CAUTION: SLIPPERY FLOORThe Arduino Library Manager installs the ArduinoJson version 6 by default.
However, using version 5 is highly recommended because version 6 is still in beta stage.
Open the Arduino Library Manager and make sure that ArduinoJson version 5 is installed.
If you pass the flag
--fpack-struct to the compiler, it will generate the following warning:
warning: ignoring packed attribute because of unpacked non-POD field
No solution has been found so far: you need to remove that flag if you want to get rid of that warning.