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<h4 class="subsection">7.6.2 Wisdom String Export/Import from Fortran</h4>
<p><a name="index-fftw_005fexport_005fwisdom_005fto_005fstring-568"></a>Dealing with FFTW's C string export/import is a bit more painful. In
particular, the <code>fftw_export_wisdom_to_string</code> function requires
you to deal with a dynamically allocated C string. To get its length,
you must define an interface to the C <code>strlen</code> function, and to
deallocate it you must define an interface to C <code>free</code>:
<pre class="example"> use, intrinsic :: iso_c_binding
interface
integer(C_INT) function strlen(s) bind(C, name='strlen')
import
type(C_PTR), value :: s
end function strlen
subroutine free(p) bind(C, name='free')
import
type(C_PTR), value :: p
end subroutine free
end interface
</pre>
<p>Given these definitions, you can then export wisdom to a Fortran
character array:
<pre class="example"> character(C_CHAR), pointer :: s(:)
integer(C_SIZE_T) :: slen
type(C_PTR) :: p
p = fftw_export_wisdom_to_string()
if (.not. c_associated(p)) stop 'error exporting wisdom'
slen = strlen(p)
call c_f_pointer(p, s, [slen+1])
...
call free(p)
</pre>
<p><a name="index-c_005fassociated-569"></a><a name="index-c_005ff_005fpointer-570"></a>
Note that <code>slen</code> is the length of the C string, but the length of
the array is <code>slen+1</code> because it includes the terminating null
character. (You can omit the ‘<samp><span class="samp">+1</span></samp>’ if you don't want Fortran to
know about the null character.) The standard <code>c_associated</code> function
checks whether <code>p</code> is a null pointer, which is returned by
<code>fftw_export_wisdom_to_string</code> if there was an error.
<p><a name="index-fftw_005fimport_005fwisdom_005ffrom_005fstring-571"></a>To import wisdom from a string, use
<code>fftw_import_wisdom_from_string</code> as usual; note that the argument
of this function must be a <code>character(C_CHAR)</code> that is terminated
by the <code>C_NULL_CHAR</code> character, like the <code>s</code> array above.
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