InterlockedCompareExchange

[taazz]

any one has any inside for the InterlockedCompareExchange function and when/how is used? From my naive point of view it seems a bit unnecessary eg what is the point in comparing newvalue to comperand and then setting the target to it? when is this going to be needed?

[rtusrghsdfhsfdhsdfhsfdhs]

It is used for lock free structures such as queues etc..

[DonAlfredo]

I use (Delphi code)

Code: [Select]

if TInterlocked.Exchange(TimersBusy, integer(True))<>integer(True) then
begin
    try
      // do something time-consuming
    finally
      TInterlocked.Exchange(TimersBusy, integer(False));
    end;
end;
a lot in timer code to prevent re-entrant-errors

[taazz]

I do understand some of the use cases for InterlockedExchange, I'm guessing that there are far more than I can see for now, but I can't understand the compare part. I'm used in ranged compares for values eg case, between etc but I can't see a use case where I would like to set a value only when it is equal to X where X is very specific ee not a range or something different than the new value. Lets simplify things a bit so we can talk on the same think if I was to re-create those two functions using critical sections for example it would go something along the lines of

Code: [Select]

function InterLockedExchange(var Target: Pointer;Source: Pointer):Pointer;
begin
  EnterCriticalSection(myCS);
  try
    result := target;
    target := Source;
  finally
    LeaveCriticalSection(myCS);
  end;
end;
function InterlockedCompareExchange(var Target: Pointer;  NewValue: Pointer;  Comperand: Pointer):Pointer;
begin
  EnterCriticalSection(myCS);
  try
    Result := target;
    if NewValue = Comperand then target := NewValue;
  finally
    LeaveCriticalSection(myCS);
  end;
end;

and that makes no sense to me at all. Any cases I could take a look? For now is categorized as "nice idea no real value".

[DonAlfredo]

AFAIK, CS-locks are (much) more expensive than a simple InterLocked[Compare]Exchange.