publish json data to mqtt

Question

I am trying to publish JSON data to mqtt with following code snippet

#include <Arduino_JSON.h>
JSONVar data;
data["temperature"] = temperature;
data["humidity"] = humidity;
data["pressure"] = pressure;
data["chipId"] = chipId;
String jsonString = JSON.stringify(data);
client.publish("esp32/data", jsonString);

But getting following error

/home/ubuntu/Arduino/libraries/PubSubClient/src/PubSubClient.h:144:12: note: candidate expects 4 arguments, 2 provided
Multiple libraries were found for "PubSubClient.h"
 Used: /home/ubuntu/Arduino/libraries/PubSubClient
Multiple libraries were found for "Wire.h"
 Used: /home/ubuntu/.arduino15/packages/esp32/hardware/esp32/1.0.4/libraries/Wire
Multiple libraries were found for "Adafruit_BME280.h"
 Used: /home/ubuntu/Arduino/libraries/Adafruit_BME280_Library
Multiple libraries were found for "Adafruit_Sensor.h"
 Used: /home/ubuntu/Arduino/libraries/Adafruit_Unified_Sensor
Multiple libraries were found for "SPI.h"
 Used: /home/ubuntu/.arduino15/packages/esp32/hardware/esp32/1.0.4/libraries/SPI
Multiple libraries were found for "Arduino_JSON.h"
 Used: /home/ubuntu/Arduino/libraries/Arduino_JSON
Multiple libraries were found for "WiFi.h"
 Used: /home/ubuntu/.arduino15/packages/esp32/hardware/esp32/1.0.4/libraries/WiFi
 Not used: /opt/arduino-1.8.10/libraries/WiFi
exit status 1
no matching function for call to 'PubSubClient::publish(const char [11], String&)'

Whats is wrong here ?

Answer 1

What's your last error message?

no matching function for call to 'PubSubClient::publish(const char [11], String&)'

Read the documentation for the publish() method in PubSubClient:

int publish (topic, payload)

Publishes a string message to the specified topic.

Parameters

topic - the topic to publish to (const char[])

payload - the message to publish (const char[])

Look at the parameters you're using. You're passing a String as payload. publish() expects payload to be a char[] (essentially the same thing as a char*) - a C language string. This is the error you're seeing.

Read the documentation for String. How do you get a char* from a String? The c_str() method.

You need to change

client.publish("esp32/data", jsonString);

to

client.publish("esp32/data", jsonString.c_str());

As for your other warnings, it looks like your Arduino libraries are a mess with multiple conflicting libraries installed; if you want to get rid of the warnings, clean up your libraries.

• rp346: is it possible to convert json to char* ?
• romkey: @roy - I've given you the steps to do that in the answer. If you want to do it more directly, you're using the Arduino_JSON library. The declaration for the stringily() method is at github.com/arduino-libraries/Arduino_JSON/blob/master/src/… and it only shows a String as a return, so you cannot do it more directly using this library.
• rp346: Arduino_JSON has this link github.com/arduino-libraries/Arduino_JSON, not much info here. Most of the example are based on ArduinoJSON.
• romkey: My comment answered your question - I may have been editing it when you were replying. You can find the answer in the declaration of stringify() injson.h