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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)  where  that  file  resides.  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 meta-
       data 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 modification; 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 file descriptor open for writing.

       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
       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() immediately 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() return.

       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.

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 3.25 of the Linux man-pages project.  A description of the
       project,  and  information  about  reporting  bugs,  can  be  found   at   http://www.ker-
       nel.org/doc/man-pages/.

Linux					    2008-11-07					 FSYNC(2)
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