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[CONNECTOR]: Update documentation to match reality. 

Updated documentation to reflect 2.6.14 netlink changes
about socket options, multicasting and group number.
Please concider for 2.6.14.

Signed-off-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Evgeniy Polyakov 2005-10-13 14:42:04 -07:00 committed by David S. Miller
parent 34cb711ba9
commit eb0d604114
1 changed files with 44 additions and 0 deletions
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44
Documentation/connector/connector.txt
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@ -131,3 +131,47 @@ Netlink itself is not reliable protocol, that means that messages can
be lost due to memory pressure or process' receiving queue overflowed,
so caller is warned must be prepared. That is why struct cn_msg [main
connector's message header] contains u32 seq and u32 ack fields.
/*****************************************/
Userspace usage.
/*****************************************/
2.6.14 has a new netlink socket implementation, which by default does not
allow to send data to netlink groups other than 1.
So, if to use netlink socket (for example using connector)
with different group number userspace application must subscribe to
that group. It can be achieved by following pseudocode:
s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
l_local.nl_family = AF_NETLINK;
l_local.nl_groups = 12345;
l_local.nl_pid = 0;
if (bind(s, (struct sockaddr *)&l_local, sizeof(struct sockaddr_nl)) == -1) {
perror("bind");
close(s);
return -1;
}
{
int on = l_local.nl_groups;
setsockopt(s, 270, 1, &on, sizeof(on));
}
Where 270 above is SOL_NETLINK, and 1 is a NETLINK_ADD_MEMBERSHIP socket
option. To drop multicast subscription one should call above socket option
with NETLINK_DROP_MEMBERSHIP parameter which is defined as 0.
2.6.14 netlink code only allows to select a group which is less or equal to
the maximum group number, which is used at netlink_kernel_create() time.
In case of connector it is CN_NETLINK_USERS + 0xf, so if you want to use
group number 12345, you must increment CN_NETLINK_USERS to that number.
Additional 0xf numbers are allocated to be used by non-in-kernel users.
Due to this limitation, group 0xffffffff does not work now, so one can
not use add/remove connector's group notifications, but as far as I know,
only cn_test.c test module used it.
Some work in netlink area is still being done, so things can be changed in
2.6.15 timeframe, if it will happen, documentation will be updated for that
kernel.