I’m encountering a problem, which I am not being able to solve despite trying several options. The problem goes like this.

I have to carry a FFT forward transformation for a Poisson solver, which I’m doing with the fftw3 routines. I’m using FORTRAN90 in a Mac with gfortran. The fft routine goes as follows:

    subroutine fft_forward(f,fout)
      use,intrinsic :: iso_c_binding
      implicit none
      include 'fftw3.f03'

      double precision,dimension(:) :: f,fout
      integer :: J,N,k,nn
      double complex,allocatable :: ffout(:)
      double precision,allocatable :: ff(:)
      integer(8) :: p

      ! Find J. Declare local arrays.
      J = size(f)-1
      N = 2*J
      allocate(ff(N),ffout(N))

      ! Load and extend data 
      ff(1) = 0.0
      ff(J+1) = 0.0
      ff(2:J) = f(2:J)
      do k = 2,J
        ff(2*(J+1)-k) = -f(k)
      enddo

      call dfftw_plan_dft_r2c_1d(p,N,ff,ffout,FFTW_ESTIMATE)
      call dfftw_execute_dft_r2c(p,ff,ffout)
      call dfftw_destroy_plan (p) 

      ! Unload data
      fout(1) = 0.0; fout(J+1) = 0.0
      fout(2:J) = - 2.0*ffout(2:J)%im

      ! Normalize data
      fout = fout/(2.0*J)
    end subroutine fft_forward

I also have a sample driver program, which goes like this:

    program driver
      implicit none
      double precision,dimension(:),allocatable :: inarray,outarray
      integer :: J,N
      interface
        subroutine fft_forward(f,fout)
        use,intrinsic :: iso_c_binding
        implicit none
        include 'fftw3.f03'
        double precision :: f(:),fout(:)
      end subroutine fft_forward
    end interface

    ! Declare local arrays.
    J = 100
    allocate(inarray(J),outarray(J))

    ! Here goes the assignment of inarray
    call random_number(inarray)

    call fft_forward(inarray,outarray)
    print*,outarray
  end program driver

As one can check, the driver compiled standalone with the fft program produces the right results. However, when I incoporate the same fft routine with my larger program, it indicates a

    Program received signal SIGSEGV: Segmentation fault - invalid memory reference.

I have not beein able to detect the error at all! All arrays are declared in a compatibel way. The compilation command line is

    gfortran -I/opt/local/include -L/opt/local/lib -lfftw3 -lm

The fftw3 package is instlled through configure-make system and installed in the dir /opt/local. My question is whether it is speciifc to a Mac machine or something drastically wrong with my code.

Madhurjya