Linked List : list_for_each

  • In this program, you are going to learn

  • How to iterate over a list ?

Topics in this section,

  • In this example, we are going to test whether a list is the last entry in list.

  • 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 "List entry: %d\n",entry->data);

        printk(KERN_INFO "Traversing the list: \n");
        list_for_each(pos, &my_list) {
                entry = list_entry(pos, struct list_node, list);
                printk(KERN_INFO "List entry: %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_for_each_entry to iterate over list of given type.

list_for_each_entry(entry, &my_list, list)

  • list_for_each to iterate over a list.

list_for_each(pos, &my_list)

  • list_entry to get the struct for this entry.

entry = list_entry(pos, struct list_node, 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);
17
18struct list_node data1, data2, data3, data4, *entry;
19
20static int __init linkedlist_init(void)
21{
22	struct list_head *pos;
23
24	pr_info("Driver loaded\n");
25
26	data1.data = 10;
27	data2.data = 20;
28	data3.data = 30;
29	data4.data = 40;
30
31	INIT_LIST_HEAD(&data1.list);
32	INIT_LIST_HEAD(&data2.list);
33	INIT_LIST_HEAD(&data3.list);
34	INIT_LIST_HEAD(&data4.list);
35
36	list_add(&data1.list, &my_list);
37	list_add(&data2.list, &my_list);
38	list_add(&data3.list, &my_list);
39	list_add(&data4.list, &my_list);
40
41	list_for_each_entry(entry, &my_list, list)
42                printk(KERN_INFO "List entry: %d\n",entry->data);
43
44	printk(KERN_INFO "Traversing the list: \n");
45	list_for_each(pos, &my_list) {
46		entry = list_entry(pos, struct list_node, list);
47		printk(KERN_INFO "List entry: %d\n", entry->data);
48	}
49
50	return 0;
51}
52
53static void __exit linkedlist_exit(void)
54{
55	struct list_node *ptr, *next;
56
57	list_for_each_entry_safe(ptr, next, &my_list, list) {
58		list_del(&ptr->list);
59	}
60
61	pr_info("Driver unloaded\n");
62}
63
64module_init(linkedlist_init);
65module_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
[ 9860.538226] Driver loaded
[ 9860.538228] List entry: 40
[ 9860.538228] List entry: 30
[ 9860.538229] List entry: 20
[ 9860.538229] List entry: 10
[ 9860.538229] Traversing the list:
[ 9860.538230] List entry: 40
[ 9860.538230] List entry: 30
[ 9860.538230] List entry: 20
[ 9860.538230] List entry: 10

  • Run rmmod to unload the module.

$ sudo rmmod linked_list

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

$ dmesg
[ 9860.538226] Driver loaded
[ 9860.538228] List entry: 40
[ 9860.538228] List entry: 30
[ 9860.538229] List entry: 20
[ 9860.538229] List entry: 10
[ 9860.538229] Traversing the list:
[ 9860.538230] List entry: 40
[ 9860.538230] List entry: 30
[ 9860.538230] List entry: 20
[ 9860.538230] List entry: 10
[ 9875.522362] 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

To iterate over a list

list_entry

To get the struct for this entry

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
See Also