ctypes

TLDR

Load a shared library

$ python -c "from ctypes import CDLL; lib = CDLL('[libname.so]')"

Call a C function

$ python -c "from ctypes import CDLL; lib = CDLL('libc.so.6'); print(lib.getpid())"

Pass string argument to C function

$ python -c "from ctypes import CDLL, c_char_p; lib = CDLL('libc.so.6'); lib.puts(c_char_p(b'Hello'))"

Define function argument and return types

$ python -c "from ctypes import CDLL, c_int; lib = CDLL('libm.so.6'); lib.abs.argtypes = [c_int]; lib.abs.restype = c_int; print(lib.abs(-42))"

Access Windows DLL

$ python -c "from ctypes import windll; windll.user32.MessageBoxW(0, 'Hello', 'Title', 0)"

SYNOPSIS

$ from ctypes import CDLL, c_int, c_char_p, POINTER

ctypes is a Python standard library module for calling functions in C shared libraries and DLLs. It provides C-compatible data types and allows calling functions with appropriate argument and return type specifications.
The module enables Python code to interface with native libraries without writing C extension modules. It handles type conversion between Python and C, pointer management, and structure/union definitions.
Common uses include accessing system libraries, using hardware interfaces, integrating legacy C code, and calling platform-specific APIs (Windows DLLs, macOS frameworks).

TYPES

c_int, c_long, c_longlong

Integer types.

c_uint, c_ulong

Unsigned integers.

c_float, c_double

Floating point types.

c_char, c_wchar

Character types.

c_char_p, c_wchar_p

String pointers (char*, wchar_t*).

c_void_p

Void pointer.

POINTER(type)

Pointer to type.

Structure, Union

Base classes for C structs/unions.

EXAMPLE

$ from ctypes import CDLL, c_double

# Load math library
libm = CDLL('libm.so.6')  # Linux
# libm = CDLL('libSystem.B.dylib')  # macOS

# Configure function
libm.sqrt.argtypes = [c_double]
libm.sqrt.restype = c_double

# Call function
result = libm.sqrt(16.0)
print(result)  # 4.0

CAVEATS

Incorrect type specifications can cause crashes or memory corruption. Pointers and memory management require careful handling. Library paths vary by OS; use find_library for portability. Performance overhead exists compared to C extensions. Not suitable for C++ libraries with name mangling.

HISTORY

ctypes was created by Thomas Heller and merged into Python's standard library in version 2.5 (2006). It was based on the earlier ctypes project available on PyPI since 2003. The module provided a crucial capability for Python to interface with native code without compilation, enabling pure-Python access to system libraries.