RFI: R to Modern Fortran Interface

This R package provides .ModernFortran(), an interface similar to .Fortran() but for Fortran 2018.

The 2018 Fortran language standard expanded support for interfacing C and Fortran. One of the additions is the introduction of C descriptors, a data structure for passing arrays between C and Fortran. This package takes advantage of that.

In contrast with .Fortran, R arrays are not passed as naked pointers, but as C descriptors that contain information about rank, shape, element size, type, and memory stride of the array that the Fortran routine can access directly. This means that additional arguments for passing the size or rank of arrays are no longer needed, which should lead to cleaner, simpler Fortran code. Additionally, logical and raw types are now supported directly.

Currently supported type conversions are:

R type	Fortran type
logical	logical(c_bool)
integer	integer(c_int)
double	real(c_double)
complex	complex(c_double_complex)
raw	integer(c_int8_t)

Installation

You can install RFI from github:

if(!requireNamespace("remotes")) install.packages("remote")
remotes::install_github("t-kalinowski/RFI")

Example

Say you have a Fortran 2018 module with a subroutine like so:

 module mod_cshift

   use iso_c_binding, only: c_int, c_double, c_double_complex, c_bool
   implicit none

contains

   subroutine my_subroutine(array, shift) bind(c)
      integer(c_int), intent(in out) :: array(:)
      integer(c_int), intent(in) :: shift

      array = cshift(array, shift)
   end subroutine my_subroutine

end module mod_cshift

(cshift by the way is Fortran intrinsic for circular shift)

From R you can compile it like so:

f_module_file <- "mod_my_fortran_module.f90"
so_file <- sub("f90$", "so", f_module_file)

compile_cmd <- sprintf(
  "gfortran -std=f2018 -shared -lgfortran -lm -lquadmath %s -o %s",
  f_module_file, so_file)

system(compile_cmd)

(you could also use the official R pathway to creating shared objects), with something like:

compile_cmd <- sprintf(
  "PKG_FFLAGS=-std=f2018 R CMD SHLIB %s -o %s",
  f_module_file, so_file)
system(compile_cmd)

Once the shared object is made, use it from R like so:

# load the fortran module
dll <- dyn.load(so_file)

# get C pointer to the Fortran subroutine
func_ptr <- getNativeSymbolInfo('my_subroutine', dll)$address

# call the subroutine with R arrays
RFI::.ModernFortran(func_ptr, array=1:5, shift=2L)
#> $array
#> [1] 3 4 5 1 2
#> 
#> $shift
#> [1] 2

Just like the other interfaces to compiled code (.Fortran,.Call, etc), you’ll probably want to wrap this in an R function for convince and input type coercion.

cshift <- function(array, shift) {
  RFI::.ModernFortran(func_ptr, 
                      array = as.integer(array), 
                      shift = as.integer(shift))$array
}

cshift(1:10, -3)
#>  [1]  8  9 10  1  2  3  4  5  6  7

For the most part, the interface to .ModernFortran tries to match that of .Fortran. One area where .ModernFortran deviates a little is in regards to duplicating objects. A mechanism is provided for selectively duplicating only some of the SEXP objects by passing an (0-based) integer vector of argument index positions to the DUP argument. For example, we know the subroutine above only modifies the first argument, array, and does not modify the second argument, shift. Accordingly, we can tell the R interface that only the first argument needs to be duplicated by passing 0L. (We include the typeof check to avoid duplicating if as.integer already duplicated)

cshift <- function(array, shift) {
  RFI::.ModernFortran(
    func_ptr,
    array = as.integer(array),
    shift = as.integer(shift),
    DUP = if(typeof(array) == "integer") 0L else FALSE
  )$array
}
cshift(1:10, 3)
#>  [1]  4  5  6  7  8  9 10  1  2  3

Tested with gfortran 9.2 and 9.3, R 3.6.3 and R 4.0.