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### 6.1 Fortran-interface routines

Nearly all of the FFTW functions have Fortran-callable equivalents. The name of the Fortran routine is the same as that of the corresponding C routine, but with the `fftw_` prefix replaced by `dfftw_`. (The single and long-double precision versions use `sfftw_` and `lfftw_`, respectively, instead of `fftwf_` and `fftwl_`.)1

For the most part, all of the arguments to the functions are the same, with the following exceptions:

- `plan` variables (what would be of type `fftw_plan` in C), must be declared as a type that is at least as big as a pointer (address) on your machine. We recommend using `integer*8`.
- Any function that returns a value (e.g. `fftw_plan_dft`) is converted into a subroutine. The return value is converted into an additional first parameter of this subroutine.2
- The Fortran routines expect multi-dimensional arrays to be in column-major order, which is the ordinary format of Fortran arrays (see Multi-dimensional Array Format). They do this transparently and costlessly simply by reversing the order of the dimensions passed to FFTW, but this has one important consequence for multi-dimensional real-complex transforms, discussed below.
- Wisdom import and export is somewhat more tricky because one cannot easily pass files or strings between C and Fortran; see Wisdom of Fortran?.
- Fortran cannot use the `fftw_malloc` dynamic-allocation routine. If you want to exploit the SIMD FFTW (see Data Alignment), you'll need to figure out some other way to ensure that your arrays are at least 16-byte aligned.
- Since Fortran 77 does not have data structures, the `fftw_iodim` structure from the guru interface (see Guru vector and transform sizes) must be split into separate arguments. In particular, any `fftw_iodim` array arguments in the C guru interface become three integer array arguments (`n`, `is`, and `os`) in the Fortran guru interface, all of whose length should be equal to the corresponding `rank` argument.

In general, you should take care to use Fortran data types that correspond to (i.e. are the same size as) the C types used by FFTW. If your C and Fortran compilers are made by the same vendor, the correspondence is usually straightforward (i.e. `integer` corresponds to `int`, `real` corresponds to `float`, etcetera). The native Fortran double/single-precision complex type should be compatible with `fftw_complex`/`fftwf_complex`. Such simple correspondences are assumed in the examples below.

#### Footnotes

1. Technically, Fortran 77 identifiers are not allowed to have more than 6 characters, nor may they contain underscores. Any compiler that enforces this limitation doesn't deserve to link to FFTW.

2. The reason for this is that some Fortran implementations seem to have trouble with C function return values, and vice versa.