Building a Dynamic Library

Source: Albertech Blog

Here's a minimal source and command line for building a Windows dynamic link library, or DLL. I like to use it as a quick template for larger projects, without all the noise you get by creating a similar project in Visual Studio.

helloworld.cpp

#include <stdio.h>
#include <wchar.h>

extern "C" {

__declspec(dllexport)
void print_hello(const wchar_t* u) {
    wprintf(L"hello: %s\n", u);
}

} // extern "C"

This is very trivial as you can see. It has a single function that expects an array of wide chars, which it outputs. A couple notes on the source.

extern "C" {} informs the compiler that we want the function names to be preserved. That is, to not "mangle" the names as is done for C++ code.

This way, when we do..

dumpbin.exe /exports helloworld.dll

..we will see and can call the function name as we typed it:

          1    0 00001000 print_helo

Also, we use __declspec(dllexport) so we do not need to use a .def file to export the function.

Compile

cl.exe /D_USRDLL /D_WINDLL helloworld.cpp /MT /link /DLL /OUT:helloworld.dll

More details about build settings for a DLL can be found here.

Bonus Round

And here is how you'd call print_hello() from Python. (What's the use of a DLL unless you have some way of using it, am I right?) Pretend we have a file helloworld.py in the same directory as our compiled helloworld.dll:

import ctypes

lib = ctypes.cdll.LoadLibrary('helloworld.dll')

# Our 'ctypes' wrapper around the DLL function -- this is where we
# convert Python types to C types and call the DLL function.
def print_hello(w):
    func = lib.print_hello
    func.argtypes = [ctypes.c_wchar_p]
    p = ctypes.c_wchar_p(w)
    func(p, len(w))

print_hello('my name is ...')