JSON For C

JSON For C is a very simple JSON parser for the C Programming Language. Parse the string, get what you need, and get out.

This Library is NOT for writting or serializing JSON files. Just reading!

Build

Building this project will create a shared object and headers to use this library.

git clone https://github.com/Ethan-Heimer/Json-For-C.git ./LibJson
cd LibJson
mkdir build && cd build
cmake ..
make

or using cmake:

add_subdirectory("${CMAKE_SOURCE_DIR}/PATH_TO_JSON_FOR_C_DIR")
...
target_link_libraries(${NAME} PRIVATE jsonlib)

The most basic example to link against this library is as follows, where the project is being built within the build directory generated.

gcc -o test -I./ -L./ main.c -libjson

API

The API for this library is very simple.

Initialization

Before Parsing, JInit Must be called. This initializes the JSON Abstract Syntax Tree for parsing and syntax Errors.

JInit();

Likewise, before the program terminates, JEnd Must be called to free the AST and other memory the library used in the background.

JEnd();

Parsing

To show off the API, the following, example.json will be used:

{
    "Hello" : "World",

    "Nested-Object" : {
        "One": 1,
        "Two" : 2, 
        "Three" : 3
    },

    "Array" : [
        10,
        "Ten",
        {
            "Ten" : 10
        }
    ]
}   

After initialization, parsing can begin. To parse a file, call JParseFile(file name)

Json* root = JParseFile("example.json");

Or a string, call JParseString(const char* string)

Json* root = JParseString("...");

JParse* will return a JSON object to be interpreted. This JSON object represents the root of the file.

If the object returned is NULL, then an error occured while parsing. The Error is printed by the library.

To free the resulting JSON Object when its no longer needed, call JDelete(Json**)

JDelete(&root);

REMEMBER: Delete The Root JSON Object, Or you will leak memory!

Getting Key Value Pairs

To grab a value from a JSON object, JGetValue(JSON object, key, value) is called.

JsonValue hello;
JGetValue(root, "Hello", &hello);

if(hello.hasValue)
    printf("Output: %s\n", hello.value->string);

The result:

Output: World

After calling JGetValue, If the key is found and the pair is a simple value, the value will be stored in JsonValue hello. hello.value->string is the text, number, or bool value found at the key.

If the key is not found, hello.hasValue is false.

JsonValue objects have flags that can be used to determine its value's type (ie. .isString, .isInt, .isFloat, .isNull, .isBool).

Getting Nested Objects

JGetValue will return the nested JSON object if the key is paired with one.

JsonValue value;
Json* nested = JGetValue(root, "Nested-Object", &value);

if(nested){
    JGetValue(nested, "Two", value);
    if(value.hasValue)
        printf("Output: %s\n", value.value->string);
}

The Result:

Output: 2

Getting Arrays

JGetValue Can also be used to get array values. The Key is the array index.

JsonValue element;
Json* array = JGetValue(root, "Array", &element)

JGetValue(array, "0", &element);

if(element.hasValue)
    printf("Output 0: %s\n", element.value->string);

JGetValue(root, "1", &element);
if(element.hasValue)
    printf("Output 1: %s\n", element.value->string);

JGetValue(root, "2", &element);
if(element.hasValue)
    printf("Output 2: %s\n", element.value->string);

The result:

Output 0: 10
Output 1: Ten

The element at index 2 is an object, so its not stored in value, but returned by JGetValue()

Looping through an array is simple. JSON Objects store the type of object it is (VALUE, OBJECT, ARRAY), and its number if children. Using that information, looping throught all elements can be achieved like so:

int children = array->childCount;
for(int i = 0; i < children; i++){
    char buffer[20];
    sprintf(buffer, "%s", i);

    JGetValue(array, buffer, &element);
    if(element.hasValue)
        printf("Value at %d is %s\n", i, element.value->string);
}

The Result:

Value at 0 is 10
Value at 1 is Ten