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### 8.87 `FLUSH` — Flush I/O unit(s)

Description:
  Flushes Fortran unit(s) currently open for output. Without the optional argument, all units are flushed, otherwise just the unit specified.
Standard:
  GNU extension
Class:
  Subroutine
Syntax:
  `CALL FLUSH(UNIT)`
Arguments:

  UNIT | (Optional) The type shall be `INTEGER`.

Note:
  Beginning with the Fortran 2003 standard, there is a `FLUSH` statement that should be preferred over the `FLUSH` intrinsic.

  The `FLUSH` intrinsic and the Fortran 2003 `FLUSH` statement have identical effect: they flush the runtime library's I/O buffer so that the data becomes visible to other processes. This does not guarantee that the data is committed to disk.

  On POSIX systems, you can request that all data is transferred to the storage device by calling the `fsync` function, with the POSIX file descriptor of the I/O unit as argument (retrieved with GNU intrinsic `FNUM`). The following example shows how:

                  ! 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"