Jansson

What it is

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.

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.

Installation

sudo apt install libjansson-dev

Getting started

The smallest useful thing you can do with it, and what each part means.

Parsing a JSON string

c

12345678910111213141516#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

c

123456789#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.

Advanced usage

Where the library earns its place over a simpler alternative.

Iterating over JSON objects

c

12345const 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

c

1234567json_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

c

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

c

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.

Errors and fixes

The failures you are most likely to hit, and what actually resolves them.

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.

Background

Why it exists, and what it was reacting to.

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.

Related libraries

Others in Serialization & Formats for C.

zlib

libxml2

cJSON

bzip2

LZ4