Linked List : Insert Front

  • In this program, you are going to learn

  • How to insert the elments to rear end of the linked list ?

Topics in this section,

  • In this example, we are going to insert the elements in the front end.

  • Add the list of header files to refer the APIs used in this program.

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/list.h>
#include <linux/slab.h>

  • Add the modules macro which lists the information about the license, author and description.

MODULE_LICENSE("GPL");
MODULE_AUTHOR("linux_usr");
MODULE_DESCRIPTION("Linked List");

  • list_head is used to initialize the list.

static struct list_head my_list;

  • INIT_LIST_HEAD is used to initialize a list_head structure.

INIT_LIST_HEAD(&my_list);

INIT_LIST_HEAD(&new_node->list);

  • Add the module init function to execute the function once when the module is loaded to the linux kernel.

static int __init linkedlist_init(void)
{
    pr_info("Driver loaded\n");

    INIT_LIST_HEAD(&my_list);

    insert_front(0);
    insert_front(1);
    insert_front(2);
    insert_front(3);

    pr_info("Linked list after insertion : \n");
    display();

    return 0;
}

  • Add module exit function which is executed once the module is unloaded from the kernel.

static void __exit linkedlist_exit(void)
{
    struct list_node *ptr, *next;

    list_for_each_entry_safe(ptr, next, &my_list, list) {
        list_del(&ptr->list);
        kfree(ptr);
    }

    pr_info("Driver unloaded\n");
}

  • insert_front function inserts a new node at the beginning with the given value into the linked list.

void insert_front(int value)
{
    struct list_node * new_node;

    new_node = kmalloc(sizeof(struct list_node), GFP_KERNEL);

    if (!new_node) {
            pr_err("Memory allocation failed\n");

            return;
    }

    new_node->data = value;
    INIT_LIST_HEAD(&new_node->list);
    list_add(&new_node->list, &my_list);
}

  • display function iterates through the linked list using list_for_each_entry. It prints the data in each node to the kernel log.

void display(void)
{
    struct list_node * ptr;

    pr_info("Linked list: ");
    list_for_each_entry(ptr, &my_list, list) {
            printk(KERN_CONT "%d -> ", ptr->data);
    }

    printk(KERN_CONT "NULL\n");
}

  • Add the module init and exit which is called when the module is loaded and unloaded.

module_init(linkedlist_init);
module_exit(linkedlist_exit);

 1#include <linux/init.h>
 2#include <linux/module.h>
 3#include <linux/kernel.h>
 4#include <linux/list.h>
 5#include <linux/slab.h>
 6
 7MODULE_LICENSE("GPL");
 8MODULE_AUTHOR("linux_usr");
 9MODULE_DESCRIPTION("Linked List");
10
11struct list_node {
12	int data;
13	struct list_head list;
14};
15
16static struct list_head my_list;
17
18void insert_front(int value)
19{
20	struct list_node *new_node;
21
22	new_node = kmalloc(sizeof(struct list_node), GFP_KERNEL);
23
24	if (!new_node) {
25		pr_err("Memory allocation failed\n");
26		return;
27	}
28
29	new_node->data = value;
30	INIT_LIST_HEAD(&new_node->list);
31	list_add(&new_node->list, &my_list);
32}
33
34void display(void)
35{
36	struct list_node *ptr;
37
38	pr_info("Linked list: ");
39	list_for_each_entry(ptr, &my_list, list) {
40		printk(KERN_CONT "%d -> ", ptr->data);
41	}
42
43	printk(KERN_CONT "NULL\n");
44}
45
46
47static int __init linkedlist_init(void)
48{
49	pr_info("Driver loaded\n");
50	
51	INIT_LIST_HEAD(&my_list);
52
53	insert_front(0);
54	insert_front(1);
55	insert_front(2);
56	insert_front(3);
57	
58	pr_info("Linked list after insertion : \n");
59	display();
60
61	return 0;
62}
63
64static void __exit linkedlist_exit(void)
65{
66	struct list_node *ptr, *next;
67
68	list_for_each_entry_safe(ptr, next, &my_list, list) {
69		list_del(&ptr->list);
70		kfree(ptr);
71	}
72
73	pr_info("Driver unloaded\n");
74}
75
76module_init(linkedlist_init);
77module_exit(linkedlist_exit);

1obj-m += insert.o
2all:
3	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
4clean:
5	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

1$ make all

API

Learning

INIT_LIST_HEAD

To initialize a list_head structure

list_head

To initialize the list

list_for_each_entry

To iterate over list of given type

list_for_each_entry_safe

To iterate over list of given type safe against removal of list entry

list_add

To insert a new entry after the specified head

list_del

To delete entry from list
See Also