Calling External C and C++ Libraries from Python

Last updated on 2025-03-31 | Edit this page

Overview

Questions

Which options are available to call from Python C and C++ libraries?
How does this work together with Numpy arrays?
How do I use this in multiple threads while lifting the GIL?

Objectives

Compile and link simple C programs into shared libraries.
Call these libraries from Python and time their executions.
Compare the performance with Numba-decorated Python code.
Bypass the GIL when calling these libraries from multiple threads simultaneously.

Calling C and C++ libraries

Simple example using either pybind11 or ctypes

External C and C++ libraries can be called from Python code using a number of options, e.g., Cython, CFFI, pybind11 and ctypes. We will discuss the last two because simple cases require the least amount of boilerplate. This may not be the case with more complex examples. Consider this simple C program, test.c, which adds up consecutive numbers:

C

#include <pybind11/pybind11.h>
namespace py = pybind11;

long long sum_range(long long high)
{
  long long i;
  long long s = 0LL;

  for (i = 0LL; i < high; i++)
      s += (long long)i;

  return s;
}

PYBIND11_MODULE(test_pybind, m) {
    m.doc() = "Export the sum_range function as sum_range";

    m.def("sum_range", &sum_range, "Adds upp consecutive integer numbers from 0 up to and including high-1");
}

You can easily compile and link it into a shared object (*.so) file with pybind11. You can install that in several ways, like pip; I prefer creating virtual environments using pipenv:

BASH

pip install pipenv
pipenv install pybind11
pipenv shell

c++ -O3 -Wall -shared -std=c++11 -fPIC `python3 -m pybind11 --includes` test.c -o test_pybind.so

which generates a shared object test_pybind.so, which can be called from a iPython shell as follows:

PYTHON

%import test_pybind
%sum_range=test_pybind.sum_range
%high=1000000000
%brute_force_sum=sum_range(high)

Now you might want to check and compare the output with the well-known formula for the sum of consecutive integers:

PYTHON

%sum_from_formula=high*(high-1)//2
%sum_from_formula
%difference=sum_from_formula-brute_force_sum
%difference

Give this script a suitable name, such as call_C_libraries.py. The same thing can be done using ctypes instead of pybind11, but the coding requires slightly more boilerplate on the Python side and slightly less on the C side. The program test.c will just contain the algorithm:

C

long long sum_range(long long high)
{
  long long i;
  long long s = 0LL;

  for (i = 0LL; i < high; i++)
      s += (long long)i;

  return s;
}

Compile and link with:

BASH

gcc -O3 -g -fPIC -c -o test.o test.c
ld -shared -o libtest.so test.o

which generates a libtest.so file.

You then need some extra boilerplate:

PYTHON

%import ctypes
%testlib = ctypes.cdll.LoadLibrary("./libtest.so")
%sum_range = testlib.sum_range
%sum_range.argtypes = [ctypes.c_longlong]
%sum_range.restype = ctypes.c_longlong
%high=1000000000
%brute_force_sum=sum_range(high)

Again, you can compare the result with the formula for the sum of consecutive integers:

PYTHON

%sum_from_formula=high*(high-1)/2
%sum_from_formula
%difference=sum_from_formula-brute_force_sum
%difference

Performance

Now we can time our compiled sum_range C library, e.g. from the iPython interface:

PYTHON

%timeit sum_range(10**7)

OUTPUT

2.69 ms ± 6.01 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

If you contrast with the Numba timing in Episode 3, you will see that the C library for sum_range is faster than the Numpy computation but significantly slower than the numba.jit-decorated function.

C versus Numba

Check if the Numba version of this conditional sum range function outperforms its C counterpart.

Inspired by a blog by Christopher Swenson.

C

long long conditional_sum_range(long long to)
{
  long long i;
  long long s = 0LL;

  for (i = 0LL; i < to; i++)
    if (i % 3 == 0)
      s += i;

  return s;
}

Show me the solution

Insert a line if i%3==0: in the code for sum_range_numba and rename it to conditional_sum_range_numba:

PYTHON

@numba.jit
def conditional_sum_range_numba(a: int):
    x = 0
    for i in range(a):
        if i%3==0:
            x += i
    return x

Let’s check how fast it runs:

%timeit conditional_sum_range_numba(10**7)

OUTPUT

8.11 ms ± 15.6 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

Compare this with the run time of the C code for conditional_sum_range. Compile and link in the usual way, assuming the file name is still test.c:

BASH

gcc -O3 -g -fPIC -c -o test.o test.c
ld -shared -o libtest.so test.o

Again, we can time our compiled conditional_sum_range C library, e.g. from the iPython interface:

PYTHON

import ctypes
testlib = ctypes.cdll.LoadLibrary("./libtest.so")
conditional_sum_range = testlib.conditional_sum_range
conditional_sum_range.argtypes = [ctypes.c_longlong]
conditional_sum_range.restype = ctypes.c_longlong
%timeit conditional_sum_range(10**7)

OUTPUT

7.62 ms ± 49.7 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

The C code is somewhat faster than the Numba-decorated Python code for this slightly more complicated example.

