asked on

WH_CBT Strange Behavior

Hello,

I'm trying to use WH_CBT hooking to get a list of any activated process, however while testing i only get several HCBT_CLICKSKIPPED message codes at first and that's its, i don't receive any other messages which i am interested in like HCBT_CREATEWND or HCBT_ACTIVATE

Here is my full implementation:
//*****************************************************************************************
library HookLib;

uses
SysUtils, Classes, Windows;

type
TClientCallBack = procedure(Code,wParam : integer) of object;

var fHookhandle: THandle;
clientFunction : TClientCallBack;

{$R *.res}

function callbackFunction(Code, wParam, lParam: Integer): Integer; stdcall;
begin
OutputDebugString(PChar('code from dll' + InttoStr(Code))); // always prints 6 (HCBT_CLICKSKIPPED)
      if (Code < 0) then begin
result := CallNextHookEx(fHookhandle, Code, wParam, lParam);
exit;
end;

clientFunction(Code,wParam); //Calls client callback function

      result := CallNextHookEx(fHookhandle, Code, wParam, lParam);

end;

function InstallCBTHook(ClientCallBackFunction: TClientCallBack):boolean; stdcall;
begin

clientfunction := ClientCallBackFunction;
      fHookhandle := SetWindowsHookEx(WH_CBT, @callbackFunction, hInstance, 0);
      if fHookhandle > 0 then result := true else result := false;

end;

procedure UninstallCBTHook();stdcall;
begin
      UnhookWindowsHookEx(fHookhandle);

end;

exports
InstallCBTHook, UninstallCBTHook;

begin
end.
//*************************************************************************

yehiaeg

ASKER

ahh this behavior was on Windows XP, i just tested the executable on Windows Vista and it caused explorer.exe to crash instantaneously, so obviously something is going wrong

ZhaawZ

>> clientfunction := ClientCallBackFunction;
You're setting this value only in 1 process (in that one which sets the hook). This .dll is then loaded into other processes, which have their own memory space - value of clientfunction is not ClientCallBackFunction in their memory.
You may test it by setting default value of variable when defining it (var test : integer = 0;), changing it's value in InstallCBTHook and then doing something like "if test = 0 then MessageBeep(0);" in callback function. You'll probably hear beeps, which mean that "test" variable has default value, although it's changed in InstallCBTHook.

I'm not sure what's the best way to solve this though. Could be solved by using memory mapped files (I haven't done that yet) or sending messages with SendMessage() to comunicate with main application.

arioh

function callbackFunction( Code : Integer; WParam : wParam; LParam : lParam ): LResult; stdcall;
begin
if Code >= 0 then
begin
OutputDebugString( PChar( 'code from dll ' + IntToStr( Code ) ) );
clientFunction( Code, WParam );
end;
result := CallNextHookEx( fHookhandle, Code, wParam, lParam );
end;

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arioh

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yehiaeg

ASKER

ok i followed what you were saying. ZhaawZ about using the memory mapped file, and arioh about the new function, however i still have the same behavior, several HCBT_CLICKSKIPPED messages and that's it:

library HookLib;

uses
SysUtils, Classes, Windows;

type
//TClientCallBack = procedure(Code,wParam : integer) of object;

PSharedRecord = ^TSharedRecord;
TSharedRecord = record
fHookhandle: THandle;
clientHwnd : HWND;
fInstallProcess: DWORD;
end;

var fMemoryFile: THandle = 0;
sharedRec: PSharedRecord = nil;
uiMyMsg: UINT = 0;
{$R *.res}

function callbackFunction(Code, wParam, lParam: Integer): Integer; stdcall;
begin

      if Code >= 0 then begin
OutputDebugString(PChar('code from dll' + InttoStr(Code)));
if sharedRec <> nil then begin
PostMessage(sharedRec^.clientHwnd, uiMyMsg, Code, wParam);

end;
end;
      result := CallNextHookEx(sharedRec^.fHookhandle, Code, wParam, lParam);

end;

function InstallCBTHook(clientHwnd : HWND): boolean ; stdcall;
begin

result := false;
fMemoryFile := CreateFileMapping($FFFFFFFF, nil, PAGE_READWRITE,0, // size: high 32-bits
SizeOf(TSharedRecord), 'auniquename');

      if (fMemoryFile <> 0 ) then begin
sharedRec := MapViewOfFile(fMemoryFile, FILE_MAP_ALL_ACCESS, 0, 0, 0);
if sharedRec <> nil then begin
sharedRec^.clientHwnd := clientHwnd ;
sharedRec^.fInstallProcess := GetCurrentProcessId;
     sharedRec^.fHookhandle := SetWindowsHookEx(WH_CBT, @callbackFunction, hInstance, 0);

result := true ;
end;
end
else result := false;

end;

procedure UninstallCBTHook();stdcall;
begin
if assigned(sharedRec) then begin
if sharedRec^.fInstallProcess = GetCurrentProcessId then begin
UnhookWindowsHookEx(sharedRec^.fHookhandle);
sharedRec^.fHookhandle := 0;
sharedRec^.clientHwnd := 0;
sharedRec^.fInstallProcess := 0;
end;
UnmapViewOfFile(sharedRec);
sharedRec := nil;
end;
if (fMemoryFile <> 0 ) then begin
CloseHandle(fMemoryFile);
fMemoryFile := 0;
end;

end;

procedure DllEntry(dwReason : DWORD);
begin
if (dwReason = Dll_Process_Attach) then begin
uiMyMsg := RegisterWindowMessage('auniquemsgname');

if uiMyMsg <> 0 then begin
fMemoryFile := OpenFileMapping(FILE_MAP_WRITE, False, 'auniquemsgname');
sharedRec := MapViewOfFile(fMemoryFile, FILE_MAP_READ, 0, 0, 0);
if sharedRec = nil then UninstallCBTHook();

end;
end
else if (dwReason = Dll_Process_Detach) then begin
UninstallCBTHook();
end;
end;

exports
InstallCBTHook, UninstallCBTHook, callbackFunction;

begin
DisableThreadLibraryCalls(hInstance);
DllProc := @DLLEntry;
DllEntry(DLL_PROCESS_ATTACH);

end.

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