Standard Fortran and C Interoperability

The Intel® Fortran Compiler supports the Fortran standardized mechanism for allowing Fortran code to reliably communicate (or interoperate) with C code.

This mechanism includes a group of features for C interoperability, enabling mixed-language programming in a more portable manner.

The Fortran standard discusses interoperability in terms of a "companion C processor." Each Fortran implementation is free to choose which C is its companion. Although the standard explicitly specifies a companion C (not C++) processor, you can use C++, as long as you use features that are compatible with C when interoperating with Fortran.

For Intel® Fortran, the supported C companion is the Intel® C++ Compiler or the Microsoft* Visual C++* Compiler on Windows*, and the Intel® C++ Compiler or gcc* on Linux* and OS X*.

The core principle of interoperability is that something should work the same way in Fortran as it does in C. In terms of interoperability, the following applies:

Fortran provides a way to reference procedures that are defined by the C programming language or procedures that are described by C prototypes, even if they are not actually defined by means of C.
Conversely, a procedure defined by a Fortran subprogram can be referenced by a function defined by means of C.
In addition, you can define global variables that are associated with C variables whose names have external linkage.
You can also declare Fortran variables, data structures and enumerations that correspond to similar declarations in C.

The following sections describe interoperability requirements for types, variables, procedures, and global data.

Example of Fortran Calling C

The following example calls a C function.

C Function Prototype Example
int C_Library_Function(void* sendbuf, int sendcount, int *recvcounts);

Fortran Module Example
`module ftn_C_2 interface integer (C_INT) function C_Library_Function & (sendbuf, sendcount, recvcounts) & BIND(C, name='C_Library_Function’) use, intrinsic :: ISO_C_BINDING implicit none type (C_PTR), value :: sendbuf integer (C_INT), value :: sendcount type (C_PTR), value :: recvcounts end function C_Library_Function end interface end module ftn_C_2`

Fortran Module Example

module ftn_C_2
   interface 
     integer (C_INT) function C_Library_Function & 
     (sendbuf, sendcount, recvcounts) & 
        BIND(C, name='C_Library_Function’) 
        use, intrinsic :: ISO_C_BINDING  
        implicit none 
        type (C_PTR), value :: sendbuf 
        integer (C_INT), value :: sendcount 
        type (C_PTR), value :: recvcounts 
     end function C_Library_Function 
   end interface 
end module ftn_C_2

Fortran Calling Sequence Example
`use, intrinsic :: ISO_C_BINDING, only: C_INT, C_FLOAT, C_LOC use ftn_C_2 ... real (C_FLOAT), target :: send(100) integer (C_INT) :: sendcount integer (C_INT), ALLOCATABLE, target :: recvcounts(100) ... ALLOCATE( recvcounts(100) ) ... call C_Library_Function(C_LOC(send), sendcount, & C_LOC(recvcounts)) ...`

Fortran Calling Sequence Example

use, intrinsic :: ISO_C_BINDING, only: C_INT, C_FLOAT, C_LOC 
use ftn_C_2 
...
real (C_FLOAT), target :: send(100) 
integer (C_INT) :: sendcount 
integer (C_INT), ALLOCATABLE, target :: recvcounts(100) 
... 
ALLOCATE( recvcounts(100) ) 
... 
call C_Library_Function(C_LOC(send), sendcount, & 
C_LOC(recvcounts)) 
...

Example of C Calling Fortran

The following example calls a Fortran subroutine called Simulation. This subroutine corresponds to the C void function simulation.

Fortran Code Example
subroutine Simulation(alpha, beta, gamma, delta, arrays) BIND(C) use, intrinsic :: ISO_C_BINDING implicit none integer (C_LONG), value :: alpha real (C_DOUBLE), intent(inout) :: beta integer (C_LONG), intent(out) :: gamma real (C_DOUBLE),dimension(*),intent(in) :: delta type, BIND(C) :: pass integer (C_INT) :: lenc, lenf type (C_PTR) :: c, f end type pass type (pass), intent(inout) :: arrays real (C_FLOAT), ALLOCATABLE, target, save :: eta(:) real (C_FLOAT), pointer :: c_array(:) ... ! Associate c_array with an array allocated in C call C_F_POINTER (arrays%c, c_array, (/arrays%lenc/) ) ... ! Allocate an array and make it available in C arrays%lenf = 100 ALLOCATE (eta(arrays%lenf)) arrays%f = c_loc(eta) ... end subroutine Simulation

Fortran Code Example

subroutine Simulation(alpha, beta, gamma, delta, arrays) BIND(C) 
   use, intrinsic :: ISO_C_BINDING 
   implicit none 
   integer (C_LONG), value :: alpha 
   real (C_DOUBLE), intent(inout) :: beta 
   integer (C_LONG), intent(out) :: gamma 
   real (C_DOUBLE),dimension(*),intent(in) :: delta
   type, BIND(C) :: pass 
      integer (C_INT) :: lenc, lenf
      type (C_PTR) :: c, f 
   end type pass 
   type (pass), intent(inout) :: arrays
   real (C_FLOAT), ALLOCATABLE, target, save :: eta(:)
   real (C_FLOAT), pointer :: c_array(:) 
   ... 
   ! Associate c_array with an array allocated in C 
   call C_F_POINTER (arrays%c, c_array, (/arrays%lenc/) ) 
   ... 
   ! Allocate an array and make it available in C 
   arrays%lenf = 100 
   ALLOCATE (eta(arrays%lenf)) 
   arrays%f = c_loc(eta) 
   ... 
end subroutine Simulation

C Struct declaration Example
struct pass {int lenc, lenf; float c, f;};

C Function Prototype Example
void simulation(long alpha, double beta, long gamma, double delta[], struct pass *arrays);

C Calling sequence Example
simulation(alpha, &beta, &gamma, delta, &arrays);