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Memory mapped files troubles

Posted on 2003-11-26
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Last Modified: 2007-12-19
I’m having a bit of trouble with a dll that uses memory mapped files. Since my application must receive key presses when out of focus, I used a small bit of delphi code I found on the net to build a keyhook. The dll in in the apps resources as a piece of rcdata, and is unpacked to a temporary file, then loaded, and finally deleted when the app is done. Or it should be. If I just open the app, then close it, the file is deleted fine. But if I trigger a keypress, then close it, nu-uh. After reading the sdk doc for DeleteFile, I figured it was the memory mapped files doing this. But I can’t figure what I am doing wrong. Here’s the applicable code:

{ Dll loading code }

var
    DllHandle, MemFile: THandle;
    Receiver: PHandle;
    HookOn, HookOff: procedure; stdcall;

...

  MemFile := CreateFileMapping($FFFFFFFF, nil, PAGE_READWRITE, 0, SizeOf(Integer), 'KeyReceiver');
  if (MemFile = 0) then
    raise Exception.Create('Error while creating file');

  Receiver := MapViewOfFile(MemFile, FILE_MAP_WRITE, 0, 0, 0);
  Receiver^ := Handle;

  // Load the keyhook
  DllHandle := LoadLibrary(PChar(DllFile)); //DllFile contains the temp name
  if DllHandle = 0 then
    raise Exception.Create('Unable to load the required DLL');

  @HookOn := GetProcAddress(DllHandle, '_HookOn@0');
  @HookOff := GetProcAddress(DllHandle, '_HookOff@0');
  if (not Assigned(HookOn)) or (not Assigned(HookOff)) then
    raise Exception.Create('Unable to locate the required DLL functions');

  HookOn;

Here’s the DLL init code:

HHOOK Keybrd;
HANDLE MemFile, DllInst;
HWND *Receiver;

...

  switch (ul_reason_for_call)
  {
    case DLL_PROCESS_ATTACH:
      DllInst = hModule;
      MemFile = OpenFileMapping(FILE_MAP_READ, FALSE, "KeyReceiver");
      if (MemFile != 0)
        Receiver = MapViewOfFile(MemFile, FILE_MAP_READ, 0, 0, 0);      
      break;
    case DLL_PROCESS_DETACH:
      if (MemFile != 0) {
        UnmapViewOfFile(Receiver);
        CloseHandle(MemFile);
      }
      break;
  }

Hook activation code:

Keybrd = SetWindowsHookEx(WH_KEYBOARD, &HookCallBack, DllInst, 0);

Deactivation code:

UnhookWindowsHookEx(Keybrd);

The code to send a message to the app:

  if (Code == HC_ACTION)
    PostMessage(*Receiver, CM_SEND_KEY, wParam, lParam);

  return CallNextHookEx(Keybrd, Code, wParam, lParam);

And the code that frees the app’s vars and deletes (or tries to) the dll:

  if Assigned(HookOff) then
    HookOff;

  if (DllHandle > 0) then
    FreeLibrary(DllHandle);

  if (MemFile > 0) then begin
    UnmapViewOfFile(Receiver);
    CloseHandle(MemFile);
  end;

  if FileExists(DllFile) then
    DeleteFile(DllFile);

The most I can find is that windows reports Access is denied when it tries to delete the file (if I have pressed a key, if not, it deletes fine). Also, the file can be deleted manually once the app is closed.

If any more info is needed, just say.

Thanks!
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Question by:SpiderVenom
4 Comments
 
LVL 6

Accepted Solution

by:
GloomyFriar earned 125 total points
ID: 9831382
Does the DllFile contain right pathname at the error point?
If so, then someone is using the dll and that is why you can't delete it.
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LVL 1

Expert Comment

by:pucko
ID: 25505444
Hmm force accept on a Question the same day?
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