Linked List : list_replace

  • In this program, you are going to learn

  • How to delete from one list and add as another’s head ?

Topics in this section,

  • In this example, we are going to replace old entry by new one.

  • 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.

struct list_node {
    int data;
    struct list_head list;
};

static LIST_HEAD(my_list);
static LIST_HEAD(new_list);

struct list_node data1, data2, data3,data4,  *entry;

  • 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");

    data1.data = 10;
    data2.data = 20;
    data3.data = 30;
    data4.data = 40;

    INIT_LIST_HEAD(&data1.list);
    INIT_LIST_HEAD(&data2.list);
    INIT_LIST_HEAD(&data3.list);
    INIT_LIST_HEAD(&data4.list);

    list_add(&data1.list, &my_list);
    list_add(&data2.list, &my_list);
    list_add(&data3.list, &my_list);
    list_add(&data4.list, &my_list);

    list_for_each_entry(entry, &my_list, list)
        printk(KERN_INFO "my_list entry 1: %d\n",entry->data);

    list_replace(&data3.list, &data5.list);

    list_for_each_entry(entry, &my_list, list)
        printk(KERN_INFO "my_list entry 2: %d\n",entry->data);

    return 0;
}

  • INIT_LIST_HEAD is used to initialize a list_head structure.

INIT_LIST_HEAD(&data1.list);
INIT_LIST_HEAD(&data2.list);
INIT_LIST_HEAD(&data3.list);
INIT_LIST_HEAD(&data4.list);

  • list_add to insert a new element after the given list head.

list_add(&data1.list, &my_list);
list_add(&data2.list, &my_list);
list_add(&data3.list, &my_list);
list_add(&data4.list, &my_list);

  • list_replace to replace old entry by new one.

list_replace(&data3.list, &data5.list);

  • 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);
        }

        list_for_each_entry_safe(ptr, next, &new_list, list) {
                list_del(&ptr->list);
        }
        pr_info("Driver unloaded\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 LIST_HEAD(my_list);
17struct list_node data1, data2, data3, data4, data5, *entry;
18
19static int __init linkedlist_init(void)
20{
21	pr_info("Driver loaded\n");
22
23	data1.data = 10;
24	data2.data = 20;
25	data3.data = 30;
26	data4.data = 40;
27	data5.data = 50;
28
29	INIT_LIST_HEAD(&data1.list);
30	INIT_LIST_HEAD(&data2.list);
31	INIT_LIST_HEAD(&data3.list);
32	INIT_LIST_HEAD(&data4.list);
33	INIT_LIST_HEAD(&data5.list);
34
35	list_add(&data1.list, &my_list);
36	list_add(&data2.list, &my_list);
37	list_add(&data3.list, &my_list);
38	list_add(&data4.list, &my_list);
39
40	list_for_each_entry(entry, &my_list, list) 
41		printk(KERN_INFO "my_list entry 1: %d\n",entry->data);
42
43	list_replace(&data3.list, &data5.list);
44
45	list_for_each_entry(entry, &my_list, list) 
46		printk(KERN_INFO "my_list entry 2: %d\n",entry->data);
47
48	return 0;
49}
50
51static void __exit linkedlist_exit(void)
52{
53	struct list_node *ptr, *next;
54
55	list_for_each_entry_safe(ptr, next, &my_list, list) {
56		list_del(&ptr->list);
57	}
58
59	pr_info("Driver unloaded\n");
60}
61
62module_init(linkedlist_init);
63module_exit(linkedlist_exit);

1obj-m += linked_list.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

  • Run Make to compile the module.

$make
make -C /lib/modules/5.4.0-150-generic/build M=$HOME/kernel_linked_list modules
make[1]: Entering directory '/usr/src/linux-headers-5.4.0-150-generic'
  CC [M]  $HOME/kernel_linked_list/linked_list.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC [M]  $HOME/kernel_linked_list/linked_list.mod.o
  LD [M]  $HOME/kernel_linked_list/linked_list.ko
make[1]: Leaving directory '/usr/src/linux-headers-5.4.0-150-generic'

  • Run ls to check if linked_list.ko is generated or not.

$ ls -l
total 36
-rw-rw-r-- 1 test test  154 Feb 26 13:18 Makefile
-rw-rw-r-- 1 test test   47 Feb 26 13:18 modules.order
-rw-rw-r-- 1 test test    0 Feb 26 13:18 Module.symvers
-rw-rw-r-- 1 test test  820 Feb 26 13:18 linked_list.c
-rw-rw-r-- 1 test test 5880 Feb 26 13:18 linked_list.ko
-rw-rw-r-- 1 test test   47 Feb 26 13:18 linked_list.mod
-rw-rw-r-- 1 test test  919 Feb 26 13:18 linked_list.mod.c
-rw-rw-r-- 1 test test 3448 Feb 26 13:18 linked_list.mod.o
-rw-rw-r-- 1 test test 3320 Feb 26 13:18 linked_list.o

  • Run insmod to load the module.

$ sudo insmod ./linked_list.ko

  • Check the kernel messages to see if the kernel module is loaded or not.

$ dmesg
[  537.955566] driver loaded
[  537.955568] my_list entry 1: 40
[  537.955568] my_list entry 1: 30
[  537.955569] my_list entry 1: 20
[  537.955569] my_list entry 1: 10
[  537.955569] my_list entry 2: 40
[  537.955569] my_list entry 2: 50
[  537.955570] my_list entry 2: 20
[  537.955570] my_list entry 2: 10

  • Run rmmod to unload the module.

$ sudo rmmod linked_list

  • Check dmesg to see if the module is unloaded from kernel.

$ dmesg
[  537.955566] driver loaded
[  537.955568] my_list entry 1: 40
[  537.955568] my_list entry 1: 30
[  537.955569] my_list entry 1: 20
[  537.955569] my_list entry 1: 10
[  537.955569] my_list entry 2: 40
[  537.955569] my_list entry 2: 50
[  537.955570] my_list entry 2: 20
[  537.955570] my_list entry 2: 10
[  541.931631] Driver unloaded

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_add

To insert a new entry after the specified head

list_for_each_entry_safe

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

list_del

To delete entry from list

list_replace

To replace old entry by new one
See Also