> 	in an driver, what is the intention of the following code?
> 	it seems odd to me that it takes & between new if_flags value and
> 	old value.  i guess it should be ifp->if_flags only.

It may be written as follows, though I don't feel it is much better...

		if ((ifp->if_flags & (IFF_UP | IFF_RUNNING)) ==
		    (IFF_UP | IFF_RUNNING) &&
		    (sc->an_if_flags & (IFF_UP | IFF_RUNNING)) ==
		    (IFF_UP | IFF_RUNNING)) {

Anyway I added a comment why IFF_PROMISC is handled separately here:

@@ -1040,6 +1040,12 @@

 	switch (command) {
 	case SIOCSIFFLAGS:
+		/*
+		 * Handle special case for IFF_PROMISC.  If only IFF_PROMISC
+		 * flag is changed, do not call an_init() to avoid initiating
+		 * reassociation to another access point.  It is really
+		 * helpful for tcpdump(8).
+		 */
 		if ((ifp->if_flags & sc->an_if_flags &
 		    (IFF_UP | IFF_RUNNING)) == (IFF_UP | IFF_RUNNING)) {
 			if (ifp->if_flags & IFF_PROMISC &&

Regards,

Atsushi