Real World Instrumentation with Python

Python’s ctypes Foreign Function Library

There is another way to get at the functions exposed by a DLL: ctypes, Python’s library for accessing external DLL objects. This section provides a general overview of ctypes.

The ctypes library gives Python programs the ability to directly access the functions within an external DLL without the need to create interface code in C. ctypes is part of the standard distribution with Python 2.5 and later, and it supports both Linux and Windows environments, with the usual caveats concerning the fundamental differences between the two platforms.

Loading External DLLs with ctypes

ctypes exports three primary interface classes: cdll, windll, and oledll. cdll is used for both Linux and Windows and supports library modules that use the cdecl calling convention. The windll class supports Windows libraries that use the stdcall convention. oledll also uses the stdcall convention, but it assumes that the library functions return an HRESULT error code. cdecl is the default for C programs, so most library objects on Linux will use this convention. On a Windows system you should check the appropriate Windows technical documentation to see what the various library objects use as their calling convention.

Using ctypes is straightforward. Here is what happens when the Windows C runtime library, msvcrt (Microsoft Visual C Run-Time), is accessed using ctypes:

>>> from ctypes import *
>>> msvcrt = cdll.msvcrt
>>> msvcrt <CDLL 'msvcrt', handle 78000000 at 97b0f0> >>> ...