v1_11_0/_sources/os/modules/json/json.rst.txt

JSON
====

JSON is a data interchange format. The description of this format can be
found from IETF RFC 4627.

.. contents::
  :local:
  :depth: 2

Description
-----------

This package helps in converting between C data types and JSON data
objects. It supports both encoding and decoding.

Data structures
---------------

Encoding
~~~~~~~~

.. code:: c

    /* Encoding functions */
    typedef int (*json_write_func_t)(void *buf, char *data,
            int len);

    struct json_encoder {
        json_write_func_t je_write;
        void *je_arg;
        int je_wr_commas:1;
        char je_encode_buf[64];
    };

Here's the data structure encoder funtions use, and it must be
initialized by the caller. The key element is *je\_write*, which is a
function pointer which gets called whenever encoding routine is ready
with encoded data. The element *je\_arg* is passed to *je\_write* as the
first argument. The rest of the structure contents are for internal
state management. This function should collect all the data encoder
function generates. It can collect this data to a flat buffer, chain of
mbufs or even stream through.

.. code:: c

    /**
     * For encode.  The contents of a JSON value to encode.
     */
    struct json_value {
        uint8_t jv_pad1;
        uint8_t jv_type;
        uint16_t jv_len;

        union {
            uint64_t u;
            float fl;
            char *str;
            struct {
                char **keys;
                struct json_value **values;
            } composite;
        } jv_val;
    };

This data structure is filled with data to be encoded. It is best to
fill this using the macros *JSON\_VALUE\_STRING()* or
*JSON\_VALUE\_STRINGN()* when value is string, *JSON\_VALUE\_INT()* when
value is an integer, and so forth.

Decoding
~~~~~~~~

.. code:: c

    /* when you implement a json buffer, you must implement these functions */

    /* returns the next character in the buffer or '\0'*/
    typedef char (*json_buffer_read_next_byte_t)(struct json_buffer *);
    /* returns the previous character in the buffer or '\0' */
    typedef char (*json_buffer_read_prev_byte_t)(struct json_buffer *);
    /* returns the number of characters read or zero */
    typedef int (*json_buffer_readn_t)(struct json_buffer *, char *buf, int n);

    struct json_buffer {
        json_buffer_readn_t jb_readn;
        json_buffer_read_next_byte_t jb_read_next;
        json_buffer_read_prev_byte_t jb_read_prev;
    };

Function pointers within this structure are used by decoder when it is
reading in more data to decode.

.. code:: c

    struct json_attr_t {
        char *attribute;
        json_type type;
        union {
            int *integer;
            unsigned int *uinteger;
            double *real;
            char *string;
            bool *boolean;
            char *character;
            struct json_array_t array;
            size_t offset;
        } addr;
        union {
            int integer;
            unsigned int uinteger;
            double real;
            bool boolean;
            char character;
            char *check;
        } dflt;
        size_t len;
        const struct json_enum_t *map;
        bool nodefault;
    };

This structure tells the decoder about a particular name/value pair.
Structure must be filled in before calling the decoder routine
*json\_read\_object()*.

+-------------+-------------------------------------------------------+
| Element     | Description                                           |
+=============+=======================================================+
| attribute   | Name of the value                                     |
+-------------+-------------------------------------------------------+
| type        | The type of the variable; see enum json\_type         |
+-------------+-------------------------------------------------------+
| addr        | Contains the address where value should be stored     |
+-------------+-------------------------------------------------------+
| dflt        | Default value to fill in, if this name is not found   |
+-------------+-------------------------------------------------------+
| len         | Max number of bytes to read in for value              |
+-------------+-------------------------------------------------------+
| nodefault   | If set, default value is not copied name              |
+-------------+-------------------------------------------------------+

API
---

.. doxygenfile:: json/include/json/json.h