Quoting Dmitry V. Levin (2016-06-14 12:03:47)
> On Mon, Jun 13, 2016 at 02:37:23PM +0000, Fabien Siron wrote:
> [...]
> > +static void
> > +send_query(const int fd)
> > +{
> > + struct {
> > + struct nlmsghdr nlh;
> > + char magic[4];
> > + } req = {
> > + .nlh = {
> > + .nlmsg_len = sizeof(req),
> > + .nlmsg_type = SOCK_DIAG_BY_FAMILY,
> > + .nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST
> > + },
> > + .magic = "abcd"
> > + };
> > +
> > + if (sendto(fd, &req, sizeof(req), MSG_DONTWAIT, NULL, 0) <= 0)
> > + perror_msg_and_skip("sendto");
>
> Why <= 0? The next statement assumes == (unsigned) sizeof(req).
Indeed.
>
> > +
> > + printf("sendto(%d, {{len=%u, type=20, flags=NLM_F_REQUEST|0x300, "
> > + "seq=0, pid=0}, \"abcd\"}, 20, MSG_DONTWAIT, NULL, 0) = %u\n",
> > + fd, (unsigned) sizeof(req), (unsigned) sizeof(req));
>
> Please do not encode values of constants into output strings verbatim.
Well, the size of the magic must be aligned on 4 characters to avoid '\0' stuff.
So it means that print req.magic will fail because of no null byte. An easy
way to do that is to use a define.
But maybe do you prefer a variable with a memcpy?
e.g.:
static void
send_query(const int fd)
{
const char magic[] = "abcd";
struct {
struct nlmsghdr nlh;
char magic[4];
} req = {
.nlh = {
.nlmsg_len = sizeof(req),
.nlmsg_type = SOCK_DIAG_BY_FAMILY,
.nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST
},
};
memcpy(req.magic, magic, 4);
...
>
> --
> ldv
>
Regards,
--
Fabien Siron
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