Unable to set line 4 to input

[seompark]

I'm trying to make a Raspberry Pi 3 REST API that provides temperature and humidity with DHT22. The whole code:

from flask import Flask, jsonify, request
from sds011 import SDS011
from adafruit_dht import DHT22
import board
import os
import time

app = Flask(__name__)
dht = DHT22(board.D4)

def get_dht_data():
    while True:
        try:
            temperature, humidity = dht.temperature, dht.humidity
            print(temperature, humidity)
            if temperature is not None and humidity is not None:
                return temperature, humidity
            else:
                raise
        except:
            time.sleep(0.5)

@app.route('/', methods=['GET'])
def status():
    temperature, humidity = get_dht_data()

    return jsonify({
        'temperature': temperature,
        'humidity': humidity
    })

if __name__ == '__main__':
    app.run(debug=True)

However, when I start server, it shows message

Unable to set line 4 to input

and temperature and humidity is always None. If I don't run flask app but just DHT code, it works.

[ladyada]

maybe you have to run as root? not sure - the code works as you said, just not in Flask

[monkeywire]

..it seems to work fine when I first boot up the raspberry pi and I can run the test code once and it works.... After that if I try to run to the test code a second time I get the same error....

[ladyada]

try killing any 'libgpiod_pulsein' processes

[GratefulCoder]

try killing any 'libgpiod_pulsein' processes

Same issue occurring here - not using Flask at all though. Worked on the first try when on fresh boot, but then subsequent executions would fail. Changing the pin and running the program had the same effect, worked the first time and subsequent attempts would fail with example:

Unable to set line 4 to input
Timed out waiting for PulseIn message
Timed out waiting for PulseIn message
Timed out waiting for PulseIn message
...

Locating the process libgpiod_pulsein ( pgrep libgpiod_pulsein for example ) and kill <PID found> did the trick.

Thanks!!!

[monkeywire]

Works, thank you!

[sczerwinski]

In my case, pgrep libgpiod_pulsein returned nothing. Reboot didn't change anything either.

Fortunately, changing the PIN to D18 worked.

Still no idea what is blocking GPIO4 though.

[jsubirat]

Short answer

Remove debug=True and enjoy. :-)

What was going on?

This problem drove me nuts but I think I found the root cause! In my case, I was encapsulating the DHT22 object in another object, like so:

...
class DHT22Custom:
    def __init__(self):
        print("**** BUILDING by {0}!".format(threading.currentThread().getName()))
        self.dht_device = adafruit_dht.DHT22(board.D17)
...

My main.py looked like:

import RPi.GPIO as GPIO
from sensor.config.app_config import create_app

if __name__ == '__main__':
    app = create_app()         # builds DHT22Custom inside
    app.run(debug=True)
    print("Cleaning up GPIO before exiting...")
    GPIO.cleanup()

And look what an interesting output I got:

root@05600e5:/app# python -m sensor.main
**** BUILDING by MainThread!
 * Serving Flask app "FlaskWS" (lazy loading)
 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: on
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
 * Restarting with stat
**** BUILDING by MainThread!
Unable to set line 17 to input

The MainThread was initializing my object twice! How was that? Well, if you look at the documentation of Flask's run(), you'll see the following:

If the :attr:`debug` flag is set the server will automatically reload
 for code changes and show a debugger in case an exception happened.

So, it seems that Flask just relaunches the application or something like this. Not clear to me, to be honest. But well, if you just remove debug, you'll see something like:

root@05600e5:/app# python -m sensor.main
**** BUILDING by MainThread!
 * Serving Flask app "FlaskWS" (lazy loading)
 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: off
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

I hope this helps. Happy coding!

[rashmendis]

try killing any 'libgpiod_pulsein' processes

This is a temporary solution right? It happens every time I stop the code and re-run it. Any solution?

[poisa]

Fortunately, changing the PIN to D18 worked.

In my case, Pi 3 A+, only pin 11 (aka GPIO17/board.D17) worked. Neither of the other pins suggested here worked. ¯_(ツ)_/¯

[Saeven]

Also seeing the same problem!

[rashmendis]

Their old library worked for me. https://github.com/adafruit/Adafruit_Python_DHT

[SmackTO]

I'm also seeing this problem on a RPi4. Also, the libgpio_pulsein task uses 25% of my CPU throughput!

The old library worked fine for me too. I'm going to revert and see what happens.

[jsubirat]

Did you guys try to remove debug=True when initializing Flask? I think it's as simple as that...

[Saeven]

Not even using Flask - just this script, is enough to make the RPI4 hang:

import time
import board
import adafruit_dht

dhtDevice = adafruit_dht.DHT22(board.D4)

[RashmiMendis]

Yeah it is happening even to their sample code. Just try to run the code. Stop it. They run it again.

…

On May 3, 2020, 7:50 AM +0530, Alexandre Lemaire ***@***.***>, wrote: Not even using Flask - just this script, is enough to make the RPI4 hang: import time import board import adafruit_dht dhtDevice = adafruit_dht.DHT22(board.D4) — You are receiving this because you commented. Reply to this email directly, view it on GitHub, or unsubscribe.