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FSYNC(2)		   Linux Programmer's Manual		      FSYNC(2)

NAME
       fsync,  fdatasync - synchronize a file's	in-core	state with storage de-
       vice

SYNOPSIS
       #include	<unistd.h>

       int fsync(int fd);

       int fdatasync(int fd);

   Feature Test	Macro Requirements for glibc (see feature_test_macros(7)):

       fsync():	_BSD_SOURCE || _XOPEN_SOURCE
		|| /* since glibc 2.8: */ _POSIX_C_SOURCE >= 200112L
       fdatasync(): _POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE	>= 500

DESCRIPTION
       fsync() transfers ("flushes") all modified in-core data of (i.e., modi-
       fied  buffer cache pages	for) the file referred to by the file descrip-
       tor 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  associ-
       ated with the file (see stat(2)).

       Calling	fsync()	 does not necessarily ensure that the entry in the di-
       rectory containing the file has also reached disk.   For	 that  an  ex-
       plicit fsync() on a file	descriptor for the directory is	also needed.

       fdatasync() is similar to fsync(), but does not flush modified metadata
       unless that metadata is needed in order to allow	a subsequent data  re-
       trieval	to  be correctly handled.  For example,	changes	to st_atime or
       st_mtime	(respectively, time of last access and time of last  modifica-
       tion;  see stat(2)) do not require flushing because they	are not	neces-
       sary for	a subsequent data read to be handled correctly.	 On the	 other
       hand, a change to the file size (st_size, as made by say	ftruncate(2)),
       would require a metadata	flush.

       The aim of fdatasync() is to reduce disk	activity for applications that
       do not require all metadata to be synchronized with the disk.

RETURN VALUE
       On  success, these system calls return zero.  On	error, -1 is returned,
       and errno is set	appropriately.

ERRORS
       EBADF  fd is not	a valid	open file descriptor.

       EIO    An error occurred	during synchronization.

       EROFS, EINVAL
	      fd is bound to a special file which does	not  support  synchro-
	      nization.

CONFORMING TO
       4.3BSD, POSIX.1-2001.

AVAILABILITY
       On  POSIX  systems  on  which fdatasync() is available, _POSIX_SYNCHRO-
       NIZED_IO	is defined in _unistd.h_ to a value greater than 0.  (See also
       sysconf(3).)

NOTES
       On  some	 UNIX  systems (but not	Linux),	fd must	be a writable file de-
       scriptor.

       In Linux	2.2 and	earlier, fdatasync() is	equivalent to fsync(), and  so
       has no performance advantage.

       The  fsync()  implementations in	older kernels and lesser used filesys-
       tems does not know how to flush	disk  caches.	In  these  cases  disk
       caches  need  to	 be disabled using hdparm(8) or	sdparm(8) to guarantee
       safe operation.

SEE ALSO
       bdflush(2), open(2), sync(2), sync_file_range(2), hdparm(8),  mount(8),
       sync(1)

COLOPHON
       This  page  is  part of release 3.74 of the Linux 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/.

Linux				  2014-08-19			      FSYNC(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | AVAILABILITY | NOTES | SEE ALSO | COLOPHON

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