So, my questions are:
1) Does anyone know what exactly happens when control-C is pressed during foreign function evaluation?
Yes. Two cases: if the program hasn't requested that Ctrl-C be trapped (using the Posix library's installHandler), then the SIGINT handler just sets a flag which causes the scheduler to kill all the running threads and exit. If the program is in a foreign call, then the scheduler won't get to run until the foreign call returns. If the program has installed a handler for SIGINT (which GHCi does), then the SIGINT handler in the RTS sets a different flag. The next time the scheduler runs, it creates a new thread for the handler and puts it on the run queue. In the case of GHCi, the handler thread raises an exception in the main thread, which is caught by the GHCi loop. Again, if the main thread is in a foreign call, none of this will happen until the call returns.
2) Does anyone know how this can be controlled in a more finegrained way?
We don't have a good solution for this at the moment. Where possible, don't do blocking foreign calls. The threadWaitRead/threadWaitWrite primitives can be used to avoid blocking if you know the file descriptor that the foreign call is going to block on. Cheers, Simon