GPIO.add_event_detect requires root

Question

I am trying to run a program using GPIO from a normal user. I checked that /dev/gpiomem has the correct permission (root:gpio 0660) and that my user is member of group gpio.

The following works correctly:

import RPi.GPIO as GPIO
from time import sleep

try:
    pin = 21
    GPIO.setmode(GPIO.BCM)
    GPIO.setup(pin, GPIO.IN)

    while True:
        pin_value = GPIO.input(pin)
        print("Pin {} has value {}".format(pin, pin_value))
        sleep(1)
finally:
    GPIO.cleanup()

When I press a button attached to pin 21 it does indeed show 1, and 0 otherwise. I run this as a standard user, so I guess it uses /dev/gpiomem (and indeed it failed before I fixed its permissions).

However, if I try to set an event on such pin

import RPi.GPIO as GPIO
from time import sleep

try:
    pin = 21
    GPIO.setmode(GPIO.BCM)
    GPIO.setup(pin, GPIO.IN)

    GPIO.add_event_detect(pin, GPIO.RISING)
    GPIO.add_event_callback(pin, lambda _: print("Pin {} was pressed".format(pin)))

    while True:
        pass
finally:
    GPIO.cleanup()

it raises the following exception:

Traceback (most recent call last):
  File "tst.py", line 9, in <module>
    GPIO.add_event_detect(pin, GPIO.RISING)
RuntimeError: Failed to add edge detection

Everything works fine using root, but I want to run this program as a standard user. Kernel is 4.14.37 and RPi.GPIO is 0.6.3. I am using a RPi 2.

It seems that adding an event on a pin still requires being root. Am I right? Am I doing anything wrong? How else could I detect a pin rising as a standard user?

Answer 1

I'm not sure what is going wrong. My Python knowledge is sketchy.

I'd dump the lambda and try the following (which works for me).

#!/usr/bin/env python

import RPi.GPIO as GPIO
import time

def callback(pin): 
   print("Pin {} was pressed".format(pin))

try:
    pin = 21
    GPIO.setmode(GPIO.BCM)
    GPIO.setup(pin, GPIO.IN)

    GPIO.add_event_detect(pin, GPIO.RISING, callback)

    while True:
        time.sleep(60)
finally:
    GPIO.cleanup()

Answer 2

I just changed the GPIO setup and it works for me (remote connection to rpi over ssh). This is my complete code. I added the pullup, changed the pin to 12 and because I was lazy used the setmode GPIO.BOARD because I just had the pin number. The shebang line does not influence, BUT I have to use python3 (3.7.3) to start the code since python (2.7.16) does return SyntaxError: invalid syntax with the line

GPIO.add_event_callback(pin, lambda _: print("Pin {} was pressed".format(pin)))

Here is my code that works. I start it with python3 main.py from the folder.

#!/usr/bin/python3
# -*- coding: utf-8 -*-

import RPi.GPIO as GPIO

try:
    pin = 12
    GPIO.setmode(GPIO.BOARD)
    GPIO.setup(pin, GPIO.IN, pull_up_down=GPIO.PUD_UP)

    GPIO.add_event_detect(pin, GPIO.RISING)
    GPIO.add_event_callback(pin, lambda _: print("Pin {} was pressed".format(pin)))

    while True:
        pass
finally:
    GPIO.cleanup()

Answer 3

Unfortunately, to interface with the GPIO pins, it is required that you use sudo. This is because it is a component that directly controls the Pi's electronics, and it is usually safer to only allow administrators to use those components, especially because they want to make it idiotproof in case it is used as a public computer.

So, the only way your program can work is like this: sudo python filename.py

Of course, you could use an extra component or a microcontroller to send the information wirelessly, but that would be annoying, so unless you want to do that, I would just use sudo.