The dimension of the transform is a positive integer value represented in an integer scalar of MKL_LONG data type. For a one-dimensional transform, the transform length is specified by a positive integer value represented in an integer scalar of MKL_LONG data type. For multi-dimensional (≥ 2) transform, the lengths of each of the dimensions are supplied in an integer array (of MKL_LONG data type).
DFTI_DIMENSION and DFTI_LENGTHS do not have a default value. To set them, use the DftiCreateDescriptor function and not the DftiSetValue function.
To better understand usage of the DFTI_DIMENSION and DFTI_LENGTHS configuration parameters, you can refer to basic examples of one-, two-, and three-dimensional transforms in your Intel MKL directory. Naming conventions for the examples are self-explanatory. For example, refer to these examples of single-precision two-dimensional transforms: