Solved

Understanding HOOKS

Posted on 2013-05-27
2
246 Views
Last Modified: 2013-06-03
I have some code that monitors an Edit control and looks for the number 7 to be entered. When a 7 is entered, an action is taken.

I am new to c++, but want to understand each and every line of this code thoroughly.

To be fair, I will, of course break this down into multiple questions.

My first question about this code is:

[1] Can we go over the GetMsgProc function? A simple line by line explanation would be most beneficial.

Thank you! Although this code does work, I want to understand WHY.



and here is the code:

#include <windows.h>
HHOOK g_hhk;
HWND g_hEdit;

LRESULT CALLBACK GetMsgProc(
    int code,
    WPARAM wParam,
    LPARAM lParam
)
{
    if (0 > code || PM_NOREMOVE == wParam) return CallNextHookEx(g_hhk,code,wParam,lParam);

    MSG* pmsg = (MSG*) lParam;

    if (pmsg->message == WM_KEYDOWN) { // this one is for us

        if (pmsg->hwnd == g_hEdit && pmsg->wParam == VK_7) { // '7' has been pressed if the monitored edit control

            // ---> ACTION!
            MessageBox(NULL, "The number was pressed!", "Notice", MB_OK);
        }
    }

    return CallNextHookEx(g_hhk,code,wParam,lParam);
}

void InitMonitoring(HWND hEdit) {

    g_hEdit = hEdit;

    g_hhk = SetWindowsHookEx(WH_GETMESSAGE,GetMsgProc,NULL,GetCurrentThreadId());
}

void StopMonitoring() {

    UnhookWindowsHookEx(g_hhk);
}

Open in new window

0
Comment
Question by:edvinson
2 Comments
 
LVL 31

Accepted Solution

by:
Zoppo earned 500 total points
ID: 39200367
Hi edvinson,

the GetMsgProc function is called every time the application calls GetMessage or PeekMessage because it's installed as a hook of type WH_GETMESSAGE. To learn more about hooks reading this is a good start: http://msdn.microsoft.com/en-us/library/windows/desktop/ms632589%28v=vs.85%29.aspx

In GetMsgProc first two things are checked:

1. If code is less than 0 the function has to call CallNextHookEx without further processing. This is proposed at http://msdn.microsoft.com/en-us/library/windows/desktop/ms644981%28v=vs.85%29.aspx

2. If wParam is PM_NOREMOVE the function even just calls CallNextHookEx without further processing because this means the application called PeekMessage without removing the message from the message queue. So this avoids the same message being processed more than once.

Next the function checks if the message is a WM_KEYDOWN and the control which recieved the message is the wanted g_hEdit and the pressed key is 7. If so the message box is shown.

In any case next the function calls CallNextHookEx to allow other maybe installed hooks to proccess the message too.

Hope that helps,

ZOPPO
0
 
LVL 1

Author Closing Comment

by:edvinson
ID: 39217995
very thorough explanation thank you very much
0

Featured Post

VMware Disaster Recovery and Data Protection

In this expert guide, you’ll learn about the components of a Modern Data Center. You will use cases for the value-added capabilities of Veeam®, including combining backup and replication for VMware disaster recovery and using replication for data center migration.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Introduction This article is a continuation of the C/C++ Visual Studio Express debugger series. Part 1 provided a quick start guide in using the debugger. Part 2 focused on additional topics in breakpoints. As your assignments become a little more …
Many modern programming languages support the concept of a property -- a class member that combines characteristics of both a data member and a method.  These are sometimes called "smart fields" because you can add logic that is applied automaticall…
The goal of the tutorial is to teach the user how to use functions in C++. The video will cover how to define functions, how to call functions and how to create functions prototypes. Microsoft Visual C++ 2010 Express will be used as a text editor an…
The viewer will learn additional member functions of the vector class. Specifically, the capacity and swap member functions will be introduced.

778 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question