FFTW: Trouble with real to complex and complex to real 2D tranfsorms

As the title states I'm using FFTW (version 3.2.2) with Fortran 90/95 to perform a 2D FFT of real data (actually a field of random numbers). I think the forward step is working (at least I am getting some ouput). However I wanted to check everything by doing the IFFT to see if I can re-construct the original input. Unfortunately when I call the complex to real routine, nothing happens and I obtain no error output, so I'm a bit confused. Here are some code snippets:

implicit none

include "fftw3.f"

! - im=501, jm=401, and lm=60

real*8    :: u(im,jm,l开发者_如何学运维m),recov(im,jm,lm)
complex*8 :: cu(1+im/2,jm)
integer*8 :: planf,planb    
real*8    :: dv

! - Generate array of random numbers
dv=4.0
call random_number(u)
u=u*dv
recov=0.0

k=30

! - Forward step (FFT)

call dfftw_plan_dft_r2c_2d(planf,im,jm,u(:,:,k),cu,FFTW_ESTIMATE)
call dfftw_execute_dft_r2c(planf,u(:,:,k),cu)
call dfftw_destroy_plan(planf)

! - Backward step (IFFT)

call dfftw_plan_dft_c2r_2d(planb,im,jm,cu,recov(:,:,k),FFTW_ESTIMATE)
call dfftw_execute_dft_c2r(planb,cu,recov(:,:,k))
call dfftw_destroy_plan(planb)

The above forward step seems to work (r2c) but the backward step does not seem to work. I checked this by differencing the u and recov arrays - which ended up not being zero. Additionally the max and min values of the recov array were both zero, which seems to indicate that nothing was changed.

I've looked around the FFTW documentation and based my implementation on the following page http://www.fftw.org/fftw3_doc/Fortran-Examples.html#Fortran-Examples . I am wondering if the problem is related to indexing, at least that's the direction I am leaning. Anyway, if any one could offer some help, that would be wonderful!

Thanks!

Not sure if this is the root of all troubles here, but the way you declare variables may be the culprit.

For most compilers (this is apparently not even a standard), Complex*8 is an old syntax for single precision: the complex variable occupies a total of 8 bytes, shared between the real and the imaginary part (4+4 bytes).

[Edit 1 following Vladimir F comment to my answer, see his link for details:] In my experience (i.e. the systems/compiler I ever used), Complex(Kind=8) corresponds to the declaration of a double precision complex number (a real and an imaginary part, both of which occupy 8 bytes).

On any system/compiler, Complex(Kind=Kind(0.d0)) should declare a double precision complex.

In short, your complex array does not have the right size. Replace occurences of Real*8 and Complex*8 by Real(kind=8) and Complex(Kind=8) (or Complex(Kind=kind(0.d0)) for a better portability), respectively.