sigfd_set
sigfd = signalfd (-1, &sigfd_set, SFD_NONBLOCK | SFD_CLOEXEC);
sigfd = signalfd (-1, &sigfd_set, 0); /* retry without flags */
sigemptyset (&sigfd_set);
sigaddset (&sigfd_set, w->signum);
sigprocmask (SIG_BLOCK, &sigfd_set, 0);
signalfd (sigfd, &sigfd_set, 0);
sigdelset (&sigfd_set, w->signum);
signalfd (sigfd, &sigfd_set, 0);
VARx(sigset_t, sigfd_set)
#define sigfd_set ((loop)->sigfd_set)