sk</SPAN></B>(2).
If <I>how</I> is <B>SIG_SETMASK</B>, the signal mask is set to
<I>newmask</I>. If <I>how</I> is <B>SIG_BLOCK</B>, the signals
specified to <I>newmask</I> are added to the current signal mask. If
<I>how</I> is <B>SIG_UNBLOCK</B>, the signals specified to <I>newmask</I>
are removed from the current signal mask. 
</P>
<P>Recall that signal masks are set on a per-thread basis, but signal
actions and signal handlers, as set with <B>sigaction</B>(2), are
shared between all threads. 
</P>
<P><B>p