Python Compile Mixed C and C++ Extension

Question

I'm attempting to compile libspng with PyBind11 so I can easily convert image data into a numpy array.

The compilation process requires compiling a few C files and then linking them to C++. However, I'm not sure how to do this with Python setuptools. I've so far been compiling all C++ or all C modules in my practice, but on clang, this results in problems when I mix the pybind11 file with the C files.

Is there a simple way that I'm missing to compile and link the C files to C++ within setup.py?

I tried building an extension for libspng, but I wasn't sure how to reference the compiled shared object file which was in the build folder.

Thanks!

Answer 1

In case it's helpful (and because I am procrastinating) I built a small demo based on the spng example

// pywrappers.cpp
#include <pybind11/pybind11.h>
#include <utility>
#include <cstdio>
extern "C" {
    #include <spng/spng.h>
}

namespace py = pybind11;

std::pair<size_t, size_t> get_size(const std::string& filename)
{
    FILE* fp = fopen(filename.c_str(), "rb");

    if (fp == nullptr)
    {
        fclose(fp);
        throw std::runtime_error("File not found");
    }

    spng_ctx* ctx = spng_ctx_new(0);

    spng_set_crc_action(ctx, SPNG_CRC_USE, SPNG_CRC_USE);

    size_t limit = 1024 * 1024 * 64;
    spng_set_chunk_limits(ctx, limit, limit);

    spng_set_png_file(ctx, fp);

    struct spng_ihdr ihdr;
    spng_get_ihdr(ctx, &ihdr);

    spng_ctx_free(ctx);
    fclose(fp);

    return std::make_pair<size_t, size_t>(ihdr.width, ihdr.height);
}


PYBIND11_MODULE(foo, m)
{
    m.def("get_size", &get_size);
}

# setup.py
from setuptools import setup, Extension
import pybind11

SPNG_SOURCE = './libspng-0.7.1' # Wherever you put the spng headers and .a

libfoo = Extension(
    'foo',
    sources=['pywrappers.cpp'],
    extra_compile_args = ['-std=c++14'],
    include_dirs=[SPNG_SOURCE, pybind11.get_include()],
    extra_link_args=['-lspng_static','-lz'],
    library_dirs=[SPNG_SOURCE]
)

if __name__ == "__main__":
    setup(
        name = 'foo',
        ext_modules=[libfoo],
    )

Build the extension as usual with e.g. python setup.py build_ext --inplace. Then for example

import foo
foo.get_size('stuff.png')