Zephyr Cbor

Zephyr CBOR (zcbor)

Zephyr integrates the zcbor library for CBOR encoding/decoding.

⭐ Recommended: Code Generation from CDDL Schema

For any non-trivial message format, use zcbor's code generator instead of hand-writing zcbor calls. It generates type-safe C structs and encode/decode functions from a CDDL schema — less error-prone, easier to maintain, and handles unions/optionals automatically.

1. Write a CDDL schema

CDDL supports two container styles — choose based on whether field order matters:

• Group/tuple (...) — ordered sequence, no keys on wire (more compact)
• Map {...} — key-value pairs, order-independent (more flexible)

; my-api.cddl

; Integer constants for a discriminated union
cmd_get_status = 1
cmd_set_config = 2

command_id /= cmd_get_status
command_id /= cmd_set_config

; Enum values
status_idle   = 0
status_active = 1
status_error  = 2

device_status /= status_idle
device_status /= status_active
device_status /= status_error

; Tuple (ordered, compact — no keys on wire)
set_config_params = (
    threshold: uint .size 2,
    enabled:   bool,
)

; Map (key-value, order-independent)
status_response = {
    status_code: device_status,
    uptime_ms:   uint,
    ? error_msg: tstr,
}

; Top-level message with optional params
My_Command = (
    command_id: command_id,
    ? params:   set_config_params,
)

2. Generate C code

pip install zcbor

zcbor code --decode --encode \
  --short-names \
  -c my-api.cddl \
  -t My_Command \
     status_response \
  --output-c src/cbor/my-api.c \
  --output-h include/cbor/my-api.h \
  --output-h-types include/cbor/my-api_types.h \
  --include-prefix "cbor/" \
  --default-max-qty 3

Key flags:

zcbor splits my-api.c into my-api_decode.c / my-api_encode.c automatically. Do not hand-edit generated files — regenerate from .cddl when the schema changes.

CMake integration — trigger regeneration when the .cddl changes:

set(zcbor_command
    zcbor code --decode --encode --short-names
    -c ${CMAKE_CURRENT_LIST_DIR}/my-api.cddl
    -t My_Command status_response
    --output-c     ${CMAKE_CURRENT_LIST_DIR}/src/cbor/my-api.c
    --output-h     ${CMAKE_CURRENT_LIST_DIR}/include/cbor/my-api.h
    --output-h-types ${CMAKE_CURRENT_LIST_DIR}/include/cbor/my-api_types.h
    --include-prefix "cbor/"
    --default-max-qty 3
)

add_custom_command(
    OUTPUT
        ${CMAKE_CURRENT_LIST_DIR}/src/cbor/my-api_decode.c
        ${CMAKE_CURRENT_LIST_DIR}/src/cbor/my-api_encode.c
        ${CMAKE_CURRENT_LIST_DIR}/include/cbor/my-api_decode.h
        ${CMAKE_CURRENT_LIST_DIR}/include/cbor/my-api_encode.h
        ${CMAKE_CURRENT_LIST_DIR}/include/cbor/my-api_types.h
    COMMAND ${zcbor_command}
    DEPENDS ${CMAKE_CURRENT_LIST_DIR}/my-api.cddl
    COMMENT "Regenerating CBOR code from CDDL schema"
)

3. Use the generated API

zcbor naming conventions for generated symbols:

• Structs: struct <TypeName> (e.g. struct My_Command)
• Union choice enums: <union_name>_<value_name>_m_c (e.g. command_id_cmd_get_status_m_c)
• Union choice field: <union_name>_choice (e.g. command_id_choice)
• Optional fields: bool <field>_present alongside the field
• Repeated fields: <field>_m[N] array + size_t <field>_m_count
• Single-field groups are flattened into their parent struct

#include "cbor/my-api_decode.h"   /* cbor_decode_My_Command() */
#include "cbor/my-api_encode.h"   /* cbor_encode_status_response() */
#include "cbor/my-api_types.h"    /* struct My_Command, device_status_r, ... */

/* --- Decode --- */
struct My_Command cmd = {0};
size_t consumed;

int err = cbor_decode_My_Command(buf, buf_len, &cmd, &consumed);
if (err != ZCBOR_SUCCESS) {
    return err;
}

switch (cmd.command_id_choice) {
    case command_id_cmd_get_status_m_c:
        break;
    case command_id_cmd_set_config_m_c:
        if (cmd.params_present) {
            uint32_t threshold = cmd.params.threshold;
            bool     enabled   = cmd.params.enabled;
        }
        break;
}

/* --- Encode --- */
struct status_response resp = {
    .status_code.device_status_choice = device_status_status_active_m_c,
    .uptime_ms   = k_uptime_get_32(),
    .error_msg_present = false,
};
uint8_t out[64];
size_t  out_len;
cbor_encode_status_response(out, sizeof(out), &resp, &out_len);

When to use code generation vs. manual

Manual API

For simple cases. All operations use a zcbor_state_t state machine and return bool (true=success).

Kconfig

CONFIG_ZCBOR=y

Quick Start: Encoding

#include <zcbor_encode.h>

uint8_t buf[64];
ZCBOR_STATE_E(zse, 0, buf, sizeof(buf), 1);  // 0 backups, 1 top-level element

bool ok = zcbor_map_start_encode(zse, 2)
       && zcbor_tstr_put_lit(zse, "temp")
       && zcbor_int32_put(zse, 22)
       && zcbor_tstr_put_lit(zse, "unit")
       && zcbor_tstr_put_lit(zse, "C")
       && zcbor_map_end_encode(zse, 2);

size_t len = zse->payload - buf;  // bytes written

Quick Start: Decoding

#include <zcbor_decode.h>

ZCBOR_STATE_D(zsd, 0, buf, buf_len, 1, 0);  // 0 backups, 1 element, 0 flags

int32_t temp;
struct zcbor_string unit;

bool ok = zcbor_map_start_decode(zsd)
       && zcbor_tstr_expect_lit(zsd, "temp")
       && zcbor_int32_decode(zsd, &temp)
       && zcbor_tstr_expect_lit(zsd, "unit")
       && zcbor_tstr_decode(zsd, &unit)
       && zcbor_map_end_decode(zsd);

References

• references/api.md — Type quick reference, complete function signatures, state initialization, error codes. Read when working with manual encode/decode functions.
• references/patterns.md — Common patterns: lists, unordered maps, byte strings, nested CBOR, multi-decode. Read when implementing specific data structures.
• references/kconfig.md — All CONFIG_ZCBOR_* options. Read when configuring zcbor features beyond CONFIG_ZCBOR=y.
• references/locations.md — Header paths, sample locations, upstream links. Read when locating source files.

Related Skills

• zephyr-net-socket: CoAP/UDP transport for CBOR payloads
• zephyr-bluetooth-le: SMP over BLE uses CBOR
• zephyr-json: Alternative text-based serialization