The directory layout for this recipe shows the now familiar structure for a superbuild:

.
├── CMakeLists.txt
├── external
│   └── upstream
│       ├── CMakeLists.txt
│       └── fftw3
│           └── CMakeLists.txt
└── src
    ├── CMakeLists.txt
    └── fftw_example.c

The code for our project, fftw_example.c, is located in the src subdirectory and will calculate the Fourier transform of a function defined in the source code.