strange clientID about arduino-mqtt HOT 6 CLOSED

My best guess is that your code failed to read 'MQTT_User' from the json-file.
I think the single u means that no username was provided. This is how it looks for me.
1502949323: New client connected from 192.168.1.77 as mega2560 (c1, k60, u'mega2560').

Good luck!

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Thanks for the support. You have even looked at my code!
I analysed the mosquitto logfile closer. I have an entry from a working connection by the same code, but different MQTT library (imroy):
1502516513: New client connected from 192.168.11.80 as sonoffth1 (c1, k15, u'mqtt').
The MQTT clientID is placed before the bracket. The user (mqtt) is inside the bracket. The password is (luckily) not in the logfile. My conclusion is:

• the ClientID is either not sent or missing
• the username is missing too

Therefore the problem is even worse, but at least consistent. Maybe Joël has an idea.
Kind regards,
Urs.

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Dear Urs,

I can ensure you that the arduino-mqtt library is correctly passing along the client-id, username and password parameters if they are correct C strings.

But in your code I can see that you are allocating the StaticJsonBuffer<700> jsonBuffer; on the stack in setup(). Without knowing the ArduinoJSON library by heart I believe that the derived C strings from the JSON object are just pointers to parts of that buffer. In other words, the memory is reused when leaving setup() and entering loop(). Now since 700 bytes are a big portion of memory the data might still be there, and thats why you probaly run into inconsistencies between the libraries as they affect the memory layout.

I hope that helps.

Best,
Joël

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Hello Joël, thanks for your feedback and for having a look at my code.
I started to suspect something like that yesterday. I'm not a programmer, especially not in C and really do not have a clue about the difference between pointers and why the heck something which is once in a variable just disappears sometimes later after a few hours of running. Really. This is stupid. But it just happened with my code.
Since I have no clue about programming, I said to myself, if I copy the variable content into another variable where there is no pointer strangeness, it might solve my problem. Here is what I did, most probably ugly as hell. I just copy the code section which handle the MQTT client ID:

In the top section:
const char* Hostname;
String mqtt_id_s;
const char* mqtt_id_p;

In the setup() section:
StaticJsonBuffer<700> jsonBuffer;
JsonObject& readc = jsonBuffer.parseObject(buf.get());
Hostname = readc["Hostname"];
mqtt_id_s = String(Hostname);
mqtt_id_p = mqtt_id_s.c_str();

And then where the MQTT connection to the broker is started:
mqttClient.connect(mqtt_id_p, mqtt_user_p, mqtt_pass_p);

So far I do not see the above mentioned problems in the log. So everything seems fine.
If someone more knowledgeable than me on this topic (and there must be thousands out there) can help to make this into better code, please drop a note.
Kind regards,
Urs.

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Converting the C string to a String object is a good approach as it will automatically allocate memory on the heap to store the data.

FYI: C has no support for string objects as other programming languages do. A const char * just points to the address of the first byte of the string. By convention any string operation will stop if a null byte is found e.g. \0. Thus, if you copy a const char * you just copy an address and not its contents. That's why Arduino provides the String object.

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Thanks for your support, I really appreciate it.
With your last hint I'm now able to make the code a little prettier, now that it works and I even understand why.

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