.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) and
.\" and Copyright 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified 21 Aug 1994 by Michael Chastain <mec@shell.portal.com>:
.\"   Removed note about old libc (pre-4.5.26) translating to 'sync'.
.\" Modified 15 Apr 1995 by Michael Chastain <mec@shell.portal.com>:
.\"   Added `see also' section.
.\" Modified 13 Apr 1996 by Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
.\"   Added remarks about fdatasync.
.\" 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 2023-03-30 "Linux man-pages 6.04"
.SH NAME
fsync, fdatasync \- synchronize a file's in-core state with storage device
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int fsync(int " fd );
.PP
.BI "int fdatasync(int " fd );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.nf
.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
.fi
.PP
.BR fdatasync ():
.nf
    _POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >= 500
.fi
.SH DESCRIPTION
.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 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.
.PP
As well as flushing the file data,
.BR fsync ()
also flushes the metadata information associated with the file (see
.BR inode (7)).
.PP
Calling
.BR fsync ()
does not necessarily ensure
that the entry in the directory containing the file has also reached disk.
For that an explicit
.BR fsync ()
on a file descriptor for the directory is also needed.
.PP
.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 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
.RI ( st_size ,
as made by say
.BR ftruncate (2)),
would require a metadata flush.
.PP
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 to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I fd
is not a valid open file descriptor.
.TP
.B EINTR
The function was interrupted by a signal; see
.BR signal (7).
.TP
.B 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,
.\" commit 088737f44bbf6378745f5b57b035e57ee3dc4750
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.
.TP
.B ENOSPC
Disk space was exhausted while synchronizing.
.TP
.BR EROFS ", " EINVAL
.I fd
is bound to a special file (e.g., a pipe, FIFO, or socket)
which does not support synchronization.
.TP
.BR ENOSPC ", " EDQUOT
.I fd
is bound to a file on NFS or another filesystem which does not allocate
space at the time of a
.BR write (2)
system call, and some previous write failed due to insufficient
storage space.
.SH VERSIONS
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.
.PP
On some UNIX systems (but not Linux),
.I fd
must be a
.I writable
file descriptor.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, 4.3BSD.
.PP
In Linux 2.2 and earlier,
.BR fdatasync ()
is equivalent to
.BR fsync (),
and so has no performance advantage.
.PP
The
.BR 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
.BR hdparm (8)
or
.BR sdparm (8)
to guarantee safe operation.
.SH SEE ALSO
.BR sync (1),
.BR bdflush (2),
.BR open (2),
.BR posix_fadvise (2),
.BR pwritev (2),
.BR sync (2),
.BR sync_file_range (2),
.BR fflush (3),
.BR fileno (3),
.BR hdparm (8),
.BR mount (8)
