Issue with Fortran-C interoperability with -O3?

Hi,

I'm writing some FORTRAN code that calls a C library, and having issues when I compile the code with -O3. The code that is breaking is this:

Subroutine create_qsched_tasks(nr_threads)
Use, Intrinsic :: ISO_C_BINDING
Use cell_list_module
  Use quicksched
Implicit None

Integer, Intent(In) :: nr_threads
Integer(Kind=C_INT), Dimension(1:number_cells) :: cell_res, sorts
Integer(Kind=C_INT) :: i, j ,k, id, p, id2, temp
Integer(Kind=C_INT), Target :: data(0:10)
Type(C_PTR) :: dat_ptr

dat_ptr = C_LOC(data(0))

qsched = f_qsched_create()
call qsched_init(qsched,Int(nr_threads, C_INT), 0)
do i=1,number_cells
  cell_res(i) = qsched_addres(qsched, qsched_owner_none, qsched_res_none)
  print *, i
  data(0) = i
  sorts(i) = qsched_addtask(qsched, type_sort, 0, dat_ptr, int(c_sizeof(type_sort), C_INT), cell_list(i)%cell_count)
  call qsched_addlock(qsched, sorts(i), cell_res(i))
end do

The qsched_addtask function runs some code, but also contains 

 printf("data is %i, size is %i\n", *(int*)data, data_size);

where data is a void* and is argument 4, and data_size is argument 5. The output from this code is:

1
data is 0, size is 4
2
data is 0, size is 4
3
data is 0, size is 4

However with -O2 I get:

1
data is 1, size is 4
2
data is 2, size is 4
3
data is 3, size is 4
 

The function itself seems to work fine, as following this code I have another statement:

do i=0, x_cells+1
  do j=0, y_cells+1
    do k=0, z_cells+1
      id = 1+i+(x_cells+2)*(j+(y_cells+2)*k)
        data(0) = id
        temp = qsched_addtask(qsched, type_vdw_self, 0, dat_ptr, int(c_sizeof(type_sort),C_INT), cell_list(id)%cell_count)
        call qsched_addlock(qsched, temp, cell_res(id))
        call qsched_addunlock(qsched, sorts(id), temp)
...

which functions completely as expected, and the value stored in data(0) is passed correctly to the C library.

The interoperable function in question is as follows:

    Integer(Kind=C_INT) Function qsched_addtask(s, types, flags, data, data_size, cost) BIND(C)
      Use, Intrinsic :: ISO_C_BINDING
      Implicit None

      Type(C_PTR), VALUE :: s
      Integer(Kind=C_INT), Intent(In), VALUE :: types
      Integer(Kind=C_INT), Intent(In), VALUE :: flags
      Type(C_PTR), VALUE :: data
      Integer(Kind=C_INT), Intent(In), VALUE :: data_size
      Integer(Kind=C_INT), Intent(In), VALUE :: cost

    End Function

Am I doing something wrong that means the compiler thinks it can optimise away the value stored in data(0) in the first case?

Thanks

Aidan Chalk