Keywords: sic-gdfv2

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## GILDAS_NULL

The subroutine `GILDAS_NULL` is the principal initialization routine for GILDAS data structures. No access to the Gildas derived type elements should be done before calling this routine, as some elements are dangling pointers. Subroutine `GILDAS_ERROR` will return an error and issue a message if the gildas data structure has not been initialized. 7

    interface
      subroutine gildas_null(hx, type)
        use image_def
        !---------------------------------------------------------------------
        ! @ public
        ! GDF API
        !   Initialize a Gildas structure and reset its content
        !   TYPE can be: 'IMAGE' (default), 'TABLE', 'UVT', 'TUV', 'VOTABLE'
        !---------------------------------------------------------------------
        type(gildas), intent(out), target :: hx ! Gildas structure
        character(len=*), intent(in), optional :: type
      end subroutine gildas_null
    end interface

It has an optional argument named `type`, which indicates which type of Gildas data structure is used. The default is `'IMAGE'`.

Calling `GILDAS_NULL(h, type = 'UVT')` will set appropriate defaults for UV data set in natural order, while `GILDAS_NULL(h, type = 'TUV')` should be used for the transposed order. Existing pointers are nullified, and then associated to their appropriate target.