On ter?a-feira, 29 de novembro de 2016 04:31:11 PST Arpit Agarwal wrote:
> In the following code in CAReceiveMessage function:
>
> if (flags & CA_IPV6)
> {
> namelen = sizeof (struct sockaddr_in6);
> level = IPPROTO_IPV6;
> type = IPV6_PKTINFO;
> len = sizeof (struct in6_pktinfo);
> }
> else
> {
> namelen = sizeof (struct sockaddr_in);
> level = IPPROTO_IP;
> type = IP_PKTINFO;
> len = sizeof (struct in6_pktinfo);
> }
>
> May I know why the len is set to sizeof (struct in6_pktinfo) in case of IPv4
> packet? Is it intentional with a purpose? Shouldn't it be set to sizeof
> (struct in_pktinfo) ? It will be helpful if someone can clear my doubt.
Hello Arpit
Yes and no. This looks like a copy & paste error and it should have been
sizeof(struct in_pktinfo). However, this is harmless. This part of the code is
simply telling the OS the size of the buffer, which was defined above as;
union control
{
struct cmsghdr cmsg;
unsigned char data[CMSG_SPACE(sizeof (struct in6_pktinfo))];
} cmsg;
If it set the len to sizeof(struct in_pktinfo), then it needs to assert
(hopefully static_assert) that the size of in6_pktinfo is greater than or
equal to the size of in_pktinfo.
Alternatively, it can do what the Windows code below does and simply pass the
size of the buffer:
union control
{
WSACMSGHDR cmsg;
uint8_t data[WSA_CMSG_SPACE(sizeof (IN6_PKTINFO))];
} cmsg;
// ....
.Control = {.buf = cmsg.data, .len = sizeof (cmsg)}
--
Thiago Macieira - thiago.macieira (AT) intel.com
Software Architect - Intel Open Source Technology Center
