Vs2015 uses loadlibrary to call DLL library

 Reference resources ：
//blog.csdn.net/luoyu510183/article/details/93666808
//blog.csdn.net/SonnAdolf/article/details/80339070
//blog.csdn.net/blade1080/article/details/81538384

Loadlibrary Way no longer needed .lib library , When we compile the dynamic library , Two files will be generated ,.dll and .lib ,lib Unlike static libraries , There are just some function symbols , The real realization is dll in . So , Generated when compiling the dynamic library lib You don't need it . however , If you don't use Loadlibrary The way , Then both files need .

Here is a small example ：

1、 Compile Library

Let's compile a library file first , Then use Loadlibrary Method call .
mydll.h

#pragma once
#include <windows.h>
#include <iostream>
using namespace std;


#define MY_DLL_EXPORTS

#ifdef MY_DLL_EXPORTS
#define MY_DLL_API __declspec(dllexport)
#else
#define MY_DLL_API __declspec(dllimport)
#endif


extern "C" MY_DLL_API int open_device(int handle);

extern "C" MY_DLL_API int open_seesion(int hseion);

mydll.cpp

#include "mydll.h"

	
extern "C" MY_DLL_API int  open_device(int handle)
{
    
	cout << "display func: open_device,"<<"handle is:"<< handle << endl;
	return 0;
}

extern "C" MY_DLL_API int  open_seesion(int session)
{
    
	cout << "display func: open_seesion,"<<"session is:"<< session << endl;
	return 0;
}

The default call method is ：

After compilation , We use it dumpbin Look at the export symbol ：

It means that the function has been exported in the Library , Use loadlibrary It needs to correspond to the exported name .

2、 Compile call app

Be careful , Your application and library files should be in the same directory .

Loadlibrary_test.cpp

#include <iostream>
#include <Windows.h>
#include "mydll.h"

using namespace std;

int main(void)
{
    
	HMODULE hMod = LoadLibrary(TEXT("lib_dll.dll"));// load dll
	if (hMod == nullptr) 
	{
    
		cout << "load dll error!" << endl;
		return -1;
	}

	typedef int(* Open_Device)(int);
	typedef int(* Open_Seesion)(int);

	Open_Device opendev = (Open_Device)GetProcAddress(hMod, "open_device");// Get function address 
	if (opendev == nullptr)
	{
    
		cout << "load open_device error!" << endl;
		return -1;
	}

	int rv = opendev(1);// Call function through function pointer 



	Open_Seesion openses = (Open_Seesion)GetProcAddress(hMod, "open_seesion");
	if (openses == nullptr)
	{
    
		cout << "load open_session error!" << endl;
		return -1;
	}

	rv = openses(2);// Call function through function pointer 



	FreeLibrary(hMod);

	system("pause");
	return 0;
}

The phenomenon ：

3、 remarks

By the way, let's talk about the relationship between calling convention and function symbols , namely _stdcall, _cdecl. And that is extern "c" Influence on function name symbol .

extern “C” + _stdcall, The function export symbol is ： _+ Function name [email protected]+ Number of parameter transfer bytes

because _stdcall Is the callee cleaning up the stack , So the function symbol contains the information of parameters

extern “C” + _cdecl, The function export symbol is : Function name

because _cdecl It is the caller who cleans up the stack , So you only need the function name

Don't use extern Under the circumstances , yes C++ Export mode of , The function symbols are as follows ：
：？+ Function name [email protected]@YG+ Return type + Parameters 1 type …[email protected]
If it is _cdecl @YG Turn into @YA
If there is no parameter, the parameter is void, with Z ending , for example ：
: ?+ Function name [email protected]@YA+ Return type +XZ
above X Express void type ,H Express int Parameter type