Arduino JSON Library

JSON structure

What is JSON

Exchanging data with other computers can be a daunting task with Arduino. No matter if you just want to pass some information to Processing, to a Web Service or something else – You always have to encode the data and decode the answer.

There always have been solutions like XML for structured data. But XML is hard to decode, complicated an takes up a lot of space. And then there is JSON.
JSON is described best on

It’s like XML, but fat-free. You use it to move data around, store things, or just generally represent your program’s state.
JSON is especially useful to exchange data efficiently with e.g. JavaScript, Java, C++, Processing or anything else.

JSON is quite widespread used so that is perfect for exchanging data with other applications. In combination with HTTP it is suitable to implement REST Web Services.

What is aJson

aJson is an Arduino library to enable JSON processing with Arduino. It easily enables you to decode, create, manipulate and encode JSON directly from and to data structures. By this you don’t have to bother with data encoding and decoding – this will aJson handle for you. aJson is a library to receive, understand, create or modify JSON strings directly in the Arduino. aJson provides functions to parse JSON strings to object models. Handle, search and create and modify JSON Object structures.

It is based on the cJSON implementation, reduced in size and mising one or two features:

  • The code has very limited support on ATmega168 – there is just not enough memory and memory fragmentation is a serious problem
  • Arrays and Lists are max 255 elements big
  • There is no proper Unicode handling in this code
  • There is an internal buffer eating up 256 bytes of RAM

You can download the latest version of aJsons from Github. If you notice any problem just file an issue so that I can deal with it.

Parsing JSON

This is some JSON from this page:

"name": "Jack (\"Bee\") Nimble",
"format": {
"type": "rect",
"width": 1920,
"height": 1080,
"interlace": false,
"frame rate": 24

To parse such a structure with aJson you simply convert it to a object tree:

aJsonObject* jsonObject = aJson.parse(json_string);

Assuming you got the JSON string in the variable json_string – as a char*. This is an object. We’re in C. We don’t have objects. But we do have structs. Therefore the objects are translated into structs, with all the drawbacks it brings. Now we can e.g. retrieve the value for name:

aJsonObject* name = aJson.getObjectItem(root, "name");

The value of name can be retrieved via:


Note that the aJsonObject has a union which holds all possible value types as overlays – you can get only useful data for the type which you have at hand. You can get the type as
which can be either aJson_False, aJson_True, aJson_NULL, aJson_Number, aJson_String, aJson_Array or aJson_Object. For aJson_Number you can use valueint for Integers or valuedouble for floating point numbers. For aJson_String you can use valuestring. For True or False, you can use valuebool.

To render the object back to a string you can simply call
char *json_String=aJson.print(jsonObject);
Finished? Delete the root (this takes care of everything else).
This deletes the objects and all values referenced by it.

Parsing streams

As you can see this will eat up lots of memory. Storing the original string and the JSON object is a bit too much for your Arduino – it will most likely use up all the memory. Therefore it is better to parse streams instead of strings.
A stream in C is a FILE* – on Arduino there are some special streams, but later adapters will be provided. So if you for example read from a FILE* stream you can simply call
aJsonObject* jsonObject = aJson.parse(file);
By that you don’t have to store the JSON string in memory.

You can of course print JSON objects directly to a FILE* stream:

Filtering while parsing

Any JSON respond can have object name/value pairs your code either does not understand or is not interested in. To avoid those values to go into your memory you can simply add filters to your parsing request. A set of filter is just a list of names you are interested in, ended by a null value. If you are only interested in “name”, “format”, “height” and “width” in the above example you can do it like:
char** jsonFilter = {"name,"format","height","width",NULL};
aJsonObject* jsonObject = aJson.parse(json_string,json_filter);
(assuming you got the JSON string in the variable json_string – as a char*)

By that only the following structure is parsed – the rest will be ignored:
"name": "Jack (\"Bee\") Nimble",
"format": {
"width": 1920,
"height": 1080,

It is good practice to always use the filtering feature to parse JSON answers, to avoid unknown objects swamping your memory.

