Have the GUI set a flag that stops the loop from looping.
eg:
```
static int keepgoing;
void stopnow()
{
keepgoing = 0;
}
void enterthread()
{
keepgoing = 1;
prepare(...)
while (keepgoing)
(
step(...)
reset(...)
sleep(...)
}
finalize(...)
}
```
Dispatch enterthread on its own thread. Then just have the GUI change keepgoing to 0 (call stopnow()). Unless of course the statement is taking hours to run ... if it is only taking a few miliseconds then the overhead of even bothering to do anything else is hardly worth it.