One open question regarding interaction with threading.  sigprocmask's behavior in a multithreaded program is unspecified.  pthread_sigmask should be used instead.  I could either expose both of these and let the caller choose, or I could make signal.sigprocmask use pthread_sigmask if it's available, and fall back to sigprocmask.  I don't see any disadvantages of doing the latter, and the former seems like it would create an attractive nuisance.  However, I don't have much experience with sigprocmask; I'm only interested in it because of its use in combination with signalfd.