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


       #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


       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 synchronization.


       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 descriptor.

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

       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), sync(1)


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