.\" $NetBSD: fsync.2,v 1.4 1995/02/27 12:32:38 cgd Exp $ .\" .\" Copyright (c) 1983, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" 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. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the University of .\" California, Berkeley and its contributors. .\" 4. 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. .\" .\" @(#)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 .