json-gen-c

‍Fast, tiny, and friendly code generator that turns your C structs into fully featured JSON serializers/deserializers.

Highlights

• Schema-first workflow – describe your structs once, generate battle-tested C code automatically.
• Zero runtime reflection – everything happens at compile time, so the generated code stays fast and lean.
• Thread-safe runtime – the parsing context uses explicit structures rather than globals.
• Batteries included – includes a lightweight sstr string helper library and ready-made array helpers.
• CI-friendly build – warnings are treated as errors and the Make targets work the same locally and in automation.

Benchmark

json-gen-c is faster than cJSON on all benchmarks — up to 47% faster marshal, 25% faster unmarshal — while providing type-safe code generation, selective parsing, and multi-format support (JSON + MessagePack + CBOR). See benchmark/RESULTS.md for full results and reproduction steps.

Contents

• Benchmark
• Overview
• Build and Install
• Quick Start
• The Format of Structs Definition File
• The JSON API
• More Resources
• Contributing & Community

Overview

json-gen-c is a program for serializing C structs to JSON and deserializing JSON to C structs. It parses struct definition files then generates C code to handle both directions.

• Document

Build and Install

make
sudo make install

The project uses a modern, efficient build system with support for parallel compilation:

# Parallel build (recommended)
make -j$(nproc)
 
# Debug build
make debug
 
# Build with sanitizers
make sanitize
 
# Show build configuration
make show-config

To build example, tests, and benchmarks

# build ./build/example/example
make example
# build ./build/test/unit_test
make test
# build ./build/benchmark/json_bench
make benchmark
# Debian/Ubuntu one-command benchmark reproduction
make benchmark-repro

make benchmark-repro installs the distro benchmark dependencies when needed, clones yyjson into benchmark/yyjson/, installs it locally under benchmark/.deps/prefix/, then builds and runs the full benchmark suite with -O2 -DNDEBUG. Both local dependency directories are gitignored on purpose.

All build artifacts are organized under the build/ directory:

• build/bin/ - Main executable
• build/lib/ - Static libraries
  
• build/example/ - Example executable
• build/test/ - Test executables
• build/benchmark/ - Benchmark executable

Alternative Build Systems

CMake (3.16+):

mkdir build-cmake && cd build-cmake
cmake ..
cmake --build . -j$(nproc)
sudo cmake --install .

Windows (CMake + MSVC):

mkdir build && cd build
cmake ..
cmake --build . --config Release
ctest -C Release --output-on-failure

Windows (CMake + MinGW):

mkdir build && cd build
cmake .. -G "MinGW Makefiles"
cmake --build .
ctest --output-on-failure

‍Note: On Windows, CMake automatically uses a built-in xxd replacement (cmake/xxd.cmake). No external tools beyond a C compiler and CMake are needed.

Meson:

meson setup builddir
ninja -C builddir
sudo ninja -C builddir install

See BUILD_SYSTEM.md for pkg-config integration, find_package(JsonGenC), and the json_gen_c_generate() CMake macro.

Package Managers

Homebrew (macOS/Linux):

brew install zltl/tap/json-gen-c

To set up the tap, see packaging/homebrew/.

Debian/Ubuntu (from source package):

# Install build dependencies
sudo apt install build-essential debhelper devscripts xxd
 
# Build the .deb
cp -r packaging/debian .
dpkg-buildpackage -us -uc -b
sudo dpkg -i ../json-gen-c_*.deb

Arch Linux (AUR):

# Using an AUR helper
yay -S json-gen-c
 
# Or manually
cd packaging/aur
makepkg -si

Quick Start

example

Define Structs

For example, create a file name struct.json-gen-c as contents below:

struct A {
    int int_val1;
    int int_val2;
    long long_val;
    double double_val;
    float float_val;
    sstr_t sstr_val;
    int int_val_array[];
    int fixed_data[10];
    B b_val;
};
 
struct B {
    int id;
};

Note that we don't use C-style string char*, a more resonable type is sstr_t. You can find more details about sstr_t in document of sstr.

Compiling Your Struct Definition File

json-gen-c -in struct.json-gen-c -out .

This generates the following files in your specified destination directory:

• json.gen.h, the header which declares your generated structures and functions.
• json.gen.c, which contains the implementation of your functions.
• sstr.h, sstr.c, the string manipulation helper functions that generated code depends on.

