netlinkUser.c

/*
Source: http://stackoverflow.com/questions/3299386/how-to-use-netlink-socket-to-communicate-with-a-kernel-module/3334782#3334782
*/

#include <sys/socket.h>
#include <linux/netlink.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define NETLINK_USER 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;
char message[MAX_PAYLOAD] = {0};

int main()
{
    sock_fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_USER);
    if (sock_fd < 0)
    {
	    printf("ERROR in user program!!!\n");
	    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 from user");

    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;

    // first time calling the kernel the kernel to give pid
    sendmsg(sock_fd, &msg, 0);

    /* Read message from kernel */

    while(1)
    {
        recvmsg(sock_fd, &msg, 0);
        printf("USER GOT:%s\n", NLMSG_DATA(nlh));
        if(strcmp(NLMSG_DATA(nlh), "./unload.sh") == 0)
            break;
        sendmsg(sock_fd, &msg, 0);
    }    
    close(sock_fd);
}