如何在Python中实现一个C接口并使用CFFI传递对象

Question

I would like to know how to use an existing C interface (a header file) and implement such an interface in Python by using the CFFI library.

For example, I have the following header file, interface.h :

void* foo(void* value);

And I want to implement it in Python. I thought the following program would do the work work, but it doesn't. It creates the function foo , but it doesn't guarantee that the implementation follows the structure defined in the header file.

import cffi

ffibuilder = cffi.FFI()

with open('interface.h') as f:
    data = ''.join([line for line in f if not line.startswith('#')])
    ffibuilder.embedding_api(data)

ffibuilder.set_source("_lib", r'''
    #include "interface.h"
''')

ffibuilder.embedding_init_code("""
    from _lib import ffi

     #include "interface.h"

    @ffi.def_extern()
    def foo(param):
        return __import__(param)

""")

ffibuilder.compile(verbose=True)

---

How to pass Python objects using CFFI?

In the above example, as a result of calling foo that returns a Python object. Then on the client-side, I have the following:

ffi = FFI()
# Load my lib file
lib = C.CDLL(dll)

h = lib.foo("math")

When I look up the value of h, it shows a number of type integer. However, I was expecting to receive an object. According to the interface, foo must return a pointer void* . How can I read that object from it? Am I doing something wrong?

Answer 1

To answer your last question: you are using ctypes , not cffi , in this example. These are different project. It should be apparent because the ffi object is from cffi , but is unused in the following lines. Instead of lib = C.CDLL() with I guess C being an alias to ctypes , you should call lib = ffi.dlopen("...") !

Answer 2

I found that the problem is that when switching from ctypes to CFFI for loading a DLL. The first requires the definition of the methods that are available in the dynamic library. Therefore, adding this fixes the problem

ffi.cdef("""
  void foo(void* value);
""")