With FreeBSD 8.0 it seems like you can't block pending but undelivered signals

[Jeremy Allison]

On Tue, Jan 08, 2013 at 11:15:39AM -0800, Richard Sharpe wrote:
> 
> OK, it seems that it is not so bad. Based on a response I got from the
> FreeBSD Hackers mailing list, and some digging, I think the answer is:
> 
>             sigaddset(&((ucontext_t *)uctx)->uc_sigmask, sig);
> 
> I will try this soon and get back to you.
> 
> >From here: http://stackoverflow.com/questions/5531006/returning-from-a-signal-handler-and-leaving-signal-masked

Oh God that is a *horrible* hack :-).