ctypes: Calling C Libraries from Python
Load shared libraries and call C functions using ctypes.
ctypes: Calling C Libraries from Python is a free Python Academy lesson on CoddyKit. This is lesson 2 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, and your progress syncs across the web and the CoddyKit app. The Python Academy course includes 4 lessons in total.
What Is ctypes?
ctypes is a standard-library module for loading shared C libraries and calling their functions from Python — no compilation required.
import ctypes
# Load the C standard library
libc = ctypes.CDLL("libc.so.6") # Linux
# libc = ctypes.CDLL("libSystem.dylib") # macOS
libc.puts(b"Hello from C!")Loading Libraries
Use ctypes.CDLL for cdecl calling convention (most C libraries) and WinDLL for Windows stdcall APIs.
import ctypes
# Platform-independent approach:
import ctypes.util
libm_name = ctypes.util.find_library("m")
libm = ctypes.CDLL(libm_name)
result = libm.sqrt
result.restype = ctypes.c_double
result.argtypes = [ctypes.c_double]
print(result(2.0)) # 1.4142...C Data Types
Map Python types to C types using ctypes classes: c_int, c_double, c_char_p, c_void_p, etc.
import ctypes
x = ctypes.c_int(42)
print(x.value) # 42
d = ctypes.c_double(3.14)
print(d.value) # 3.14
s = ctypes.c_char_p(b"hello")
print(s.value) # b"hello"Setting argtypes and restype
Always declare argtypes (parameter types) and restype (return type) to avoid silent crashes from type mismatches.
import ctypes, ctypes.util
libm = ctypes.CDLL(ctypes.util.find_library("m"))
libm.pow.argtypes = [ctypes.c_double, ctypes.c_double]
libm.pow.restype = ctypes.c_double
print(libm.pow(2.0, 10.0)) # 1024.0Passing Pointers
Use ctypes.byref(var) or ctypes.pointer(var) to pass a pointer to a C variable.
import ctypes
libc = ctypes.CDLL(None) # None = current process
value = ctypes.c_int(0)
# Passing pointer to C function:
# libc.some_func(ctypes.byref(value))
print(value.value)Structs
Define C structs by subclassing ctypes.Structure and listing _fields_.
import ctypes
class Point(ctypes.Structure):
_fields_ = [
("x", ctypes.c_double),
("y", ctypes.c_double),
]
p = Point(1.0, 2.0)
print(p.x, p.y) # 1.0 2.0Arrays
Create C arrays with TypeClass * N.
import ctypes
IntArray5 = ctypes.c_int * 5
arr = IntArray5(10, 20, 30, 40, 50)
for v in arr:
print(v, end=" ") # 10 20 30 40 50Callbacks (Function Pointers)
Use ctypes.CFUNCTYPE to create a C-callable Python function to pass as a callback.
import ctypes
CompareFunc = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_int, ctypes.c_int)
def my_compare(a, b):
return a - b
c_compare = CompareFunc(my_compare)
# Pass c_compare to a C function expecting a comparatorLoading Windows DLLs
On Windows, use ctypes.WinDLL for DLLs using stdcall convention (e.g., the Windows API).
import ctypes # Windows only
kernel32 = ctypes.WinDLL("kernel32", use_last_error=True)
kernel32.GetTickCount.restype = ctypes.c_ulong
print(kernel32.GetTickCount())Error Handling
Check ctypes.get_last_error() (Windows) or use Python-level try/except for errors returned as special values.
import ctypes
# Many C functions return -1 or NULL on error:
result = some_c_function()
if result == -1:
raise OSError("C function failed")
# For errno on POSIX:
import os
if result < 0:
raise OSError(os.strerror(ctypes.get_errno()))Practical Example: SHA-256 with OpenSSL
Call OpenSSL's SHA256 directly via ctypes without any Python binding library.
import ctypes, ctypes.util
libcrypto = ctypes.CDLL(ctypes.util.find_library("crypto"))
SHA256 = libcrypto.SHA256
SHA256.argtypes = [ctypes.c_char_p, ctypes.c_size_t, ctypes.c_char_p]
SHA256.restype = ctypes.c_char_p
buf = ctypes.create_string_buffer(32)
SHA256(b"hello", 5, buf)
print(buf.raw.hex())Quick Check
Why should you always set argtypes and restype on a ctypes function?
Recap
ctypes loads shared C libraries without compilation. Declare argtypes and restype on every function. Use Structure for C structs, TypeClass * N for arrays, and CFUNCTYPE for callbacks.
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Load shared libraries and call C functions using ctypes. 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.
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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