Re: [lwip-users] [PATCH] fix warning for gcc and possible unaligned access

[Pedro Alves]

address@hidden wrote:

> Hi,
>  
> Why not just replace the lines
>
>     *(struct ip_addr2 *)&sipaddr = hdr->sipaddr;
>     *(struct ip_addr2 *)&dipaddr = hdr->dipaddr;
>
> with
>
>     sipaddr = *(struct ip_addr *)&hdr->sipaddr;
>     dipaddr = *(struct ip_addr *)&hdr->dipaddr;

Because if hdr->sipaddr is unaligned and the architecture doesn't 
support unaligned accesses, like many RISCs do, the result is undefined.

> PS.
> Haven't tested it, and dont now why one uses "struct ip_addr" and 
> "struct ip_addr2"
> DS.
Don't know that either.

Cheers,
Pedro Alves


> /Beach/
>  
>  
>
>
> On 4/18/06, Pedro Alves <address@hidden 
> <mailto:address@hidden>> wrote:
> > This patch fixes this catched by gcc:
> >
> > $ make
> > (...)/lwip/src/netif/etharp.c
> > (...)/lwip/src/netif/etharp.c: In function 'etharp_arp_input':
> > (...)/lwip/src/netif/etharp.c:505: warning: dereferencing type-punned
> > pointer will break strict-aliasing rules
> > (...)/lwip/src/netif/etharp.c:506: warning: dereferencing type-punned
> > pointer will break strict-aliasing rules
> >
> > We could also use memcpy here, but that seems overkill for copying 8 
> bytes.
> >
> > Cheers,
> > Pedro Alves
> >
> > Index: src/netif/etharp.c
> > ===================================================================
> > RCS file: /sources/lwip/lwip/src/netif/etharp.c,v
> > retrieving revision 1.94
> > diff -u -r1.94 etharp.c
> > --- src/netif/etharp.c    29 Mar 2006 13:16:40 -0000    1.94
> > +++ src/netif/etharp.c    18 Apr 2006 10:41:47 -0000
> > @@ -487,10 +487,11 @@
> >   }
> >
> >   hdr = p->payload;
> > -
> >   /* get aligned copies of addresses */
> > -  *(struct ip_addr2 *)&sipaddr = hdr->sipaddr;
> > -  *(struct ip_addr2 *)&dipaddr = hdr->dipaddr;
> > +  for(i=0; i<sizeof(hdr->sipaddr); i++)
> > +    ((u8_t*)&sipaddr)[i] = ((u8_t*)&hdr->sipaddr)[i];
> > +  for(i=0; i<sizeof(hdr->dipaddr); i++)
> > +    ((u8_t*)&dipaddr)[i] = ((u8_t*)&hdr->dipaddr)[i];
> >
> >   /* this interface is not configured? */
> >   if (netif->ip_addr.addr == 0) {
> >
> >
> >
> > _______________________________________________
> > lwip-users mailing list
> > address@hidden <mailto:address@hidden>
> > http://lists.nongnu.org/mailman/listinfo/lwip-users
> >
>
>  
>
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