Question

I have a Win32 Thread (no TThread) that runs alle the time and iterates over a static array. The mainthread can modify fields of the array. What is the best way to make this thread-safe without components like TThreadList (for a no-vcl application), only with Windows Critical Sections (TRTLCriticalSection)?

Code:

type
  T = record
    Idx: Integer;
    Str: string;
    Num: Real;
    Enabled: Boolean;
  end;

var
  A: Array[0..9] of T;
  Cnt: Integer;
  CS: TRTLCriticalSection;

procedure thread;
var
  I: Integer;
begin
  while True do
  begin
    for I := Low(A) to High(A) do
    begin
      if A[I].Enabled then
      begin
        //modify some fields from A[I]

        Inc(A[I].Idx);
        if A[I].Idx >= 10 then
        begin
          A[I].Enabled := False;
          InterlockedDecrement(Cnt);
        end;
      end;
    end;
    if Cnt = 0 then Sleep(1);
  end;
end;

procedure Add(...); //called only from mainthread

  function GetFreeField: Integer;
  begin
    for Result := Low(A) to High(A) do
      if not A[Result].Enabled then Exit;
    Result := -1;
  end;

var
  I: Integer;
begin
  I := GetFreeField;
  if I = -1 then Exit;

  //set fields A[I]

  A[I].Enabled := True;
  InterlockedIncrement(Cnt);
end;

At the beginning the array is initialized with enabled = false and cnt = 0.

Is the following modification sufficient?

procedure thread;
var
  I: Integer;
begin
  while True do
  begin
    for I := Low(A) to High(A) do
    begin
      EnterCriticalSection(CS);
      if A[I].Enabled then
      begin
        LeaveCriticalSection(CS);
        //modify some fields from A[I]

        Inc(A[I].Idx);
        if A[I].Idx >= 10 then
        begin
          EnterCriticalSection(CS);
          A[I].Enabled := False;
          LeaveCriticalSection(CS);

          InterlockedDecrement(Cnt);
        end;
      end
      else
        LeaveCriticalSection(CS);
    end;
    if Cnt = 0 then Sleep(1);
  end;
end;

procedure Add(...); //called only from mainthread
var
  I: Integer;
begin
  I := GetFreeField;
  if I = -1 then Exit;

  //set fields A[I]

  EnterCriticalSection(CS);
  A[I].Enabled := True;
  LeaveCriticalSection(CS);

  InterlockedIncrement(Cnt);
end;

Answer 1

It looks to me as though your design is that:

1. The main thread only ever switches the Enabled flag from False to True.
2. The worker thread only ever switches the flag in the opposite direction.
3. No code other than what we see here accesses the array.

If that is true, the original code without the critical section is already thread safe. At least it is on hardware that uses a strong memory model. For example the Intel x86 or x64 architectures. The Enabled boolean acts as a synchronisation barrier between the threads.

However, your entire design looks flawed to me. The while True loop and the Sleep causes me some alarm. That thread is going run repeatedly for no good reason. Surely you should only be executing the code in the thread when the main thread has made modifications to the array. I’d prefer the use of a signal (for example a Windows event) to wake up the thread.