gfortran.texi (Interoperability with C): Fix ordering in menu and add new subsection about pointers.
2010-08-10 Daniel Kraft <d@domob.eu> * gfortran.texi (Interoperability with C): Fix ordering in menu and add new subsection about pointers. (Interoperable Subroutines and Functions): Split off the pointer part. (working with Pointers): New subsection with extended discussion of pointers (especially procedure pointers). From-SVN: r163073
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Daniel Kraft
Daniel Kraft
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@ -1,3 +1,11 @@
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2010-08-10 Daniel Kraft <d@domob.eu>
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* gfortran.texi (Interoperability with C): Fix ordering in menu
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and add new subsection about pointers.
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(Interoperable Subroutines and Functions): Split off the pointer part.
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(working with Pointers): New subsection with extended discussion
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of pointers (especially procedure pointers).
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2010-08-09 Thomas Koenig <tkoenig@gcc.gnu.org>
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PR fortran/44235
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@ -1933,10 +1933,11 @@ and their use is highly recommended.
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@menu
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* Intrinsic Types::
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* Further Interoperability of Fortran with C::
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* Derived Types and struct::
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* Interoperable Global Variables::
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* Interoperable Subroutines and Functions::
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* Working with Pointers::
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* Further Interoperability of Fortran with C::
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@end menu
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Since Fortran 2003 (ISO/IEC 1539-1:2004(E)) there is a
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@ -2059,7 +2060,8 @@ matches the Fortran declaration
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integer(c_int) :: j
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@end smallexample
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Note that pointer arguments also frequently need the @code{VALUE} attribute.
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Note that pointer arguments also frequently need the @code{VALUE} attribute,
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see @ref{Working with Pointers}.
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Strings are handled quite differently in C and Fortran. In C a string
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is a @code{NUL}-terminated array of characters while in Fortran each string
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@ -2096,7 +2098,7 @@ literal has the right type; typically the default character
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kind and @code{c_char} are the same and thus @code{"Hello World"}
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is equivalent. However, the standard does not guarantee this.
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The use of pointers is now illustrated using the C library
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The use of strings is now further illustrated using the C library
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function @code{strncpy}, whose prototype is
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@smallexample
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@ -2128,8 +2130,13 @@ example, we ignore the return value:
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end
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@end smallexample
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C pointers are represented in Fortran via the special derived type
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@code{type(c_ptr)}, with private components. Thus one needs to
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The intrinsic procedures are described in @ref{Intrinsic Procedures}.
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@node Working with Pointers
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@subsection Working with Pointers
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C pointers are represented in Fortran via the special opaque derived type
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@code{type(c_ptr)} (with private components). Thus one needs to
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use intrinsic conversion procedures to convert from or to C pointers.
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For example,
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@ -2147,14 +2154,131 @@ For example,
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@end smallexample
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When converting C to Fortran arrays, the one-dimensional @code{SHAPE} argument
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has to be passed. Note: A pointer argument @code{void *} matches
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@code{TYPE(C_PTR), VALUE} while @code{TYPE(C_PTR)} matches @code{void **}.
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has to be passed.
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If a pointer is a dummy-argument of an interoperable procedure, it usually
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has to be declared using the @code{VALUE} attribute. @code{void*}
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matches @code{TYPE(C_PTR), VALUE}, while @code{TYPE(C_PTR)} alone
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matches @code{void**}.
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Procedure pointers are handled analogously to pointers; the C type is
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@code{TYPE(C_FUNPTR)} and the intrinsic conversion procedures are
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@code{C_F_PROC_POINTER} and @code{C_FUNLOC}.
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@code{C_F_PROCPOINTER} and @code{C_FUNLOC}.
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The intrinsic procedures are described in @ref{Intrinsic Procedures}.
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Let's consider two examples of actually passing a procedure pointer from
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C to Fortran and vice versa. Note that these examples are also very
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similar to passing ordinary pointers between both languages.
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First, consider this code in C:
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@smallexample
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/* Procedure implemented in Fortran. */
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void get_values (void (*)(double));
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/* Call-back routine we want called from Fortran. */
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void
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print_it (double x)
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@{
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printf ("Number is %f.\n", x);
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@}
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/* Call Fortran routine and pass call-back to it. */
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void
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foobar ()
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@{
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get_values (&print_it);
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@}
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@end smallexample
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A matching implementation for @code{get_values} in Fortran, that correctly
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receives the procedure pointer from C and is able to call it, is given
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in the following @code{MODULE}:
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@smallexample
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MODULE m
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IMPLICIT NONE
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! Define interface of call-back routine.
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ABSTRACT INTERFACE
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SUBROUTINE callback (x)
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USE, INTRINSIC :: ISO_C_BINDING
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REAL(KIND=C_DOUBLE), INTENT(IN), VALUE :: x
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END SUBROUTINE callback
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END INTERFACE
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CONTAINS
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! Define C-bound procedure.
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SUBROUTINE get_values (cproc) BIND(C)
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USE, INTRINSIC :: ISO_C_BINDING
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TYPE(C_FUNPTR), INTENT(IN), VALUE :: cproc
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PROCEDURE(callback), POINTER :: proc
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! Convert C to Fortran procedure pointer.
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CALL C_F_PROCPOINTER (cproc, proc)
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! Call it.
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CALL proc (1.0_C_DOUBLE)
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CALL proc (-42.0_C_DOUBLE)
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CALL proc (18.12_C_DOUBLE)
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END SUBROUTINE get_values
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END MODULE m
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@end smallexample
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Next, we want to call a C routine that expects a procedure pointer argument
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and pass it a Fortran procedure (which clearly must be interoperable!).
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Again, the C function may be:
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@smallexample
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int
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call_it (int (*func)(int), int arg)
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@{
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return func (arg);
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@}
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@end smallexample
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It can be used as in the following Fortran code:
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@smallexample
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MODULE m
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USE, INTRINSIC :: ISO_C_BINDING
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IMPLICIT NONE
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! Define interface of C function.
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INTERFACE
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INTEGER(KIND=C_INT) FUNCTION call_it (func, arg) BIND(C)
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USE, INTRINSIC :: ISO_C_BINDING
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TYPE(C_FUNPTR), INTENT(IN), VALUE :: func
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INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
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END FUNCTION call_it
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END INTERFACE
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CONTAINS
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! Define procedure passed to C function.
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! It must be interoperable!
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INTEGER(KIND=C_INT) FUNCTION double_it (arg) BIND(C)
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INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
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double_it = arg + arg
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END FUNCTION double_it
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! Call C function.
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SUBROUTINE foobar ()
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TYPE(C_FUNPTR) :: cproc
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INTEGER(KIND=C_INT) :: i
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! Get C procedure pointer.
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cproc = C_FUNLOC (double_it)
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! Use it.
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DO i = 1_C_INT, 10_C_INT
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PRINT *, call_it (cproc, i)
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END DO
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END SUBROUTINE foobar
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END MODULE m
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@end smallexample
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@node Further Interoperability of Fortran with C
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@subsection Further Interoperability of Fortran with C
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