Change in a Fortran array results in change of another when using C pointers

Question

I have C-type SIMD allocations using FFTW3 of two arrays f and g using the same plans. I have simply set f to 1, then g to f, then zeroed f. This also ends up zeroing g.

Why does this arise and what are some ways I can ensure that any derived arrays by slicing for example, don't get modified.

I would like f and g to point to two different blocks of two-dimensioanl double precision memory. I have not used the usual explicit shape (double precision, dimension(n,n)) definitions as the FFTW3 documentation states that arrays allocated in this manner are faster to deal with.

program main

use,intrinsic::iso_c_binding
implicit none
include 'fftw3.f03'

integer,parameter::n=16
real(C_DOUBLE),pointer::f(:,:),g(:,:)
type(C_PTR)::p

p=fftw_alloc_real(int(n**2,C_SIZE_T))
!i am thinking of these as FFTW plans that store only the stencil to
!allocate space starting from addresses given by real(C_DOUBLE),pointers above.

call c_f_pointer(p,f,[n,n])
call c_f_pointer(p,g,[n,n])

f=1.0d0
print*,sum(f)
g=f

f=0.0d0
print*,sum(g)

call fftw_free(p)
end program

The output is

256.00000000000000     
0.0000000000000000

You have set f and g to point to the same thing. Do you understand the effects of that (ie., how pointers in Fortran work)? — francescalus
– francescalus, Commented Jul 15, 2018 at 10:54
Does the use of the same plan p do this? I have used the array declaration statements ALLOCATABLE,DIMENSION() within fortran always. I do have a general knowledge of pointers but not specific to Fortran. — sai
– sai, Commented Jul 15, 2018 at 11:02
c_f_pointer does nothing special (ie., it knows nothing about FFTW plans). The two calls just point f and g (in this case) to the same target as the C pointer p. — francescalus
– francescalus, Commented Jul 15, 2018 at 11:04
Could you clarify, in the question (as an edit) what you want to happen? That is, how you intend to use the various memory aspects/pointers. As it is, we're probably guessing about what you mean by re-using FFTW plans (consider also the next reader with the same problem who isn't using FFTW but something else). — francescalus
– francescalus, Commented Jul 15, 2018 at 11:11

francescalus · Accepted Answer · 2018-07-15 11:54:49Z

2

The allocation of memory is done by fftw_alloc_real. You call that only once, so only one block of memory is allocated. c_f_pointer does not perform any allocation.

c_f_pointer takes a C pointer and pointer associates a Fortran pointer with the target of the C pointer. When you

call c_f_pointer(p,f,[n,n])
call c_f_pointer(p,g,[n,n])

you are associating both f and g with the same lump of memory, that pointed to by p.

Simply allocate two distinct parts, with two calls to fftw_alloc_real and point f to one and g to the other.

answered Jul 15, 2018 at 11:54

francescalus

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