gcc/fortran/doc/gfortran/intrinsic-procedures/flush.rst

   1 ..
   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. index:: FLUSH, file operation, flush
   7
   8 .. _flush:
   9
  10 FLUSH --- Flush I/O unit(s)
  11 ***************************
  12
  13 .. function:: FLUSH(UNIT)
  14
  15   Flushes Fortran unit(s) currently open for output. Without the optional
  16   argument, all units are flushed, otherwise just the unit specified.
  17
  18   :param UNIT:
  19     (Optional) The type shall be ``INTEGER``.
  20
  21   Standard:
  22     GNU extension
  23
  24   Class:
  25     Subroutine
  26
  27   Syntax:
  28     .. code-block:: fortran
  29
  30       CALL FLUSH(UNIT)
  31
  32   Note:
  33     Beginning with the Fortran 2003 standard, there is a ``FLUSH``
  34     statement that should be preferred over the ``FLUSH`` intrinsic.
  35
  36     The ``FLUSH`` intrinsic and the Fortran 2003 ``FLUSH`` statement
  37     have identical effect: they flush the runtime library's I/O buffer so
  38     that the data becomes visible to other processes. This does not guarantee
  39     that the data is committed to disk.
  40
  41     On POSIX systems, you can request that all data is transferred  to  the
  42     storage device by calling the ``fsync`` function, with the POSIX file
  43     descriptor of the I/O unit as argument (retrieved with GNU intrinsic
  44     ``FNUM``). The following example shows how:
  45
  46     .. code-block:: fortran
  47
  48         ! Declare the interface for POSIX fsync function
  49         interface
  50           function fsync (fd) bind(c,name="fsync")
  51           use iso_c_binding, only: c_int
  52             integer(c_int), value :: fd
  53             integer(c_int) :: fsync
  54           end function fsync
  55         end interface
  56
  57         ! Variable declaration
  58         integer :: ret
  59
  60         ! Opening unit 10
  61         open (10,file="foo")
  62
  63         ! ...
  64         ! Perform I/O on unit 10
  65         ! ...
  66
  67         ! Flush and sync
  68         flush(10)
  69         ret = fsync(fnum(10))
  70
  71         ! Handle possible error
  72         if (ret /= 0) stop "Error calling FSYNC"