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       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.,
       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
       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.


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


       EBADF  fd is not a valid open file descriptor.

       EIO    An error occurred during synchronization.

              fd  is  bound  to  a  special  file  which  does   not   support


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


       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).)


       On  some  UNIX  systems  (but  not  Linux),  fd must be a writable file

       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.


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


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