Re: [OFFTOPIC?] Re: 2.2.x kernels missend odd-sized ICMP pa

Horst von Brand (vonbrand@sleipnir.valparaiso.cl)
Mon, 12 Apr 1999 07:31:28 -0400

Vojtech Pavlik <vojtech-lists@twilight.ucw.cz> said:
> On Sun, Apr 11, 1999 at 09:51:35PM -0400, Horst von Brand wrote:
> > (This is an excerpt from ping.c:in_cksum in netkit-0.10, sent to me by Petr
> > Vandrovec):

> > register u_short *w = addr;
> > register int sum = 0;
> > u_short answer = 0;
> > ....
> > *(u_char*)(&answer) = *(u_char*)w;
> > sum += answer;

> > This is a dumb way to write this.
> >
> > egcs-19990405 generates code that gets the right value, others (including
> > egcs-1.0.3 used to build RedHat-5.2, AFAIKS) break, as they don't respect
> > the initialization and put a random byte in the higher half.
> >
> > A better way to write the above is just:
> >
> > sum += *(u_char*)w;

> But, isn't this a compiler bug anyway? It shouldn't ignore the initialization
> in this case - actually I wonder why it would decide to.

Yes, it is a genuine compiler bug. One that can be worked around by writing
cleaner code, in this particular case ;-)

-- 
Horst von Brand                             vonbrand@sleipnir.valparaiso.cl
Casilla 9G, Viņa del Mar, Chile                               +56 32 672616


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