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fftw3: Wisdom File Export/Import from Fortran

 
 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.
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