multithreading - Delphi - timer inside thread generates AV

Question

I have the following thread code which executes correct first time. After that from time to time I get an AV on the Execute method of the thread, e.g

Debug Output: TProcesses.Execute Access violation at address 00409C8C in module 'ListenOutputDebugString.exe'. Read of address 08070610 Process ListenOutputDebugString.exe (740)

I don't know what is generating this AV...

unit Unit3;

interface

uses
  Classes,
  StdCtrls,
  Windows,
  ExtCtrls,
  SysUtils,
  Variants,
  JvExGrids,
  JvStringGrid;

type
  TProcesses = class(TThread)
  private
    { Private declarations }
    FTimer :  TTimer;
    FGrid  :  TJvStringGrid;
    FJobFinished : Boolean;
    procedure OverrideOnTerminate(Sender: TObject);
    procedure DoShowData;
    procedure DoShowErrors;
    procedure OverrideOnTimer(Sender: TObject);
  protected
    procedure Execute; override;
  public
    constructor Create(aGrid : TJvStringGrid);overload;
  end;

implementation

{TProcesses }

var SharedMessage : String;
    ErrsMess      : String;
    lp            : Integer;

constructor TProcesses.Create(aGrid : TJvStringGrid);
begin
 FreeOnTerminate := True;
 FTimer := TTimer.Create(nil);
 FTimer.OnTimer := OverrideOnTerminate;
 FTimer.OnTimer := OverrideOnTimer;
 FTimer.Interval := 10000;
 FGrid := aGrid;
 inherited Create(false);
 FTimer.Enabled := true;
 FJobFinished := true;
end;

procedure TProcesses.DoShowData;
var wStrList : TStringList;
    wi,wj : Integer;
begin
// FMemo.Lines.Clear;
 for wi := 1 to FGrid.RowCount-1 do
  for wj := 0 to FGrid.ColCount-1 do
   FGrid.Cells[wj,wi] := '';
 try
  try
  wStrList := TStringList.Create;
  wStrList.Delimiter := ';';
  wStrList.StrictDelimiter := true;
  wStrList.DelimitedText := SharedMessage;
//  outputdebugstring(PChar('Processes list '+SharedMessage));
  FGrid.RowCount := wStrList.Count div 4;
  for wi := 0 to wStrList.Count-1 do
    FGrid.Cells[(wi mod 4), (wi div 4)+1] := wStrList[wi];
  Except on e:Exception do
   OutputDebugString(Pchar('TProcesses.DoShowData '+e.Message));
  end;
 finally
  FreeAndNil(wStrList);
 end;
end;

procedure TProcesses.DoShowErrors;
begin
// FMemo.Lines.Add('Error '+ ErrsMess);
 FGrid.Cells[1,1] := 'Error '+ ErrsMess;
 ErrsMess := '';
end;

procedure TProcesses.Execute;
  function EnumProcess(hHwnd: HWND; lParam : integer): boolean; stdcall;
  var
    pPid : DWORD;
    title, ClassName : string;
  begin
    //if the returned value in null the
    //callback has failed, so set to false and exit.
    if (hHwnd=NULL) then
    begin
      result := false;
    end
    else
    begin
      //additional functions to get more
      //information about a process.
      //get the Process Identification number.
      GetWindowThreadProcessId(hHwnd,pPid);
      //set a memory area to receive
      //the process class name
      SetLength(ClassName, 255);
      //get the class name and reset the
      //memory area to the size of the name
      SetLength(ClassName,
                GetClassName(hHwnd,
                             PChar(className),
                             Length(className)));
      SetLength(title, 255);
      //get the process title; usually displayed
      //on the top bar in visible process
      SetLength(title, GetWindowText(hHwnd, PChar(title), Length(title)));
      //Display the process information
      //by adding it to a list box
      SharedMessage := SharedMessage +
        (className +' ;'+//'Class Name = ' +
         title +' ;'+//'; Title = ' +
         IntToStr(hHwnd) +' ;'+ //'; HWND = ' +
         IntToStr(pPid))+' ;'//'; Pid = ' +
         ;//         +#13#10;
      Result := true;
    end;
  end;
begin
if FJobFinished  then
 begin
  try
   FJobFinished := false;
  //define the tag flag
   lp := 0; //globally declared integer
  //call the windows function with the address
  //of handling function and show an error message if it fails
  SharedMessage := '';
  if EnumWindows(@EnumProcess,lp) = false then
   begin
      ErrsMess := SysErrorMessage(GetLastError);
      Synchronize(DoShowErrors);
   end
   else
    Synchronize(DoShowData);
   FJobFinished := true;
  Except on e:Exception do
   OutputDebugString(Pchar('TProcesses.Execute '+e.Message));
  end;
 end
end;

procedure TProcesses.OverrideOnTerminate(Sender: TObject);
begin
 FTimer.Enabled := false;
 FreeAndNil(FTimer);
end;

procedure TProcesses.OverrideOnTimer(Sender: TObject);
begin
  Self.Execute;
end;

end.

Answer 1

I would never use timer in a thread. Instead I would create a system event and wait for it in the thread's execution loop for a specified time with the WaitForSingleObject function. This function waits until the specified object (in this case the event) is in the signalled state or the time-out interval elapses.

The principle is easy, you'll create the event in the non-signalled state and keep it in that state until the thread is going to be terminated. This will result the WaitForSingleObject function to timeout every time what blocks your thread execution loop for the time specified in the function call. Once you'll decide to terminate your thread you just set the thread's termination flag (on which you should ask as much as you can) and set that event to the signalled state what causes the WaitForSingleObject function to return immediately.

Here is an example which simulates a thread timer (with 2 seconds interval = 2000ms used as a second parameter in WaitForSingleObject function calls):

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TTimerThread = class(TThread)
  private
    FTickEvent: THandle;
  protected
    procedure Execute; override;
  public
    constructor Create(CreateSuspended: Boolean);
    destructor Destroy; override;
    procedure FinishThreadExecution;
  end;

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    FTimerThread: TTimerThread;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  ReportMemoryLeaksOnShutdown := True;
  FTimerThread := TTimerThread.Create(False);
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  FTimerThread.FinishThreadExecution;
end;

{ TTimerThread }

constructor TTimerThread.Create(CreateSuspended: Boolean);
begin
  inherited;
  FreeOnTerminate := True;
  FTickEvent := CreateEvent(nil, True, False, nil);
end;

destructor TTimerThread.Destroy;
begin
  CloseHandle(FTickEvent);
  inherited;
end;

procedure TTimerThread.FinishThreadExecution;
begin
  Terminate;
  SetEvent(FTickEvent);
end;

procedure TTimerThread.Execute;
begin
  while not Terminated do
  begin
    if WaitForSingleObject(FTickEvent, 2000) = WAIT_TIMEOUT then
    begin
      Synchronize(procedure
        begin
          Form1.Tag := Form1.Tag + 1;
          Form1.Caption := IntToStr(Form1.Tag);
        end
      );
    end;
  end;
end;

end.