.\" Hey Emacs! This file is -*- nroff -*- source.
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.\" Copyright (C) 1996 Andries Brouwer (aeb@cwi.nl)
.\" Copyright (C) 1996 Markus Kuhn.
.\"
.\" [This version merged from two independently written pages - aeb]
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.\" 1996-04-12  Andries Brouwer <aeb@cwi.nl>
.\" 1996-04-13  Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
.\"
.TH FDATASYNC 2 1996-04-13 "Linux 1.3.86" "Linux Programmer's Manual"
.SH NAME
fdatasync \- synchronize a file's in-core data with that on disk
.SH SYNOPSIS
.B #include <unistd.h>
.sp
.BI "int fdatasync(int " fd );
.SH DESCRIPTION
.B fdatasync
flushes all data buffers of a file to disk (before the system
call returns).  It resembles
.B fsync
but is not required to update the metadata such as access time.

Applications that access databases or log files often write a tiny
data fragment (e.g., one line in a log file) and then call
.B fsync
immediately in order to ensure that the written data is physically
stored on the harddisk. Unfortunately,
.B fsync
will always initiate two write operations: one for the newly written
data and another one in order to update the modification time stored
in the inode. If the modification time is not a part of the transaction
concept
.B fdatasync
can be used to avoid unnecessary inode disk write operations.
.SH "RETURN VALUE"
On success, zero is returned.  On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EBADF
.I fd
is not a valid file descriptor open for writing.
.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 BUGS
Currently (Linux 2.2)
.B fdatasync
is equivalent to
.BR fsync .
.SH AVAILABILITY
On POSIX systems on which
.B fdatasync
is available,
.B _POSIX_SYNCHRONIZED_IO
is defined in <unistd.h> to a value greater than 0. (See also
.BR sysconf (3).)
.\" POSIX 1003.1-2001: It shall be defined to -1 or 0 or 200112L.
.\" -1: unavailable, 0: ask using sysconf().
.\" glibc defines them to 1.
.SH "CONFORMING TO"
POSIX1b (formerly POSIX.4)
.SH "SEE ALSO"
.BR fsync (2)
.br
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 220-223 and 343.