.\"	$NetBSD: fsync.2,v 1.4 1995/02/27 12:32:38 cgd Exp $
.\"
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.\"     @(#)fsync.2	8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt FSYNC 2
.Os BSD 4.2
.Sh NAME
.Nm fsync
.Nd "synchronize a file's in-core state with that on disk"
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn fsync "int fd"
.Sh DESCRIPTION
.Fn Fsync
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
Note that while
.Fn fsync
will flush all data from the host
to the drive (i.e. the "permanent storage
device"), the
drive itself may not physically
write the data to the
platters for quite some time
and it may be written in an
out-of-order sequence.
.Pp
Specifically, if the drive loses power
or the OS crashes,
the application
may find that only some or none of their data was
written.  The disk drive may also re-order
the data so that later writes
may be present while earlier writes are not.
.Pp
This is not a theoretical
edge case.  This scenario is easily reproduced
with real world workloads and drive
power failures.
.Pp
For applications that require tighter guarantess about
the integrity of their data, MacOS X provides the
F_FULLFSYNC fcntl.  The F_FULLFSYNC fcntl asks the
drive to flush all buffered data to permanent
storage.  Applications such as databases that require
a strict ordering of writes should use F_FULLFSYNC to
ensure their data is written in the order they expect.
Please see
.Xr fcntl 2
for more detail.
.Pp
.Sh RETURN VALUES
A 0 value is returned on success.  A -1 value indicates
an error.
.Sh ERRORS
The
.Fn fsync
fails if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa fd
is not a valid descriptor.
.It Bq Er EINVAL
.Fa fd
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 sync 8 ,
.Xr update 8 ,
.Xr fcntl 2
.Sh HISTORY
The
.Fn fsync
function call appeared in
.Bx 4.2 .