Passing arrays between languages

Fortran stores array elements in ascending storage units in column-major order. C stores array elements in row-major order. Fortran array indexes start at 1, while C array indexes start at 0.

The following example shows how a two-dimensional array that is declared by A(3,2) is stored in Fortran and C.

Table 1. Corresponding array layouts for Fortran and C. The Fortran array reference `A(X,Y,Z)` can be expressed in C as `a[Z-1][Y-1][X-1]`. Keep in mind that although C passes individual scalar array elements by value, it passes arrays by reference.
	Fortran Element Name	C Element Name
Lowest storage unit	A(1,1)	A[0][0]
	A(2,1)	A[0][1]
	A(3,1)	A[1][0]
	A(1,2)	A[1][1]
	A(2,2)	A[2][0]
Highest storage unit	A(3,2)	A[2][1]

To pass all or part of a Fortran array to another language, you can use Fortran 90/Fortran 95 array notation:

REAL, DIMENSION(4,8) :: A, B(10)

! Pass an entire 4 x 8 array.
CALL CFUNC( A )
! Pass only the upper-left quadrant of the array.
CALL CFUNC( A(1:2,1:4) )
! Pass an array consisting of every third element of A.
CALL CFUNC( A(1:4:3,1:8) )
! Pass a 1-dimensional array consisting of elements 1, 2, and 4 of B.
CALL CFUNC( B( (/1,2,4/) ) )

Where necessary, the Fortran program constructs a temporary array and copies all the elements into contiguous storage. In all cases, the C routine needs to account for the column-major layout of the array.

Any array section or noncontiguous array is passed as the address of a contiguous temporary unless an explicit interface exists where the corresponding dummy argument is declared as an assumed-shape array or a pointer. To avoid the creation of array descriptors (which are not supported for interlanguage calls) when calling non-Fortran procedures with array arguments, either do not give the non-Fortran procedures any explicit interface, or do not declare the corresponding dummy arguments as assumed-shape or pointers in the interface:

! This explicit interface must be changed before the C function
! can be called.
INTERFACE
  FUNCTION CFUNC (ARRAY, PTR1, PTR2)
    INTEGER, DIMENSION (:) :: ARRAY          ! Change this : to *.
    INTEGER, POINTER, DIMENSION (:) :: PTR1  ! Change this : to *
                                             ! and remove the POINTER
                                             ! attribute.
    REAL, POINTER :: PTR2                    ! Remove this POINTER
                                             ! attribute or change to TARGET.
  END FUNCTION
END INTERFACE