Delphi Memory Mapped Files Notice new data? - delphi

Question

althoug my questionn might be noob-ish, let me explain:
I recently started playing with MMF, created 2 processes wich access the same memory Pointer, Process1 writes an integer to MMF, Process2 has a button, which onClick, it displays the first integer in MMF.
What i want to do is, when i "send", write data from Process1 to MMF, Process2 Notices this request ontime, and displays the data on exact time, and so on with new data written.
I'm not sure whether a Thread checking for changes in MMF would be ok, sounds Dirty.
Hope somebody could point me out a solution, because i'm out of ideas :(.
Here's a piece of code:
procedure OpenMap;
var
llInit: Boolean;
lInt: Integer;
begin
if Hmapping<>0 then Exit;
HMapping := CreateFileMapping($FFFFFFFF, nil, PAGE_READWRITE,
0, MAPFILESIZE, pchar('wowsniff'));
// Check if already exists
llInit := (GetLastError() <> ERROR_ALREADY_EXISTS);
if (hMapping = 0) then
exit;
PMapData := MapViewOfFile(HMapping, FILE_MAP_ALL_ACCESS, 0, 0, 0);
if PMapData = nil then
exit;
if (llInit) then
begin
// Init block to #0 if newly created
FillChar(PMapData^, MAPFILESIZE, 0);
end
end;
procedure TForm1.Button3Click(Sender: TObject);
begin
LockMap;
PDword(PMapData)^:=Strtoint(edit1.Text);
UnlockMap;
end;

Answer 1

Use a named event object for that, either via TEvent or CreateEvent(). Both processes can create the same event name (just like they are creating the same mapping name), then Process 1 can signal the event whenever it writes new data, and Process 2 can wait for the event to be signaled before reading the data (for real-time reading, you should use a thread for the waiting/reading).
You can use a named mutex object, via TMutex or CreateMutex(), to implement your lock/unlock functionality when reading/writing the data.
Try something like this:
Process 1 :
procedure OpenMap;
var
llInit: Boolean;
begin
llInit := False;
if hMapEvent = 0 then
begin
hMapEvent := CreateEvent(nil, True, False, PChar('wowsniffDataReady'));
if hMapEvent = 0 then RaiseLastOSError;
end;
if hMapLock = 0 then
begin
hMapLock := CreateMutex(nil, False, PChar('wowsniffDataLock'));
if hMapLock = 0 then RaiseLastOSError;
end;
if hMapping = 0 then
begin
hMapping := CreateFileMapping(INVALID_HANDLE_VALUE, nil, PAGE_READWRITE, 0, MAPFILESIZE, PChar('wowsniff'));
if hMapping = 0 then RaiseLastOSError;
// Check if already exists
llInit := (GetLastError() <> ERROR_ALREADY_EXISTS);
end;
if PMapData = nil then
begin
PMapData := MapViewOfFile(hMapping, FILE_MAP_WRITE, 0, 0, MAPFILESIZE);
if PMapData = nil then RaiseLastOSError;
if llInit then
begin
// Init block to #0 if newly created
ZeroMemory(PMapData, MAPFILESIZE);
end;
end;
end;
procedure TForm1.Button3Click(Sender: TObject);
begin
LockMap;
try
PDword(PMapData)^ := StrToInt(Edit1.Text);
SetEvent(hMapEvent);
finally
UnlockMap;
end;
end;
procedure TForm1.LockMap;
var
llRet: DWORD;
begin
llRet := WaitForSingleObject(hMapLock, 5000);
if llRet = WAIT_OBJECT_0 then Exit;
if llRet <> WAIT_FAILED then SetLastError(llRet);
RaiseLastOSError;
end;
procedure TForm1.UnlockMap;
begin
ReleaseMutex(hMapLock);
end;
Process 2:
type
TMyThread = class(TThread)
private
hTerminate: THandle;
hMapLock: THandle;
hMapEvent: THandle;
hMapping: THandle;
PMapData: Pointer;
protected
procedure Execute; override;
procedure DoTerminate; override;
procedure TerminatedSet; override;
public
constructor Create;
destructor Destroy; override;
end;
constructor TMyThread.Create;
begin
inherited Create(False);
hTerminate := CreateEvent(nil, True, False, nil);
if hTerminate = 0 then RaiseLastOSError;
end;
destructor TMyThread.Destroy;
begin
if hTerminate <> 0 then CloseHandle(hTerminate)
end;
procedure TMyThread.TerminatedSet;
begin
SetEvent(hTerminate);
end;
procedure TMyThread.Execute;
var
llInit: Boolean;
llRet, llValue: DWORD;
llHandles: array[0..1] of THandle;
begin
hMapEvent := CreateEvent(nil, True, False, PChar('wowsniffDataReady'));
if hMapEvent = 0 then RaiseLastOSError;
hMapLock := CreateMutex(nil, False, PChar('wowsniffDataLock'));
if hMapLock = 0 then RaiseLastOSError;
hMapping := CreateFileMapping(INVALID_HANDLE_VALUE, nil, PAGE_READWRITE, 0, MAPFILESIZE, PChar('wowsniff'));
if hMapping = 0 then RaiseLastOSError;
// Check if already exists
llInit := (GetLastError() <> ERROR_ALREADY_EXISTS);
PMapData := MapViewOfFile(hMapping, FILE_MAP_WRITE, 0, 0, MAPFILESIZE);
if PMapData = nil then RaiseLastOSError;
if llInit then
begin
// Init block to #0 if newly created
FillChar(PMapData^, MAPFILESIZE, 0);
end;
llHandles[0] := hMapEvent;
llHandles[1] := hTerminate;
while not Terminated do
begin
llRet := WaitForMultipleObjects(2, PWOHandleArray(#llHandles), False, INFINITE);
case llRet of
WAIT_OBJECT_0+0:
begin
llRet := WaitForSingleObject(hMapLock, 5000);
if llRet = WAIT_OBJECT_0 then
begin
try
llValue := PDword(PMapData)^;
ResetEvent(hMapEvent);
finally
ReleaseMutex(hMapLock);
end;
// use llValue as needed...
Continue;
end;
end;
WAIT_OBJECT_0+1:
begin
Exit;
end;
end;
if llRet <> WAIT_FAILED then SetLastError(llRet);
RaiseLastOSError;
end;
end;
procedure TMyThread.DoTerminate;
begin
if PMapData <> nil then UnmapViewOfFile(PMapData);
if hMapping <> 0 then CloseHandle(hMapping);
if hMapEvent <> 0 then CloseHandle(hMapEvent);
if hMapLock <> 0 then CloseHandle(hMapLock);
inherited;
end;