#ok:
write(1, "got signal.\n", 12got signal.
)           = 12
wait4(-1, 0x7fff97d3635c, WNOHANG, NULL) = -1 ECHILD (No child processes)
rt_sigprocmask(SIG_BLOCK, [USR1], [USR1], 8) = 0
rt_sigaction(SIGUSR1, {0x4ba320, [], SA_RESTORER, 0x7ff73ec382e0}, {0x4ba320, [], SA_RESTORER, 0x7ff73ec382e0}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [USR1], NULL, 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [USR1], NULL, 8) = 0
rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0

#not ok:
write(1, "got signal.\n", 12got signal.
)           = 12
wait4(-1, 0x7fff97d3635c, WNOHANG, NULL) = -1 ECHILD (No child processes)
rt_sigprocmask(SIG_BLOCK, [USR1], [USR1], 8) = 0
rt_sigaction(SIGUSR1, {0x4ba320, [], SA_RESTORER, 0x7ff73ec382e0}, {0x4ba320, [], SA_RESTORER, 0x7ff73ec382e0}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [USR1], NULL, 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [USR1], NULL, 8) = 0
rt_sigaction(SIG_0, NULL, {0x8ab3b8, [], SA_NOCLDSTOP}, 8) = -1 EINVAL (Invalid argument)