Calling C functions using ctypes

When the dynamic/shared library is successfully loaded to the Python object, the common pattern is to store it as a module-level variable with the same name as the name of loaded library. The functions can be accessed as object attributes, so calling them is like calling a Python function from any other imported module, for example:

>>> import ctypes
>>> from ctypes.util import find_library
>>> libc = ctypes.cdll.LoadLibrary(find_library('c'))
>>> libc.printf(b"Hello world!\n")
Hello world!
13

Unfortunately, all the built-in Python types except integers, strings, and bytes are incompatible with C datatypes and thus must be wrapped in the corresponding classes provided by the ctypes module. Here is the full list of compatible datatypes that come from the ctypes documentation:

ctypes type	C type	Python type
c_bool	_Bool	bool
c_char	char	1-character bytes object
c_wchar	wchar_t	1-character string
c_byte	char	int
c_ubyte	unsigned char	int
c_short	short	int
c_ushort	unsigned short	int
c_int	int	int
c_uint	unsigned int	int
c_long	long	int
c_ulong	unsigned long	int
c_longlong	__int64 or long long	int
c_ulonglong	unsigned __int64 or unsigned long long	int
c_size_t	size_t	int
c_ssize_t	ssize_t or Py_ssize_t	int
c_float	float	float
c_double	double	float
c_longdouble	long double	float
c_char_p	char * (NUL terminated)	bytes object or None
c_wchar_p	wchar_t * (NUL terminated)	string or None
c_void_p	void *	int or None

 

As you can see, the preceding table does not contain dedicated types that would reflect any of the Python collections as C arrays. The recommended way to create types for C arrays is to simply use the multiplication operator with the desired basic ctypes type as follows:

>>> import ctypes
>>> IntArray5 = ctypes.c_int * 5
>>> c_int_array = IntArray5(1, 2, 3, 4, 5)
>>> FloatArray2 = ctypes.c_float * 2
>>> c_float_array = FloatArray2(0, 3.14)
>>> c_float_array[1]
3.140000104904175

As syntax works for every basic ctypes type.

Let's look at how Python functions are passed as C callbacks in the next section.