Set the hardware clock in Python?

Question

How do I set the hardware clock with Python on embedded Linux systems?

Answer 1

This uses ioctl to set the hardware clock as requested (but not the system clock). It avoids extra processes but is more involved. I am using pytz and dateutil to handle local/utc conversions. Feel free to use the code (3-clause BSD License). Get the clock with get_hwclock() and set it with set_hwclock()...

from collections import namedtuple
from datetime import datetime
from fcntl import ioctl
import struct
from dateutil.tz import tzutc
from pytz import timezone


# From `uapi/asm-generic/ioctl.h`
_IOC_NRBITS = 8
_IOC_TYPEBITS = 8
_IOC_SIZEBITS = 14
_IOC_DIRBITS = 2

_IOC_NRMASK = (1 << _IOC_NRBITS) - 1
_IOC_TYPEMASK = (1 << _IOC_TYPEBITS) - 1
_IOC_SIZEMASK = (1 << _IOC_SIZEBITS) - 1
_IOC_DIRMASK = (1 << _IOC_DIRBITS) - 1

_IOC_NRSHIFT = 0
_IOC_TYPESHIFT = _IOC_NRSHIFT + _IOC_NRBITS
_IOC_SIZESHIFT = _IOC_TYPESHIFT + _IOC_TYPEBITS
_IOC_DIRSHIFT = _IOC_SIZESHIFT + _IOC_SIZEBITS

_IOC_NONE = 0
_IOC_WRITE = 1
_IOC_READ = 2


def _IOC(dir, type, nr, size):
    return ((dir << _IOC_DIRSHIFT) |
            (type << _IOC_TYPESHIFT) |
            (nr << _IOC_NRSHIFT) |
            (size << _IOC_SIZESHIFT))


def _IOC_TYPECHECK(t):
    return len(t)


def _IO(type, nr):
    return _IOC(_IOC_NONE, type, nr, 0)


def _IOR(type, nr, size):
    return _IOC(_IOC_READ, type, nr, _IOC_TYPECHECK(size))


def _IOW(type, nr, size):
    return _IOC(_IOC_WRITE, type, nr, _IOC_TYPECHECK(size))


def to_utc(dtobj):
    if dtobj.tzinfo is None:
        dtobj = timezone("UTC").localize(
            dtobj.replace(tzinfo=None) - tzlocal().utcoffset(dtobj))
    return dtobj.astimezone(timezone("UTC"))


class RtcTime(namedtuple(
    # man(4) rtc
    "RtcTime",
    "tm_sec tm_min tm_hour "
    "tm_mday tm_mon tm_year "
    "tm_wday tm_yday tm_isdst"  # Last row is unused.
)):

    _fmt = 9 * "i"

    def __new__(cls, tm_sec=0, tm_min=0, tm_hour=0,
                tm_mday=0, tm_mon=0, tm_year=0,
                tm_wday=0, tm_yday=0, tm_isdst=0):
        return super(RtcTime, cls).__new__(cls, tm_sec, tm_min, tm_hour,
                                            tm_mday, tm_mon, tm_year,
                                            tm_wday, tm_yday, tm_isdst)

    def __str__(self):
        return self.to_datetime().isoformat()

    @classmethod
    def from_datetime(cls, dtobj):
        dt = to_utc(dtobj)
        return cls(tm_sec=dt.second, tm_min=dt.minute, tm_hour=dt.hour,
                   tm_mday=dt.day, tm_mon=dt.month - 1, tm_year=dt.year - 1900)

    def to_datetime(self):
        # From `hwclock.c`.
        return datetime(
            year=self.tm_year + 1900, month=self.tm_mon + 1, day=self.tm_mday,
            hour=self.tm_hour, minute=self.tm_min, second=self.tm_sec,
            tzinfo=tzutc())

    def pack(self):
        return struct.pack(self._fmt, *self)

    @classmethod
    def unpack(cls, buffer):
        return cls._make(struct.unpack(cls._fmt, buffer))


# From `uapi/linux/rtc.h`
rtc_time = RtcTime().pack()
RTC_RD_TIME = _IOR(ord("p"), 0x09, rtc_time)   # 0x80247009
RTC_SET_TIME = _IOW(ord("p"), 0x0a, rtc_time)  # 0x4024700a
del rtc_time


def get_hwclock(devrtc="/dev/rtc"):
    with open(devrtc) as rtc:
        ret = ioctl(rtc, RTC_RD_TIME, RtcTime().pack())
    return RtcTime.unpack(ret).to_datetime()


def set_hwclock(dt, devrtc="/dev/rtc"):
    with open(devrtc) as rtc:
        ioctl(rtc, RTC_SET_TIME, RtcTime.from_datetime(dt).pack())

Answer 2

An updated version on Ubuntu 16.04:

import subprocess
import shlex

subprocess.call(shlex.split("timedatectl set-ntp false"))  # May be necessary
subprocess.call(shlex.split("sudo date -s '2 OCT 2006 18:00:00'"))
subprocess.call(shlex.split("sudo hwclock -w"))

Important note: you may need to change the time/date settings to manually set (set-ntp false) or else it will immediately change it back to the current time.

hwclock -w sets the hardware clock based on the current system time (set by date)

It is required that date & hwclock are run as sudo as well.

Answer 3

Use Python's os.system function to call the hwclock command.

Answer 4

Probably no easy way other than doing an os.system() call.

import os
os.system('hwclock --set %s' % date_str)

or using the 'date' command

import os
os.system('date -s %s' % date_str)

or if you are dying to do some c coding, wrapping the system calls with swig... but I think that would be more work than its worth.