Introduction

A high performance JSON library written in ANSI C.

Features

• Fast: can read or write gigabytes per second JSON data on modern CPUs.
• Portable: complies with ANSI C (C89) for cross-platform compatibility.
• Strict: complies with RFC 8259 JSON standard, ensuring strict number format and UTF-8 validation.
• Extendable: offers options to allow comments, trailing commas, NaN/Inf, and custom memory allocator.
• Accuracy: can accurately read and write int64, uint64, and double numbers.
• Flexible: supports unlimited JSON nesting levels, \u0000 characters, and non null-terminated strings.
• Manipulation: supports querying and modifying using JSON Pointer, JSON Patch and JSON Merge Patch.
• Developer-Friendly: easy integration with only one h and one c file.

Limitations

• An array or object is stored as a data structure such as linked list, which makes accessing elements by index or key slower than using an iterator.
• Duplicate keys are allowed in an object, and the order of the keys is preserved.
• JSON parsing result is immutable, requiring a mutable copy for modification.

Performance

Benchmark project and dataset: yyjson_benchmark

The simdjson’s new On Demand API is faster if most JSON fields are known at compile-time.
This benchmark project only checks the DOM API, a new benchmark will be added later.

AWS EC2 (AMD EPYC 7R32, gcc 9.3)

twitter.json	parse (GB/s)	stringify (GB/s)
yyjson(insitu)	1.80	1.51
yyjson	1.72	1.42
simdjson	1.52	0.61
sajson	1.16
rapidjson(insitu)	0.77
rapidjson(utf8)	0.26	0.39
cjson	0.32	0.17
jansson	0.05	0.11

iPhone (Apple A14, clang 12)

twitter.json	parse (GB/s)	stringify (GB/s)
yyjson(insitu)	3.51	2.41
yyjson	2.39	2.01
simdjson	2.19	0.80
sajson	1.74
rapidjson(insitu)	0.75
rapidjson(utf8)	0.30	0.58
cjson	0.48	0.33
jansson	0.09	0.24

More benchmark reports with interactive charts (update 2020-12-12)

Platform	CPU	Compiler	OS	Report
Intel NUC 8i5	Core i5-8259U	msvc 2019	Windows 10 2004	Charts
Intel NUC 8i5	Core i5-8259U	clang 10.0	Ubuntu 20.04	Charts
Intel NUC 8i5	Core i5-8259U	gcc 9.3	Ubuntu 20.04	Charts
AWS EC2 c5a.large	AMD EPYC 7R32	gcc 9.3	Ubuntu 20.04	Charts
AWS EC2 t4g.medium	Graviton2 (ARM64)	gcc 9.3	Ubuntu 20.04	Charts
Apple iPhone 12 Pro	A14 (ARM64)	clang 12.0	iOS 14	Charts

For better performance, yyjson prefers:

• A modern processor with:
  • high instruction level parallelism
  • excellent branch predictor
  • low penalty for misaligned memory access
• A modern compiler with good optimizer (e.g. clang)

Sample Code

Read JSON string

const char *json = "{\"name\":\"Mash\",\"star\":4,\"hits\":[2,2,1,3]}";

// Read JSON and get root
yyjson_doc *doc = yyjson_read(json, strlen(json), 0);
yyjson_val *root = yyjson_doc_get_root(doc);

// Get root["name"]
yyjson_val *name = yyjson_obj_get(root, "name");
printf("name: %s\n", yyjson_get_str(name));
printf("name length:%d\n", (int)yyjson_get_len(name));

// Get root["star"]
yyjson_val *star = yyjson_obj_get(root, "star");
printf("star: %d\n", (int)yyjson_get_int(star));

// Get root["hits"], iterate over the array
yyjson_val *hits = yyjson_obj_get(root, "hits");
size_t idx, max;
yyjson_val *hit;
yyjson_arr_foreach(hits, idx, max, hit) {
    printf("hit%d: %d\n", (int)idx, (int)yyjson_get_int(hit));
}

// Free the doc
yyjson_doc_free(doc);

// All functions accept NULL input, and return NULL on error.

