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Showing content from https://arduinojson.org/v6/api/basicjsondocument/shrinktofit/ below:

BasicJsonDocument<T>::shrinkToFit() | ArduinoJson 6

Description

BasicJsonDocument<T>::shrinkToFit() reduces the capacity of the memory pool to match the current usage. Use this function to release some memory.

There is no reverse operation to resize the memory pool; you can only shrink it.

Unlike JsonDocument::garbageCollect(), this function doesn’t reclaim leaked memory blocks.

This function was added in ArduinoJson 6.14.0

Example

Here is how you can deserialize using as a lot of memory, and then release what’s unused.

DynamicJsonDocument doc(8192);
deserializeJson(doc, json);
doc.shrinkToFit();

On ESP8266, you can call ESP.getMaxFreeBlockSize() to know how much memory you can use:

DynamicJsonDocument doc(ESP.getMaxFreeBlockSize() - 512);
deserializeJson(doc, json);
doc.shrinkToFit();

Similarly, on ESP32, you can call ESP.getMaxAllocHeap():

DynamicJsonDocument doc(ESP.getMaxAllocHeap() - 1024);
deserializeJson(doc, json);
doc.shrinkToFit();

These two functions return the size of the largest block of free memory. The result may be significantly lower than the total available memory (ESP.getFreeHeap()) if the heap is fragmented.