Using FFTW3 plan for repeated use

Question

As the documentation says that fftw3 plan can be reused with different values if the array remains same. I'm using the following statement to create the plan p in Fortran.

call dfftw_plan_dft_r2c_1d(plan,N,fin,fout,FFTW_ESTIMATE)

However, the above statement is used through a subroutine away from the main program and will be repeatedly called with different values of fin with the same size. My present strategy is as follows:

subroutine fft_forward(fin,fout)
  implicit none
  integer(8),save :: plan
  integer,save    :: first=0
  double precision,dimension(:) :: fin
  double complex,dimension(:)   :: fout
  integer :: N
  
  N = size(fin)
  if (first == 0) then
    call dfftw_plan_dft_r2c_1d(plan,N,fin,fout,FFTW_ESTIMATE)
    first = 1
  endif
  call dfftw_execute_dft_r2c(plan,fin,fout)
  ...
  ...
end subroutine

Is the right way to do? I'm confused about saving the variable plan during repeated calling of the subroutine.