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### 6.2 FFTW Constants in Fortran

When creating plans in FFTW, a number of constants are used to specify options, such as `FFTW_MEASURE` or `FFTW_ESTIMATE`. The same constants must be used with the wrapper routines, but of course the C header files where the constants are defined can't be incorporated directly into Fortran code.

Instead, we have placed Fortran equivalents of the FFTW constant definitions in the file `fftw3.f`, which can be found in the same directory as `fftw3.h`. If your Fortran compiler supports a preprocessor of some sort, you should be able to `include` or `#include` this file; otherwise, you can paste it directly into your code.

In C, you combine different flags (like `FFTW_PRESERVE_INPUT` and `FFTW_MEASURE`) using the ```|``' operator; in Fortran you should just use ```+``'. (Take care not to add in the same flag more than once, though.)