Python Programming/Extending with ctypes

ctypes[1] is a foreign function interface module for Python (included with Python 2.5 and above), which allows you to load in dynamic libraries and call C functions. This is not technically extending Python, but it serves one of the primary reasons for extending Python: to interface with external C code.

Basics

A library is loaded using the ctypes.CDLL function. After you load the library, the functions inside the library are already usable as regular Python calls. For example, if we wanted to forego the standard Python print statement and use the standard C library function, printf, you would use this:

from ctypes import *
libName = 'libc.so' # If you're on a UNIX-based system
libName = 'msvcrt.dll' # If you're on Windows
libc = CDLL(libName)
libc.printf("Hello, World!\n")

Of course, you must use the libName line that matches your operating system, and delete the other. If all goes well, you should see the infamous Hello World string at your console.

Getting Return Values

ctypes assumes, by default, that any given function's return type is a signed integer of native size. Sometimes you don't want the function to return anything, and other times, you want the function to return other types. Every ctypes function has an attribute called restype. When you assign a ctypes class to restype, it automatically casts the function's return value to that type.

Common Types

ctypes name	C type	Python type	Notes
None	void	None	the None object
c_bool	C99 _Bool	bool
c_byte	signed char	int
c_char	signed char	str	length of one
c_char_p	char *	str
c_double	double	float
c_float	float	float
c_int	signed int	int
c_long	signed long	long
c_longlong	signed long long	long
c_short	signed short	long
c_ubyte	unsigned char	int
c_uint	unsigned int	int
c_ulong	unsigned long	long
c_ulonglong	unsigned long long	long
c_ushort	unsigned short	int
c_void_p	void *	int
c_wchar	wchar_t	unicode	length of one
c_wchar_p	wchar_t *	unicode