Fortran BIND(C) optional arguments

bind_c_optional.c

void do_the_thing(int*, double*, double*, double*);
extern const int blah;

int main(int argc, char **argv)
{
    int n = 5;
    double x[5] = {1, 2, 3, 4, 5};
    double y[5] = {10, 20, 30, 40, 50};
    double z[5];

    do_the_thing(&n, x, y, z);

    do_the_thing(&n, x, 0, z);

    return 0;
}

bind_c_optional.f90

module foo

  use iso_c_binding

  implicit none
  private

  public :: do_the_thing

  integer(c_int), public, bind(c) :: blah = 12

contains

  subroutine do_the_thing(n, x, y, z) bind(c)
    integer(c_int), intent(in) :: n
    real(c_double), dimension(n), intent(in) :: x
    real(c_double), dimension(n), intent(in), optional :: y
    real(c_double), dimension(n), intent(out) :: z


    if (present(y)) then
      print *, "Y is present"
    else
      print *, "Y is not present"
    end if

    z = x
    if (present(y)) then
      z = z + y
    end if
  end subroutine do_the_thing

end module foo

bind_c_optional.py

import cffi

ffi = cffi.FFI()
ffi.cdef("""
void do_the_thing(int*, float*, float*, float*);
extern const int blah;
""")

api = ffi.dlopen('./libfoo.so')

n = ffi.new('int[1]', [5])
x = ffi.new('float[]', [1, 2, 3, 4, 5])
y = ffi.new('float[]', [10, 20, 30, 40, 50])
z = ffi.new('float[]', 5)

api.do_the_thing(n, x, y, z)
api.do_the_thing(n, x, ffi.NULL, z)

print(api.blah)

howto.txt

$ gfortran bind_c_optional.f90 bind_c_optional.c
$ ./a.out
 Y is present
 Y is not present
12

$ gfortran -shared -fPIC -o libfoo.so bind_c_optional.f90
$ python bind_c_optional.py 
 Y is present
 Y is not present
Traceback (most recent call last):
  File "bind_c_optional.py", line 19, in <module>
    print(api.blah)
  File "/usr/lib/python2.7/dist-packages/cffi/api.py", line 460, in __getattr__
    make_accessor(name)
  File "/usr/lib/python2.7/dist-packages/cffi/api.py", line 456, in make_accessor
    make_accessor_locked(name)
  File "/usr/lib/python2.7/dist-packages/cffi/api.py", line 450, in make_accessor_locked
    raise AttributeError(name)
AttributeError: blah

$ nm libfoo.so | grep -E "blah|do_the_thing"
0000000000201048 D blah
00000000000007d5 T do_the_thing

$ gcc bind_c_optional.c -lfoo -L.
$ LD_LIBRARY_PATH=. ./a.out 
 Y is present
 Y is not present
12