与系统调用，/proc，sysfs等类似，netlink也是一种用于用户进程与内核通信的机制，它是基于BSD套接字协议，

使用AF_NETLINK地址簇。

与系统调用，proc，sysfs文件系统等方式相比，netlink具有简单，支持双向通信的特点，并支持消息多播机制。

当我们编写内核驱动并需要与用户进程通信时，我们便能利用netlink来实现这个通信机制。hostapd(一个无线

AP的dameon)中就是采用netlink接口(nl80211)与内核进行通信，下文将通过一个实例来说明如何在自己的内核

模块中支持netlink通信。

1. 编写内核模块

首先编写内核模块文件：netlink_hello_mod.c，并增加相应的makefile，代码如下：

netlink_hello_mod.c:

#include

#include

#include

#include

#define MY_NETLINK_TYPE 31//max is 32, see netlink.h

struct sock *my_nl_sock = NULL;

static void hello_nl_recv_msg(struct sk_buff *skb)

{

struct nlmsghdr *nlh;

int pid;

struct sk_buff *skb_out;

int msg_size;

char *msg = "Hello from kernel";

int res;

printk(KERN_INFO "Entering: %s\n", __FUNCTION__);

msg_size = strlen(msg);

nlh = (struct nlmsghdr *)skb->data;

printk(KERN_INFO "Netlink receive msg: %s\n", (char *)nlmsg_data(nlh));

pid = nlh->nlmsg_pid;

skb_out = nlmsg_new(msg_size, 0);

if (!skb_out) {

printk(KERN_INFO "alloc nlmsg failed\n");

return;

}

nlh = nlmsg_put(skb_out, 0, 0, NLMSG_DONE, msg_size, 0);

NETLINK_CB(skb_out).dst_group = 0;

strncpy(nlmsg_data(nlh), msg, msg_size);

res = nlmsg_unicast(my_nl_sock, skb_out, pid);

if (res < 0)

printk(KERN_INFO "nlmsg unicast failed\n");

return;

}

static int __init hello_init(void)

{

struct netlink_kernel_cfg cfg = {

.groups = 0,

.input = hello_nl_recv_msg

};

printk("Entering: %s:\n", __FUNCTION__);

my_nl_sock = netlink_kernel_create(&init_net, MY_NETLINK_TYPE, &cfg);

if (!my_nl_sock) {

printk(KERN_ALERT "netlink create sock failed\n");

return -10;

}

return 0;

}

static void __exit hello_exit(void)

{

printk(KERN_INFO "Exiting hello module\n");

netlink_kernel_release(my_nl_sock);

my_nl_sock = NULL;

}

module_init(hello_init);

module_exit(hello_exit);

MODULE_LICENSE("Dual BSD/GPL");

MODULE_AUTHOR("Yangjin");

MODULE_DESCRIPTION("netlink hello module");

Makefile:

obj-m = netlink_hello_mod.o

KVERSION = $(shell uname -r)

PWD = $(shell pwd)

all:

make -C /lib/modules/$(KVERSION)/build M=$(PWD) modules

clean:

make -C /lib/modules/$(KVERSION)/build M=$(PWD) clean

2. 编写用户进程代码

用户进程主要使用netlink的socket与内核进行通信，首先向内核发送一条消息，然后再接收内核的

消息，代码如下:

#include

#include

#include

#include

#include

#define MY_NETLINK_TYPE 31

#define MAX_PAYLOAD 1024/* maximum payload size*/

struct sockaddr_nl src_addr, dest_addr;

struct nlmsghdr *nlh = NULL;

struct iovec iov;

int sock_fd;

struct msghdr msg;

int

main(int argc, char *argv[])

{

sock_fd = socket(PF_NETLINK, SOCK_DGRAM, MY_NETLINK_TYPE);

if (sock_fd < 0)

return -1;

memset(&src_addr, 0, sizeof(src_addr));

src_addr.nl_family = AF_NETLINK;

src_addr.nl_pid = getpid(); /* self pid */

bind(sock_fd, (struct sockaddr *)&src_addr, sizeof(src_addr));

memset(&dest_addr, 0, sizeof(dest_addr));

memset(&dest_addr, 0, sizeof(dest_addr));

dest_addr.nl_family = AF_NETLINK;

dest_addr.nl_pid = 0; /* For Linux Kernel */

dest_addr.nl_groups = 0; /* unicast */

nlh = (struct nlmsghdr *)malloc(NLMSG_SPACE(MAX_PAYLOAD));

memset(nlh, 0, NLMSG_SPACE(MAX_PAYLOAD));

nlh->nlmsg_len = NLMSG_SPACE(MAX_PAYLOAD);

nlh->nlmsg_pid = getpid();

nlh->nlmsg_flags = 0;

strcpy(NLMSG_DATA(nlh), "Hello");

iov.iov_base = (void *)nlh;

iov.iov_len = nlh->nlmsg_len;

msg.msg_name = (void *)&dest_addr;

msg.msg_namelen = sizeof(dest_addr);

msg.msg_iov = &iov;

msg.msg_iovlen = 1;

printf("Sending message to kernel\n");

sendmsg(sock_fd, &msg, 0);

printf("Waiting for message from kernel\n");

/* Read message from kernel */

recvmsg(sock_fd, &msg, 0);

printf("Received message payload: %s\n", NLMSG_DATA(nlh));

close(sock_fd);

return 0;

}

3. 测试结果

先make一下编译生成我们的内核模块文件：netlink_helloc_mod.ko，然后加载该模块：

: insmod netlink_hello_mod.ko

加载后可用lsmod查看是否加载成功，然后编译并运行用户进程hello_user，可在终端

中看到如下结果：

Sending message to kernel

Waiting for message from kernel

Received message payload: Hello from kernel

上面是用户进程的输出，可用dmesg命令查看内核模块的输出(或者直接查看/var/log/kern).