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.\"     @(#)fsync.2	8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd August 17, 2016
.Dt FSYNC 2
.Os
.Sh NAME
.Nm fdatasync, 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.
.El
.Sh SEE ALSO
.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 12.0
.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.