Local hook

[Natter]

Hi,

I need to make a local hook. The second parameter in SetWindowsHookEx is the handler function for this hook. What is the correct way to write this parameter?

[Antonio Linares]

chatgpt explains it really well:

When creating a local hook with the SetWindowsHookEx function, the second parameter must be a pointer to a callback function that matches the required signature for the type of hook you're setting. Here’s the general process:

Steps:
1. Identify the Hook Type: Choose the appropriate hook type (e.g., WH_KEYBOARD, WH_MOUSE, etc.). The hook type determines the callback function's signature.

2. Define the Callback Function: Write a function that matches the signature required for the hook type.

3. Pass the Callback Function's Address: The callback function’s address should be passed as the second parameter.

Example for a WH_KEYBOARD Hook
Here’s an example of setting a local keyboard hook:

Define the Callback Function
The signature for a keyboard hook callback is:

Code: Select all | Expand

LRESULT CALLBACK KeyboardProc(
    int nCode,      // Hook code
    WPARAM wParam,  // Virtual-key code
    LPARAM lParam   // Key state information
);
 

Code: Select all | Expand

#include <windows.h>

HHOOK hHook;

LRESULT CALLBACK KeyboardProc(int nCode, WPARAM wParam, LPARAM lParam) {
    if (nCode >= 0) { // Process the message
        if (wParam == WM_KEYDOWN) {
            // Handle key press
            KBDLLHOOKSTRUCT* pKeyBoard = (KBDLLHOOKSTRUCT*)lParam;
            printf("Key pressed: %d\n", pKeyBoard->vkCode);
        }
    }
    return CallNextHookEx(hHook, nCode, wParam, lParam);
}

void SetHook() {
    hHook = SetWindowsHookEx(WH_KEYBOARD, KeyboardProc, NULL, GetCurrentThreadId());
    if (hHook == NULL) {
        printf("Failed to install hook!\n");
    }
}

void RemoveHook() {
    UnhookWindowsHookEx(hHook);
}
 

Key Points
The callback function must use the CALLBACK (or WINAPI) calling convention.
For local hooks (dwThreadId specified as the current thread), the hMod parameter in SetWindowsHookEx should be NULL.
Pass the callback function's name (e.g., KeyboardProc) as the second parameter.

[Natter]

Thank you !