Thread-safe in delphi

by

Writing a thread safe code in Delphi involves the basic care you would have in any other language, which means to deal with race conditions. A race condition happens when different threads access the same data. A good way to deal with that is to declare an instance of TCriticalSection and wrap the dangerous code in it.

The code below shows a getter and a setter of a property that, by hypotesis, has a race condition.

constructor TMyThread.Create;
begin
  CriticalX := TCriticalSection.Create;
end;

destructor TMyThread.Destroy; override;
begin
  FreeAndNil(CriticalX);
end;

function TMyThread.GetX: string;
begin
  CriticalX.Enter;
  try
    Result := FX;
  finally
    CriticalX.Leave;
  end;
end;

procedure TMyThread.SetX(const value: string);
begin
  CriticalX.Enter;
  try
    FX := Value;
  finally
    CriticalX.Leave;
  end;
end;

Notice the use of a single instance of TCriticalSection (CriticalX) to serialize the access to the data member FX.

However, with Delphi you have an aditional consideration! VCL is not thread safe, so in order to avoid VCL race conditions, any operation that results in screen changing must run in the main thread. You get that by calling such a code inside a Synchronize method. Considering the class above, you should do something like this:

procedure TMyThread.ShowX;
begin
  Synchronize(SyncShowX);
end;

procedure TMyThread.SyncShowX;
begin
  ShowMessage(IntToStr(FX));
end;

If you have Delphi 2010 or later, there is an easier way that makes use of anonymous methods:

procedure TMyThread.ShowX;
begin
  Synchronize(procedure begin
    ShowMessage(IntToStr(FX));
  end);
end;

I hope this helps!

More Related Contents:

  1. Difference between -pthread and -lpthread while compiling
  2. What does threadsafe mean?
  3. How Do I Choose Between the Various Ways to do Threading in Delphi?
  4. Simple Thread Sample Delphi
  5. Is it better to use TThread’s “Synchronize” or use Window Messages for IPC between main and child thread?
  6. What is the difference between a process and a thread?
  7. What is the difference between concurrency, parallelism and asynchronous methods?
  8. Multi-threading in VBA
  9. What is a mutex?
  10. Is this an idiomatic worker thread pool in Go?
  11. Redirecting stdout and stderr to a PyQt4 QTextEdit from a secondary thread
  12. Thread context switch Vs. process context switch
  13. Where do I get a thread-safe CollectionView?
  14. Binary to Base64 (Delphi)
  15. Odd even number printing using thread
  16. Workaround for ncurses multi-thread read and write
  17. iphone ios running in separate thread
  18. Inflate a view in a background thread
  19. How to manage db connections on server?
  20. How efficient is locking and unlocked mutex? What is the cost of a mutex?
  21. Multithreaded & SIMD vectorized Mandelbrot in R using Rcpp & OpenMP
  22. How does x86 pause instruction work in spinlock *and* can it be used in other scenarios?
  23. “Collection was mutated while being enumerated” on executeFetchRequest
  24. Is Stopwatch.ElapsedTicks threadsafe?
  25. Prolog find all paths Implementation
  26. Is Cipher thread-safe?
  27. What advantage(s) does dispatch_sync have over @synchronized?
  28. Can we have race conditions in a single-thread program?
  29. Does multi-threading improve performance? How?
  30. SqlConnection Thread-Safe?

Leave a Comment