Fantastic articles athena. I will definitely read 'em all.

Multithreading still scares me a bit and the only way to overcome that is by rolling up the sleeves haha... I would like to use them in my engine, to separate rendering and audio from the rest of the code.

I really need to write an article too btw. There aren't that many and new site does a good job at presenting them.

Originally Posted by chronozphere

I really need to write an article too btw. There aren't that many and new site does a good job at presenting them.

Feel free! We need a lot more beginner articles. Either an introduction to a specific API or a completely non-API specific article about the basics of game programming.

Thanks

It's not quite as scary as it may seem at first... although I speak from the point of view of someone who nearly got fired because the first time they used multi-threading was in a business critical application which kept crashing because I messed up

After that, no programming problem has ever seemed quite so scary

nice stuff. i have written one slightly less popular article as it would seem but am working on another as will would tell you with the newbie-frindly project or something

AthenaOfDelphi,
Do you have any comments on suspend and resume procedures being deprecated starting from D2009. Since you are using these methods in your examples it may confuse some people because stuff normally gets deprecated when better ways of doing things become available.

Dan, apologies for the delay... well, what can I say... I didn't realise they were deprecated, certainly my compiler doesn't complain about anything (I get the feeling from reading one or two articles it should complain).

Whilst I understand the theories I've read, I can honestly say I've (a) never used the suspend/resume mechanism for synchronising threads and (b) never had any problems using suspend/resume. I generally use it for job processing type scenarios where the thread put's itself to sleep until it is needed. Replacing these calls with an eventing mechanisms (wait_for_single_object IIRC) shouldn't be too much of a problem. Something like this:-

Code:

unit classSuspendableThread;

interface

uses
  sysUtils, classes, windows;

type
  TSuspendableThread = class(TThread)
  protected
    fWakeUpEvent        : THandle;
    fSleeping           : boolean;

    procedure goToSleep;
  public
    constructor create(createSuspended:boolean);
    destructor Destroy; override;

    procedure terminate;

    procedure wakeUp;

    property sleeping:boolean read fSleeping;
  end;

implementation

constructor TSuspendableThread.create(createSuspended:boolean);
var
  temp    : string;
  nameBuf : PWideChar;
begin
  inherited create(createSuspended);

  temp:='WUE-'+intToHex(self.ThreadId,8);
  nameBuf:=strAlloc(length(temp));
  strPCopy(nameBuf,temp);
  fWakeupEvent:=createEvent(nil,false,false,nameBuf);
  strDispose(nameBuf);
end;

destructor TSuspendableThread.destroy;
begin
  closeHandle(fWakeupEvent);

  inherited;
end;

procedure TSuspendableThread.terminate;
begin
  TThread(self).terminate;

  if (fSleeping) then
  begin
    wakeup;
  end;
end;

procedure TSuspendableThread.wakeUp;
begin
  if (fSleeping) then
  begin
    try
      setEvent(fWakeupEvent);
    except
    end;
  end;
end;

procedure TSuspendableThread.goToSleep;
begin
  fSleeping:=true;
  waitForSingleObject(fWakeupEvent,INFINITE);
  fSleeping:=false;
end;

end.

This class works, call 'wakeUp' instead of resume (although resume, or it's replacement start would still be needed if you created it suspended). You will of course note that the 'gotoSleep' method which replaces suspend is now protected. Calling it from anywhere other than the execute method will hold the calling thread.

This solution is far from perfect. The Windows API calls keep a count of suspends and resumes which this class does not. I suspect there are numerous ways of solving the puzzle and your solution will depend very much on usage.

Do I think deprecating them is a good idea... it would be a cold day in hell if I said yes. They are legitimate mechanisms for controlling threads (and as I say, ones I've used myself on numerous occasions with no problems). So, I've just gone looking for an answer. I think this excerpt sums it up nicely (taken from this article by Peter Richie):-

For time eternal there has been a way to suspend (and by association resume) another thread. It wasn't until .NET 2.0 that someone took leadership and conceded that suspending another thread is not a safe thing to do. .NET 2.0 deprecates System.Threading.Thread.Suspend() and Resume() (although, Resume() isn't dangerous by association...).

Basically, Suspend() has no care that the thread may actually be managing one or more complex invariants that can't be changed atomically. Normally with complex invariants synchronization is used to ensure while one thread is modifying the individual components of an invariant another thread doesn't try to access it. This is important because the validity of an invariant is rarely atomic; meaning changing an invariant may take several steps and may be in an unknown or invalid state between the first and the last step (a date is a good example, setting the day and month is two steps; until both steps are complete the date might be invalid). Suspending a thread circumvents the synchronization primitives and cuts the thread off at the knees.

Basically, calling suspend from outside a thread is bad... well all I can say is "there's a suprise". I've never done it and I wouldn't encourage you to do it for the reasons Peter gives.... the caller of suspend has no idea about the status of the thread. So, maybe a better solution would to have been to change visibility of the suspend method so that it was only accessible to the thread itself.

Who knows, but I certainly don't agree with removing them as it's going to mean we have some changes to do at work when we finally port to an up to date Delphi version.