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  • it is a know bug in Arduino toolchain. copy the callback function before the first use. before client declaration. Commented Mar 22, 2018 at 6:34
  • @Juraj I've changed it, but I found the same error Commented Mar 23, 2018 at 12:23
  • update the code in the question Commented Mar 23, 2018 at 12:35
  • have you tried adding void callback(char* topic, byte* payload, unsigned int length); (exactly that one line with ; at the end) before PubSubClient client(MQTT_SERVER, 1883, callback, wifiClient); Commented Mar 23, 2018 at 23:02
  • @Juraj - that's not a bug, that's how C++ rolls - you can't "reference" a function in code before you "declare" it - also, moving callback will mean the callback function will try to "reference" client before it is declared, so, similar (but not exactly the same) error will occur - declaring the function callback before client will fix the issue Commented Mar 23, 2018 at 23:06