Timmy Brolin wrote:
Pedro Alves wrote:
[EMAIL PROTECTED] 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.
Are you sure that it can become unaligned?
There is code in place in lwip to guarantee header alignment...
Sure they can, that's why the copying is beeing made.
I missed the fact that ip_addr and ip_addr2 are both pack_struct'ed.
The first time I looked I got the impression that struct ip_addr2 was
packed and struct ip_addr wasn't.
Beach's suggestion removes the gcc warning and is used throughout lwip,
but is it really safe?
-*(struct ip_addr2 *)&sipaddr = hdr->sipaddr;
-*(struct ip_addr2 *)&dipaddr = hdr->dipaddr;
+sipaddr = *(struct ip_addr *)&hdr->sipaddr;
+dipaddr = *(struct ip_addr *)&hdr->dipaddr;
The only safe way I've heard that ensures correct conversion is using
unions like this:
/* These tell the
* compiler that the type punning between the two pointer classes needs to
* be taken into account when optimizing.
*/
#define IP_ADDR_TO_IP_ADDR2(FROM, TO) \
do { \
union ip_add_pun { \
struct ip_addr ip; \
struct ip_addr2 ip2; \
} d; \
d.ip = (FROM); \
(TO) = d.ip2; \
} while (0)
#define IP_ADDR2_TO_IP_ADDR(FROM, TO) \
do { \
union ip_add_pun { \
struct ip_addr ip; \
struct ip_addr2 ip2; \
} d; \
d.ip2 = (FROM); \
(TO) = d.ip; \
} while (0)
IP_ADDR2_TO_IP_ADDR(hdr->sipaddr, sipaddr);
IP_ADDR2_TO_IP_ADDR(hdr->dipaddr, dipaddr);
Can anyone comment on why we need struct ip_addr and struct ip_addr2?
Cheers,
Pedro Alves
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