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.\"     @(#)fsync.2	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: April 18 2019 $
.Dt FSYNC 2
.Os
.Sh NAME
.Nm fsync ,
.Nm fdatasync
.Nd synchronize a file's in-core state with that on disk
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn fsync "int fd"
.Ft int
.Fn fdatasync "int fd"
.Sh DESCRIPTION
The
.Fn fsync
function causes all modified data and attributes of
.Fa fd
to be moved to a permanent storage device.
This normally results in all in-core modified copies
of buffers for the associated file to be written to a disk.
.Pp
The
.Fn fdatasync
function is similar to
.Fn fsync
except that it only guarantees modified data
.Pq and metadata necessary to read that data
is committed to storage.
Other file modifications may be left unsynchronized.
.Pp
.Fn fsync
and
.Fn fdatasync
should be used by programs that require a file to be in a known state,
for example, in building a simple transaction facility.
.Pp
If
.Fn fsync
or
.Fn fdatasync
fail with
.Er EIO ,
the state of the on-disk data may have been only partially written.
To guard against potential inconsistency, future calls will continue failing
until all references to the file are closed.
.Sh RETURN VALUES
.Rv -std fsync fdatasync
.Sh ERRORS
The
.Fn fsync
and
.Fn fdatasync
functions fail if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa fd
is not a valid descriptor.
.It Bq Er EINVAL
.Fa fd
does not refer to a file which can be synchronized.
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.El
.Sh SEE ALSO
.Xr sync 2 ,
.Xr sync 8
.Sh STANDARDS
The
.Fn fsync
and
.Fn fdatasync
functions conform to
.St -p1003.1-2008 .
.Sh HISTORY
The
.Fn fsync
system call first appeared in
.Bx 4.1c ,
and the
.Fn fdatasync
function has been available since
.Ox 5.4 .
.Sh BUGS
The
.Fn fdatasync
function is currently a wrapper around
.Fn fsync ,
so it synchronizes more state than necessary.