I want to expose a buffer protocol for a object, just like in this example of the Cython documentation, however I need to do this using CFFI and I wasn't able to find any examples to expose a buffer protocol.
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 (e.g. 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'))