使用CFFI的缓冲区协议

Question

我想公开一个对象的缓冲协议，就像在 Cython文档的示例中一样，但是我需要使用CFFI来做到这一点，而我找不到任何公开缓冲协议的示例。

Answer 1

My reading of the question is that you have some data you've got from a CFFI interface and want to expose it using the standard Python buffer protocol (which lots of C extensions use for quick access to array data).

The good news ffi.buffer() command (which, in fairness, I didn't know about until OP mentioned it!) exposes both a Python interface and the C-API side buffer protocol. It is restricted to viewing the data as an unsigned char/byte array though. Fortunately, using other Python objects (eg a memoryview it's possible to view it as other types).

Remainder of the post is an illustrative example:

# buf_test.pyx
# This is just using Cython to define a couple of functions that expect
# objects with the buffer protocol of different types, as an easy way
# to prove it works. Cython isn't needed to use ffi.buffer()!
def test_uchar(unsigned char[:] contents):
    print(contents.shape[0])
    for i in range(contents.shape[0]):
        contents[i]=b'a'

def test_double(double[:] contents):
    print(contents.shape[0])
    for i in range(contents.shape[0]):
        contents[i]=1.0

... and the Python file using cffi

import cffi
ffi = cffi.FFI()

data = ffi.buffer(ffi.new("double[20]")) # allocate some space to store data
         # alternatively, this could have been returned by a function wrapped
         # using ffi

# now use the Cython file to test the buffer interface
import pyximport; pyximport.install()
import buf_test

# next line DOESN'T WORK - complains about the data type of the buffer
# buf_test.test_double(obj.data) 

buf_test.test_uchar(obj.data) # works fine - but interprets as unsigned char

# we can also use casts and the Python
# standard memoryview object to get it as a double array
buf_test.test_double(memoryview(obj.data).cast('d'))