Init with small code example

.gitignore

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+
+# Created by https://www.gitignore.io/api/c
+# Edit at https://www.gitignore.io/?templates=c
+
+### C ###
+# Prerequisites
+*.d
+
+# Object files
+*.o
+*.ko
+*.obj
+*.elf
+
+# Linker output
+*.ilk
+*.map
+*.exp
+
+# Precompiled Headers
+*.gch
+*.pch
+
+# Libraries
+*.lib
+*.a
+*.la
+*.lo
+
+# Shared objects (inc. Windows DLLs)
+*.dll
+*.so
+*.so.*
+*.dylib
+
+# Executables
+*.exe
+*.out
+*.app
+*.i*86
+*.x86_64
+*.hex
+
+# Debug files
+*.dSYM/
+*.su
+*.idb
+*.pdb
+
+# Kernel Module Compile Results
+*.mod*
+*.cmd
+.tmp_versions/
+modules.order
+Module.symvers
+Mkfile.old
+dkms.conf
+
+# End of https://www.gitignore.io/api/c

kernel/Makefile

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+
+obj-m += netlink-test.o
+
+all:
+	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
+
+clean:
+	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

kernel/netlink-test.c

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+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <net/sock.h>
+#include <net/netlink.h>
+
+#define NETLINK_TESTPROTO (30)
+
+static struct sock *my_nl_sock;
+
+
+static void nl_test_recv(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 received msg payload:%s\n",(char*)nlmsg_data(nlh));
+pid = nlh->nlmsg_pid; /*pid of sending process */
+
+skb_out = nlmsg_new(msg_size,0);
+
+if(!skb_out)
+{
+
+    printk(KERN_ERR "Failed to allocate new skb\n");
+    return;
+
+} 
+nlh=nlmsg_put(skb_out,0,0,NLMSG_DONE,msg_size,0);  
+NETLINK_CB(skb_out).dst_group = 0; /* not in mcast group */
+strncpy(nlmsg_data(nlh),msg,msg_size);
+
+res=nlmsg_unicast(my_nl_sock,skb_out,pid);
+
+if(res<0)
+    printk(KERN_INFO "Error while sending bak to user\n");
+
+}
+
+static struct netlink_kernel_cfg nl_cfg = {
+	.groups = 0,
+	.input = nl_test_recv,
+};
+
+static int nl_test_init(void)
+{	
+
+	my_nl_sock = netlink_kernel_create(&init_net, NETLINK_TESTPROTO, &nl_cfg);
+	
+	if (!my_nl_sock) {
+        	printk(KERN_ERR "%s: receive handler registration failed\n", __func__);
+        	return -ENOMEM;
+	}
+
+	printk(KERN_INFO "Hello :)\n");
+	return 0;
+}
+
+static void nl_test_exit(void)
+{
+	if (my_nl_sock)
+		netlink_kernel_release(my_nl_sock);
+
+	printk(KERN_INFO "Goodbye :(\n");
+}
+
+module_init(nl_test_init);
+module_exit(nl_test_exit);
+
+MODULE_AUTHOR("Mario Huettel <mario.huettel@gmx.net>");
+MODULE_LICENSE("GPL v2");
+MODULE_DESCRIPTION("Netlink communication test");

user/main.c

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+
+#include <sys/socket.h>
+#include <linux/netlink.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdio.h>
+
+#define NETLINK_USER 30
+
+#define MAX_PAYLOAD 1024 /* maximum payload size*/
+
+int main()
+{
+
+struct sockaddr_nl src_addr, dest_addr;
+struct nlmsghdr *nlh = NULL;
+struct iovec iov;
+int sock_fd;
+struct msghdr msg;
+
+
+sock_fd=socket(PF_NETLINK, SOCK_RAW, NETLINK_USER);
+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(&msg, 0, sizeof(msg));
+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", (char *)NLMSG_DATA(nlh));
+close(sock_fd);
+}