scripting locks

[SteveL]

Is there an accepted way to enforce concurrency in TDM scripts? For example, the kind of situation where you want to share a single scripted procedure between many AI, but want only one of them to be executing the procedure at any given time.

 

Naive example:

float someoneDoingX = 0;

void doX(entity ent)
{
if (someoneDoingX == 0)
{
	someoneDoingX = 1;
	// make ent do something;
	someoneDoingX = 0;
}
}

 

Obviously this code can run into trouble if entity A starts to doX, but then entity B starts to doX too and changes the global flag in between entity A's flagcheck and entity A's first update of the flag.

[grayman]

What you're doing is correct. If you expect to wait for the AI to finish what you've asked him to do, you'll need a sys.wait(N) (where N is the number of seconds) statement after the statement that makes him do it. That will hold the lock until the wait() expires.

[SteveL]

Thanks for the reply grayman. My code isn't quite thread safe though is it? If two AIs hit the trigger simultaneously, both could perform the check on someoneDoingX before either of them updates it. I guess the chances are vanishingly small.

[Obsttorte]

float someoneDoingX = 0;

void doX(entity ent)
{
       if (someoneDoingX == 0 && someoneDoingX = 1;)
       {
               // make ent do something;
               someoneDoingX = 0;
       }
}

[grayman]

If you're worried about thread safety, create a dummy entity named "Lock" in your map, and do this:

 

void doX (entity ent)
{
 if ( !$Lock.getFloatKey("someoneDoingX") )
 {
	  $Lock.setKey("someoneDoingX",1);
	  // make ent do something
	  $Lock.setKey("someoneDoingX",0);
 }
}

void main()
{
    $Lock.setKey("someoneDoingX",0);
}	

 

And you don't even need to create a separate $Lock entity for this. You could put the key/value on any entity in your map, as long as it's one that's going to be around for the entire mission.