How to shift zero frequency with 2d fftw data

Hi,gays! I do  SFFTW_PLAN_DFT_R2C_2D(FFTW_PLAN,nx,ny,density,fft,FFTW_ESTIMATE) and then get a complex array fft(ny/2+1,nx)! I try to shift the  zero frequency to the center but failed! Who can tell me how to  shift zero frequency on  fft(ny/2+1,nx)?

Thanks!