriptor.
+
+In Linux 2.2 and earlier,
+.BR fdatasync ()
+is equivalent to
+.BR fsync (),
+and so has no performance advantage.
+
+The
+.BR fsync ()
+implementations in older kernels and lesser used filesystems
+does not know how to flush disk caches.
+In these cases disk caches need to be disabled using
+.BR hdparm (8)
+or
+.BR sdparm (8)
+to guarantee safe operation.
+.SH SEE ALSO
.BR bdflush (2),
.BR open (2),
-.BR sync (2)
+.BR sync (2),
+.BR sync_file_range (2),
+.BR hdparm (8),
.BR mount (8),
-.BR sync (8),
-.BR update (8)
+.BR sync (1)
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