libfjson

Overview

fjson is a recursive Finite-State Machine JSON parser which operates on streams (byte per byte!) by keeping track internally of the current state of parsing.
The compiled library is under 6 kilobytes.

API

fjson's APIs are just 6

• fjson_new() Allocates a new fjson_t structure.
• fjson_free() Frees the fjson_t structure.
• fjson_get_value_by_key() Given an element with type FJSON_TYPE_OBJECT, and a key, it returns the value (if present).
• fjson_putbyte() Accepts one byte and processes it. Returns 0 on still-parsing, -1 on error, 1 on successful parsing.
• fjson_putbuf() If you want to process entire buffers (even partial json documents are okay) you can call this function. It just uses fjson_putbyte() internally.
• fjson_free_element() Recursively frees the fjson_element_t structure.

How to use it

This is a Finite State Machine parser that operates byte per byte.
This means that you can literally start to download a json document and parsing it on-the-fly without the need of caching it.
It is easy:

1. Allocate the fjson_t structure
2. When you have one byte or a buffer, just process it using the fjson_putbyte() or fjson_putbuf() APIs.
3. Once finished the parsing you can access the fjson_t->el which is the parsed fjson_element_t structure and then you can free the fjson_t structure.
4. Walk the fjson_element_t structure and search for what you want.
5. Free the fjson_element_t structure by using the fjson_free_element() API.

The fjson_element_t structure

This structure contains a fjson_type_t enumerator and a union that contains:

• char *str in case the type is FJSON_TYPE_STRING.
• double num in case the type is FJSON_TYPE_NUMBER.
• struct fjson_array_s *array in case the type is FJSON_TYPE_ARRAY. This struct is a linked list of other elements.
• struct fjson_pair_s *pairs in case the type is FJSON_TYPE_OBJECT. This struct is a linked list of key-value, where the key is always a fjson_element_t with type FJSON_TYPE_STRING and the value is just a fjson_element_t with variable type.
• unsigned char bool_val in case the TYPE is FJSON_TYPE_BOOLEAN. If it equals to 0 the boolean value in the JSON element is false, else it is true.

When the type is FJSON_TYPE_NULL there is nothing inside the union to be checked for.

Missing & TODO

• There is no support for unicode characters ("\uxxxx").
• Remove typedef for struct fjson_element_s, fjson_array_s and fjson_pair_s.

Thanks

A special thanks to my friend Davide, that gave me the idea and the method to develop it.