The fastest JSON library in C

pushedAt 1 week ago



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 CPU.
  • Portable: compliance with ANSI C (C89).
  • Standard: strict compliance with RFC 8259 and ECMA-404 standard.
  • Safe: complete JSON form, number format and UTF-8 validation.
  • Accuracy: can read and write int64, uint64 and double number accurately.
  • No Limit: support unlimited JSON level, \u0000 string.
  • Extendable: options to allow comments, trailing commas, nan/inf, custom memory allocator.
  • Developer Friendly: only one h and one c file, easy to use API.


  • An array or object is stored as some data structure like linked list, access elements with index or key is slower than iterator.
  • Duplicate keys are allowed in an object, and the order of the keys is preserved.
  • JSON parsing result is immutable, a mutable copy is required for modification.


Benchmark project and dataset: yyjson_benchmark

The simdjson's new On Demand API is faster if most JSON fields is known at compile time. This benchmark project only checks the DOM API, 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 optimization.



Just copy yyjson.h and yyjson.c to your project and start using it. Since yyjson is ANSI C compatible, no other configuration is needed typically.

yyjson has been tested with the following compilers: gcc, clang, msvc, icc, tcc. If you find a compile error, please report a bug.

yyjson includes all features by default, but you can trim out some of them by adding compile flags to get higher performance and/or smaller binary. See compile flags for details.


Clone repository and create build directory:

git clone https://github.com/ibireme/yyjson.git
mkdir build
cd build

Build static library:

cmake .. 
cmake --build .

Build static library and run tests:

cmake --build .

Supported CMake options:

  • -DBUILD_SHARED_LIBS=ON Build shared library instead of static library.
  • -DYYJSON_BUILD_TESTS=ON Build all tests.
  • -DYYJSON_DISABLE_READER=ON Disable JSON reader if you don't need it.
  • -DYYJSON_DISABLE_WRITER=ON Disable JSON writer if you don't need it.
  • -DYYJSON_DISABLE_FAST_FP_CONV=ON Disable fast floating-point number conversion.
  • -DYYJSON_DISABLE_NON_STANDARD=ON Disable non-standard JSON support at compile time.

See compile flags for details.

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

// 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

Read JSON file with options

// Read JSON file, allowing comments and 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

Write JSON file with options

// Read the JSON file as a mutable doc
yyjson_mut_doc *doc = yyjson_mut_read_file("/tmp/config.json", 0, NULL, 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)) {

// Write the json pretty, 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



  • Add documentation page.
  • Add GitHub workflow for CI and codecov.
  • Add more tests: valgrind, sanitizer.
  • Support JSON Pointer to query value from document.
  • Add fuzzer.
  • 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.

ucloud ads