7.6.1 Wisdom File Export/Import from Fortran

The easiest way to export and import wisdom is to do so using fftw_export_wisdom_to_filename and fftw_wisdom_from_filename. The only trick is that these require you to pass a C string, which is an array of type CHARACTER(C_CHAR) that is terminated by C_NULL_CHAR. You can call them like this:

       integer(C_INT) :: ret
       ret = fftw_export_wisdom_to_filename(C_CHAR_'my_wisdom.dat' // C_NULL_CHAR)
       if (ret .eq. 0) stop 'error exporting wisdom to file'
       ret = fftw_import_wisdom_from_filename(C_CHAR_'my_wisdom.dat' // C_NULL_CHAR)
       if (ret .eq. 0) stop 'error importing wisdom from file'

Note that prepending ‘C_CHAR_’ is needed to specify that the literal string is of kind C_CHAR, and we null-terminate the string by appending ‘// C_NULL_CHAR’. These functions return an integer(C_INT) (ret) which is 0 if an error occurred during export/import and nonzero otherwise.

It is also possible to use the lower-level routines fftw_export_wisdom_to_file and fftw_import_wisdom_from_file, which accept parameters of the C type FILE*, expressed in Fortran as type(C_PTR). However, you are then responsible for creating the FILE* yourself. You can do this by using iso_c_binding to define Fortran intefaces for the C library functions fopen and fclose, which is a bit strange in Fortran but workable.