C / C++ / MFC

// Testdll.cpp : Defines the entry point for the DLL application.
//

#include "stdafx.h"
#include "Testdll.h"

#pragma data_seg(".HOOKDATA")//Shared data among all instances.
HHOOK hook = NULL;
HWND hwnd = NULL;
#pragma data_seg()

#pragma comment(linker, "/SECTION:.HOOKDATA,RWS")//linker directive

HINSTANCE hinstance = NULL;

BOOL APIENTRY DllMain( HANDLE hModule, 
                       DWORD  ul_reason_for_call, 
                       LPVOID lpReserved
					 )
{
    switch (ul_reason_for_call)
	{
		case DLL_PROCESS_ATTACH:
		case DLL_THREAD_ATTACH:
		case DLL_THREAD_DETACH:
		case DLL_PROCESS_DETACH:
			break;
    }

	hinstance = (HINSTANCE)hModule;
	hook = NULL;

    return TRUE;
}

Testdll_API void installhook(HWND h)
{
	hook = NULL;
	hwnd = h;
	hook = SetWindowsHookEx(WH_KEYBOARD,hookproc,hinstance,NULL);
	if(hook==NULL)
		MessageBox(NULL,"Unable to install hook","Error!",MB_OK);
}
Testdll_API void removehook()
{
	UnhookWindowsHookEx(hook);
}


Testdll_API LRESULT CALLBACK hookproc(int ncode,WPARAM wparam,LPARAM lparam)
{
	if(ncode>=0)
	{
		if((lparam & 0x80000000) == 0x00000000)//Check whether key was pressed(not released).
		{
			hwnd = FindWindow("#32770","TestDialog");//Find application window handle
			PostMessage(hwnd,WM_USER+755,wparam,lparam);//Send info to app Window.
		}
	}
	return ( CallNextHookEx(hook,ncode,wparam,lparam) );//pass control to next hook in the hook chain.
}

// Testdll.h : Defines the entry point for the DLL application.
//

#ifdef TESTDLL_EXPORTS
#define TESTDLL_API __declspec(dllexport)
#else
#define TESTDLL_API __declspec(dllimport)
#endif

TESTDLL_API void installhook(HWND h);//This function installs the Keyboard hook.
TESTDLL_API void removehook();//This function removes the previously installed hook.
TESTDLL_API LRESULT CALLBACK hookproc(int ncode,WPARAM wparam,LPARAM lparam);//This function is called when the keyboard is operated.

installhook(this->GetSafeHwnd());