The new mask is the union of the current mask and the specified set.
The new mask is the intersection of the current mask and the complement of the specified set.
The current mask is replaced by the specified set.
If oset is not null, it is set to the previous value of the signal mask. When set is null, the value of how is insignificant and the mask remains unset providing a way to examine the signal mask without modification.
The system quietly disallows SIGKILL or SIGSTOP to be blocked.