# μCOBS
[](https://crates.io/crates/ucobs)
[](https://docs.rs/ucobs)
[](LICENSE)
**The** COBS implementation for Rust.
[Consistent Overhead Byte Stuffing](https://en.wikipedia.org/wiki/Consistent_Overhead_Byte_Stuffing)
(COBS) encodes arbitrary bytes so that `0x00` never appears in the output,
allowing `0x00` to serve as an unambiguous frame delimiter. Overhead is exactly
1 byte per 254 input bytes (worst case).
## Features
- **`no_std`, zero-alloc** — runs anywhere, from 8-bit MCUs to servers
- **~140 lines of implementation** — small, auditable, easy to verify
- **100% spec-compliant** — Cheshire & Baker, IEEE/ACM Transactions on Networking, 1999
- **Proven interoperability** — cross-validated against `corncobs`, `cobs`, and Python `cobs`
- **Insanely tested** — canonical vectors, property-based tests, fuzz targets,
cross-crate interop, randomized payloads, and more coming
## Performance
Benchmarked against the two other `no_std` COBS crates using
[Criterion](https://github.com/bheisler/criterion.rs) across three payload
patterns: **all zeros** (worst case — every byte triggers a code emit),
**no zeros**, and **mixed** (0x00–0xFF cycling). Throughput in MB/s, higher
is better.
### Encode
| 64 B | zeros | 595 | 748 | 180 |
| 64 B | mixed | 1,853 | 1,011 | 1,734 |
| 64 B | nonzero | 1,884 | 1,012 | 2,110 |
| 256 B | zeros | 654 | 817 | 184 |
| 256 B | mixed | 1,888 | 1,002 | 1,887 |
| 256 B | nonzero | 1,964 | 1,004 | 1,968 |
| 4096 B | zeros | 675 | 845 | 188 |
| 4096 B | mixed | 1,734 | 1,016 | 1,746 |
| 4096 B | nonzero | 1,775 | 1,020 | 2,245 |
### Decode
| 64 B | zeros | 595 | 440 | 732 |
| 64 B | mixed | 7,041 | 504 | 7,991 |
| 64 B | nonzero | 9,900 | 502 | 9,807 |
| 256 B | zeros | 656 | 472 | 742 |
| 256 B | mixed |17,024 | 645 | 16,207 |
| 256 B | nonzero |18,456 | 647 | 19,549 |
| 4096 B | zeros | 675 | 483 | 768 |
| 4096 B | mixed |17,522 | 656 | 22,959 |
| 4096 B | nonzero |37,987 | 623 | 40,005 |
### Code size
Measured from `.text` section of release-optimized symbols (`encode` + `decode`
combined):
| μCOBS | 541 B |
| `cobs` 0.3 | 537 B |
| `corncobs` 0.1 | 637 B |
All three crates are tiny. The difference is negligible for any target.
### Crate properties
| `no_std` | always | opt-in | always |
| Zero-alloc | yes | opt-in | yes |
| `unsafe`-free | yes | yes | yes |
| `const fn` encode | yes | no | no |
| Implementation | ~120 LOC | ~790 LOC | ~645 LOC |
> Measured on Intel N100, Rust 1.93.1, Linux 6.12. Run `just bench` to
> reproduce. Full Criterion reports are generated in `target/criterion/report/`.
## When to use μCOBS
| Minimal code to audit | **~120 LOC** | ~790 LOC | ~645 LOC |
| Compile-time (`const fn`) encode | **yes** | no | no |
| Fastest encode (zero-heavy data) | **3–4×** faster | baseline | slowest |
| Fastest decode (large payloads) | competitive | slow | **slight edge** |
| Dead-simple API (3 functions) | **yes** | yes | more surface area |
| In-place / streaming encode | no | no | **yes** |
| `no_std` + zero-alloc by default | **yes** | opt-in | **yes** |
| Thorough test suite | **106 tests, fuzz, proptest** | basic | basic |
**Choose μCOBS** if you want the smallest, most auditable COBS implementation
with a minimal API and strong performance — especially on embedded targets or
payloads containing many zero bytes.
**Choose `corncobs`** if you need in-place encoding, an iterator-based API, or
your workload is dominated by large non-zero payloads where it holds a slight
throughput edge.
**Choose `cobs`** if you need `std` convenience features and don't care about
`no_std` or encode performance.
