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

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

       #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

       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)	 where
       that  file  resides.  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.

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

       EBADF  fd is not a valid file descriptor open for writing.

       EIO    An error occurred during synchronization.

	      fd is bound to a special file which does	not  support  synchro‐

       4.3BSD, POSIX.1-2001.

       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

       Applications that access databases or log files often write a tiny data
       fragment (e.g., one line in a log file) and then call  fsync()  immedi‐
       ately  in order to ensure that the written data is physically stored on
       the harddisk.  Unfortunately, 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 fdatasync() can be used
       to avoid unnecessary inode disk write operations.

       If the underlying hard disk has write caching enabled,  then  the  data
       may  not	 really	 be  on	 permanent  storage when fsync() / fdatasync()

       When an ext2 file system is mounted with	 the  sync  option,  directory
       entries are also implicitly synced by fsync().

       On  kernels  before  2.4,  fsync() on big files can be inefficient.  An
       alternative might be to use the O_SYNC flag to open(2).

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

       bdflush(2),  open(2), sync(2), sync_file_range(2), hdparm(8), mount(8),
       sync(8), update(8)

       This page is part of release 3.22 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				  2008-11-07			      FSYNC(2)

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