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Old 11-26-2009, 12:08 AM   #1
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Registered: Nov 2009
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Help regarding Netlink sockets for 2.6 kernels: Kernel module

I'm a netlink newbie developing a kernel module (as stated above) for 2.6.x kernels.
I'm simply truing to pass a(any) message between the user space and the kernel.

The changes to the netlink APIs from kernel to kernel are confusing.
Below is my kernel code:

#define NETLINK_TEST 17
#define VFW_GROUP 0
#define MSG_SIZE NLMSG_SPACE(1024)

static struct sock *nl_sk = NULL;

static void nltest_rcv(struct sock *sk, int len)
struct sk_buff *nl_skb;
struct nlmsghdr *nl_hdr;
int pid;
while ((nl_skb = skb_dequeue(&sk->sk_receive_queue)) != NULL) {
nl_hdr = (struct nlmsghdr *)nl_skb->data;
pid = nl_hdr->nlmsg_pid;
printk(KERN_ALERT "*** Message from user with PID: (pid = %d) is %s\n", pid, (char*)NLMSG_DATA(nl_hdr));
nl_skb = alloc_skb(MSG_SIZE, in_interrupt() ? GFP_ATOMIC : GFP_KERNEL);
skb_put(nl_skb, MSG_SIZE);
nl_hdr = (struct nlmsghdr *)nl_skb->data;
nl_hdr->nlmsg_len = MSG_SIZE;
nl_hdr->nlmsg_pid = pid;
nl_hdr->nlmsg_flags = 0;
strcpy(NLMSG_DATA(nl_hdr), "HELLO HELLO HELLO");
NETLINK_CB(nl_skb).pid = 0;
NETLINK_CB(nl_skb).dst_pid = pid;
NETLINK_CB(nl_skb).dst_group = VFW_GROUP;
netlink_unicast(nl_sk, nl_skb, pid, 0);

/* LXR reference:
struct sock * netlink_kernel_create
(struct net *net, int unit, unsigned int groups,void (*input)(struct sk_buff *skb),struct mutex *cb_mutex, struct module *module)

static int __init nltest_init(void)
struct net *net;
printk(KERN_ALERT "INIT/START: nltest\n");
nl_sk = netlink_kernel_create(net,NETLINK_TEST, VFW_GROUP, nltest_rcv, 0, THIS_MODULE);
if (!nl_sk) {
printk(KERN_ALERT "ERROR: nltest - netlink_kernel_create() failed\n");
return -1;
return 0;
static void __exit nltest_exit(void)
printk(KERN_ALERT "EXIT: nltest\n");


when I run a make on this file, it shows the following errors:

root@ubuntu:/home/p# make
make -C /lib/modules/2.6.27-15-generic/build M=/home/p modules
make[1]: Entering directory `/usr/src/linux-headers-2.6.27-15-generic'
CC [M] /home/priyanka/kern.o
/home/p/kern.c: In function ‘nltest_rcv’:
/home/p/kern.c:37: error: ‘struct netlink_skb_parms’ has no member named ‘dst_pid’
/home/p/kern.c: In function ‘nltest_init’:
/home/p/kern.c:54: warning: passing argument 4 of ‘netlink_kernel_create’ from incompatible pointer type
make[2]: *** [/home/p/kern.o] Error 1
make[1]: *** [_module_/home/p] Error 2
make[1]: Leaving directory `/usr/src/linux-headers-2.6.27-15-generic'
make: *** [all] Error 2

The two errors are for
1. netlink_kernel_create function
2. struct netlink_skb_parms

Can anyone help me figure out how to solve these errors?

Old 11-26-2009, 04:49 AM   #2
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The two errors are for
1. netlink_kernel_create function
2. struct netlink_skb_parms
I don't know much about Netlink, but if these functions are pre-defined why don't u check their declarations in their corresponding header files i.e. the ones which u have included in the above shown code!

error: ‘struct netlink_skb_parms’ has no member named ‘dst_pid’
/home/p/kern.c: In function ‘nltest_init’:
/home/p/kern.c:54: warning: passing argument 4 of ‘netlink_kernel_create’ from incompatible pointer type
From the errors u are receiving it seems u have passed the wrong parameters !

Last edited by TheIndependentAquarius; 11-26-2009 at 05:00 AM.


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