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+.\" Copyright (c) 1983, 1993
+.\"	The Regents of the University of California.  All rights reserved.
+.\" Copyright (c) 2016 The FreeBSD Foundation, Inc.
+.\"
+.\" Parts of this documentation were written by
+.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
+.\" from the FreeBSD Foundation.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.Dd March 30, 2020
+.Dt FSYNC 2
+.Os
+.Sh NAME
+.Nm fdatasync ,
+.Nm fsync
+.Nd "synchronise changes to a file"
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In unistd.h
+.Ft int
+.Fn fdatasync "int fd"
+.Ft int
+.Fn fsync "int fd"
+.Sh DESCRIPTION
+The
+.Fn fsync
+system call
+causes all modified data and attributes of the file referenced by
+the file descriptor
+.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
+system call causes all modified data of
+.Fa fd
+to be moved to a permanent storage device.
+Unlike
+.Fn fsync ,
+the system call does not guarantee that file attributes or
+metadata necessary to access the file are committed to the permanent storage.
+.Pp
+The
+.Fn fsync
+system call
+should be used by programs that require a file to be
+in a known state, for example, in building a simple transaction
+facility.
+If the file metadata has already been committed, using
+.Fn fdatasync
+can be more efficient than
+.Fn fsync .
+.Pp
+Both
+.Fn fdatasync
+and
+.Fn fsync
+calls are cancellation points.
+.Sh RETURN VALUES
+.Rv -std fsync
+.Sh ERRORS
+The
+.Fn fsync
+and
+.Fn fdatasync
+calls fail if:
+.Bl -tag -width Er
+.It Bq Er EBADF
+The
+.Fa fd
+argument
+is not a valid descriptor.
+.It Bq Er EINVAL
+The
+.Fa fd
+argument
+refers to a socket, not to a file.
+.It Bq Er EIO
+An I/O error occurred while reading from or writing to the file system.
+.It Bq Er EINTEGRITY
+Corrupted data was detected while reading from the file system.
+.El
+.Sh SEE ALSO
+.Xr fsync 1 ,
+.Xr sync 2 ,
+.Xr syncer 4 ,
+.Xr sync 8
+.Sh HISTORY
+The
+.Fn fsync
+system call appeared in
+.Bx 4.2 .
+The
+.Fn fdatasync
+system call appeared in
+.Fx 11.1 .
+.Sh BUGS
+The
+.Fn fdatasync
+system call currently does not guarantee that enqueued
+.Xr aio 4
+requests for the file referenced by
+.Fa fd
+are completed before the syscall returns.