Question

I want to test wrapping a dll with ctypes. I’ve written the following test code and compiled it using Code::Blocks and Cygwin to a dll.

#define DLL_EXPORT extern "C" __declspec(dllexport)

DLL_EXPORT int sum(int a, int b) {
    return a + b;
}

Note: This is the whole code. Maybe there is something missing ?

Now, I copy the TestDll.dll to my Desktop and start the Python interpreter. But when I want to load it, the interpreter just exits !

C:\Users\niklas\Desktop>python
Python 2.7.2 (default, Jun 12 2011, 15:08:59) [MSC v.1500 32 bit (Intel)] on win 32
Type "help", "copyright", "credits" or "license" for more information.
>>> import ctypes
>>> dll = ctypes.WinDLL('TestDll')

C:\Users\niklas\Desktop>

However, loading any other library works, or giving an error if the library could not be found.
Can you tell me what I’m doing wrong here ?

Using Cygwin g++ 3.4.4

Answer 1

Check objdump -p TestDll.dll | grep dll to see if you’ve linked in “cygwin1.dll” and nm TestDll.dll | grep Dll to see if you have a DllMain. The following command should build the DLL correctly:

g++ testdll.c -mno-cygwin -shared -o TestDll.dll

Also, you need to use CDLL for the cdecl calling convention, not WinDLL:

>>> import ctypes
>>> dll = ctypes.CDLL('TestDll')
>>> dll.sum(4, 5)
9

Edit: I compiled with i686-w64-mingw32-g++.exe (4.5.3) from the Cygwin repository, but I used to use the default Cygwin gcc without a problem, given the -mno-cygwin option.