Timer : One Shot

  • In this program, you are going to learn

  • How to create the kernel timer ?

  • How to modify the timer’s timeout ?

Topics in this section,

  • A timer is a specialized type of clock used for measuring specific time intervals.

  • timer_setup used to initialize and set up a timer object.

timer_setup(&my_timer, timer_callback, 0);

  • mod_timer used to modify a timer’s timeout. This is a more efficient way to update the expires field of an active timer (if the timer is inactive it will be activated). For example,

mod_timer(&my_timer, jiffies + msecs_to_jiffies(TIMEOUT));

  • del_timer used to stop a running timer and prevent it’s callback function from being executed. For example

del_timer(&my_timer);

  • Here is the part by part explanation of the source code.

  • Add the list of headers to the refer the APIs used in this source code.

#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/timer.h>
#include <linux/jiffies.h>
#include <linux/err.h>

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

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Linux_usr");
MODULE_DESCRIPTION("One Shot Timer");

  • Initialize the functions and variables to be using in this program.

static struct timer_list my_timer;

static void timer_callback(struct timer_list *timer)

  • Create the module init function to execute when the module is loaded to kernel.

static int __init timer_init(void)
{
    pr_info("Hello world\n");

    timer_setup(&my_timer, timer_callback, 0);
    mod_timer(&my_timer, jiffies + msecs_to_jiffies(TIMEOUT));

    return 0;
}

  • Create the module exit function which executes once when the module is unloaded from the kernel.

static void __exit timer_exit(void)
{
    del_timer(&my_timer);
    pr_info("Good bye, world\n");
}

  • Create the timer APIs which will be executed once the timer is called.

static void timer_callback(struct timer_list *timer)
{
    pr_info("Inside Timer Callback function\n");
}

  • Add the module init and exit functions to execute it once the module is loaded and unloaded.

module_init(timer_init);
module_exit(timer_exit);

 1#include <linux/init.h>
 2#include <linux/kernel.h>
 3#include <linux/module.h>
 4#include <linux/fs.h>
 5#include <linux/timer.h>
 6#include <linux/jiffies.h>
 7#include <linux/err.h>
 8
 9#define TIMEOUT 5000 
10
11static struct timer_list my_timer;
12
13MODULE_LICENSE("GPL");
14MODULE_AUTHOR("Linux_usr");
15MODULE_DESCRIPTION("One Shot Timer");
16
17static void timer_callback(struct timer_list *timer)
18{
19	pr_info("Inside Timer Callback function\n");
20}
21
22static int __init timer_init(void)
23{
24	pr_info("Hello world\n");
25
26	timer_setup(&my_timer, timer_callback, 0);
27	mod_timer(&my_timer, jiffies + msecs_to_jiffies(TIMEOUT));
28
29	return 0;
30}
31
32static void __exit timer_exit(void)
33{
34	del_timer(&my_timer);
35	pr_info("Good bye, world\n");
36}
37
38module_init(timer_init);
39module_exit(timer_exit);

1obj-m += timer.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 kernel module.

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

  • Check if timer.ko is generated or not using ls command.

$ 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 timer.c
-rw-rw-r-- 1 test test 5880 Feb 26 13:18 timer.ko
-rw-rw-r-- 1 test test   47 Feb 26 13:18 timer.mod
-rw-rw-r-- 1 test test  919 Feb 26 13:18 timer.mod.c
-rw-rw-r-- 1 test test 3448 Feb 26 13:18 timer.mod.o
-rw-rw-r-- 1 test test 3320 Feb 26 13:18 timer.o

  • Run insmod command to load the module in kernel.

$ sudo insmod ./timer.ko

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

$ dmesg
[ 1173.096954] Hello world
[ 1178.154636] Inside Timer Callback function

  • Run rmmod to unload the kernel module.

$ sudo rmmod timer

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

$ dmesg
[ 1173.096954] Hello world
[ 1178.154636] Inside Timer Callback function
[ 1184.041014] Good bye, world

API

Learning

timer_setup

To initialize the timer

mod_timer

To modify a timer’s timeout

del_timer

To stop a running timer

msecs_to_jiffies

To convert milliseconds