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The following code works fine. It gives out the message when the user presses a key. But there are certain things I am not aware of. What is the role of Message Loop here ? I read that calling SetWindowsHookEx(...) registers a function with the windows and windows calls the appropriate function automatically when a event of registered type happens. No doubt that i don't see the output if don't give the message loop it's space.

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

HINSTANCE hinst = NULL;
static HHOOK handleKeyboardHook = NULL;
static LRESULT CALLBACK LowLevelKeyboardProc(int nCode, WPARAM wParam, LPARAM lParam);

void setWinHook() {
  handleKeyboardHook = SetWindowsHookEx(WH_KEYBOARD_LL, LowLevelKeyboardProc,NULL, 0);
if(handleKeyboardHook == NULL) {
    cout << "is NULL";
} else {
    cout << "is not NULL";
}
  cout<<("Inside function setWinHook !");
}

static LRESULT CALLBACK LowLevelKeyboardProc(int nCode, WPARAM wParam, LPARAM lParam) {
 cout << ("You pressed a key !\n");
  return CallNextHookEx(handleKeyboardHook, nCode, wParam, lParam);
}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
 handleKeyboardHook = SetWindowsHookEx(WH_KEYBOARD_LL, LowLevelKeyboardProc, hInstance, 0);
 MSG msg;
 while(GetMessage(&msg, NULL, 0, 0))
 {
   TranslateMessage(&msg);
   DispatchMessage(&msg);
 }
 return msg.wParam;
}

EDIT

Does exiting the program (closing the console window) unregister the hook ?

EDIT 2

what role does Dispatch Message play here ? According to doc it dispatches a message to window procedure,but here even if i exclude that,it doesn't effect the output.

BenMorel
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Suhail Gupta
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  • this seems to be an unfinished piece of code, so I'd say that the message loop is not useful YET :) – SirDarius May 29 '12 at 08:17
  • @SirDarius why do you say it to be unfinished ? I guess b'coz i didn't unregister the hook ? – Suhail Gupta May 29 '12 at 08:43
  • well, there is the `setWinHook` function that isn't used in your code, and as some other commentor said, an empty message loop is a good starting point ;) – SirDarius May 29 '12 at 10:01

3 Answers3

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All events in Windows, even the low-level keyboard event used in your example, is sent using the normal message events. So for the program to be able to sense keyboard events, it has to use an event loop processing messages.

Some programmer dude
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2

Without a loop, the program would exit immediately, and the hook would be removed at once too. You cannot register a hook and exit — the system would become a mess if buggy programs were leaving too many forgotten hooks after them. Once your process dies, the hook is scheduled for removal.

I don't remember about low-level keyboard hook, but callbacks of many other hooks are only called inside GetMessage/PeekMessage, and not on some other thread, so just an infinite loop won't suffice — it has to be a message loop.

hamstergene
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  • @SuhailGupta It's a classic pattern for message loop (not the most correct by the way if you consult MSDN). Author has probably copy-pasted it without meaning anything but to have a message loop. It is a good idea to always have it, in case you will start displaying GUI at some point. – hamstergene May 29 '12 at 08:47
  • to whom does `DispatchMessage` dispatch the message in this code ? – Suhail Gupta May 29 '12 at 08:49
  • @SuhailGupta In this code, to nobody. There are probably no messages returned by `GetMessage` anyway. – hamstergene May 29 '12 at 08:51
  • to whom it transfers generally ? – Suhail Gupta May 29 '12 at 08:51
  • _"There are probably no messages returned by GetMessage anyway"_ Doesn't it receive my keyboard event ? Why do you say so ? – Suhail Gupta May 29 '12 at 08:53
  • @SuhailGupta Hook events are not delivered as `MSG` messages. They are handled inside `GetMessage`. This function only returns `MSG` structures for posted messages (PostMessage), it does not return anything for sent ones (SendMessage) nor for hook events nor for whatever else is happening there. – hamstergene May 29 '12 at 08:59
  • then what is GetMessage doing ? – Suhail Gupta May 29 '12 at 09:08
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"what role does Dispatch Message play here ? According to doc it dispatches a message to window procedure,but here even if i exclude that,it doesn't effect the output."

DispatchMessage is pretty useless cos console window doesnt receive much messages. Only message received is when window loses focus.

neo85
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