JSMN for mbed

This is an adaption of Serge Zaitsev's jsmn to work directly on the mbed-os platform with the mbed cli.

Installation

You can add this library to your project using the mbed-cli:

mbed add https://github.com/productize/mbed-jsmn

JSMN

jsmn (pronounced like 'jasmine') is a minimalistic JSON parser in C. It can be easily integrated into resource-limited or embedded projects.

You can find more information about JSON format at json.org

Library sources are available at https://github.com/zserge/jsmn

The web page with some information about jsmn can be found at http://zserge.com/jsmn.html

Philosophy

Most JSON parsers offer you a bunch of functions to load JSON data, parse it and extract any value by its name. jsmn proves that checking the correctness of every JSON packet or allocating temporary objects to store parsed JSON fields often is an overkill.

JSON format itself is extremely simple, so why should we complicate it?

jsmn is designed to be robust (it should work fine even with erroneous data), fast (it should parse data on the fly), portable (no superfluous dependencies or non-standard C extensions). And of course, simplicity is a key feature - simple code style, simple algorithm, simple integration into other projects.

Features

• compatible with C89
• no dependencies (even libc!)
• highly portable (tested on x86/amd64, ARM, AVR)
• about 200 lines of code
• extremely small code footprint
• API contains only 2 functions
• no dynamic memory allocation
• incremental single-pass parsing
• library code is covered with unit-tests

Design

The rudimentary jsmn object is a token. Let's consider a JSON string:

'{ "name" : "Jack", "age" : 27 }'

It holds the following tokens:

• Object: { "name" : "Jack", "age" : 27} (the whole object)
• Strings: "name", "Jack", "age" (keys and some values)
• Number: 27

In jsmn, tokens do not hold any data, but point to token boundaries in JSON string instead. In the example above jsmn will create tokens like: Object [0..31], String [3..7], String [12..16], String [20..23], Number [27..29].

Every jsmn token has a type, which indicates the type of corresponding JSON token. jsmn supports the following token types:

• Object - a container of key-value pairs, e.g.: { "foo":"bar", "x":0.3 }
• Array - a sequence of values, e.g.: [ 1, 2, 3 ]
• String - a quoted sequence of chars, e.g.: "foo"
• Primitive - a number, a boolean (true, false) or null

Besides start/end positions, jsmn tokens for complex types (like arrays or objects) also contain a number of child items, so you can easily follow object hierarchy.

This approach provides enough information for parsing any JSON data and makes it possible to use zero-copy techniques.

API

Token types are described by jsmntype_t:

typedef enum {
	JSMN_UNDEFINED = 0,
	JSMN_OBJECT = 1,
	JSMN_ARRAY = 2,
	JSMN_STRING = 3,
	JSMN_PRIMITIVE = 4
} jsmntype_t;

Note: Unlike JSON data types, primitive tokens are not divided into numbers, booleans and null, because one can easily tell the type using the first character:

• 't', 'f' - boolean
• 'n' - null
• '-', '0'..'9' - number

Token is an object of jsmntok_t type:

typedef struct {
	jsmntype_t type; // Token type
	int start;       // Token start position
	int end;         // Token end position
	int size;        // Number of child (nested) tokens
} jsmntok_t;

Note: string tokens point to the first character after the opening quote and the previous symbol before final quote. This was made to simplify string extraction from JSON data.

All job is done by jsmn_parser object. You can initialize a new parser using:

jsmn_parser parser;
jsmntok_t tokens[10];

jsmn_init(&parser);

// js - pointer to JSON string
// tokens - an array of tokens available
// 10 - number of tokens available
jsmn_parse(&parser, js, strlen(js), tokens, 10);

This will create a parser, and then it tries to parse up to 10 JSON tokens from the js string.

A non-negative return value of jsmn_parse is the number of tokens actually used by the parser. Passing NULL instead of the tokens array would not store parsing results, but instead the function will return the value of tokens needed to parse the given string. This can be useful if you don't know yet how many tokens to allocate.

If something goes wrong, you will get an error. Error will be one of these:

• JSMN_ERROR_INVAL - bad token, JSON string is corrupted
• JSMN_ERROR_NOMEM - not enough tokens, JSON string is too large
• JSMN_ERROR_PART - JSON string is too short, expecting more JSON data

If you get JSON_ERROR_NOMEM, you can re-allocate more tokens and call jsmn_parse once more. If you read json data from the stream, you can periodically call jsmn_parse and check if return value is JSON_ERROR_PART. You will get this error until you reach the end of JSON data.

Other info

This software is distributed under MIT license, so feel free to integrate it in your commercial products.