Write JSON string

// Create a mutable doc
yyjson_mut_doc *doc = yyjson_mut_doc_new(NULL);
yyjson_mut_val *root = yyjson_mut_obj(doc);
yyjson_mut_doc_set_root(doc, root);

// Set root["name"] and root["star"]
yyjson_mut_obj_add_str(doc, root, "name", "Mash");
yyjson_mut_obj_add_int(doc, root, "star", 4);

// Set root["hits"] with an array
int hits_arr[] = {2, 2, 1, 3};
yyjson_mut_val *hits = yyjson_mut_arr_with_sint32(doc, hits_arr, 4);
yyjson_mut_obj_add_val(doc, root, "hits", hits);

// To string, minified
const char *json = yyjson_mut_write(doc, 0, NULL);
if (json) {
    printf("json: %s\n", json); // {"name":"Mash","star":4,"hits":[2,2,1,3]}
    free((void *)json);
}

// Free the doc
yyjson_mut_doc_free(doc);

Read JSON file with options

// Read JSON file, allowing comments and trailing commas
yyjson_read_flag flg = YYJSON_READ_ALLOW_COMMENTS | YYJSON_READ_ALLOW_TRAILING_COMMAS;
yyjson_read_err err;
yyjson_doc *doc = yyjson_read_file("/tmp/config.json", flg, NULL, &err);

// Iterate over the root object
if (doc) {
    yyjson_val *obj = yyjson_doc_get_root(doc);
    yyjson_obj_iter iter;
    yyjson_obj_iter_init(obj, &iter);
    yyjson_val *key, *val;
    while ((key = yyjson_obj_iter_next(&iter))) {
        val = yyjson_obj_iter_get_val(key);
        printf("%s: %s\n", yyjson_get_str(key), yyjson_get_type_desc(val));
    }
} else {
    printf("read error (%u): %s at position: %ld\n", err.code, err.msg, err.pos);
}

// Free the doc
yyjson_doc_free(doc);

Write JSON file with options

// Read the JSON file as a mutable doc
yyjson_doc *idoc = yyjson_read_file("/tmp/config.json", 0, NULL, NULL);
yyjson_mut_doc *doc = yyjson_doc_mut_copy(idoc, NULL);
yyjson_mut_val *obj = yyjson_mut_doc_get_root(doc);

// Remove null values in root object
yyjson_mut_obj_iter iter;
yyjson_mut_obj_iter_init(obj, &iter);
yyjson_mut_val *key, *val;
while ((key = yyjson_mut_obj_iter_next(&iter))) {
    val = yyjson_mut_obj_iter_get_val(key);
    if (yyjson_mut_is_null(val)) {
        yyjson_mut_obj_iter_remove(&iter);
    }
}

// Write the json pretty, escape unicode
yyjson_write_flag flg = YYJSON_WRITE_PRETTY | YYJSON_WRITE_ESCAPE_UNICODE;
yyjson_write_err err;
yyjson_mut_write_file("/tmp/config.json", doc, flg, NULL, &err);
if (err.code) {
    printf("write error (%u): %s\n", err.code, err.msg);
}

// Free the doc
yyjson_doc_free(idoc);
yyjson_mut_doc_free(doc);

Documentation

The latest (unreleased) documentation can be accessed in the doc directory.
The pre-generated Doxygen HTML for the release version can be viewed here:

• Home Page
  • Build and test
  • API and sample code
  • Data structure
  • Changelog

Packaging status

Built With yyjson

A non-exhaustive list of projects that expose yyjson to other languages or
use yyjson internally for a major feature. If you have a project that uses
yyjson, feel free to open a PR to add it to this list.

Project	Language	Description
py_yyjson	Python	Python bindings for yyjson
orjson	Python	JSON library for Python with an optional yyjson backend
cpp-yyjson	C++	C++ JSON library with a yyjson backend
reflect-cpp	C++	C++ library for serialization through automated field name retrieval from structs
yyjsonr	R	R binding for yyjson
Ananda	Swift	JSON model decoding based on yyjson
duckdb	C++	DuckDB is an in-process SQL OLAP Database Management System
fastfetch	C	A neofetch-like tool for fetching system information and displaying them in a pretty way
Zrythm	C	Digital Audio Workstation that uses yyjson to serialize JSON project files
bemorehuman	C	Recommendation engine with a focus on uniqueness of the person receiving the rec
mruby-yyjson	mruby	Efficient JSON parsing and serialization library for mruby using yyjson
YYJSON.jl	Julia	Julia bindings for yyjson

TODO for v1.0

• [x] Add documentation page.
• [x] Add GitHub workflow for CI and codecov.
• [x] Add more tests: valgrind, sanitizer, fuzzing.
• [x] Support JSON Pointer to query and modify JSON.
• [x] Add RAW type for JSON reader and writer.
• [x] Add option to limit real number output precision.
• [ ] Add option to support JSON5 (if feasible).
• [ ] Add functions to diff two JSON documents.
• [ ] Add documentation on performance optimizations.
• [ ] Ensure ABI stability.

License

This project is released under the MIT license.