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POSIX Alarm Timers example using Python ctypes
#!/usr/bin/env python
# POSIX Alarm Timer example using Python ctypes
# Author: David Coles <>
# Date: 2012-03-19
# To the extent possible under law, the author(s) have dedicated all copyright
# and related and neighboring rights to this software to the public domain
# worldwide. This software is distributed without any warranty.
# POSIX Alarm Timers in Linux (see are one
# way to get a system to wake from suspend. The basic idea is to use the
# normal POSIX Timer interface to get the kernel to set an alarm on the RTC so
# an application doesn't have to deal with RTC clock drift or fight other
# applications for use of the RTC alarm.
# Requires Linux 3.0 (or greater) and must be run by with CAP_WAKE_ALARM or as
# root to allow the creation of a CLOCK_REALTIME_ALARM clock.
import ctypes
import errno
import os
import threading
import sys
# C libraries
libc = ctypes.CDLL("")
librt = ctypes.CDLL("", use_errno=True)
# Most of the stuff here is all the structs and types required to make this
# work with ctypes. Basically this is all taken straight out of bits/siginfo.h
# on Linux and thus is highly non-portable and otherwise a really bad idea.
# ... but it does let you do it from Python. ;)
# Types
clockid_t = ctypes.c_int32
pid_t = ctypes.c_int32
timer_t = ctypes.c_void_p
time_t = ctypes.c_long
# Clock IDs
# Notification IDs
SIGEV_SIGNAL = ctypes.c_int(0)
SIGEV_NONE = ctypes.c_int(1)
SIGEV_THREAD = ctypes.c_int(2)
# The sigval union from <signal.h>
class Union_sigval(ctypes.Union):
_fields_ = [
("sival_int", ctypes.c_int),
("sival_ptr", ctypes.c_void_p),
# The SIGEV_THREAD callback funtion type
sigev_notify_function = ctypes.CFUNCTYPE(None, Union_sigval)
# The sigevent structure from <signal.h>
class Struct__sigev_thread(ctypes.Structure):
_fields_ = [
("sigev_notify_function", sigev_notify_function),
("sigev_notify_attributes", ctypes.c_void_p),
class Struct__sigev_un(ctypes.Union):
_anonymous_ = ("_sigev_thread", )
_fields_ = [
("_pad", ctypes.c_int * 13), # or 12 on 64-bit...
("_tid", pid_t),
("_sigev_thread", Struct__sigev_thread),
class Struct_sigevent(ctypes.Structure):
_anonymous_ = ("_sigev_un", )
_fields_ = [
("sigev_value", Union_sigval),
("sigev_signo", ctypes.c_int),
("sigev_notify", ctypes.c_int),
("_sigev_un", Struct__sigev_un),
# The timespec struct from <time.h>
class Struct_timespec(ctypes.Structure):
_fields_ = [
("tv_sec", time_t),
("tv_nsec", ctypes.c_long)
# The itimerspec struct from <time.h>
class Struct_itimerspec(ctypes.Structure):
_fields_ = [
("it_interval", Struct_timespec), # Interval for periodic timer
("it_value", Struct_timespec), # First expiration
if __name__ == "__main__":
# Event will be set when we get timer callback
done = threading.Event()
# Callback function
def callback(sigval_value):
# How we signal the event
ev = Struct_sigevent()
ev.sigev_signo = 0
ev.sigev_notify = SIGEV_THREAD
ev.sigev_notify_function = sigev_notify_function(callback)
ev.sigev_notify_attributes = None
# Create a new POSIX Alarm Timer
timerid = ctypes.c_int(0)
ret = librt.timer_create(CLOCK_REALTIME_ALARM, ctypes.byref(ev),
if ret == -1:
errnum = ctypes.get_errno()
print("ERROR: timer_create: %s"%(os.strerror(errnum)))
if errnum == errno.EPERM:
print("You probably aren't root or someone with CAP_WAKE_ALARM")
print("Maybe try `sudo ./`")
elif errnum == errno.EINVAL:
print("This system might not support CLOCK_REALTIME_ALARM")
# Get the current time...
value = Struct_itimerspec()
ret = librt.clock_gettime(CLOCK_REALTIME_ALARM,
if ret == -1:
print("ERROR: clock_gettime: %s"%(os.strerror(ctypes.get_errno())))
# ...add 1 minute too it...
value.it_interval.tv_sec = 0
value.it_interval.tv_nsec = 0
value.it_value.tv_sec += 60
value.it_value.tv_nsec = 0
# ...and use it to arm a our timer.
ret = librt.timer_settime(timerid, 0x0, ctypes.byref(value), None)
if ret == -1:
print("ERROR: timer_settime: %s"%(os.strerror(ctypes.get_errno())))
print("Please suspend your computer... It should wake in 1 minute.")
print("Woken by CLOCK_REALTIME_ALARM!")
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