Fixed memory allocation
ArduinoJson uses a preallocated memory pool to store the object tree, this is done by the
Before using any function of the library you need to create a
StaticJsonBuffer. Then you can use this instance to create arrays and objects, or parse a JSON string.
StaticJsonBuffer has a template parameter that determines its capacity. For example, the following line creates a
StaticJsonBuffer with a capacity of 200 bytes:
The bigger the buffer is, the more complex the object tree can be, but also the more memory you need.
How to determine the buffer size?
So the big question you should have in mind right now is How can I determine the size?.
There are basically two approaches here:
- either you can predict the content of the object tree,
- or, you know how much memory is available.
In the first case, you know some constraints on the object tree. For instance, let’s say that you know in advance (and by that I mean “at compilation time”) that you want to generate an object with 3 values, one of them being an array with 2 values, like the following:
To determine the memory usage of this object tree, you use the two macros
JSON_OBJECT_SIZE(n), both take the number of elements as an argument.
For the example above, it would be:
const int BUFFER_SIZE = JSON_OBJECT_SIZE(3) + JSON_ARRAY_SIZE(2); StaticJsonBuffer<BUFFER_SIZE> jsonBuffer;
If you’re in this situation, ArduinoJson Assistant will be of great help.
In the second case, let’s say you dynamically generate a JSON object tree of a random complexity so you can’t put a limit based on that. But on the other hand, you don’t want your program to crash because the object tree doesn’t fit in memory. The solution here is to determine how much memory is available, or in other words how much memory you can afford for the JSON object tree.
Why choose fixed allocation?
This fixed allocation approach may seem a bit strange, especially if you are a desktop application developer used to dynamic allocation, but it makes a lot of sense in an embedded context:
- the code is smaller
- it uses less memory
- it doesn’t create memory fragmentation
- it is predictable
Don’t forget that the memory is “freed” as soon as the
StaticJsonBuffer is out of scope, like any other variable. It only holds the memory for a short amount of time.
Dynamic memory allocation
This library also supports dynamic memory allocation.
However, usage of this memory model make sense only for devices with more that 10KB or RAM
To switch to dynamic memory allocation, simply replace:
Memory is released in
DynamicJsonBuffer’s destructor, so you don’t have to do anything special.
Object size for 8-bit AVR
|JsonArray of N element||4 + 8 * N|
|JsonObject of N element||4 + 10 * N|
Object size on ESP8266
|JsonArray of N element||8 + 12 * N|
|JsonObject of N element||8 + 16 * N|
Object size on ESP8266 if
JsonFloat is changed to
|JsonArray of N element||8 + 24 * N|
|JsonObject of N element||8 + 32 * N|
Use ArduinoJson Assistant to compute the size required for your project.
Where to go next?
In the ArduinoJson ebook, the chapter “Inside ArduinoJson” explains how
DynamicJsonBuffer work, and how to choose between them.
The book also contains a quick C++ course to catch up with memory management in general. It explains the differences between “stack,” “heap” and “global” memories. It also debunks wrong assumption that memory management is done by the developer.