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### 5.4 Data consistency and durability

This section contains a brief overview of data and metadata consistency and durability issues when doing I/O.

With respect to durability, GNU Fortran makes no effort to ensure that data is committed to stable storage. If this is required, the GNU Fortran programmer can use the intrinsic `FNUM` to retrieve the low level file descriptor corresponding to an open Fortran unit. Then, using e.g. the `ISO_C_BINDING` feature, one can call the underlying system call to flush dirty data to stable storage, such as `fsync` on POSIX, `_commit` on MingW, or `fcntl(fd, F_FULLSYNC, 0)` on Mac OS X. The following example shows how to call fsync:

      ! Declare the interface for POSIX fsync function
      interface
        function fsync (fd) bind(c,name="fsync")
        use iso_c_binding, only: c_int
          integer(c_int), value :: fd
          integer(c_int) :: fsync
        end function fsync
      end interface

      ! Variable declaration
      integer :: ret

      ! Opening unit 10
      open (10,file="foo")

      ! ...
      ! Perform I/O on unit 10
      ! ...

      ! Flush and sync
      flush(10)
      ret = fsync(fnum(10))

      ! Handle possible error
      if (ret /= 0) stop "Error calling FSYNC"

With respect to consistency, for regular files GNU Fortran uses buffered I/O in order to improve performance. This buffer is flushed automatically when full and in some other situations, e.g. when closing a unit. It can also be explicitly flushed with the `FLUSH` statement. Also, the buffering can be turned off with the `GFORTRAN_UNBUFFERED_ALL` and `GFORTRAN_UNBUFFERED_PRECONNECTED` environment variables. Special files, such as terminals and pipes, are always unbuffered. Sometimes, however, further things may need to be done in order to allow other processes to see data that GNU Fortran has written, as follows.

The Windows platform supports a relaxed metadata consistency model, where file metadata is written to the directory lazily. This means that, for instance, the `dir` command can show a stale size for a file. One can force a directory metadata update by closing the unit, or by calling `_commit` on the file descriptor. Note, though, that `_commit` will force all dirty data to stable storage, which is often a very slow operation.

The Network File System (NFS) implements a relaxed consistency model called open-to-close consistency. Closing a file forces dirty data and metadata to be flushed to the server, and opening a file forces the client to contact the server in order to revalidate cached data. `fsync` will also force a flush of dirty data and metadata to the server. Similar to `open` and `close`, acquiring and releasing `fcntl` file locks, if the server supports them, will also force cache validation and flushing dirty data and metadata.

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