MessagePack Format

To generate MessagePack (binary) serialization instead of JSON:

json-gen-c --format msgpack -in struct.json-gen-c -out .

This generates msgpack.gen.h and msgpack.gen.c with msgpack_pack_* / msgpack_unpack_* functions. The struct definitions and sstr helper are identical regardless of format.

CBOR Format

To generate CBOR (RFC 8949) binary serialization:

json-gen-c --format cbor -in struct.json-gen-c -out .

This generates cbor.gen.h and cbor.gen.c with cbor_pack_* / cbor_unpack_* functions. Same struct definitions and API patterns as JSON and MessagePack.

C++ Wrapper (Optional)

json-gen-c --cpp-wrapper -in struct.json-gen-c -out .

This generates json_gen_c.gen.hpp — a C++17 header with RAII wrapper classes inside namespace jgc. Each class wraps a generated C struct with:

• Default constructor (_init), destructor (_clear), move and copy semantics
• Typed get/set accessors (strings as std::string, enums as their C enum type)
• marshal(), unmarshal(), and unmarshal_into() member functions
• Equality operators and c_struct() for C interop

#include "json_gen_c.gen.hpp"
 
jgc::Person p;
p.set_name("Alice");
p.set_age(30);
 
std::string json = p.marshal();                   // serialize
jgc::Person p2 = jgc::Person::unmarshal(json);    // deserialize
assert(p == p2);                                   // compare
struct ::Person& c = p.c_struct();                 // C interop

Can be combined with --format to also generate MessagePack or CBOR alongside.

Rust Module (Optional)

json-gen-c --rust -in struct.json-gen-c -out .

This generates json_gen_c.gen.rs — a self-contained Rust module with native serde-compatible structs and enums. Add serde and serde_json to your Cargo.toml, then:

mod json_gen_c_gen; // or rename as you like
use json_gen_c_gen::*;
 
let p = Person { name: "Alice".into(), age: "30".into() };
let json = serde_json::to_string(&p).unwrap();         // serialize
let p2: Person = serde_json::from_str(&json).unwrap();  // deserialize
assert_eq!(p, p2);

Type mapping: int→i32, long→i64, float→f32, double→f64, sstr_t→String, enums→Rust enums, oneof→#[serde(tag)] enums, optional→Option<T>, arrays→Vec<T>/[T; N], maps→HashMap<String, V>.

Go Source (Optional)

json-gen-c --go -in struct.json-gen-c -out .

This generates json_gen_c.gen.go — a self-contained Go source file using encoding/json struct tags. No external dependencies required:

package main
 
import (
    "encoding/json"
    "fmt"
    "your_module/gen"
)
 
func main() {
    p := gen.Person{Name: "Alice", Age: "30"}
    data, _ := json.Marshal(p)              // serialize
    var p2 gen.Person
    json.Unmarshal(data, &p2)               // deserialize
    fmt.Println(p2.Name)                    // "Alice"
}

Type mapping: int→int32, long→int64, float→float32, double→float64, sstr_t→string, enums→type E string, oneof→custom marshal/unmarshal, optional→*T + omitempty, arrays→[]T/[N]T, maps→map[string]V.

Use Your Generated Codes

To Serialize Structs to JSON

struct A a;
A_init(&a);
// set values to a ...
// ...
sstr_t json_str = sstr_new();
json_marshal_A(&a, json_str);
 
printf("marshal a to json> %s\n", sstr_cstr(json_str));
 
sstr_free(json_str);
A_clear(&a);

To Serialize Array of Structs to JSON

struct A a[3];
for (i = 0; i < 3; ++i) {
    A_init(&a[i]);
    // set values to a[i] ...
}
 
sstr_t json_str = sstr_new();
json_marshal_array_A(a, 3, json_str);
 
printf("marshal a[] to json> %s\n", sstr_cstr(json_str));
 
for (i = 0; i < 3; ++i) {
    A_clear(&a[i]);
}

To Deserialize JSON to Structs

// const char *p_str = "{this is a json string}";
// sstr_t json_str = sstr(pstr);
 
struct A a;
A_init(&a);
json_unmarshal_A(json_str, &a); // json_str is a type of sstr_t
// ...
A_clear(&a);

