Use Fortran subroutine in R? Undefined symbol

Question

This is a follow up to my previous question. I wrapped my Fortran code in a module and now it compiles when I run:

R CMD SHLIB ./Fortran/Fpi.f90

Answer 1

Aparently I cannot wrap the subroutines in a module. This fortran code is working:

subroutine dboard(darts, dartsscore)
  implicit none
  integer, intent(in)           :: darts
  double precision, intent(out) :: dartsscore
  double precision              :: x_coord, y_coord
  integer                       :: score, n

score = 0
do n = 1, darts
  call random_number(x_coord)
  call random_number(y_coord)

  if ((x_coord**2 + y_coord**2) <= 1.0d0) then
  score = score + 1
  end if
end do

dartsscore = 4.0d0*score/darts

end subroutine dboard

subroutine pi(avepi, DARTS, ROUNDS)
  implicit none
  double precision, intent(out)   ::  avepi
  integer, intent(in)             ::  DARTS, ROUNDS
  integer                         ::  MASTER, rank, i, n
  integer, allocatable            ::  seed(:)
  double precision                ::  pi_est, homepi, pirecv, pisum

interface 
   subroutine dboard(darts, dartsscore)
      implicit none
      integer, intent(in)           :: darts
      double precision, intent(out) :: dartsscore
   end subroutine dboard
end interface

! we set it to zero in the sequential run
rank = 0
! initialize the random number generator
! we make sure the seed is different for each task
call random_seed()
call random_seed(size = n)
allocate(seed(n))
seed = 12 + rank*11
call random_seed(put=seed(1:n))
deallocate(seed)

avepi = 0
do i = 0, ROUNDS-1
  call dboard(darts, pi_est)
  ! calculate the average value of pi over all iterations
  avepi = ((avepi*i) + pi_est)/(i + 1)
end do
end subroutine pi    

and this is the R code:

system("R CMD SHLIB ./Fortran/Fpi.f90")

mypi <- function(DARTS, ROUNDS) {
  dyn.load("./Fortran/Fpi.so")
  retvals <- .Fortran("pi", avepi = as.numeric(1), DARTS =  as.integer(DARTS), ROUNDS =  as.integer(ROUNDS))
  return(retvals$avepi)
}

mypi(DARTS = 50000, ROUNDS = 10)

Answer 2

Your problem comes down to the declaration of dboard:

   double precision                ::  pi_est, homepi, pirecv, pisum, dboard

Here you are saying that dboard is an external function, rather than a module procedure. This explains why there is a symbol dboard_ coming into play. You want to remove that:

double precision                ::  pi_est, homepi, pirecv, pisum

and instead rely, in pi on the module procedure-ness of dboard: pi already knows about it without this declaration.

Now, beyond that, because pi is in a module there is going to be some name mangling going on for that subroutine itself. I'd solve this problem by making pi itself a (C) interoperable procedure.

Module Fpi 
  IMPLICIT NONE
contains 
  subroutine pi(avepi, DARTS, ROUNDS) bind(C)
    use, intrinsic :: iso_c_binding, only : c_double, c_int
    real(c_double), intent(out)   ::  avepi
    integer(c_int), intent(in)    ::  DARTS, ROUNDS
    ...

and then using .C rather than .Fortran.

You can keep pi and dboard in the module, and this latter needn't even be interoperable.

Answer 3

Try to fix the name of the fortran call inside the R function. You typed "pi" where it should be "Fpi". Also, why not bring the function to your path instead of passing a long path inside the function?

mypi <- function(DARTS, ROUNDS) {
  dyn.load("./Fortran/Fpi.so")
  retvals <- .Fortran("Fpi", DARTS = as.integer(DARTS) , ROUNDS = as.integer(ROUNDS), answer = as.numeric(1))
  return(retvals$answer)
}

mypi(DARTS = 50000, ROUNDS = 10)