Interfacing Fortran, C, C++, and Python: Fortran, C and C++ talking to each other

Learning goals

• Calling Fortran libraries from C(++)
• Calling C(++) libraries from Fortran

Example

Imagine you are on a desert island and wish to compute $\pi$. You have a computer with you with compilers installed but no math libraries and no Wikipedia.

SPOILER BELOW

Here is one way of doing it - throwing darts (generating random points in an interval and checking whether the points are within a given radius):

Discuss briefly how you can approximate $\pi$ from such an experiment and how you would program it.

In this workshop we will implement this example in 3 different languages (C++, Fortran, Python) and demonstrate how to call this functionality across languages.

Later we will combine these 3 implementations in a Python package and learn few useful tricks.

Questions

• Have you ever tried to call C functions from Fortran or vice versa?
• What were the problems?
• What is name mangling? Related question: How does GitHub avoid name collisions between projects?
• How have you worked around the problem of name mangling?

Problems before iso_c_binding arrived

• Codes needed to work around name mangling.
• Codes needed to figure out bit representations of data types.

Exercise 3: testing the algorithm

Before you continue, create a fresh directory for this session.

Test the following Python code (Python newcomers please sit next to an experienced Python developer):

import random


def _distance_to_origin_squared(x, y):
    return x*x + y*y


def approximate_pi(num_points):
    random.seed(0)

    num_inside = 0
    for _ in range(num_points):
        x = random.uniform(0.0, 1.0)
        y = random.uniform(0.0, 1.0)
        if _distance_to_origin_squared(x, y) < 1.0:
            num_inside += 1

    # we multiply by 4 to get the full circle
    # from the 4 segments
    return 4.0*num_inside/float(num_points)

Questions

• Does it converge to the right number?
• Does it take longer when you increase num_points?
• Why is there no math.sqrt when computing distances?

Exercise 4: testing the isolated C++ and Fortran implementations

Let us now fetch an example from the web:

$ git clone --branch exercise/cxx-fortran https://github.com/bast/python-cffi-demo.git cxx-fortran

Let us inspect the file structure:

$ cd cxx-fortran

We see the following files:

.
|-- CMakeLists.txt
`-- island
    |-- main.cpp
    |-- main.f90
    |-- pi.cpp
    |-- pi.f90
    |-- pi.h
    `-- pi.py

Your task is to configure and build the code with:

$ mkdir build
$ cd build
$ cmake ..
$ make

• Test the binaries built under build/bin/.
• Also have a look at CMakeLists.txt which defines the configuration of the code and see if you can make sense of it.

Exercise 5: C++ and Fortran talking to each other

Your task now is to let C++ also call the Fortran implementation and to let Fortran also call the C++ implementation.

For this first uncomment the two following lines:

• island/main.f90
• island/main.cpp

Then try to recompile - you will observe that the code has unmet dependencies. Try to fix these (use out-commented code for hints). Finally, verify that the binaries work and that they indeed call both implementations.

Discuss with the group how it works.

Questions

• Why did we need to do more work on the Fortran side?
• How does C++ know the type information?