Passing Numpy arrays to C libraries

Now let us consider a more complex example. Instead of computing the sum of numbers up to an upper limit, let us compute that for an array of upper limits. This operation will return an array of sums. How difficult is it to modify our C and Python code to get this done? You just need to replace &sum_range with py::vectorize(sum_range):

C

PYBIND11_MODULE(test_pybind, m) {
    m.doc() = "pybind11 example plugin"; // optional module docstring

    m.def("sum_range", py::vectorize(sum_range), "Adds upp consecutive integer numbers from 0 up to and including high-1");
}

Now let’s see what happens if we pass to test_pybind.so an array instead of an integer.

The code:

PYTHON

%import test_pybind
%sum_range=test_pybind.sum_range
%ys=range(10)
%sum_range(ys)

gives

OUTPUT

array([ 0,  0,  1,  3,  6, 10, 15, 21, 28, 36])

It does not crash! You can check that the array is correct upon subtracting the previous sum from each sum (except the first):

PYTHON

%out=sum_range(ys)
%out[1:]-out[:-1]

which gives

OUTPUT

array([0, 1, 2, 3, 4, 5, 6, 7, 8])

which are the elements of ys except the last, as expected.

Call the C library from multiple threads simultaneously.

We can show that a C library compiled using pybind11 can be run as multithreaded. Try the following from an iPython shell:

PYTHON

%high=int(1e9)
%timeit(sum_range(high))

OUTPUT

274 ms ± 1.03 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

Now try a straightforward parallelization of 20 calls of sum_range over two threads, hence at 10 calls per thread. This should take about 10 * 274 ms = 2.74 s for a parallelization free of overheads. Running:

PYTHON

import threading as T
import time
def timer():
    start_time = time.time()
    for x in range(10):
        t1 = T.Thread(target=sum_range, args=(high,))
        t2 = T.Thread(target=sum_range, args=(high,))
        t1.start()
        t2.start()
        t1.join()
        t2.join()
    end_time = time.time()
    print(f"Time elapsed = {end_time-start_time:.2f}s")
timer()

gives

Time elapsed = 5.59s

i.e., more than twice the time we expected. In fact, the sum_range was run sequentially instead of parallelly. We then need to add a single declaration to test.c: py::call_guard<py::gil_scoped_release>():

C

PYBIND11_MODULE(test_pybind, m) {
    m.doc() = "pybind11 example plugin"; // optional module docstring

    m.def("sum_range", py::vectorize(sum_range), "Adds upp consecutive integer numbers from 0 up to and including high-1");
}

as follows:

C

PYBIND11_MODULE(test_pybind, m) {
    m.doc() = "pybind11 example plugin"; // optional module docstring

    m.def("sum_range", &sum_range, "A function which adds upp numbers from 0 up to and including high-1", py::call_guard<py::gil_scoped_release>());
}

Now compile again:

BASH

c++ -O3 -Wall -shared -std=c++11 -fPIC `python3 -m pybind11 --includes` test.c -o test_pybind.so

Import again the rebuilt shared object (only possible after quitting and relaunching the iPython interpreter), and time again.

This code:

PYTHON

import test_pybind
import time
import threading as T

sum_range=test_pybind.sum_range
high=int(1e9)

def timer():
    start_time = time.time()
    for x in range(10):
        t1 = T.Thread(target=sum_range, args=(high,))
        t2 = T.Thread(target=sum_range, args=(high,))
        t1.start()
        t2.start()
        t1.join()
        t2.join()
    end_time = time.time()
    print(f"Time elapsed = {end_time-start_time:.2f}s")
timer()

gives:

OUTPUT

Time elapsed = 2.81s

as you would expect for two sum_range modules running in parallel.

Key Points

Multiple options are available to call external C and C++ libraries, and the best choice depends on the complexity of your problem.
Obviously, there is an extra compile-and-link step, but the execution will be much faster than pure Python.
Also, the GIL will be circumvented in calling these libraries.
Numba might also offer you the speed-up you want with even less effort.