How Do You Programmatically Set the Hardware Clock on Linux?

Question

Linux provides the stime(2) call to set the system time. However, while this will update the system's time, it does not set the BIOS hardware clock to match the new system time.

Linux systems typically sync the hardware clock with the system time at shutdown and at periodic intervals. However, if the machine gets power-cycled before one of these automatic syncs, the time will be incorrect when the machine restarts.

How do you ensure that the hardware clock gets updated when you set the system time?

Why would the programming language matter? Do you have different answers for different languages/ — Kristopher Johnson, Jan 14 '15 at 01:41

score 18 · Accepted Answer · edited Nov 04 '19 at 16:47

18

Check out the rtc man-page for details, but if you are logged in as root, something like this:

#include <linux/rtc.h>
#include <sys/ioctl.h>


    struct       rtc_time {
        int         tm_sec;      
        int         tm_min;      
        int         tm_hour;     
        int         tm_mday;     
        int         tm_mon;      
        int         tm_year;     
        int         tm_wday; /* unused */
        int         tm_yday; /* unused */
        int         tm_isdst;/* unused */
    };

int fd;
struct rtc_time rt;
/* set your values here */
fd = open("/dev/rtc", O_RDONLY);
ioctl(fd, RTC_SET_TIME, &rt);
close(fd);

edited Nov 04 '19 at 16:47

flederwiesel

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answered Oct 14 '08 at 17:59

Zan Lynx

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Root not needed, only CAP_SYS_TIME capability according to rtc(4). – Erikw Nov 12 '14 at 07:11
Hello, i have 3 questions regarding trhe answer. 1. Why the own rtc_time and not the one from rtc.h 2. O_RDONLY ? is that not only for read? 3. if i try the code I get from ioctl -1 and errno 22. What does that mean? – Offler Jun 16 '21 at 14:10
@Offler This answer was not meant as a copy and paste ready to use solution. I included the struct to show how it worked. For a real example view the source code to the `hwclock` program. – Zan Lynx Jun 16 '21 at 15:14

Kristopher Johnson · Answer 2 · 2008-10-22T17:42:24.807

8

After calling stime(), do this:

system("/sbin/hwclock --systohc");

See the hwclock(8) man page for more information.

edited Oct 22 '08 at 17:42

answered Oct 14 '08 at 17:43

Kristopher Johnson

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Note that if your rtc is setup to use UTC time you need to add `--utc` or `--localtime` otherwise. – Julien Hirel Aug 22 '13 at 15:23

Martin York · Answer 3 · 2008-10-24T11:55:15.947

2

I would also like to point out the hardware clock is only accurate to a point (I have seen systems that will loose/gain a couple of seconds a day).

Have you considered setting up the network time daemon to sync your clock against a time server?

Admittedly this syncing does not happen until after the daemon starts so keeping your hardware clock in sync also helps between the power up and the point the time daemon syncs against the time server.

edited Oct 24 '08 at 11:55

answered Oct 14 '08 at 17:48

Martin York

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Most of those variations, from my experience, are due to low voltage on the motheboard battery. I would suggest checking it before setting up an innecesary service if its not mission critical. – Manuel Ferreria Oct 22 '08 at 18:05

score 2 · Answer 4 · answered Oct 14 '08 at 17:52

2

I would start by reading the source code of hwclock.

answered Oct 14 '08 at 17:52

iny

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You might be writing for an embedded system and don't have room for another program. Or maybe you just want to know for curiosity's sake. – Zan Lynx Oct 14 '08 at 18:01
That was an answer to the original question. I don't know why it is needed. – iny Oct 14 '08 at 18:02
@iny, I meant "Why read the source to hwclock, when you could simply call hwclock?" Zan had a good answer to that. – Kristopher Johnson Oct 14 '08 at 18:05
1

Running "strace hwclock" is probably the fastest way to satisfy one's curiosity. – sigjuice Mar 19 '09 at 08:35

score -2 · Answer 5 · edited Feb 01 '18 at 17:41

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If the text editor is different than kubuntu (kate is the default in editor), use your own with the sudo command in terminal.

run terminal
copy and paste this command sudo kate /etc/default/rcS press Enter
enter user password (your login password) press Enter
text editor will open on the desktop
change the line UTC=yes to UTC=no
and click save (at top of text editor tab bar)
reboot

edited Feb 01 '18 at 17:41

Werner

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answered Oct 30 '10 at 08:03

brian carr

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Appears to bear no relation to the question asked. – cosimo193 Nov 17 '20 at 17:21

How Do You Programmatically Set the Hardware Clock on Linux?

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