Python "shutdown / reboot raspberry pi" script, using a single button

Question

I got a piece of code in Python from here: http://www.raspberry-pi-geek.com/Archive/2013/01/Adding-an-On-Off-switch-to-your-Raspberry-Pi

And I wanted to improve on it.

Since this is the first time I work with Python, I got stuck on understanding what actually happens.

Here is the code:

# Import the modules to send commands to the system and access GPIO pins
from subprocess import call
import RPi.GPIO as gpio
from time import sleep

gpio.setmode(gpio.BCM) # Set pin numbering to board numbering
gpio.setup(22, gpio.IN) # Set up pin 22 as an input

rebootBool = 0

# Define a function to keep script running
def main(pin):
    while True:
        #gpio.remove_event_detect(pin)
        gpio.add_event_detect(22, gpio.RISING, callback=confirmation, bouncetime=200) # Set up an interrupt to look for button presses
        sleep(5000000)

def confirmation(pin):
    gpio.remove_event_detect(pin)
    gpio.add_event_detect(22, gpio.RISING, callback=shutdown, bouncetime=200)
    sleep(3) # if button has been pressed again within 3 seconds, shut down will happen
    main(22)

def reboot(pin):
    rebootBool = 1
    call('reboot', shell=False)
    exit(0)

# Define a function to run when an interrupt is called
def shutdown(pin):
    gpio.remove_event_detect(pin)
    gpio.add_event_detect(22, gpio.RISING, callback=reboot, bouncetime=200)
    sleep(3) # if the button has been pressed for a third time, within 3 seconds, Pi will reboot
    if rebootBool == 0: # Just to make sure a halt is not called after the 3 seconds have passed, if reboot is called
        call('halt', shell=False)
    exit(0)

main(22) # Run the loop function to keep script running

What I want to do is this:

• If button is pressed once, a confirmation function is executed, in which it resets the button to call the shutdown function, if it's pressed within 3 seconds.
  • Else, resume with the main loop
• In the shutdown function, if it's pressed again (within 3 seconds), it resets to call the reboot function.
  • Else go on, and shut down

What happens is this:

If I press the button twice or three times, it tells me that gpio.add_event_detect is already defined, when it tries to define it in main(). So it doesn't change it, and if I press it one more time, it calls the shutdown function.

What I don't understand is:

Why does it want to define the gpio event in main, when the actual function is either reboot or shutdown (and it should call either reboot or shutdown)?

Answer 1

Because a callback function is running in a separate thread. This means, for example, when you call any callback function in the main loop it still works(main loop), and thus it happens when you call a function gpio.add_event_detect from a callback function(thread #1) and from the main loop(thread #2) at the same time. It's typical race condition. You can check it by running this code:

#!/usr/bin/python
# Import the modules to send commands to the system and access GPIO pins
from subprocess import call
import RPi.GPIO as gpio
from time import sleep

gpio.setmode(gpio.BCM) # Set pin numbering to board numbering
gpio.setup(22, gpio.IN, pull_up_down=gpio.PUD_DOWN) # Set up pin 22 as an input

rebootBool = 0

# Define a function to keep script running
def main(pin):
    while True:
        print "main loop"
        gpio.remove_event_detect(22)
        gpio.add_event_detect(22, gpio.RISING, callback=confirmation, bouncetime=200) # Set up an interrupt to look for button presses
        sleep(1)
        #raw_input()

def confirmation(pin):
    print "confirmation1"
    sleep(5)
    print "confirmation2"
    gpio.remove_event_detect(22)
    gpio.add_event_detect(22, gpio.RISING, callback=shutdown, bouncetime=200)
    sleep(5) # if button has been pressed again within 3 seconds, shut down will happen
    gpio.remove_event_detect(22)
    main(22)

def reboot(pin):
    rebootBool = 1
    print "reboot"
    #call('reboot', shell=False)
    exit(0)

# Define a function to run when an interrupt is called
def shutdown(pin):
    print "shutdown"
    gpio.remove_event_detect(pin)
    gpio.add_event_detect(22, gpio.RISING, callback=reboot, bouncetime=200)
    sleep(3) # if the button has been pressed for a third time, within 3 seconds, Pi will reboot
    if rebootBool == 0: # Just to make sure a halt is not called after the 3 seconds have passed, if reboot is called
        #call('halt', shell=False)
        print "halt"
    exit(0)

main(22) # Run the loop function to keep script running