## Image Slot Handling

  SUBROUTINE GDF_GEIS (IS,ERROR)

         GDF  GEt Image Slot
              IS      I       Slot number                     Output
              ERROR   L       Error flag                      Output

  GDF_GEIS returns a unique image slot number IS, for further use in all other GDF_xxxx routines.
  SUBROUTINE GDF_CLIS (IS,ERROR)

         GDF  CLose Image Slot
              IS      I       Slot number                     Input
              ERROR   L       Error flag                      Output

  GDF_CLIS close an image slot IS, disconnecting all associated memory slot after flushing all pending updates on the image. The image slot IS stays reserved, but available for further use.
  SUBROUTINE GDF_FRIS (IS,ERROR)

         GDF  FRee Image Slot
              IS      I       Slot number                     Input
              ERROR   L       Error flag                      Output

  GDF_CLIS free an image slot IS, disconnecting all associated memory slot after flushing all pending updates on the image. The image slot IS should no longer be referenced in any call after this operation.
  INTEGER FUNCTION GDF_STIS(IS)

         GDF  STatus of Image Slot
              IS      I       Slot number                     Input

  Return status of slot: -1 No such slot, 0 Empty (Not allocated), 1 Read (Opened for ReadOnly), 2 Full (Allocated but not opened), 3 Write (Opened for ReadWrite).