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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

              Glibc 2.16 and later:
                  No feature test macros need be defined
              Glibc up to and including 2.15:
                  _BSD_SOURCE || _XOPEN_SOURCE
                      || /* since glibc 2.8: */
          _POSIX_C_SOURCE >= 200112L
              _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 per-
          manent storage device) so that all changed information can
          be retrieved even if the system crashes or is rebooted.
          This includes writing through or flushing a disk cache if
          present.  The call blocks until the device reports that the
          transfer has completed.

          As well as flushing the file data, fsync() also flushes the
          metadata information associated with the file (see

          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 modi-
          fied 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
          inode(7)) do not require flushing because they are not nec-
          essary for a subsequent data read to be handled correctly.

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          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 appli-
          cations 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.

               fd is not a valid open file descriptor.

          EIO  An error occurred during synchronization.  This error
               may relate to data written to some other file descrip-
               tor on the same file.  Since Linux 4.13, errors from
               write-back will be reported to all file descriptors
               that might have written the data which triggered the
               error.  Some filesystems (e.g., NFS) keep close track
               of which data came through which file descriptor, and
               give more precise reporting.  Other filesystems (e.g.,
               most local filesystems) will report errors to all file
               descriptors that were open on the file when the error
               was recorded.

               Disk space was exhausted while synchronizing.

          EROFS, EINVAL
               fd is bound to a special file (e.g., a pipe, FIFO, or
               socket) which does not support synchronization.

          ENOSPC, EDQUOT
               fd is bound to a file on NFS or another filesystem
               which does not allocate space at the time of a write(2)
               system call, and some previous write failed due to
               insufficient storage space.

          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

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          fsync(), and so has no performance advantage.

          The fsync() implementations in older kernels and lesser used
          filesystems do 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.

          sync(1), bdflush(2), open(2), posix_fadvise(2), pwritev(2),
          sync(2), sync_file_range(2), fflush(3), fileno(3),
          hdparm(8), mount(8)

          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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