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Hook functions cause NT 4.0 to crash. Why?

Posted on 1997-02-27
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Last Modified: 2010-04-10
NT 4.0 always crashes using these functions. Why ?

xy.cpp
------

#include <windows.h>
#include <winuser.h>
#include <stdio.h>

extern "C" void sysmon(void);
extern "C" LRESULT CallWndProc(int nCode,WPARAM wParam, LPARAM lParam);

int main(void)
{
printf("\nHidOpApp started...");
HOOKPROC hproc;
static HINSTANCE hDLL;
static HHOOK hmsg;
if ((hDLL=LoadLibrary((LPCTSTR)"xy.dll"))==NULL)
   printf("%c\nDLL not found !",7);
   else
if ((hproc=(HOOKPROC)GetProcAddress(hDLL,"_CBTProc"))==NULL)
   printf("%c\nFunction not found in DLL !",7);
   else
if ((hmsg=SetWindowsHookEx(WH_CBT,hproc,hDLL,0))==NULL)
   printf("%c\nHook not installed !",7);
FreeLibrary(hDLL);
printf("\nHidOpApp finished.\nPress RETURN ...");
while (getchar()==-1);
return 0;
}

xy.dll
------

#include <stdio.h>
#include <owl\owlpch.h>

extern "C" LRESULT _export CallWndProc(int nCode,WPARAM  wParam,LPARAM lParam);

BOOL WINAPI DllEntryPoint(HINSTANCE hinstDll,uint32 fdwReason,LPVOID lpvReserved)
      {
      if (fdwReason==DLL_PROCESS_ATTACH) printf("\nProcess Attach DLL");
         else
      if (fdwReason==DLL_THREAD_ATTACH) printf("\nThread Attach DLL");
      else
      if (fdwReason==DLL_PROCESS_DETACH) printf("\nProcess Detach DLL");
      else
      if (fdwReason==DLL_THREAD_ATTACH) printf("\nThread Detach DLL");
      else printf("\nUnknown action DLL");
   return TRUE;
   }

extern "C" LRESULT _export CBTProc(int nCode,WPARAM wParam,LPARAM lParam)
      {
      printf("\nCallWndProc wurde aufgerufen !");
      if (nCode==HC_ACTION) printf("\nCWP must act !");
         else printf("\nCWP mustn't act !");
      if (wParam==0) printf("\nMessage sent by this process !");
         else printf("\nMessage sent by other process !");
      return 0;
      }
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Question by:Torsten021897
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6 Comments
 
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Accepted Solution

by:
serge010697 earned 50 total points
ID: 1161877
If NT crashes during while (getchar()==-1); this is because you called
FreeLibrary before. You don't have to do that, because your CBTProc doesn't exist after that. Call it right before return statement.

0
 

Author Comment

by:Torsten021897
ID: 1161878
It's not because of the while(getchar()==-1) loop, I've tried that ! Whatever kind of loop you install, it spills out lots of Dr. Watsons, and the system just hangs until you kill all the Dr. Watsons and sometimes the Explorer too. I think the fault lies more in that the hook can't properly access the hook procedure CBTProc.
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Expert Comment

by:serge010697
ID: 1161879
One more thing I've noticed: you forget to use WINAPI prefix for CBTProc.
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Author Comment

by:Torsten021897
ID: 1161880
The help-topic on hooks describes this function just as it is, and if I include the WINAPI keyword, I can't do an _export from the DLL !
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Expert Comment

by:serge010697
ID: 1161881
Where exactly NT crashes?
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Author Comment

by:Torsten021897
ID: 1161882
NT crashes as soon as I try to invoke any other commands after the 'SetWindowsHookEx(...);' statement has been executed and I try to open a new Window, or try to change to another application while the program is running.
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