Error -23 in mqtt_connect

[Bastien42]

Good afternoon,

My device communicates using MQTT a few times a day. After many connections I noticed that communications stopped. Every connection attempt would fail with:

Error in mqtt_connect: -23.

Error code 23 corresponds to "file table overflow", therefore, I assumed that it was linked to a mishandling of sockets when connection failed.

I have now made sure that mqtt_abort() is called in any case where connection may fail and when disconnecting, and I am currently testing those changes.

Are there other causes to error -23 in mqtt_connect() ?

Best regards

Bastien

[Håkon Alseth]

Hi,

 

The socket is closed when you call mqtt_disconnect(), as this will then subsequently call close():

https://github.com/nrfconnect/sdk-zephyr/blob/main/subsys/net/lib/mqtt/mqtt_transport_socket_tcp.c#L167

Similar for TLS:

https://github.com/nrfconnect/sdk-zephyr/blob/main/subsys/net/lib/mqtt/mqtt_transport_socket_tls.c#L249

 

It is very important that you close a socket before initiating a new connection, to avoid overflowing the fd table.

 

Kind regards,

Håkon

[Bastien42]

Thanks for this rapid answer.

Okay, I will implement that.

Is mqtt_abort() even useful in my case ? Should I try to use mqtt_disconnect only ?

[Håkon Alseth]

This function will only disconnect if the mqtt state is not idle:

https://github.com/nrfconnect/sdk-zephyr/blob/main/subsys/net/lib/mqtt/mqtt.c#L689-L702

 

I would recommend that you call mqtt_disconnect() directly to ensure socket cleanup.

 

Kind regards,

Håkon

[Bastien42]

Okay, I tried it, i still seem to have an edge case where sockets are not closed properly.

Do you know how to properly monitor the fd table ? I think that this will help me to get to the problem.

[Håkon Alseth]

Hi,

 

Bastien42 said:

Okay, I tried it, i still seem to have an edge case where sockets are not closed properly.

Please note that each mqtt_connect() will effectively setup a new fd:

https://github.com/nrfconnect/sdk-zephyr/blob/main/subsys/net/lib/mqtt/mqtt_transport_socket_tcp.c#L21-L30

 

I would recommend that you check if your firmware has any scenarios where it re-connects without closing/disconnecting the former socket successfully.

Bastien42 said:

Do you know how to properly monitor the fd table ? I think that this will help me to get to the problem.

Looking at zephyr/sys/fdtable.h, it does not seem that there is an API directly to do this, unfortunately.

 

Kind regards,

Håkon