Wrapping c++ functions in python with ctypes on windows : function not found
c++, ctypes, python, swig, visual-studio-2012
Solution
The error reported by your Python is very clear.
function 'plop' not found
This means that the DLL does not export a function of that name. So, you need to export the function. Either with a .def file, or using `__declspec(dllexport)`:
extern "C"
{
__declspec(dllexport) int plop();
}
To inspect your DLL to debug issues like this, use `dumpbin` or Dependency Walker.
Note that since you do not specify calling convention, the default of `__cdecl` is used. That means that `cdll` is correct on the `ctypes` side.
Problem
I need to run a series of python scripts calculating various scripts, that are working fine, but one of them runs very slowly and has to be done in C++. The C++ code is ready, but I need to find a way to call the C++ function from the python and get the return value. I found information about SWIG, but didn't get it to work on Windows with Visual Studio (I have to do it in VS on Windows, because of other constraints). I found ctypes much easier for my very simple function with standart input and output values. So I did many tests with ctypes, tried all the examples I could find online, and each and every time, what happens is that I can load the dll (built with visual studio 2012 compiler) but when I try calling the function in python, it just goes : ``` Traceback (most recent call last): File "test.py", line 3, in <module> print maLib.plop() File "C:\Python27\lib\ctypes\__init__.py", line 378, in __getattr__ func = self.__getitem__(name) File "C:\Python27\lib\ctypes\__init__.py", line 383, in __getitem__ func = self._FuncPtr((name_or_ordinal, self)) AttributeError: function 'plop' not found ``` The python code is (test.py) : ``` from ctypes import cdll lib = cdll.LoadLibrary('test') print lib.plop() ``` The C++ code is (test.cpp, compiled as test.dll, Visual Studio set to build as dll) : ``` extern "C" { int plop(); } int plop() { return 4; } ``` As you can see, I tried to make it as simple as possible, to avoid details making it fail. I have read the python ctypes help, and tutorials on how to use ctypes, trying exactly the same codes as them, but I had to adapt a bit because I am using Visual Studio / windows and most of the other users are using linux. All of the files are in the same folder. I have tried multiple ways of loading the library : LoadLibrary('./name.dll'), WinDLL('name.dll'), giving the function a different name, void return type, etc... Do you think I should use SWIG ? Is my problem an obvious beginner mistake ? I am a student, and new to most of what I'm using, but I put a lot of effort in this, even if I just need it for a particular single task. When using SWIG, I had to make a wrapper with function pointers, or references, so I thought this was the problem, which made me want to try a simpler solution. I am using : Python 2.7.7, Visual Studio 2012, Windows 8.1 Many thanks in advance