On 8/4/25 10:31 AM, Yuao Ma wrote:
Hi all,
I am trying to understand the effect of the lower argument in the c_f_pointer
procedure. If I am correct, should the following test case pass?
If FPTR is an array, its shape is specified by SHAPE; the lower
bounds are specified by LOWER if it is present, otherwise each lower bound is
equal to 1.
Keep in mind that Fortran arrays are stored in column-major order, meaning that
elements in the same column are stored next to each other in memory. A C ptr
does not carry this information, so this is why shape and lower bound are
needed. Fortran pointers are 'smart' in this way.
I think your test case should pass.
! { dg-do run }
program lower
use iso_c_binding
type(c_ptr) :: x
integer, target :: array(12)
integer, pointer :: ptr(:, :)
integer :: myshape(2)
integer :: mylower(2)
array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
x = c_loc(array)
myshape = [3, 4]
mylower = [2, 2]
call c_f_pointer(x, ptr, shape=myshape, lower=mylower)
if (any(lbound(ptr) /= [2, 2])) stop 1
if (any(ubound(ptr) /= [4, 5])) stop 2
if (any(shape(ptr) /= [3, 4])) stop 3
if (ptr(2, 2) /= 1) stop 4
if (ptr(4, 5) /= 12) stop 5
end program lower
Thanks,
Yuao
