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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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| 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> [<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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| <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 ("Number is %f.\n", x); |
| } |
| |
| /* Call Fortran routine and pass call-back to it. */ |
| void |
| foobar () |
| { |
| get_values (&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> [<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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