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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 3.25 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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