sets up information about the current stack frame of an executing cygwin
process. Any function which uses 'sigframe thisframe' should be signal
aware. It should detect when a signal has arrived and return
-immediately.
+immediately. This method is also used throughout the DLL to ensure
+accurate frame info for the executing function. So, you'll see it
+sprinkled liberally throughout the DLL, usually at places where
+empirical tests have indicated problems finding this address via the
+brute force method stack walking method employed in setup_handler.
So, if mainframe is active, that means that we have good information
about the state of the main thread. Cygwin uses the stack frame info