Intel® Math Kernel Library 2018 Developer Reference - C
The configuration parameter DFTI_PRECISION denotes the floating-point precision in which the transform is to be carried out. A setting of DFTI_SINGLE stands for single precision, and a setting of DFTI_DOUBLE stands for double precision. The data must be presented in this precision, the computation is carried out in this precision, and the result is delivered in this precision.
DFTI_PRECISION does not have a default value. Set it explicitly by calling the DftiCreateDescriptor function.
To better understand configuration of the precision of transforms, you can refer to these examples in your Intel MKL directory: