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<title>The GNU Fortran Compiler: Working with Pointers</title>

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<a name="Working-with-Pointers"></a>
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<p>
Next: <a href="Further-Interoperability-of-Fortran-with-C.html#Further-Interoperability-of-Fortran-with-C" accesskey="n" rel="next">Further Interoperability of Fortran with C</a>, Previous: <a href="Interoperable-Subroutines-and-Functions.html#Interoperable-Subroutines-and-Functions" accesskey="p" rel="prev">Interoperable Subroutines and Functions</a>, Up: <a href="Interoperability-with-C.html#Interoperability-with-C" accesskey="u" rel="up">Interoperability with C</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Working-with-Pointers-1"></a>
<h4 class="subsection">7.1.5 Working with Pointers</h4>

<p>C pointers are represented in Fortran via the special opaque derived type
<code>type(c_ptr)</code> (with private components).  Thus one needs to
use intrinsic conversion procedures to convert from or to C pointers.
</p>
<p>For some applications, using an assumed type (<code>TYPE(*)</code>) can be an
alternative to a C pointer; see
<a href="Further-Interoperability-of-Fortran-with-C.html#Further-Interoperability-of-Fortran-with-C">Further Interoperability of Fortran with C</a>.
</p>
<p>For example,
</p>
<div class="smallexample">
<pre class="smallexample">  use iso_c_binding
  type(c_ptr) :: cptr1, cptr2
  integer, target :: array(7), scalar
  integer, pointer :: pa(:), ps
  cptr1 = c_loc(array(1)) ! The programmer needs to ensure that the
                          ! array is contiguous if required by the C
                          ! procedure
  cptr2 = c_loc(scalar)
  call c_f_pointer(cptr2, ps)
  call c_f_pointer(cptr2, pa, shape=[7])
</pre></div>

<p>When converting C to Fortran arrays, the one-dimensional <code>SHAPE</code> argument
has to be passed.
</p>
<p>If a pointer is a dummy-argument of an interoperable procedure, it usually
has to be declared using the <code>VALUE</code> attribute.  <code>void*</code>
matches <code>TYPE(C_PTR), VALUE</code>, while <code>TYPE(C_PTR)</code> alone
matches <code>void**</code>.
</p>
<p>Procedure pointers are handled analogously to pointers; the C type is
<code>TYPE(C_FUNPTR)</code> and the intrinsic conversion procedures are
<code>C_F_PROCPOINTER</code> and <code>C_FUNLOC</code>.
</p>
<p>Let us consider two examples of actually passing a procedure pointer from
C to Fortran and vice versa.  Note that these examples are also very
similar to passing ordinary pointers between both languages. First,
consider this code in C:
</p>
<div class="smallexample">
<pre class="smallexample">/* Procedure implemented in Fortran.  */
void get_values (void (*)(double));

/* Call-back routine we want called from Fortran.  */
void
print_it (double x)
{
  printf (&quot;Number is %f.\n&quot;, x);
}

/* Call Fortran routine and pass call-back to it.  */
void
foobar ()
{
  get_values (&amp;print_it);
}
</pre></div>

<p>A matching implementation for <code>get_values</code> in Fortran, that correctly
receives the procedure pointer from C and is able to call it, is given
in the following <code>MODULE</code>:
</p>
<div class="smallexample">
<pre class="smallexample">MODULE m
  IMPLICIT NONE

  ! Define interface of call-back routine.
  ABSTRACT INTERFACE
    SUBROUTINE callback (x)
      USE, INTRINSIC :: ISO_C_BINDING
      REAL(KIND=C_DOUBLE), INTENT(IN), VALUE :: x
    END SUBROUTINE callback
  END INTERFACE

CONTAINS

  ! Define C-bound procedure.
  SUBROUTINE get_values (cproc) BIND(C)
    USE, INTRINSIC :: ISO_C_BINDING
    TYPE(C_FUNPTR), INTENT(IN), VALUE :: cproc

    PROCEDURE(callback), POINTER :: proc

    ! Convert C to Fortran procedure pointer.
    CALL C_F_PROCPOINTER (cproc, proc)

    ! Call it.
    CALL proc (1.0_C_DOUBLE)
    CALL proc (-42.0_C_DOUBLE)
    CALL proc (18.12_C_DOUBLE)
  END SUBROUTINE get_values

END MODULE m
</pre></div>

<p>Next, we want to call a C routine that expects a procedure pointer argument
and pass it a Fortran procedure (which clearly must be interoperable!).
Again, the C function may be:
</p>
<div class="smallexample">
<pre class="smallexample">int
call_it (int (*func)(int), int arg)
{
  return func (arg);
}
</pre></div>

<p>It can be used as in the following Fortran code:
</p>
<div class="smallexample">
<pre class="smallexample">MODULE m
  USE, INTRINSIC :: ISO_C_BINDING
  IMPLICIT NONE

  ! Define interface of C function.
  INTERFACE
    INTEGER(KIND=C_INT) FUNCTION call_it (func, arg) BIND(C)
      USE, INTRINSIC :: ISO_C_BINDING
      TYPE(C_FUNPTR), INTENT(IN), VALUE :: func
      INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
    END FUNCTION call_it
  END INTERFACE

CONTAINS

  ! Define procedure passed to C function.
  ! It must be interoperable!
  INTEGER(KIND=C_INT) FUNCTION double_it (arg) BIND(C)
    INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
    double_it = arg + arg
  END FUNCTION double_it

  ! Call C function.
  SUBROUTINE foobar ()
    TYPE(C_FUNPTR) :: cproc
    INTEGER(KIND=C_INT) :: i

    ! Get C procedure pointer.
    cproc = C_FUNLOC (double_it)

    ! Use it.
    DO i = 1_C_INT, 10_C_INT
      PRINT *, call_it (cproc, i)
    END DO
  END SUBROUTINE foobar

END MODULE m
</pre></div>

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Next: <a href="Further-Interoperability-of-Fortran-with-C.html#Further-Interoperability-of-Fortran-with-C" accesskey="n" rel="next">Further Interoperability of Fortran with C</a>, Previous: <a href="Interoperable-Subroutines-and-Functions.html#Interoperable-Subroutines-and-Functions" accesskey="p" rel="prev">Interoperable Subroutines and Functions</a>, Up: <a href="Interoperability-with-C.html#Interoperability-with-C" accesskey="u" rel="up">Interoperability with C</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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