call functions from a shared fortran library in python

Question

I would like to call some functions from a Fortran shared library in Python. I have found some links on the net and read them, and according what I found, I should do

libadd = cdll.LoadLibrary('./libbin.so') 

to load the shared object. However, this shared object includes some symbols from another shared library. I read the help of cdll however it does not seem possible to load several shared object files at the same time. How may I call functions from this Fortran library, which is most probably compiled by the Intel Fortran compiler?

Answer 1

You'll need to know the signatures of the functions in the shared object. Do you have the source code, or some reference which explains the function names and argument types?

For example, I have this source code (mult.f90):

integer function multiply(a, b)
    integer, intent(in) :: a, b
    multiply = a * b
end function multiply

.. and to demonstrate how you can load and use multiple shared objects at once, I also have (add.f90):

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

Compile, examine symbols:

% gfortran-4.4 -shared -fPIC -g -o mult.so mult.f90
% gfortran-4.4 -shared -fPIC -g -o add.so add.f90
% nm -ao mult.so | grep multiply
mult.so:00000000000005cc T multiply_

Notice the symbol name in the shared object has an underscore appended. Since I have the source, I know that the signature is multiply_(int *a, int *b), so it is easy to invoke that function from ctypes:

from ctypes import byref, cdll, c_int

mult = cdll.LoadLibrary('./mult.so')
add = cdll.LoadLibrary('./add.so')
a = c_int(2)
b = c_int(4)
print mult.multiply_(byref(a), byref(b))
print add.addtwo_(byref(a), byref(b))

Output:

8
6

Answer 2

I would add to @sameplebias answer, that one can use the iso_c_binding module to force (any) fortran compiler to produce the correct C signature. Example of usage:

module fmesh_wrapper

use iso_c_binding, only: c_double, c_int
use fmesh, only: mesh_exp

implicit none

contains

subroutine c_mesh_exp(r_min, r_max, a, N, mesh) bind(c)
real(c_double), intent(in) :: r_min
real(c_double), intent(in) :: r_max
real(c_double), intent(in) :: a
integer(c_int), intent(in) :: N
real(c_double), intent(out) :: mesh(N)
call mesh_exp(r_min, r_max, a, N, mesh)
end subroutine

! wrap more functions here
! ...

end module

this will have the following C signature:

void c_mesh_exp(double *r_min, double *r_max, double *a, int *N,
        double *mesh);

and then you can call it from Python as usual. The advantage of this approach is that it works on all platforms (without using any special compiler options).