Creating JSON Objects from code

If you want to see how you’d build this struct in code?

aJsonObject *root,*fmt;
aJson.addItemToObject(root, "name", aJson.createString("Jack (\"Bee\") Nimble"));
aJson.addItemToObject(root, "format", fmt = aJson.createObject());
aJson.addStringToObject(fmt,"type", "rect");
aJson.addNumberToObject(fmt,"width", 1920);
aJson.addNumberToObject(fmt,"height", 1080);
aJson.addFalseToObject (fmt,"interlace");
aJson.addNumberToObject(fmt,"frame rate", 24);

The root object has: Object Type and a Child
The Child has name “name”, with value “Jack (“Bee”) Nimble”, and a sibling:
Sibling has type Object, name “format”, and a child.
That child has type String, name “type”, value “rect”, and a sibling:
Sibling has type Number, name “width”, value 1920, and a sibling:
Sibling has type Number, name “height”, value 1080, and a sibling:
Sibling hs type False, name “interlace”, and a sibling:
Sibling has type Number, name “frame rate”, value 24

If you want to create an array it works nearly the same way:

aJsonObject* root = aJson.createArray();
aJsonObject* day;
aJson.addItemToArray(root, day);
aJson.addItemToArray(root, day);
aJson.addItemToArray(root, day);
aJson.addItemToArray(root, day);
aJson.addItemToArray(root, day);
aJson.addItemToArray(root, day);
aJson.addItemToArray(root, day);

The whole library (nicely provided by cJSON) is optimized for easy usage. You can create and modify the object as easy as possible.

aJson Data Structures

aJson stores JSON objects in struct objects:

// The aJson structure:
typedef struct aJsonObject {
char *name; // The item's name string, if this item is the child of, or is in the list of subitems of an object.
struct aJsonObject *next, *prev; // next/prev allow you to walk array/object chains. Alternatively, use GetArraySize/GetArrayItem/GetObjectItem
struct aJsonObject *child; // An array or object item will have a child pointer pointing to a chain of the items in the array/object.
char type; // The type of the item, as above.
union {
char *valuestring; // The item's string, if type==aJson_String
char valuebool; //the items value for true & false
int valueint; // The item's number, if type==aJson_Number
float valuefloat; // The item's number, if type==aJson_Number
} aJsonObject;

By default all values are 0 unless set by virtue of being meaningful.

Note that the aJsonObject has a union ‘value’ which holds all possible value types as overlays – you can get only useful data for the type which you have at hand. You can get the type as


which can be either aJson_False, aJson_True, aJson_NULL, aJson_Number, aJson_String, aJson_Array or aJson_Object. For aJson_Number you can use valueint or valuedouble. If you’re expecting an int, read valueint, if not read valuedouble. For aJson_String you can use valuestring, for True or False, you can use valuebool.

The pointers next/prev are a double linked list of siblings. next takes you to your sibling, prev takes you back from your sibling to you.
Only objects and arrays have a “child“, and it’s the head of the doubly linked list. A “child” entry will have prev==NULL, but next potentially points on. The last sibling has next=NULL. The type expresses Null/True/False/Number/String/Array/Object, all of which are defined in aJson.h

Any entry which is in the linked list which is the child of an object will have a “string” which is the “name” of the entry. When I said “name” in the above example, that’s “string”. “string” is the JSON name for the ‘variable name’ if you will.

Now you can trivially walk the lists, recursively, and parse as you please. You can invoke aJson.parse to get aJson to parse for you, and then you can take the root object, and traverse the structure (which is, formally, an N-tree), and tokenise as you please.

Lists in aJson

Lists are easily handled in aJson, to create a list you can simply use the provided API functions:


You simply pass a array of the respective type: char*[], int[] and so on.

aJSON doesn’t make any assumptions about what order you create things in. You can attach the objects, as above, and later add children to each of those objects with


or remove them with aJson.deleteItemFromArray() – which also deletes the objects, or aJson.detachItemFromArray() – which does not free the memory

As soon as you call aJson.print(), it renders the structure to text.

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