Error -23 in mqtt_connect

RE: Error -23 in mqtt_connect

Håkon Alseth — Mon, 16 Mar 2026 10:02:25 GMT

Hi,

As seen from the code, mqtt_abort() wraps around mqtt_disconnect() with a if-sentence:

[quote user="hkn"]

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.

[/quote]

Kind regards,

Håkon

RE: Error -23 in mqtt_connect

Bastien42 — Mon, 16 Mar 2026 09:55:43 GMT

Hi,

I believe that mqtt_disconnect() may return without closing the socket in case of errors, in such cases calling mqtt_abort() will close the socket.

Best regards

Bastien Neukomm

RE: Error -23 in mqtt_connect

Håkon Alseth — Wed, 04 Mar 2026 11:11:29 GMT

Hi,

[quote user="Bastien42"]I am currently using a nRF9160 with NCS 2.7.0 and I don't think that CONFIG_ZVFS_POLL_MAX exists in that version.[/quote]

Check the value of config "CONFIG_POSIX_MAX_FDS".

As you mention, if left untouched, it should not be a problem, however; if you are concurrently maintaining several asynchronous connections; please see the restrictions in the release notes.

Kind regards,

Håkon

RE: Error -23 in mqtt_connect

Bastien42 — Tue, 03 Mar 2026 15:13:11 GMT

Hi,

I am currently using a nRF9160 with NCS 2.7.0 and I don't think that CONFIG_ZVFS_POLL_MAX exists in that version. I suppose that i have a default value for the corresponding parameter

Best regards

Bastien

RE: Error -23 in mqtt_connect

Håkon Alseth — Tue, 03 Mar 2026 09:37:57 GMT

Hi,

Yes, other concurrent connections should be considered.

Please see here for the restrictions of the modem firmware when it comes to concurrent connections, especially if they are secure sockets:

https://nsscprodmedia.blob.core.windows.net/prod/software-and-other-downloads/dev-kits/nrf9160-dk/release_notes_modemfirmware/mfw_nrf9160_1.3.7_release_notes.txt

What is your current "CONFIG_ZVFS_POLL_MAX" set to? You can try to increase this slightly, in case it is the kernel's fd table that is overflowing.

Kind regards,

Håkon

RE: Error -23 in mqtt_connect

Bastien42 — Mon, 02 Mar 2026 13:44:40 GMT

Yes sure, do connections to nRF Cloud via MQTT can also be source of problems ?

RE: Error -23 in mqtt_connect

Håkon Alseth — Mon, 02 Mar 2026 13:41:25 GMT

Hi,

[quote user="Bastien42"]Okay, I tried it, i still seem to have an edge case where sockets are not closed properly.[/quote]

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.

[quote user="Bastien42"]Do you know how to properly monitor the fd table ? I think that this will help me to get to the problem.[/quote]

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

Kind regards,

Håkon

RE: Error -23 in mqtt_connect

Bastien42 — Mon, 02 Mar 2026 09:26:55 GMT

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.

RE: Error -23 in mqtt_connect

Håkon Alseth — Mon, 23 Feb 2026 13:07:26 GMT

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

RE: Error -23 in mqtt_connect

Bastien42 — Mon, 23 Feb 2026 13:00:25 GMT

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 ?

RE: Error -23 in mqtt_connect

Håkon Alseth — Mon, 23 Feb 2026 12:57:07 GMT

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

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

Kind regards,

Håkon