Intel® Math Kernel Library 2018 Developer Reference - C
By default, the computational functions overwrite the input data with the output result. That is, the default setting of the configuration parameter DFTI_PLACEMENT is DFTI_INPLACE. You can change that by setting it to DFTI_NOT_INPLACE.
When the configuration parameter is set to DFTI_NOT_INPLACE, the input and output data sets must have no common elements.
To better understand usage of the DFTI_PLACEMENT configuration parameter, refer to the following example in your Intel MKL directory: