The HDF_VD_GETID function returns the VData reference number for the next VData in a Hierarchical Data Format file after the specified VDataId .
Set VDataId to -1 to return the first VData ID in the file.
Result = HDF_VD_GETID( FileHandle, VDataId )
The HDF file handle returned by a previous call to HDF_OPEN.
The VData reference number, generally obtained by HDF_VD_GETID or HDF_VD_LONE. Set this argument to -1 to return the first VData in the file.