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():
	   Glibc 2.16 and later:
	       No feature test macros need be defined
	   Glibc up to and including 2.15:
	       _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
       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  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 (e.g., a pipe,  FIFO,  or  socket)
	      which does not support synchronization.

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, 4.3BSD.

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
       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 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
       sync(1), bdflush(2), open(2),  posix_fadvise(2),	 pwritev(2),  sync(2),
       sync_file_range(2), fflush(3), fileno(3), hdparm(8), mount(8)

COLOPHON
       This  page  is  part of release 4.10 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
       https://www.kernel.org/doc/man-pages/.



Linux				  2017-03-13			      FSYNC(2)