On Fri, 2006-03-31 at 21:00 -0500, Jason Stephenson wrote: > Stephen Ryan wrote: > > hostmask = (1 << (32 - n)) - 1 > > netmask = ~ hostmask > > Doh! That's so obvious, so obviously, I overlooked it. ;)
Well, yes, of course :-) > > > > 1 << (32 - n) in binary is (n-1) '0' bits, a '1', then (32 - n) '0' > > bits. Subtracting 1 from that gives n '0' bits followed by (32 - n) '1' > > bits. The 'not' operator flips all the bits for the netmask. > > > > This works for /1 through /32 networks, even though some of those are > > nonsensical. A /0 might break this because of overflow (1 << (32 -n) > > overflows a 32-bit integer); theoretically, it should work even for /0 > > so long as 1 << (32-n) returns 0 (32-bit gcc 4.0 on my Athlon64 desktop > > computes this correctly, but complains 'warning: left shift count >= > > width of type' while compiling. Anyway, if you're running a /0, you've > > got other, bigger problems. > > Using gcc 3.4.4 on a 32-bit Pentium III, I get no warnings when > compiling your test program, even with -Wall. When it runs, 0 gives the > same result as 32, so it overflows (silently) on my machine. Ooh, here's something interesting. I first tried a test with constants, and got the warning: left shift count >= width of type" out of gcc. Then I rewrote the thing to use a loop, and I got correct results out of it. (This is all on an Athlon64X2.) When you described getting the same results for 0 and 32, I tried it again on my wife's laptop, a Celeron M, and got the same results you did; just out of curiosity, I looked it up, and gcc generates a 'sall' instruction to do the actual bit-shift, which on the 80386, PIII and Celeron M (according to Intel's documentation, your test, and my test, respectively) all ignore everything but the low 5 bits on the shift count. The 64-bit Athlon64 apparently doesn't, though, even though I'm supposedly running in 32-bit compatibility mode. Huh. Yet another demonstration of the dangers of of assuming specific bit-sizes for int, I suppose. _______________________________________________ gnhlug-discuss mailing list gnhlug-discuss@mail.gnhlug.org http://mail.gnhlug.org/mailman/listinfo/gnhlug-discuss