To Deserialize JSON to Array of Structs

// const char *p_str = "[this is a json string]";
// sstr_t json_str = sstr(pstr);
 
struct A *a = NULL;
int len = 0;
json_unmarshal_array_A(json_str, &a, &len);
// ...
int i;
for (i = 0; i < len; ++i) {
    A_clear(&a[i]);
}
free(a);

To Selectively Deserialize Top-Level Fields

struct User user;
User_init(&user);
 
uint64_t mask[User_FIELD_MASK_WORD_COUNT] = {0};
JSON_GEN_C_FIELD_MASK_SET(mask, User_FIELD_email);
JSON_GEN_C_FIELD_MASK_SET(mask, User_FIELD_scores);
 
json_unmarshal_selected_User(json_str, &user,
                             mask, User_FIELD_MASK_WORD_COUNT);

Selective unmarshal only updates the chosen top-level fields that are present in the JSON input. Unselected top-level fields stay unchanged.

Selected fields are treated as whole-field replacements: if a selected field is a string, array, map, nested struct, or oneof, its previous stored value is cleared before the new JSON value is parsed.

To Selectively Deserialize Nested Sub-Fields

Use the _deep variant to pass sub-field masks into nested structs:

struct User user;
User_init(&user);
 
// Select top-level "profile" field
uint64_t mask[User_FIELD_MASK_WORD_COUNT] = {0};
JSON_GEN_C_FIELD_MASK_SET(mask, User_FIELD_profile);
 
// Only parse Profile.name inside the nested struct
uint64_t inner[Profile_FIELD_MASK_WORD_COUNT] = {0};
JSON_GEN_C_FIELD_MASK_SET(inner, Profile_FIELD_name);
 
struct json_nested_mask nested[] = {
    { User_FIELD_profile, inner, Profile_FIELD_MASK_WORD_COUNT, NULL, 0 }
};
json_unmarshal_selected_User_deep(json_str, &user,
                                  mask, User_FIELD_MASK_WORD_COUNT,
                                  nested, 1);

When nested_masks is NULL or no entry matches a field, the nested struct is parsed in full (same as the non-deep API). Sub-masks can be chained recursively via the sub_masks / sub_mask_count members for deeper nesting levels.

Field-mask constants use the generated C field names, even when @json aliases change the JSON key names.

Build System

For detailed build system documentation, see BUILD_SYSTEM.md. If you are new to the project, the friendly walkthrough in docs/GETTING_STARTED.md covers installation, schema authoring, and integration.

The Format of Structs Definition File

Define a struct like:

struct <struct_name> {
    <field_type> <field_name> []?;
    <field_type> <field_name> []?;
    ...
};

The field type can be one of the following:

• int
• long
• float
• double
• sstr_t
• bool
• an enum name
• a struct name
• a oneof name (tagged union)
• map<sstr_t, V> where V is any of the above types

If a field is a dynamic array, just append [] after the field name. For fixed-size arrays, use [N] where N is a positive integer (e.g., int data[10];).

Map fields

Map fields marshal to/from JSON objects. The key type is always sstr_t (JSON keys are strings). Example:

map<sstr_t, int> scores;       // single map: {"alice":95,"bob":87}
map<sstr_t, int> tags[];       // array of maps: [{"x":1},{"y":2}]

In generated C code, each map is represented as a dynamic array of key-value entries:

struct json_map_int { struct json_map_entry_int* entries; int len; };

Tagged unions (oneof)

Tagged unions (discriminated unions) represent a value that can be one of several types, identified by a discriminator tag field in JSON:

struct Circle {
    float radius;
}
 
struct Rectangle {
    float width;
    float height;
}
 
oneof Shape {
    @tag "type"
    Circle circle;
    Rectangle rectangle;
}

• @tag "field" sets the JSON discriminator key (defaults to "type" if omitted).
• Each variant references a previously defined struct.
• JSON uses a flattened representation: {"type":"circle","radius":5.0}.

<tt>@deprecated</tt> Annotation

Mark fields, enum values, or oneof variants as deprecated:

struct Config {
    int timeout_ms;
    @deprecated int timeout_sec;  // use timeout_ms instead
};
 
enum Status { ACTIVE, @deprecated INACTIVE, ARCHIVED };

Deprecated items remain fully functional in marshal/unmarshal. The generated C code annotates them with compiler deprecation attributes so downstream code that accesses them gets a warning. See doc/schema-evolution.md for migration patterns and compatibility rules.

