DynamicJsonBuffer are designed to be throwaway memory pools, they are not intended to be reused.
As a consequence, using a global
JsonBuffer is not possible.
Make sure you read: How to reuse a
ArduinoJson is designed to do one thing and to do it well: the JSON serialization.
So before trying to use a global
JsonBuffer, ask yourself first:
Am I really using ArduinoJson for serialization, or am I pushing it beyond its initial intent?.
In particular, you should not use
JsonArray to store the internal state of your program as this would be terribly inefficient. Instead, write your own data structure and use ArduinoJson only in serialization functions, as shown in What’s the best way to use the library?
The only legitimate usage of a global buffer is to have a
StaticJsonBuffer in the
.data segment to that the compiler will issue an error if there is not enough memory. While this seems like a good idea, it wastes a lot of RAM as it reserves memory that is used only a fraction of the time. Again, make sure you read: How to reuse a