Call fortran function from Python with ctypes

Question

I am looking to use ctypes to call some old fortran libraries which were written by my boss a few years ago. I followed the example given in this previous question, and I get the results as expected.

However, when I modify the code, to get slightly closer to the situation I face, so that

integer function addtwo(a, b)
  integer, intent(in) :: a, b
  addtwo = a + b
end function

becomes

real function addtwo(a, b)
  integer, intent(in) :: a, b
  addtwo = a + b
end function

i.e., the function is now real, not integer, the value returned is always 0. Can anyone explain what's going on and how I should get around this?

(PS. I'm using a 64-bit gfortran compiler on mac os snow leopard)

EDIT: The function I'm struggling with looks like:

real function ykr(seed)

  integer, intent(in) :: seed
  real ykr0
  ykr= real(seed)
end function

Really, ykr calls another function, ykr0, recursively, but since I'm struggling even with this basic aspect I'm ignoring that for now. I can't see what's different between this code and the above, but calling ykr_(byref(c_int(4))) returns 0, not 4 as expected...

Why aren't you using the Fortran FFI stuff that comes with NumPy? — Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams, Commented Mar 26, 2012 at 3:57
You mean f2py? It's causing a lot of headaches on my system and I decided this would be a good short-term fix. In theory! — samb8s
– samb8s, Commented Mar 26, 2012 at 3:59
that does, but implemented into main code (see edit), and I'm still not getting the expected result. I can't see any difference between those two functions, though. — samb8s
– samb8s, Commented Mar 26, 2012 at 4:16
this looks interesting docs.python.org/library/ctypes.html#return-types — Kyss Tao
– Kyss Tao, Commented Mar 26, 2012 at 4:26

Kyss Tao · Accepted Answer · 2012-03-26 04:34:00Z

5

Add the line

ykr_.restype = c_float

in your python code, before ykr_(byref(c_int(4))). This sets the return type for the function to float (or real in Fortan language).

In the original post, this was not necessary since int was assumed as default.

answered Mar 26, 2012 at 4:34

Kyss Tao

5314 silver badges13 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Call fortran function from Python with ctypes

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related