## Examples
### Basic encode and decode
```rust
// Encode: [0x11, 0x00, 0x33] → [0x02, 0x11, 0x02, 0x33]
let mut buf = [0u8; 16];
let n = ucobs::encode(&[0x11, 0x00, 0x33], &mut buf).unwrap();
assert_eq!(&buf[..n], &[0x02, 0x11, 0x02, 0x33]);
// Decode reverses it
let mut out = [0u8; 16];
let m = ucobs::decode(&buf[..n], &mut out).unwrap();
assert_eq!(&out[..m], &[0x11, 0x00, 0x33]);
```
### Buffer sizing
Use `max_encoded_len` to determine the required destination buffer size:
```rust
let data = [0x01, 0x02, 0x03];
let max = ucobs::max_encoded_len(data.len()); // 4 bytes
let mut buf = [0u8; 4];
let n = ucobs::encode(&data, &mut buf).unwrap();
assert_eq!(n, 4); // fits exactly
```
If the destination is too small, `encode` returns `None` rather than panicking:
```rust
let mut tiny = [0u8; 1];
assert_eq!(ucobs::encode(&[0x01, 0x02], &mut tiny), None);
```
### Framing for transport
Append a `0x00` sentinel after encoding to delimit frames on a wire. Strip
it before decoding:
```rust
let data = [0x11, 0x00, 0x33];
// Encode and append sentinel
let mut frame = [0u8; 16];
let n = ucobs::encode(&data, &mut frame).unwrap();
frame[n] = 0x00;
let wire = &frame[..n + 1];
// Receive: strip sentinel, then decode
let cobs_data = &wire[..wire.len() - 1];
let mut out = [0u8; 16];
let m = ucobs::decode(cobs_data, &mut out).unwrap();
assert_eq!(&out[..m], &data);
```
### Parsing a stream of frames
Split a byte stream on `0x00` to extract individual COBS frames:
```rust
// Two frames separated by 0x00 sentinels
let stream = [
0x02, 0x11, 0x02, 0x33, 0x00, // frame 1: [0x11, 0x00, 0x33]
0x03, 0xAA, 0xBB, 0x00, // frame 2: [0xAA, 0xBB]
];
let mut out = [0u8; 16];
.collect();
let n = ucobs::decode(frames[0], &mut out).unwrap();
assert_eq!(&out[..n], &[0x11, 0x00, 0x33]);
let n = ucobs::decode(frames[1], &mut out).unwrap();
assert_eq!(&out[..n], &[0xAA, 0xBB]);
```
### Compile-time encoding
`encode` is `const fn`, so you can build COBS-encoded tables at compile
time with zero runtime cost:
```rust
const PING: [u8; 2] = {
let mut buf = [0u8; 2];
match ucobs::encode(&[0x01], &mut buf) {
Some(_) => buf,
None => panic!("buffer too small"),
}
};
const ACK: [u8; 3] = {
let mut buf = [0u8; 3];
match ucobs::encode(&[0x06, 0x00], &mut buf) {
Some(_) => buf,
None => panic!("buffer too small"),
}
};
// Baked into the binary — no runtime encoding needed
assert_eq!(PING, [0x02, 0x01]);
assert_eq!(ACK, [0x02, 0x06, 0x01]);
```
### Error handling
Both `encode` and `decode` return `Option<usize>` — `None` on failure,
never panic:
```rust
let mut buf = [0u8; 8];
// Destination too small
assert_eq!(ucobs::encode(&[1, 2, 3], &mut buf[..1]), None);
// Malformed input (zero byte in encoded stream)
assert_eq!(ucobs::decode(&[0x00], &mut buf), None);
// Truncated frame (code byte promises more data than exists)
assert_eq!(ucobs::decode(&[0x05, 0x11], &mut buf), None);
// Empty input is valid
assert_eq!(ucobs::encode(&[], &mut buf), Some(1)); // encodes to [0x01]
assert_eq!(ucobs::decode(&[], &mut buf), Some(0)); // decodes to []
```
## API
| `encode` | `const fn(src: &[u8], dest: &mut [u8]) -> Option<usize>` | COBS-encode `src` into `dest`. Returns byte count or `None` if buffer too small. |
| `decode` | `(src: &[u8], dest: &mut [u8]) -> Option<usize>` | Decode COBS data. Returns byte count or `None` if input is malformed. |
| `max_encoded_len` | `const fn(src_len: usize) -> usize` | Maximum encoded size for a given input length. |
**Note:** The encoder does *not* append a trailing `0x00` sentinel. Append it
yourself when framing for transport.
## Testing
μCOBS has one of the most thorough test suites of any COBS implementation —
106 tests across 7 categories:
- **Canonical vectors** — every example from Cheshire & Baker 1999 (the original
COBS paper), plus the full Wikipedia vector set
- **External corpora** — vectors drawn from `cobs-c`, `nanocobs`, `cobs2-rs`,
Jacques Fortier's C implementation, and the Python `cobs` package
- **Cross-crate interop** — byte-for-byte validation against both `corncobs` and
`cobs` crates on randomized payloads up to 4 KB
- **Property-based tests** — 6 proptest suites covering round-trip correctness,
no-zero-in-output invariants, encoding properties, decoding properties,
structural/algebraic properties, and boundary-region behavior
- **Fuzz targets** — 3 `cargo-fuzz` harnesses (decode, round-trip, small-dest)
for continuous coverage of edge cases
- **Boundary tests** — exhaustive coverage of 254/255-byte block boundaries,
multi-block splits, buffer-exact fits, and off-by-one dest sizes
- **Compile-time verification** — `const` assertions that validate encode
correctness at compile time
```sh
# Unit + property + interop tests
just test-unit
# All quality gates (tests, clippy, fmt, doc, fuzz, miri)
just test
# Fuzz testing only (requires nightly + cargo-fuzz)
just test-fuzz
```
## License
[MIT](LICENSE) — Stephen Waits