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
[edit | edit source]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
[edit | edit source]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
[edit | edit source]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 |