Jansson

Language: C

Data / JSON

Jansson was created to provide a lightweight and easy-to-use library for handling JSON in C. It is designed with safety, clarity, and portability in mind, making it popular in C projects that require JSON parsing, configuration files, or data exchange with web services.

Jansson is a C library for encoding, decoding, and manipulating JSON data. It provides simple and intuitive APIs to parse JSON strings, build JSON objects, arrays, and write JSON to files or memory.

Installation

linux: sudo apt install libjansson-dev

mac: brew install jansson

windows: Download precompiled binaries from https://digip.org/jansson/

Usage

Jansson allows you to parse JSON strings into objects, manipulate them (add/remove keys, iterate arrays), and serialize JSON objects back into strings or files. It handles memory management safely and supports Unicode.

Parsing a JSON string

#include <jansson.h>
#include <stdio.h>

int main() {
    const char *text = "{\"name\": \"Alice\", \"age\": 30}";
    json_error_t error;
    json_t *root = json_loads(text, 0, &error);
    if(!root) {
        fprintf(stderr, "Error parsing JSON: %s\n", error.text);
        return 1;
    }
    const char *name = json_string_value(json_object_get(root, "name"));
    printf("Name: %s\n", name);
    json_decref(root);
    return 0;
}

Parses a JSON string into a `json_t` object, retrieves the 'name' value, prints it, and frees memory.

Creating a JSON object

#include <jansson.h>

json_t *root = json_object();
json_object_set_new(root, "name", json_string("Bob"));
json_object_set_new(root, "age", json_integer(25));
char *text = json_dumps(root, JSON_INDENT(2));
printf("%s\n", text);
free(text);
json_decref(root);

Builds a JSON object programmatically, serializes it as a string, prints it, and frees memory.

Iterating over JSON objects

const char *key;
json_t *value;
json_object_foreach(root, key, value) {
    printf("%s: %s\n", key, json_string_value(value));
}

Iterates over all key-value pairs in a JSON object.

Working with JSON arrays

json_t *arr = json_array();
json_array_append_new(arr, json_string("apple"));
json_array_append_new(arr, json_string("banana"));
for(size_t i = 0; i < json_array_size(arr); i++) {
    printf("%s\n", json_string_value(json_array_get(arr, i)));
}
json_decref(arr);

Creates a JSON array, appends items, iterates over them, and frees memory.

Writing JSON to file

FILE *fp = fopen("data.json", "w");
json_dumpf(root, fp, JSON_INDENT(2));
fclose(fp);

Writes a JSON object to a file in a human-readable format.

Parsing JSON from file

json_error_t error;
json_t *root = json_load_file("data.json", 0, &error);

Loads a JSON object directly from a file and handles parsing errors.

Error Handling

JSON parsing error: Inspect `json_error_t` for line and column information to fix invalid JSON syntax.

Memory leaks: Always decrement references using `json_decref` for each `json_t` object created.

Type mismatch: Use functions like `json_is_string`, `json_is_integer`, etc., before extracting values to avoid invalid type access.

Best Practices

Always check the return value of `json_loads` or `json_load_file` to catch parsing errors.

Use `json_decref` to free JSON objects when no longer needed.

Prefer `json_object_set_new` and `json_array_append_new` to automatically manage memory for new items.

Use `json_dumps` or `json_dumpf` with indentation options for readable JSON output.

Handle Unicode strings carefully; Jansson internally uses UTF-8 encoding.

Official Docs Github