JSON

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

Description

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

Data structures

Encoding

/* 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.

/**
 * 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

/* 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.

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

List of Functions

Functions for encoding:

Function Description
json_encode_object_start This function starts the encoded JSON object.
json_encode_object_key This function writes out a name for a field, followed by ":" character.
json_encode_object_entry This function writes out a name for a field, followed by ":" character, and the value itself.
json_encode_object_finish This function finalizes the encoded JSON object.

Functions for decoding:

Function Description
json_read_object This function reads in JSON data stream, while looking for name/value pairs described in given attribites.