.\"	$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
.Fo fsync
.Fa "int fildes"
.Fc
.Sh DESCRIPTION
.Fn Fsync
causes all modified data and attributes of
.Fa fildes
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 guarantees
about the integrity of their data,
Mac OS 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 that their data
is written in the order they expect.
Please see
.Xr fcntl 2
for more detail.
.Pp
.Sh RETURN VALUES
.Rv -std fsync
.Sh ERRORS
The
.Fn fsync
system call will fail if:
.Bl -tag -width Er
.\" ==========
.It Bq Er EBADF
.Fa fildes
is not a valid descriptor.
.\" ==========
.It Bq Er EINTR
Its execution is interrupted by a signal.
.\" ==========
.It Bq Er EINVAL
.Fa fildes
refers to a file type (e.g., a socket)
that does not support this operation.
.\" ==========
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.El
.Pp
If a queued I/O operation fails,
.Fn fsync
may fail with any of the errors defined for
.Xr read 2
or
.Xr write 2 .
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr read 2 ,
.Xr sync 2 ,
.Xr write 2 ,
.Xr sync 8 ,
.Xr update 8
.Sh HISTORY
The
.Fn fsync
function call appeared in
.Bx 4.2 .