Exit from DLL loop by the event generated in VB

[Jeong]

In my program(VB code), call dll procedure(coded by C).
If click "stop" button in VB, must quit from the loop of DLL.
How can detect in dll(c code) whether "stop" button is clicked in VB?
In other words, even if control is passed to DLL, if VB generate some event, DLL must detect it?
Could anyone who know let me know the way?
Thanks.

I'm a senior programmer.
Working at CamSight, Dental imaging solutions industry.