cffi: C Foreign Function Interface
Use cffi for more Pythonic C interop with inline C declarations.
cffi: C Foreign Function Interface is a free Python Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Python Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What Is cffi?
cffi (C Foreign Function Interface) lets you call C code from Python using C declarations rather than ctypes classes. It is more Pythonic and often safer.
# pip install cffi
from cffi import FFI
ffi = FFI()
ffi.cdef("double sqrt(double x);")
libm = ffi.dlopen(None) # current process
print(libm.sqrt(2.0)) # 1.4142...ABI vs API Mode
ffi.dlopen() is ABI mode — calls an existing shared library at runtime. API mode compiles a C wrapper and is faster and more robust.
from cffi import FFI
ffi = FFI()
# ABI mode (no compile step):
ffi.cdef("int abs(int x);")
libc = ffi.dlopen("libc.so.6") # Linux
print(libc.abs(-42)) # 42cdef Declarations
Use ffi.cdef() to declare C function signatures, structs, and typedefs that you want to use from Python.
from cffi import FFI
ffi = FFI()
ffi.cdef("""
typedef struct {
double x;
double y;
} Point;
double distance(Point a, Point b);
""")
# lib = ffi.dlopen("mylib.so")Creating C Structs
Use ffi.new() to allocate a C struct and set its fields from Python.
from cffi import FFI
ffi = FFI()
ffi.cdef("typedef struct { int x; int y; } Vec2;")
lib = ffi.dlopen(None)
v = ffi.new("Vec2 *")
v.x = 3
v.y = 4
print(v.x, v.y) # 3 4Strings and Buffers
Use ffi.new("char[]", n) for mutable buffers and ffi.string(ptr) to convert a C string pointer to Python bytes.
from cffi import FFI
ffi = FFI()
buf = ffi.new("char[]", b"hello world")
print(ffi.string(buf)) # b"hello world"
# Mutable buffer for output:
out = ffi.new("char[128]")
# c_func(out, 128) # fill the buffer
# result = ffi.string(out).decode()API Mode with verify
API mode compiles a small C wrapper module for better type safety and performance. Use ffi.set_source() and build offline.
from cffi import FFI
ffi = FFI()
ffi.cdef("int add(int a, int b);")
ffi.set_source("_mylib",
"""
int add(int a, int b) { return a + b; }
"""
)
# Build: python build_mylib.py
# from _mylib import ffi, lib
# print(lib.add(3, 4)) # 7Callbacks from C to Python
Use ffi.callback() to wrap a Python function as a C function pointer that can be passed to a C library.
from cffi import FFI
ffi = FFI()
ffi.cdef("typedef int (*compare_fn)(int, int);")
@ffi.callback("int(int, int)")
def compare(a, b):
return a - b
# pass compare to a C sort functionMemory Management
Objects allocated with ffi.new() are owned by Python and freed automatically. Use ffi.gc(ptr, destructor) to attach a custom destructor.
from cffi import FFI
ffi = FFI()
# ptr freed when Python GC collects it:
buf = ffi.new("int[10]")
buf[0] = 42
print(buf[0]) # 42
# Custom destructor:
# ptr = ffi.gc(lib.alloc(), lib.free)cffi vs ctypes
cffi uses C declaration strings (more natural for C developers) while ctypes uses Python classes. cffi's API mode is faster; ctypes requires no compilation.
# ctypes approach:
import ctypes
libc = ctypes.CDLL("libc.so.6")
libc.abs.restype = ctypes.c_int
libc.abs.argtypes = [ctypes.c_int]
print(libc.abs(-5)) # 5
# cffi approach:
from cffi import FFI; ffi = FFI()
ffi.cdef("int abs(int);")
lib = ffi.dlopen(None)
print(lib.abs(-5)) # 5Using cffi with NumPy
Pass NumPy array data directly to C via cffi using ffi.cast() and the array's ctypes.data pointer.
import numpy as np
from cffi import FFI
ffi = FFI()
arr = np.array([1.0, 2.0, 3.0])
ptr = ffi.cast("double *", arr.ctypes.data)
# Now ptr can be passed to a C function expecting double*Error Handling with cffi
C functions typically signal errors via return values. Check return values and use ffi.errno for POSIX error codes.
import errno as errno_mod
from cffi import FFI
ffi = FFI()
ffi.cdef("int open(const char *path, int flags);")
lib = ffi.dlopen(None)
fd = lib.open(b"/nonexistent", 0)
if fd == -1:
err = ffi.errno
raise OSError(err, "open failed")Quick Check
What method does cffi use to declare the C function signatures you want to call?
Recap
cffi uses ffi.cdef() to declare C signatures and ffi.dlopen() for ABI-mode library calls. API mode compiles a wrapper for better performance. Use ffi.new() for C memory and ffi.callback() for Python-to-C callbacks.
Frequently asked questions
Is the “cffi: C Foreign Function Interface” lesson free?
Yes — the full text of “cffi: C Foreign Function Interface” is free to read here on the web, and the Python Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Python Academy course, upgrade to CoddyKit PRO.
What will I learn in “cffi: C Foreign Function Interface”?
Use cffi for more Pythonic C interop with inline C declarations. You practise Python Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Python Academy?
No prior experience is required. Python Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “cffi: C Foreign Function Interface” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Python Academy lesson?
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All lessons in this course
- Why C Extensions? Use Cases and Trade-offs
- ctypes: Calling C Libraries from Python
- cffi: C Foreign Function Interface
- Writing a Python C Extension Module