Oneof types can be used as fields in structs:

struct Drawing {
    sstr_t name;
    Shape shape;       // scalar oneof
    Shape shapes[];    // dynamic array of oneof
}

The JSON API

// initialize a struct
// always return 0
int <struct_name>_init(struct <struct_name> *obj);
 
// uninitialize a struct
// always return 0
int <struct_name>_clear(struct <struct_name> *obj);
 
// marshal a struct to json string.
// return 0 if success.
int json_marshal_<struct_name>(struct <struct_name>*obj, sstr_t out);
 
// marshal an array of struct to json string.
// return 0 if success.
int json_marshal_array_<struct_name>(struct <struct_name>*obj, int len, sstr_t out);
 
// unmarshal a json string to a struct.
// return 0 if success.
int json_unmarshal_<struct_name>(sstr_t in, struct <struct_name>*obj);
 
// unmarshal a json string to array of struct
// return 0 if success.
int json_unmarshal_array_<struct_name>(sstr_t in, struct <struct_name>**obj, int *len);
 
// generated top-level field indices
enum <struct_name>_field_index {
    <struct_name>_FIELD_<field_name> = 0,
    ...
    <struct_name>_FIELD_COUNT = N
};
 
// number of uint64_t words needed for the field mask
#define <struct_name>_FIELD_MASK_WORD_COUNT ...
 
// selectively unmarshal chosen top-level fields
int json_unmarshal_selected_<struct_name>(
    sstr_t in,
    struct <struct_name> *obj,
    const uint64_t *field_mask,
    int field_mask_word_count);
 
// selectively unmarshal with nested sub-field masks
int json_unmarshal_selected_<struct_name>_deep(
    sstr_t in,
    struct <struct_name> *obj,
    const uint64_t *field_mask,
    int field_mask_word_count,
    const struct json_nested_mask *nested_masks,
    int nested_mask_count);

Use the generated helper macros to manage mask bits:

JSON_GEN_C_FIELD_MASK_SET(mask_words, field_index);
JSON_GEN_C_FIELD_MASK_CLEAR(mask_words, field_index);
JSON_GEN_C_FIELD_MASK_TEST(mask_words, field_index);

For json_unmarshal_selected_<struct_name>() and _deep():

• passing NULL or too few mask words returns an error
• unselected top-level fields are left unchanged
• selected fields that appear in the JSON input replace the old stored field value
• field indices use C member names, not aliased JSON key names
• _deep additionally accepts json_nested_mask entries for sub-field selection within nested structs

Editor Support

VS Code Extension

The editors/vscode/ directory contains a VS Code extension providing:

• Syntax highlighting — TextMate grammar for .json-gen-c files (keywords, types, annotations, etc.)
• Language Server — Real-time diagnostics, code completion, and hover information

To enable the language server, ensure json-gen-c is on your PATH (or set jsonGenC.serverPath in VS Code settings), then install the extension. It launches json-gen-c --lsp automatically.

LSP for Other Editors

Any LSP-capable editor can use the built-in language server:

json-gen-c --lsp

This runs an LSP server over stdin/stdout (JSON-RPC 2.0). Configure your editor to launch it as a language server for .json-gen-c files.

More Resources

• docs/GETTING_STARTED.md – step-by-step tutorial with code snippets.
• example/ – real schemas plus sample host programs.
• Online reference – API documentation generated via Doxygen.
• doc/json-gen-c.1 – manual page installed with the CLI.
• doc/schema-evolution.md – schema evolution guide (compatibility rules, migration patterns, @deprecated).

Contributing & Community

We welcome issues, ideas, documentation updates, and code contributions.

• Read the CONTRIBUTING.md guide for local setup, coding style, and pull-request tips.
• File bugs or feature requests via GitHub Issues with clear reproduction details.
• Share what you build! Open a discussion or PR if you want your project added to a future showcase section.

Thanks for helping json-gen-c grow. Happy hacking!

License

Codes of json-gen-c are licensed under GPL-3.0, except for the codes it generated. The copy right of the codes generated by json-gen-c is owned by the user who wrote the struct definition file, same as the copy right of a PDF file generated by Latex is owned by the user who wrote the tex file.