What does poll function do in mqtt_simple sample program in nRF9160?

[Yusuke]

I have been studying MQTT these days.

I'm testing the program below.
https://github.com/NordicPlayground/fw-nrfconnect-nrf/blob/master/samples/nrf9160/mqtt_simple/src/main.c

In this sample program, poll function is used on line 370. I guess it waits for an incoming data for an assigned duration, CONFIG_MQTT_KEEPALIVE.

However, it does not wait at all even though no data comes. Why? I'm confused.

[Jan Tore Guggedal]

Hi,

The poll function is documented here: https://github.com/NordicPlayground/nrfxlib/blob/master/bsdlib/include/nrf_socket.h#L846

Which version of NCS are you using? v0.3.0 didn't fully support poll() and returned immediately even though there's no event on the socket, while newly released v0.4.0 does. Also latest master branch supports poll() on sockets. 

Is anything printed after poll() returns? 

Best regards,

Jan Tore

[Yusuke]

Thank you for your reply.

I'm using this code below: tag/v0.4.0.
https://github.com/NordicPlayground/fw-nrfconnect-nrf

after poll function. this message comes up.

	while (1) {
		err = poll(&fds, 1, K_SECONDS(CONFIG_MQTT_KEEPALIVE));
		if (err < 0) {
			printk("ERROR: poll %d\n", errno);
			break;
		}

        printk("after poll\n"); // ADD THIS LINE

		err = mqtt_live(&client);
		if (err != 0) {
			printk("ERROR: mqtt_live %d\n", err);
			break;
		}

***** Booting Zephyr OS v1.14.99-ncs1 *****
The MQTT simple sample started
LTE Link Connecting ...
LTE Link Connected!
IPv4 Address found 0xxxxxxxxx
after poll
[mqtt_evt_handler:169] MQTT client connected!
Subscribing to: mySubTopic len 10
after poll
[mqtt_evt_handler:219] SUBACK packet id: 1234
after poll
after poll
after poll
after poll

I see it's working. "After poll" shows up every 60sec

By the way, in this polling, MCU is sleeping? I care about power consumption during polling.

[Jan Tore Guggedal]

Hi,

One "after poll" print every 60 seconds sounds reasonable as that's the default value of CONFIG_MQTT_KEEP_ALIVE. 

Yes, the thread calling poll() is put to sleep while it waits for an event on the socket or until it times out.

Best regards,

Jan Tore

[Yusuke]

Thanks!