mirror of https://github.com/mkerrisk/man-pages
142 lines
4.3 KiB
Groff
142 lines
4.3 KiB
Groff
.\" Hey Emacs! This file is -*- nroff -*- source.
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.\"
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.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) and
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.\" and Copyright 2006 Michael Kerrisk <mtk-manpages@gmx.net>
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.\" Modified 21 Aug 1994 by Michael Chastain <mec@shell.portal.com>:
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.\" Removed note about old libc (pre-4.5.26) translating to 'sync'.
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.\" Modified 15 Apr 1995 by Michael Chastain <mec@shell.portal.com>:
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.\" Added `see also' section.
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.\" Modified 13 Apr 1996 by Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
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.\" Added remarks about fdatasync.
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.\" Modified 31 Jan 1997 by Eric S. Raymond <esr@thyrsus.com>
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.\" Modified 18 Apr 2001 by Andi Kleen
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.\" Fix description to describe what it really does; add a few caveats.
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.\" 2006-04-28, mtk, substantial rewrite of various parts.
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.\"
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.TH FSYNC 2 2006-04-28 "Linux 1.3.85" "Linux Programmer's Manual"
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.SH NAME
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fsync, fdatasync \- synchronize a file's in-core state with storage device
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.SH SYNOPSIS
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.B #include <unistd.h>
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.sp
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.BI "int fsync(int " fd );
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.sp
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.BI "int fdatasync(int " fd );
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.SH DESCRIPTION
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.BR fsync ()
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transfers ("flushes") all modified in-core data of
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(i.e., modified buffer cache pages for) the
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file referred to by the file descriptor
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.I fd
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to the disk device (or other permanent storage device)
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where that file resides.
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The call blocks until the device reports that the transfer has completed.
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It also flushes metadata information associated with the file (see
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.BR stat (2)).
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Calling
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.BR fsync ()
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does not necessarily ensure
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that the entry in the directory containing the file has also reached disk.
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For that an explicit
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.BR fsync ()
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on a file descriptor for the directory is also needed.
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.BR fdatasync ()
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is similar to
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.BR fsync (),
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but does not flush modified metadata unless that metadata
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is needed in order to allow a subsequent data retrieval to be
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correctly handled.
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For example, changes to
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.I st_atime
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or
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.I st_mtime
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(respectively, time of last access and
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time of last modification; see
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.BR stat (2))
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do not require flushing because they are not necessary for
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a subsequent data read to be handled correctly.
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On the other hand, a change to the file size
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.RI ( st_size ,
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as made by say
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.BR ftruncate (2)),
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would require a metadata flush.
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The aim of
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.BR fdatasync (2)
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is to reduce disk activity for applications that do not
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require all metadata to be synchronised with the disk.
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.SH "RETURN VALUE"
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On success, zero is returned.
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On error, \-1 is returned, and
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.I errno
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is set appropriately.
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.SH ERRORS
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.TP
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.B EBADF
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.I fd
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is not a valid file descriptor open for writing.
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.TP
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.B EIO
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An error occurred during synchronization.
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.TP
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.BR EROFS ", " EINVAL
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.I fd
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is bound to a special file which does not support synchronization.
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.SH NOTES
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If the underlying hard disk has write caching enabled, then
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the data may not really be on permanent storage when
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.BR fsync ()
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/
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.BR fdatasync ()
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return.
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.\" See
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.\" .BR hdparm (8)
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.\" for how to disable that cache for IDE disks.
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.LP
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When an ext2 file system is mounted with the
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.I sync
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option, directory entries are also implicitly synced by
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.BR fsync ().
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.LP
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On kernels before 2.4,
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.BR fsync ()
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on big files can be inefficient.
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An alternative might be to use the
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.I O_SYNC
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flag to
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.BR open (2).
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.SH "CONFORMING TO"
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4.3BSD, POSIX.1-2001
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.SH "SEE ALSO"
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.BR bdflush (2),
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.BR open (2),
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.BR sync (2),
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.BR sync_file_range (2),
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.BR hdparm (8),
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.BR mount (8),
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.BR sync (8),
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.BR update (8)
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