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<h4 class="subsection">7.6.1 Wisdom File Export/Import from Fortran</h4>
<p><a name="index-fftw_005fimport-wisdom_005ffrom_005ffilename-566"></a><a name="index-fftw_005fexport_005fwisdom_005fto_005ffilename-567"></a>The easiest way to export and import wisdom is to do so using
<code>fftw_export_wisdom_to_filename</code> and
<code>fftw_wisdom_from_filename</code>. The only trick is that these
require you to pass a C string, which is an array of type
<code>CHARACTER(C_CHAR)</code> that is terminated by <code>C_NULL_CHAR</code>.
You can call them like this:
<pre class="example"> 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'
</pre>
<p>Note that prepending ‘<samp><span class="samp">C_CHAR_</span></samp>’ is needed to specify that the
literal string is of kind <code>C_CHAR</code>, and we null-terminate the
string by appending ‘<samp><span class="samp">// C_NULL_CHAR</span></samp>’. These functions return an
<code>integer(C_INT)</code> (<code>ret</code>) which is <code>0</code> if an error
occurred during export/import and nonzero otherwise.
<p>It is also possible to use the lower-level routines
<code>fftw_export_wisdom_to_file</code> and
<code>fftw_import_wisdom_from_file</code>, which accept parameters of the C
type <code>FILE*</code>, expressed in Fortran as <code>type(C_PTR)</code>.
However, you are then responsible for creating the <code>FILE*</code>
yourself. You can do this by using <code>iso_c_binding</code> to define
Fortran intefaces for the C library functions <code>fopen</code> and
<code>fclose</code>, which is a bit strange in Fortran but workable.
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