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cffi

Learn to call C code with cffi.

What is cffi?

cffi (C Foreign Function Interface) is an alternative to ctypes. It lets you write C header declarations directly in Python.

You create an FFI object, declare C signatures with ffi.cdef, then load the library with ffi.dlopen. Two advantages over ctypes: signatures are written in C syntax (more natural) and type checking happens at parse time. cffi is also faster than ctypes on PyPy.

C code with cffi

Using cffi takes three steps: create FFI(), declare the signature with ffi.cdef('int abs(int);'), load the library with lib = ffi.dlopen(None).

You then call the function with lib.abs(-9). Since the signature is in C syntax it is easy to copy from header files. Errors in cdef are caught at parse time; there are no runtime crashes like with ctypes. This makes cffi safer for large C APIs.

Practice Call strlen with cffi

Import with from cffi import FFI. Create an FFI() object. Declare the signature with ffi.cdef('size_t strlen(const char*);'). Load the library with ffi.dlopen(None). Print the result of lib.strlen(b"dunya").

Starter code

main.py
# call a C function with cffi
# print the result

Sample solution

main.py
from cffi import FFI

ffi = FFI()
ffi.cdef("size_t strlen(const char*);")
lib = ffi.dlopen(None)
print(lib.strlen(b"dunya"))

Expected output

5

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Match cffi concepts

Term Meaning
FFI Interface object
cdef C declaration
dlopen Loads a library
cffi ctypes alternative

2 What does cffi provide?

main.py
from cffi import FFI
ffi = FFI()
ffi.cdef('int abs(int);')
  • Running SQL from Python
  • Writing C declarations in Python Correct answer
  • Generating web pages
  • File compression

Explanation: answer: writing C declarations in Python. cffi is designed for C interfaces.

3 Complete the cffi call

Question 
ffi = FFI()
ffi.____('int abs(int);')
Answer 
ffi = FFI()
ffi.cdef('int abs(int);')

Explanation: cdef declares a C signature.

All lessons in Expert

1 Metaprogramming

  1. type()
  2. Metaclass
  3. __init_subclass__
  4. __class_getitem__
  5. Decorate class
  6. Summary - Metaprogramming

2 Descriptors

  1. Descriptor protocol
  2. Data descriptor
  3. Non-data descriptor
  4. @property
  5. __set_name__
  6. Summary - Descriptors

3 AST

  1. ast.parse
  2. ast.walk
  3. NodeVisitor
  4. NodeTransformer
  5. compile
  6. Summary - AST

4 ctypes and CFFI

  1. ctypes.CDLL
  2. ctypes types
  3. argtypes and restype
  4. cffi
  5. Structure
  6. Summary - ctypes and CFFI

5 Multiprocessing

  1. Process
  2. start and join
  3. Queue
  4. Pool
  5. Manager
  6. Summary - Multiprocessing

6 Threading and asyncio

  1. threading.Thread
  2. Lock
  3. GIL
  4. concurrent.futures
  5. asyncio vs threading
  6. Summary - Threading and asyncio

7 Performance

  1. timeit
  2. cProfile
  3. sys.getsizeof
  4. slots and tuple
  5. numpy vectorization
  6. Summary - Performance

8 Expert final

  1. Project design
  2. Metaprogramming
  3. Multiprocessing
  4. ctypes
  5. Putting it together