c - 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?

Answer 1

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);