.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) and .\" and Copyright 2006 Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" Modified 21 Aug 1994 by Michael Chastain : .\" Removed note about old libc (pre-4.5.26) translating to 'sync'. .\" Modified 15 Apr 1995 by Michael Chastain : .\" Added `see also' section. .\" Modified 13 Apr 1996 by Markus Kuhn .\" Added remarks about fdatasync. .\" Modified 31 Jan 1997 by Eric S. Raymond .\" Modified 18 Apr 2001 by Andi Kleen .\" Fix description to describe what it really does; add a few caveats. .\" 2006-04-28, mtk, substantial rewrite of various parts. .\" 2012-02-27 Various changes by Christoph Hellwig .\" .TH FSYNC 2 2014-08-19 "Linux" "Linux Programmer's Manual" .SH NAME fsync, fdatasync \- synchronize a file's in-core state with storage device .SH SYNOPSIS .B #include .sp .BI "int fsync(int " fd ); .sp .BI "int fdatasync(int " fd ); .sp .in -4n Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .in .sp .BR fsync (): _BSD_SOURCE || _XOPEN_SOURCE .br || /* since glibc 2.8: */ _POSIX_C_SOURCE\ >=\ 200112L .\" _POSIX_C_SOURCE\ >=\ 200112L only since glibc 2.8 .br .BR fdatasync (): _POSIX_C_SOURCE\ >=\ 199309L || _XOPEN_SOURCE\ >=\ 500 .SH DESCRIPTION .BR fsync () transfers ("flushes") all modified in-core data of (i.e., modified buffer cache pages for) the file referred to by the file descriptor .I fd to the disk device (or other permanent storage device) so that all changed information can be retrieved even after the system crashed or was rebooted. This includes writing through or flushing a disk cache if present. The call blocks until the device reports that the transfer has completed. It also flushes metadata information associated with the file (see .BR stat (2)). Calling .BR fsync () does not necessarily ensure that the entry in the directory containing the file has also reached disk. For that an explicit .BR fsync () on a file descriptor for the directory is also needed. .BR fdatasync () is similar to .BR fsync (), but does not flush modified metadata unless that metadata is needed in order to allow a subsequent data retrieval to be correctly handled. For example, changes to .I st_atime or .I st_mtime (respectively, time of last access and time of last modification; see .BR stat (2)) do not require flushing because they are not necessary for a subsequent data read to be handled correctly. On the other hand, a change to the file size .RI ( st_size , as made by say .BR ftruncate (2)), would require a metadata flush. The aim of .BR fdatasync () is to reduce disk activity for applications that do not require all metadata to be synchronized with the disk. .SH RETURN VALUE On success, these system calls return zero. On error, \-1 is returned, and .I errno is set appropriately. .SH ERRORS .TP .B EBADF .I fd is not a valid open file descriptor. .TP .B EIO An error occurred during synchronization. .TP .BR EROFS ", " EINVAL .I fd is bound to a special file which does not support synchronization. .SH CONFORMING TO 4.3BSD, POSIX.1-2001. .SH AVAILABILITY On POSIX systems on which .BR fdatasync () is available, .B _POSIX_SYNCHRONIZED_IO is defined in .I to a value greater than 0. (See also .BR sysconf (3).) .\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L. .\" -1: unavailable, 0: ask using sysconf(). .\" glibc defines them to 1. .SH NOTES On some UNIX systems (but not Linux), .I fd must be a .I writable file descriptor. In Linux 2.2 and earlier, .BR fdatasync () is equivalent to .BR fsync (), and so has no performance advantage. The .BR fsync () implementations in older kernels and lesser used filesystems does not know how to flush disk caches. In these cases disk caches need to be disabled using .BR hdparm (8) or .BR sdparm (8) to guarantee safe operation. .SH SEE ALSO .BR bdflush (2), .BR open (2), .BR sync (2), .BR sync_file_range (2), .BR hdparm (8), .BR mount (8), .BR sync (1) .SH COLOPHON This page is part of release 3.74 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%http://www.kernel.org/doc/man\-pages/.