yyjson 0.7.0
A high performance C JSON library.

Build Codecov License Version

A high performance JSON library written in ANSI C.


  • 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.


  • 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.


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.

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);
// 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_arr_foreach(hits, idx, max, hit) {
printf("hit%d: %d\n", (int)idx, (int)yyjson_get_int(hit));
// Free the doc
// All functions accept NULL input, and return NULL on error.
yyjson_api_inline yyjson_val * yyjson_obj_get(yyjson_val *obj, const char *key)
Definition: yyjson.h:4959
yyjson_api_inline int yyjson_get_int(yyjson_val *val)
Definition: yyjson.h:4760
#define yyjson_arr_foreach(arr, idx, max, val)
Definition: yyjson.h:1753
yyjson_api_inline const char * yyjson_get_str(yyjson_val *val)
Definition: yyjson.h:4772
yyjson_api_inline yyjson_val * yyjson_doc_get_root(yyjson_doc *doc)
Definition: yyjson.h:4628
yyjson_api_inline void yyjson_doc_free(yyjson_doc *doc)
Definition: yyjson.h:4640
yyjson_api_inline size_t yyjson_get_len(yyjson_val *val)
Definition: yyjson.h:4776
yyjson_api_inline yyjson_doc * yyjson_read(const char *dat, size_t len, yyjson_read_flag flg)
Definition: yyjson.h:827
Definition: yyjson.h:4362
Definition: yyjson.h:4357

Write JSON string

// Create a mutable doc
// 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_api_inline bool yyjson_mut_obj_add_val(yyjson_mut_doc *doc, yyjson_mut_val *obj, const char *key, yyjson_mut_val *val)
Definition: yyjson.h:6766
yyjson_api_inline yyjson_mut_val * yyjson_mut_arr_with_sint32(yyjson_mut_doc *doc, const int32_t *vals, size_t count)
Definition: yyjson.h:5748
yyjson_api_inline bool yyjson_mut_obj_add_int(yyjson_mut_doc *doc, yyjson_mut_val *obj, const char *key, int64_t val)
Definition: yyjson.h:6697
yyjson_api_inline yyjson_mut_val * yyjson_mut_obj(yyjson_mut_doc *doc)
Definition: yyjson.h:6349
yyjson_api void yyjson_mut_doc_free(yyjson_mut_doc *doc)
yyjson_api_inline char * yyjson_mut_write(const yyjson_mut_doc *doc, yyjson_write_flag flg, size_t *len)
Definition: yyjson.h:1230
yyjson_api_inline void yyjson_mut_doc_set_root(yyjson_mut_doc *doc, yyjson_mut_val *root)
Definition: yyjson.h:5186
yyjson_api_inline bool yyjson_mut_obj_add_str(yyjson_mut_doc *doc, yyjson_mut_val *obj, const char *key, const char *val)
Definition: yyjson.h:6717
yyjson_api yyjson_mut_doc * yyjson_mut_doc_new(const yyjson_alc *alc)
Definition: yyjson.h:5119
Definition: yyjson.h:5071

Read JSON file with options

// Read JSON file, allowing comments and trailing commas
yyjson_doc *doc = yyjson_read_file("/tmp/config.json", flg, NULL, &err);
// Iterate over the root object
if (doc) {
yyjson_obj_iter_init(obj, &iter);
yyjson_val *key, *val;
while ((key = yyjson_obj_iter_next(&iter))) {
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
static const yyjson_read_flag YYJSON_READ_ALLOW_TRAILING_COMMAS
Definition: yyjson.h:649
yyjson_api_inline bool yyjson_obj_iter_init(yyjson_val *obj, yyjson_obj_iter *iter)
Definition: yyjson.h:4988
yyjson_api_inline yyjson_val * yyjson_obj_iter_get_val(yyjson_val *key)
Definition: yyjson.h:5021
uint32_t yyjson_read_flag
Definition: yyjson.h:622
yyjson_api_inline const char * yyjson_get_type_desc(yyjson_val *val)
Definition: yyjson.h:4728
yyjson_read_code code
Definition: yyjson.h:728
yyjson_api_inline yyjson_val * yyjson_obj_iter_next(yyjson_obj_iter *iter)
Definition: yyjson.h:5011
yyjson_api yyjson_doc * yyjson_read_file(const char *path, yyjson_read_flag flg, const yyjson_alc *alc, yyjson_read_err *err)
size_t pos
Definition: yyjson.h:732
const char * msg
Definition: yyjson.h:730
static const yyjson_read_flag YYJSON_READ_ALLOW_COMMENTS
Definition: yyjson.h:652
Definition: yyjson.h:1821
Definition: yyjson.h:726

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);
// Remove null values in root object
yyjson_mut_val *key, *val;
while ((key = yyjson_mut_obj_iter_next(&iter))) {
if (yyjson_mut_is_null(val)) {
// Write the json pretty, escape unicode
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_api yyjson_mut_doc * yyjson_doc_mut_copy(yyjson_doc *doc, const yyjson_alc *alc)
yyjson_api_inline bool yyjson_mut_is_null(yyjson_mut_val *val)
Definition: yyjson.h:5201
const char * msg
Definition: yyjson.h:1024
yyjson_api_inline yyjson_mut_val * yyjson_mut_obj_iter_next(yyjson_mut_obj_iter *iter)
Definition: yyjson.h:6282
yyjson_api_inline yyjson_mut_val * yyjson_mut_obj_iter_remove(yyjson_mut_obj_iter *iter)
Definition: yyjson.h:6299
yyjson_api_inline yyjson_mut_val * yyjson_mut_doc_get_root(yyjson_mut_doc *doc)
Definition: yyjson.h:5182
yyjson_api_inline yyjson_mut_val * yyjson_mut_obj_iter_get_val(yyjson_mut_val *key)
Definition: yyjson.h:6294
static const yyjson_write_flag YYJSON_WRITE_ESCAPE_UNICODE
Definition: yyjson.h:967
yyjson_api bool yyjson_mut_write_file(const char *path, const yyjson_mut_doc *doc, yyjson_write_flag flg, const yyjson_alc *alc, yyjson_write_err *err)
yyjson_api_inline bool yyjson_mut_obj_iter_init(yyjson_mut_val *obj, yyjson_mut_obj_iter *iter)
Definition: yyjson.h:6257
yyjson_write_code code
Definition: yyjson.h:1022
static const yyjson_write_flag YYJSON_WRITE_PRETTY
Definition: yyjson.h:964
uint32_t yyjson_write_flag
Definition: yyjson.h:954
Definition: yyjson.h:3179
Definition: yyjson.h:1020


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


  • [x] Add documentation page.
  • [x] Add GitHub workflow for CI and codecov.
  • [x] Add more tests: valgrind, sanitizer.
  • [x] Support JSON Pointer to query value from document.
  • [x] Add fuzzer.
  • [x] Add RAW type for JSON reader and writer.
  • [ ] Add streaming API for JSON reader and writer.
  • [ ] Add documentation about performance.
  • [ ] Optimize performance for 32-bit processor.


This project is released under the MIT license.