I found a memory leak in the library!

This is very unlikely. You’re probably using the library incorrectly.

The typical problem comes from reusing a JsonBuffer several time. Each time you call parseArray(), parseObject(), createArray() and createObject(), you consume memory in the JsonBuffer. To avoid running out of memory, you should discard unused data as soon as possible.

The recommended practice is to do the JSON handling in a dedicated function, with a local JsonBuffer that will be automatically reclaimed when the function exits. This means that you cannot return a JsonArray or a JsonObject from that function, because they would contain dangling pointers to what used to be the JsonBuffer. Instead, you should convert the content of the JsonArray to a custom array or vector; or the content of the JsonObject to your own data structure.

This seems like a constraint, but remember that you’re programming for an embedded platform with very limited resources, and that requires special techniques.

A positive side effect of following this recommendation is that the code is safe and memory efficient. It also encourages the separation of responsibilities: the function is only in charge of the JSON serialization and no specific JSON data leaks elsewhere is the program.

See:

• What’s the best way to use the library
• Why shouldn’t I use a global JsonBuffer?
• The first parsing succeeds, why do the next ones fail?
• ArduinoJson: Avoiding pitfalls