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<h4 class="subsection">7.6.3 Wisdom Generic Export/Import from Fortran</h4>
<p>The most generic wisdom export/import functions allow you to provide
an arbitrary callback function to read/write one character at a time
in any way you want. However, your callback function must be written
in a special way, using the <code>bind(C)</code> attribute to be passed to a
C interface.
<p><a name="index-fftw_005fexport_005fwisdom-572"></a>In particular, to call the generic wisdom export function
<code>fftw_export_wisdom</code>, you would write a callback subroutine of the form:
<pre class="example"> subroutine my_write_char(c, p) bind(C)
use, intrinsic :: iso_c_binding
character(C_CHAR), value :: c
type(C_PTR), value :: p
<em>...write c...</em>
end subroutine my_write_char
</pre>
<p>Given such a subroutine (along with the corresponding interface definition), you could then export wisdom using:
<p><a name="index-c_005ffunloc-573"></a>
<pre class="example"> call fftw_export_wisdom(c_funloc(my_write_char), p)
</pre>
<p><a name="index-c_005floc-574"></a><a name="index-c_005ff_005fpointer-575"></a>The standard <code>c_funloc</code> intrinsic converts a Fortran
<code>bind(C)</code> subroutine into a C function pointer. The parameter
<code>p</code> is a <code>type(C_PTR)</code> to any arbitrary data that you want
to pass to <code>my_write_char</code> (or <code>C_NULL_PTR</code> if none). (Note
that you can get a C pointer to Fortran data using the intrinsic
<code>c_loc</code>, and convert it back to a Fortran pointer in
<code>my_write_char</code> using <code>c_f_pointer</code>.)
<p>Similarly, to use the generic <code>fftw_import_wisdom</code>, you would
define a callback function of the form:
<p><a name="index-fftw_005fimport_005fwisdom-576"></a>
<pre class="example"> integer(C_INT) function my_read_char(p) bind(C)
use, intrinsic :: iso_c_binding
type(C_PTR), value :: p
character :: c
<em>...read a character c...</em>
my_read_char = ichar(c, C_INT)
end function my_read_char
....
integer(C_INT) :: ret
ret = fftw_import_wisdom(c_funloc(my_read_char), p)
if (ret .eq. 0) stop 'error importing wisdom'
</pre>
<p>Your function can return <code>-1</code> if the end of the input is reached.
Again, <code>p</code> is an arbitrary <code>type(C_PTR</code> that is passed
through to your function. <code>fftw_import_wisdom</code> returns <code>0</code>
if an error occurred and nonzero otherwise.
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