jcon

Streaming JSON emitter for bare-metal C. Single static writer, zero heap, character-at-a-time output via a caller-supplied putc.

Is this for you?

Probably yes if you want to emit JSON from an MCU (telemetry, config dumps, log payloads), your output sink (UART, ring buffer, file) can accept one byte at a time, and you're willing to trade features for size.

Probably no if you need to parse JSON, emit from multiple writers concurrently, or want the library to sanitize untrusted string data.

Design choices

• Zero heap. State is one file-scope static struct. Size scales with JCON_MAX_DEPTH.
• Sink-agnostic. You supply int putc(char). The library never owns a buffer.
• Stateful sinks via ctx. jcon_start takes a void *ctx that the library threads into every putc call. Use it to point at a UART handle, a ring buffer, or whatever your sink needs — no globals required. Pass NULL if you don't need it.
• No string escaping. Strings are emitted verbatim between "..."; caller is responsible for ensuring the contents are valid inside a JSON string body. If you need escaping, do it before calling. This keeps the core small and avoids pulling in a UTF-8/escape ruleset you might not want.
• Sticky errors. The first putc failure latches; subsequent emits no-op. Check once at jcon_end() — no per-call branching in caller code.
• Root is an object by default. Use jcon_start_array when you need an array at the top level (handy for streaming NDJSON-style arrays of records).
• Not thread-safe, not reentrant. One global writer; don't call jcon_* from inside putc.
• Typed emitters with optional _Generic dispatch. Call jcon_add_int / jcon_add_string / etc. explicitly, or use jcon_add(name, value) to select the right one at compile time.
• Misuse asserts in debug, latches in release. Mismatched start/end, depth overflow, and NULL keys trip assert() under debug; with NDEBUG they silently latch JCON_ERR_USAGE.

Requirements

• C11 or later (_Generic).
• snprintf, assert, and the standard fixed-width int headers.
• No other dependencies.

Compile-time knobs

Macro	Default	Effect
JCON_MAX_DEPTH	16	Max nesting depth.
JCON_INDENT	" "	Indent unit for pretty-print mode.
JCON_ENABLE_FLOAT	off	Enables jcon_add_float / jcon_add_double.
NDEBUG	standard	Turns misuse asserts into silent JCON_ERR_USAGE.

Shape of the API

static int uart_putc(void *ctx, char c) {
    (void)ctx;
    return uart_write(&c, 1) == 1 ? 0 : -1;
}

jcon_start(false, uart_putc, NULL);
jcon_add("id", 42);
jcon_array_start("log");
    jcon_add(NULL, "boot");
jcon_array_end();
jcon_end();

→

{
  "id": 42,
  "log": [
    "boot"
  ]
}

Full API in include/jcon.h; usage patterns and edge cases in tests/test_jcon.c.

Build and integrate

As a drop-in source file

Compile src/jcon.c with your project and put include/ on the header search path. Works in any toolchain that has a C11 compiler.

Via the bundled Makefile (host test builds)

make          # run debug, release (NDEBUG), and float test configurations
make lib      # build/jcon.o for consumer linking

As a CMake target

add_subdirectory(path/to/jcon)
target_link_libraries(my_app PRIVATE jcon::jcon)

CMake options: -DJCON_MAX_DEPTH=<N> (default 16), -DJCON_ENABLE_FLOAT=ON.

As a Zephyr module

Add jcon to your west.yml, then in prj.conf:

CONFIG_JCON=y

The module exposes CONFIG_JCON_MAX_DEPTH and CONFIG_JCON_ENABLE_FLOAT via Kconfig. A runnable native_sim sample lives at samples/zephyr/hello_jcon/.

License

MIT. See LICENSE.