The Difference between to usages of SetWindowsHookEx - c

Question

First: I'm using Visual Studio 2010 on Windows XP - 32 Bit.
Right now I'm trying to write a DLL which will enable another application to work with low level keyboard hooks.
Even though I got it to work - I now want to understand why.
The non-working code:
#include <Windows.h>
#include <stdio.h>
static HINSTANCE hinst;
static HHOOK kbdHook = NULL;
LRESULT CALLBACK LowLevelKeyboardProc(int nCode, WPARAM wParam, LPARAM lParam)
{
printf(":"); fflush(stdout);
return CallNextHookEx(NULL, nCode, wParam, lParam);
}
DWORD WINAPI ThreadProc(LPVOID lpParameter)
{
MSG msg;
while (GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return 0;
}
BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved)
{
switch (fdwReason)
{
case DLL_PROCESS_ATTACH:
hinst = hinstDLL;
CreateThread(NULL, 0, ThreadProc, NULL, 0, NULL);
kbdHook = SetWindowsHookEx(WH_KEYBOARD_LL, LowLevelKeyboardProc, hinst, 0);
break;
case DLL_PROCESS_DETACH:
UnhookWindowsHookEx(kbdHook);
break;
default:
break;
}
return TRUE;
}
The working code:
#include <Windows.h>
#include <stdio.h>
static HINSTANCE hinst;
static HHOOK kbdHook = NULL;
LRESULT CALLBACK LowLevelKeyboardProc(int nCode, WPARAM wParam, LPARAM lParam)
{
printf(":"); fflush(stdout);
return CallNextHookEx(NULL, nCode, wParam, lParam);
}
DWORD WINAPI ThreadProc(LPVOID lpParameter)
{
MSG msg;
kbdHook = SetWindowsHookEx(WH_KEYBOARD_LL, LowLevelKeyboardProc, GetModuleHandle(NULL), 0);
while (GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return 0;
}
BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved)
{
switch (fdwReason)
{
case DLL_PROCESS_ATTACH:
hinst = hinstDLL;
CreateThread(NULL, 0, ThreadProc, NULL, 0, NULL);
break;
case DLL_PROCESS_DETACH:
UnhookWindowsHookEx(kbdHook);
break;
default:
break;
}
return TRUE;
}
The only difference is that I moved the SetWindowsHookEx-call from DllMain to ThreadProc.
The question: Why does this make all the difference?

Answer 1

This is all explained in the documentation for LowLevelKeyboardProc callback function:
This hook is called in the context of the thread that installed it. The call is made by sending a message to the thread that installed the hook. Therefore, the thread that installed the hook must have a message loop.
Your non-working code installs the hook on a thread, that doesn't run a message loop.