Name

fsync, fdatasync — synchronize a file's in-core state with storage device

Synopsis

#include <unistd.h>
int fsync( int fd);
 
int fdatasync( int fd);
 
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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
fdatasync():
_POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >= 500

DESCRIPTION

fsync() transfers ("flushes") all modified in-core data of (i.e., modified buffer cache pages for) the file referred to by the file descriptor 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)).

Calling 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_atime or 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 ftruncate(2)), would 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.

RETURN VALUE

On success, these system calls return zero. On error, −1 is returned, and errno is set to indicate the error.

ERRORS

EBADF

fd is not a valid open file descriptor.

EIO

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.

ENOSPC

Disk space was exhausted while synchronizing.

EROFS, EINVAL

fd is bound to a special file (e.g., a pipe, FIFO, or socket) which does not support synchronization.

ENOSPC, EDQUOT

fd is 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.

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, 4.3BSD.

On POSIX systems on which fdatasync() is available, _POSIX_SYNCHRONIZED_IO is defined in <unistd.h> to a value greater than 0. (See also sysconf(3).)

NOTES

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 advantage.

The fsync() implementations 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 operation.

SEE ALSO

sync(1), bdflush(2), open(2), posix_fadvise(2), pwritev(2), sync(2), sync_file_range(2), fflush(3), fileno(3), hdparm(8), mount(8)

COLOPHON

This page is part of release 5.11 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 https://www.kernel.org/doc/man−pages/.


  Copyright 1993 Rickard E. Faith (faithcs.unc.edu) and
and Copyright 2006 Michael Kerrisk <mtk.manpagesgmail.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.

Permission is granted to copy and distribute modified versions of this
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
the use of the information contained herein.  The author(s) may not
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.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
%%%LICENSE_END

Modified 21 Aug 1994 by Michael Chastain <mecshell.portal.com>:
  Removed note about old libc (pre-4.5.26) translating to 'sync'.
Modified 15 Apr 1995 by Michael Chastain <mecshell.portal.com>:
  Added `see also' 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>