build.py

from cffi import FFI
ffibuilder = FFI()

ffibuilder.set_source("_square",
   r"""
   #include "square.h"
   """,
   sources=["square.c"])

with open("square.h") as f:
    ffibuilder.cdef(f.read())

if __name__ == "__main__":
    ffibuilder.compile(verbose=True)

parse_c_declares.py

def get_c_declares(filename, fake_include):
    from pycparser import parse_file, c_ast, c_generator

    folder = path.dirname(filename)
    ast = parse_file(filename, use_cpp=True, cpp_path="gcc",
                     cpp_args=["-E", "-I{}".format(folder), "-I{}".format(fake_include), "-DSIM_DEBUG"])
    declares = []
    generator = c_generator.CGenerator()

    for child in ast.ext:
        if isinstance(child, c_ast.FuncDef):
            continue
        declares.append(generator.visit(child))
    code = ";\n".join(declares) + ";"
    return code

square.c

#include "square.h"

float glb_f;

float calc_area(sq a) {
    float s;
    s = a->length * a->width + glb_f;
    return s;
}

square.h

struct Square {
    float length;
    float width;
};

typedef struct Square *sq;
extern float glb_f;
float calc_area(sq a);

test.py

import _square

ffi = _square.ffi
lib = _square.lib

lib.glb_f = 10.0
rect = ffi.new("sq", {"length":10, "width":20})
rect.width = 30

print(lib.calc_area(rect))