ArduinoJson 6 Deserialisation issue - unexpected output

[bill-i-am-code]

Describe the issue
Deserialisation is changing source data, and producing unexpected results.

Troubleshooter report
Here is the report generated by the ArduinoJson Troubleshooter:

1. The program uses ArduinoJson 6
2. The issue happens at run time
3. The issue concerns deserialization
4. deserializeJson() returns Ok
5. Some values are missing
6. The JSON document is not large

Environment
Here is the environment that I'm using':

• Microconroller: ESP32
• Core/runtime: ESP32 core for Arduino v2.0.10
• IDE: Arduino IDE 1.8.19

Reproduction
I have a program which captures some data as a short JSON string and stores in logHistory[i].logDataString e.g.
{"dbAverage":36.1,"dbMin":36.1,"dbMax":36.1,"timestamp":"2025-11-03 15:45:14"}

I have a webpage to monitor all values in the array logHistory[i].logDataString and want it to show them as a single JSON document e.g.

{"logHistory":[{"logData":{"dbAverage":36.1,"dbMin":36.1,"dbMax":36.1,"timestamp":"2025-11-03 15:45:30"},"submitted":true},{"logData":{"dbAverage":43,"dbMin":35.3,"dbMax":56.6,"timestamp":"2025-11-03 15:45:35"},"submitted":true},{"logData":{"dbAverage":40.2,"dbMin":35.4,"dbMax":52.2,"timestamp":"2025-11-03 15:45:40"},"submitted":true},{"logData":{"dbAverage":40.4,"dbMin":35.7,"dbMax":48.9,"timestamp":"2025-11-03 15:45:45"},"submitted":true}]}

#define LOG_HISTORY_SIZE 30         // number of historic log values to store
struct LogHistory
{
    char logDataString[80];
    bool submitted;
};
struct LogHistory logHistory[LOG_HISTORY_SIZE];
    server.on("/logHistory", HTTP_GET, [](AsyncWebServerRequest *request)
              { 
                AsyncResponseStream *response = request->beginResponseStream("application/json");
                DynamicJsonDocument doc(4096);
                char buff[80];
                for (int i = 0; i < LOG_HISTORY_SIZE; i++)
                {
                     PRINTS(i);
                     PRINTS(" ");
                     PRINTLN(logHistory[i].logDataString)
                    strlcpy(buff, logHistory[i].logDataString, 80); //if not sent via a buttfer, logDataString seems to get modified when deserialized
                    if (logHistory[i].logDataString[0] !='\0')
                    {
                   // Create a temporary document to parse the embedded JSON string
                    DynamicJsonDocument temp(768);  
                    DeserializationError error = deserializeJson(temp, buff);
                    if (!error) {
                    // Copy parsed object into main document
                    doc["logHistory"][i]["logData"] = temp.as<JsonObject>(); 
                    doc["logHistory"][i]["submitted"] = logHistory[i].submitted;
                    } else {
                    PRINTS("JSON parse error at index ");
                    PRINTS(i);
                    PRINTS(": ");
                    PRINTLN(error.c_str());
                    }
                    }

                }
                //serializeJson(doc, Serial);
                serializeJson(doc, *response);
                request->send(response); });

Before adding strlcpy(buff, logHistory[i].logDataString, 80); I was using deserializeJson(temp, logHistory[i].logDataString); but found that after deserialisation the value of logHistory[i].logDataString was null

The above code does not modify logHistory[i].logDataString but does not produce the desired output.
dbAverage key is missing, and all values of timestamp are the same

Actual output:

These 4 values of logHistory[i].logDataString

15:45:32:987 -> 0 {"dbAverage":36.1,"dbMin":36.1,"dbMax":36.1,"timestamp":"2025-11-03 15:45:14"}
15:45:32:993 -> 1 {"dbAverage":43,"dbMin":35.3,"dbMax":56.6,"timestamp":"2025-11-03 15:45:19"}
15:45:33:002 -> 2 {"dbAverage":40.2,"dbMin":35.4,"dbMax":52.2,"timestamp":"2025-11-03 15:45:25"}
15:45:33:007 -> 3 {"dbAverage":40.4,"dbMin":35.7,"dbMax":48.9,"timestamp":"2025-11-03 15:45:30"}

result in doc like this

{"logHistory":[{"logData":{"":36.1,"dbMin":36.1,"dbMax":36.1,"timestamp":"2025-11-03 15:45:30"},"submitted":true},{"logData":{"":43,"dbMin":35.3,"dbMax":56.6,"timestamp":"2025-11-03 15:45:30"},"submitted":true},{"logData":{"":40.2,"dbMin":35.4,"dbMax":52.2,"timestamp":"2025-11-03 15:45:30"},"submitted":true},{"logData":{"":40.4,"dbMin":35.7,"dbMax":48.9,"timestamp":"2025-11-03 15:45:30"},"submitted":true}]}

[bblanchon]

Hi @bill-i-am-code,

I think you fell into one of the pitfalls of the zero-copy mode of ArduinoJson 6.
Indeed, in old versions of ArduinoJson, when you pass a char[] as the input, the parser uses a memory optimization that uses the input buffer to store strings.

The solution is to avoid the zero-copy mode.
To do that, either cast the input to const char* or upgrade to ArduinoJson 7.

Also, with ArduinoJson 7, you wouldn't need an additional JsonDocument to perform the deserialization because you could pass doc["logHistory"][i] as the first argument of deserializeJson().
See online demo: https://wandbox.org/permlink/QELt4Aueg2PoTbgk

Then, you could make the code a little more efficient by first creating a JsonArray and then calling add<JsonVariant>().
See online demo: https://wandbox.org/permlink/m8NJIQUqvQE81EMC

Best regards,
Benoit

[bill-i-am-code]

Thank you for your prompt response and as ever the fantastic library. Casting to Const char* has worked as needed.

Best Regards

Bill