Why do I need __declspec(dllexport) to make some functions accessible from ctypes?

Question

So I am learning to make use of Python ctypes module.
Here is simple C file which I compiled with gcc -shared (version 4.8) on Windows to get sample .dll:

Answer 1

gcc seems to export functions by default, you can use any PE viewer like PE Explorer (View > Export) to view the exported functions:

But, If you try to compile this code with VC++, it won't export this function for you, you'll see that there is not exported function:

You need to ask it to export this function:

__declspec(dllexport) int addition(int a, int b){
    return a+b;
}

As for calling conventions, the rule is simple:

If your function uses __stdcall, as most Win32API, you need to import the DLL with WinDLL('mylib.dll') or windll.mylib, example:

> type mylib.c
__declspec(dllexport) int __stdcall addition(int a, int b) {
    return a+b;
}

***********************************************************************

> cl mylib.c /link /dll /out:mylib.dll
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 14.00.50727.762 for 80x86

Copyright (C) Microsoft Corporation.  All rights reserved.

mylib.c
Microsoft (R) Incremental Linker Version 8.00.50727.762
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:mylib.exe
/dll
/out:mylib.dll
mylib.obj
   Creating library mylib.lib and object mylib.exp

***********************************************************************

> python
>>> from ctypes import *
>>>
>>> WinDLL('mylib.dll').addition(1, 2)
3
>>> windll.mylib.addition(1, 2)
3
>>>

If your function uses __cdecl, witch is the default calling convention, you need to import the DLL with CDLL('mylib.dll') or cdll.mylib', example:

> type mylib.c
// `__cdecl` is not needed, since it's the default calling convention
__declspec(dllexport) int addition(int a, int b){
    return a+b;
}

***********************************************************************

> cl mylib.c /link /dll /out:mylib.dll
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 14.00.50727.762 for 80x86

Copyright (C) Microsoft Corporation.  All rights reserved.

mylib.c
Microsoft (R) Incremental Linker Version 8.00.50727.762
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:mylib.exe
/dll
/out:mylib.dll
mylib.obj
   Creating library mylib.lib and object mylib.exp

***********************************************************************

> python
>>> from ctypes import *
>>>
>>> CDLL('mylib.dll').addition(1, 2)
3
>>> cdll.mylib.addition(1, 2)
3
>>>