CI License: MIT C99 Zero Allocation

Minimal CBOR encoder/decoder for embedded systems.

C99 | Zero dependencies | Zero allocations | RFC 8949 subset | Portable

IoT devices send sensor data over MQTT, CoAP, or BLE. JSON is the default choice, but on a microcontroller it is expensive: string escaping, float-to-text conversion, quotes around every key, and a parser that needs to handle arbitrary nesting.

CBOR (RFC 8949) is a binary format that is often 30-50% smaller than JSON, trivial to encode, and fast to decode. microcbor provides a compact C99 implementation for small firmware and constrained runtimes.

/* Encode a telemetry message */
uint8_t buf[64];
mcbor_enc_t enc;
mcbor_enc_init(&enc, buf, sizeof(buf));
mcbor_enc_map(&enc, 3);
mcbor_enc_str(&enc, "temp"); mcbor_enc_float(&enc, 23.4f);
mcbor_enc_str(&enc, "hum"); mcbor_enc_uint(&enc, 55);
mcbor_enc_str(&enc, "ok"); mcbor_enc_bool(&enc, true);
/* Send buf[0..mcbor_enc_size(&enc)] over MQTT */
mqtt_publish("sensor/data", buf, mcbor_enc_size(&enc));

The same message in JSON would be {"temp":23.4,"hum":55,"ok":true} (35 bytes). In CBOR it fits in 26 bytes. On a LoRa link with a 51-byte payload limit, that difference matters.

• Sequential encoder that writes into a caller-provided buffer.
• Sequential decoder with typed convenience helpers.
• Zero-copy byte and text references for decode paths.
• Sticky overflow detection so encode chains can be checked once at the end.
• Compact integer encoding with automatic width selection.
• RFC 8949 subset focused on embedded telemetry workloads.
• No heap allocation and no external dependencies.

Implemented:

• Unsigned and signed 32-bit integers
• Booleans and null
• Float32
• Text strings and byte strings
• Definite-length arrays and maps
• Recursive skip for unknown or forward-compatible fields

Not implemented:

• 64-bit integers
• Indefinite-length items
• Tags
• Dynamic allocation

uint8_t buf[128];
mcbor_enc_t enc;
mcbor_enc_init(&enc, buf, sizeof(buf));
mcbor_enc_map(&enc, 2);
mcbor_enc_str(&enc, "device"); mcbor_enc_str(&enc, "sensor-01");
mcbor_enc_str(&enc, "temp"); mcbor_enc_float(&enc, 22.5f);
uint32_t size = mcbor_enc_size(&enc);
/* buf[0..size] contains valid CBOR */

mcbor_dec_t dec;
mcbor_dec_init(&dec, buf, size);
uint32_t count;
mcbor_dec_map(&dec, &count);
for (uint32_t i = 0; i < count; i++) {
 char key[24];
 uint32_t klen;
 mcbor_dec_str(&dec, key, sizeof(key), &klen);
 if (strcmp(key, "temp") == 0) {
 float temp;
 mcbor_dec_float(&dec, &temp);
 } else {
 mcbor_dec_skip(&dec);
 }
}

Minimal GCC build:

gcc -std=c99 -Wall -Wextra -Wpedantic -Werror -Iinclude src/mcbor.c tests/test_all.c -lm -o test_all
./test_all

Using the bundled test makefile:

cd tests
make

On Windows, use MinGW, MSYS2, or WSL if gcc and make are not already installed.

• include/mcbor.h public API
• src/mcbor.c implementation
• tests/test_all.c unit and round-trip tests
• docs/API_REFERENCE.md API notes
• docs/DESIGN.md design decisions
• docs/PORTING_GUIDE.md integration notes

microcbor is suitable for publishing as a focused embedded utility library. The main remaining quality lever after publishing is automated CI execution on GitHub.

MIT License. Copyright (c) 2026 Vanderhell. See LICENSE.