Call c function from python wouldn't work

Question

I am almost sure that I did this once a year ago... Not it just wouldn't work. Weird. I must be making a minor mistake somewhere... Please help!

I have the following toy c code:

// testdll.c
int sum(int a, int b)
{
    return(a+b);
}

And then, since I am using Windows 7, I used WinSDK 7.1's x64 C/C++ compiler to compile it:

cl testdll.c /TC /LD

The output is testdll.dll.

Then, in my Python 3.3, I used:

In [12]: import ctypes

In [13]: lib = ctypes.cdll.LoadLibrary('./testdll.dll')

In [14]: lib
Out[14]: <CDLL './testdll.dll', handle f7000000 at a43ea58>

In [15]: lib.sum
Traceback (most recent call last):

  File "<ipython-input-15-309017dbbec8>", line 1, in <module>
    lib.sum

  File "C:\WinPython2\python-2.7.6.amd64\lib\ctypes\__init__.py", line 378, in __getattr__
    func = self.__getitem__(name)

  File "C:\WinPython2\python-2.7.6.amd64\lib\ctypes\__init__.py", line 383, in __getitem__
    func = self._FuncPtr((name_or_ordinal, self))

AttributeError: function 'sum' not found

It can't find this function! It's driving me crazy. Since I used pure C, and I used /TC during compiling, it shouldn't be a name mangling issue.

Any idea would be appreciated. Thank you all so much!

EDIT 2014/02/13 I tried to compile it with gcc also, but with no luck. Same old problem happens. I used dir() in python, and realized everything should be in it - just not the correct name, by which I mean it can't be called via fun.sum. It is able to recognize that the result type of the function is an int.

In [34]: dir(lib)
Out[34]: 
['_FuncPtr',
 '__class__',
 '__delattr__',
 '__dict__',
 '__doc__',
 '__format__',
 '__getattr__',
 '__getattribute__',
 '__getitem__',
 '__hash__',
 '__init__',
 '__module__',
 '__new__',
 '__reduce__',
 '__reduce_ex__',
 '__repr__',
 '__setattr__',
 '__sizeof__',
 '__str__',
 '__subclasshook__',
 '__weakref__',
 '_func_flags_',
 '_func_restype_',
 '_handle',
 '_name']

In [35]: lib._func_restype_
Out[35]: ctypes.c_long

Answer 1

One way to export a symbol is via an option to the linker:

cl testdll.c /LD /link /export:sum

This doesn't scale. A better option is to use the __declspec(dllexport) modifier in the declaration. Refer to Exporting from a DLL in the Visual studio docs.

Apply it conditionally as follows:

#ifdef BUILD_TESTDLL
#define TESTAPI __declspec(dllexport)
#else
#define TESTAPI __declspec(dllimport)
#endif

/* declaration */
TESTAPI int sum(int a, int b);

/* definition */
int sum(int a, int b)
{
    return(a+b);
}

When linking via the import lib, declarations should use __declspec(dllimport), which is the default for TESTAPI. When building the DLL, define BUILD_TESTDLL in the project settings or on the command-line via /D.

Finally, for the ultimate in flexibility, use a .def file such as the following:

LIBRARY TESTDLL
EXPORTS
    sum @ 10 NONAME 
    sum_alias=sum @ 20

This lets you export a function using a different name, or export by ordinal only. Add it as a source file as follows:

cl testdll.c testdll.def /LD

Then in Python, for example:

>>> from ctypes import *
>>> lib = cdll.testdll
>>> lib.sum_alias(1, 2)
3
>>> lib[10](1, 2)      
3
>>> lib[20](1, 2)
3

---

BTW, ctypes doesn't preload the exports from a DLL. dir(lib) won't show any function pointers at first. They're cached when accessed:

>>> from ctypes import *
>>> lib = cdll.testdll
>>> sorted(vars(lib))
['_FuncPtr', '_handle', '_name']
>>> lib.sum_alias(1, 2)
3
>>> sorted(vars(lib))
['_FuncPtr', '_handle', '_name', 'sum_alias']