How to check for pending CancelRequest from C/C++ function

Hi all,

I writing some functions for postgresql extension in C/C++ and I would
like to be able to check for a pending CancelRequest from the frontend
so I can take appropriate action to honor this request.

1. How to I check if a request is pending?
2. What is the appropriate action?
    I assume this should be something like:
    a. cleanup as needed
    b. elog(ERROR, "terminated at user cancel request.");

Is there an example of this in contrib or elsewhere that you can point
me to?

Thanks,
   -Steve