Section (2) fsync
fsync, fdatasync — synchronize a file_zsingle_quotesz_s in-core state with storage device
(flushes) all modified in-core data of (i.e., modified
buffer cache pages for) the file referred to by the file
fd to the
disk device (or other permanent storage device) so that all
changed information can be retrieved even if the system
crashes or is rebooted. This includes writing through or
flushing a disk cache if present. The call blocks until the
device reports that the transfer has completed.
As well as flushing the file data,
fsync() also flushes the metadata
information associated with the file (see inode(7)).
fsync() does not
necessarily ensure that the entry in the directory containing
the file has also reached disk. For that an explicit
fsync() on a file descriptor
for the directory is also needed.
fdatasync() is similar to
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
st_mtime (respectively, time
of last access and time of last modification; see inode(7)) 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 (
st_size, as made by say
require a metadata flush.
The aim of
fdatasync() is to
reduce disk activity for applications that do not require all
metadata to be synchronized with the disk.
On success, these system calls return zero. On error,
−1 is returned, and
is set appropriately.
fdis not a valid open file descriptor.
An error occurred during synchronization. This error may relate to data written to some other file descriptor on the same file. Since Linux 4.13, errors from write-back will be reported to all file descriptors that might have written the data which triggered the error. Some filesystems (e.g., NFS) keep close track of which data came through which file descriptor, and give more precise reporting. Other filesystems (e.g., most local filesystems) will report errors to all file descriptors that were open on the file when the error was recorded.
Disk space was exhausted while synchronizing.
- EROFS, EINVAL
fdis bound to a special file (e.g., a pipe, FIFO, or socket) which does not support synchronization.
- ENOSPC, EDQUOT
fdis bound to a file on NFS or another filesystem which does not allocate space at the time of a write(2) system call, and some previous write failed due to insufficient storage space.
On POSIX systems on which
fdatasync() is available,
_POSIX_SYNCHRONIZED_IO is defined in
to a value greater than 0. (See also sysconf(3).)
On some UNIX systems (but not Linux),
fd must be a
writable file descriptor.
In Linux 2.2 and earlier,
fdatasync() is equivalent to
fsync(), and so has no performance
in older kernels and lesser used filesystems do not know how
to flush disk caches. In these cases disk caches need to be
disabled using hdparm(8) or sdparm(8) to guarantee safe
This page is part of release 5.04 of the Linux
man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
Copyright 1993 Rickard E. Faith (faithcs.unc.edu) and
and Copyright 2006 Michael Kerrisk <mtk.manpagesgmail.com>
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Modified 21 Aug 1994 by Michael Chastain <mecshell.portal.com>:
Removed note about old libc (pre-4.5.26) translating to _zsingle_quotesz_sync_zsingle_quotesz_.
Modified 15 Apr 1995 by Michael Chastain <mecshell.portal.com>:
Added `see also_zsingle_quotesz_ section.
Modified 13 Apr 1996 by Markus Kuhn <mskuhncip.informatik.uni-erlangen.de>
Added remarks about fdatasync.
Modified 31 Jan 1997 by Eric S. Raymond <esrthyrsus.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 <hchlst.de>