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

NAME

       fsync, fdatasync - synchronize a file's in-core state with storage device

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., modified buffer cache pages for) the file referred to by the file
       descriptor 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 stat(2)).

       Calling fsync() does not necessarily ensure that the entry in the directory containing the  file  has  also  reached  disk.   For  that	an
       explicit 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
       retrieval to be correctly handled.  For example, changes to st_atime or st_mtime (respectively, time of last access and time of last  modi-
       fication;  see  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 (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 synchronization.

CONFORMING TO

       4.3BSD, POSIX.1-2001.

AVAILABILITY

       On POSIX systems on which fdatasync() is available, _POSIX_SYNCHRONIZED_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 descriptor.

       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 filesystems 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(8), update(8)

COLOPHON

       This page is part of release 3.53 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2012-02-27								  FSYNC(2)