Crate corncobs

corncobs: Corny COBS encoding/decoding in Rust

This crate provides Consistent Overhead Byte Stuffing (COBS) support for Rust programs, with a particular focus on resource-limited embedded no_std targets:

• Provides both fast (buffer-to-buffer) and small (in-place or iterator-based) versions of both encode and decode routines.

• Provides a const fn for computing the maximum encoded size for a given input size, so you can define fixed-size buffers precisely without magic numbers.

• Has pretty good test coverage, Criterion benchmarks, and a honggfuzz fuzz testing suite to try to ensure code quality.

When to use this crate

COBS lets us take an arbitrary blob of bytes and turn it into a slightly longer blob that doesn’t contain a certain byte, except as a terminator at the very end. corncobs implements the version of this where the byte is zero. That is, corncobs can take a sequence of arbitrary bytes, and turn it into a slightly longer sequence that doesn’t contain zero except at the end.

The main reason you’d want to do this is framing. If you’re transmitting a series of messages over a stream, you need some way to tell where the messages begin and end. There are many ways to do this – such as by transmitting a length before every message – but most of them don’t support sync recovery. Sync recovery lets a receiver tune in anywhere in a stream and figure out (correctly) where the next message boundary is. The easiest way to provide sync recovery is to use a marker at the beginning/end of each message that you can reliably tell apart from the data in the messages. To find message boundaries in an arbitrary data stream, you only need to hunt for the end of the current message and start parsing from there. COBS can do this by ensuring that the message terminator character (0) only appears between messages.

Unlike a lot of framing methods (particularly SLIP), COBS guarantees an upper bound to the size of the encoded output: the original length, plus two bytes, plus one byte per 254 input bytes. corncobs provides the max_encoded_len function for sizing buffers to allow for worst-case encoding overhead, at compile time.

corncobs can be used in several different ways, each with different costs and benefits.

• Encoding
  • encode_buf: from one slice to another; efficient, but requires 2x the available RAM.
  • encode_iter: incremental, using an iterator; somewhat slower, but requires no additional memory. (This can be useful in a serial interrupt handler.)
• Decoding
  • decode_buf: from one slice to another; efficient, but requires 2x the available RAM.
  • decode_in_place: in-place in a slice; nearly as efficient, but overwrites incoming data.

Design decisions / tradeoffs

corncobs is optimized for a fast and simple implementation. To get best performance on normal data, it leaves something out: validation.

Specifically: corncobs will decode invalid COBS data that contains zeroes in unexpected places mid-message. It could reject such data by scanning for zeroes. We chose not to do this for performance reasons, and justify it with the following points.

First: we don’t have to do this to maintain memory safety. Several C implementations of COBS do data validation in an attempt to avoid buffer overruns or out-of-bounds accesses. We’re not writing in C and don’t have this problem to worry about.

Second: it really does improve performance, by about 5x in benchmarks. This is because, by lifting the requirement to inspect every byte hunting for zeroes, we can use copy_from_slice to move data around, which calls optimized memory-move routines for the target architecture that are basically always much faster than moving bytes.

Third: COBS does not guarantee integrity. Spurious zeroes in the middle of a message is only one way your input data could be corrupted. Your application needs to handle all possible corruption, which means having an integrity check on the COBS-decoded data, such as a CRC.

If you feed corncobs random invalid data, it will either return unexpectedly short decoded results (which will fail your next-level integrity check), or it will return an Err. It will not crash, corrupt memory, or panic!, and we have tests to demonstrate this.

Cargo features

No features are enabled by default. Embedded programmers do not need to specify default-features = false when using corncobs because who said std should be the default anyhow? People with lots of RAM, that’s who.

Features:

• std: if you’re on one of them “big computers” with “infinite memory” and can afford the inherent nondeterminism of dynamic memory allocation, this feature enables routines for encoding to-from Vec, and an Error impl for CobsError.

Tips for using COBS

If you’re designing a protocol or message format and considering using COBS, you have some options.

Optimizing for size: COBS encoding has the least overhead when the data being encoded contains 0x00 bytes, at least one for every 254 bytes sent. In practice, most data formats achieve this. However…

Optimizing for speed: COBS encode/decode, and particularly the corncobs implementation, goes fastest when data contains as few 0x00 bytes as possible – ideally none. If you can adjust the data you’re encoding to avoid zero, you can achieve higher encode/decode rates. For instance, in one of my projects that sends RGB video data, I just declared that red/green/blue value 1 is the same as 0, and made all the 0s into 1s, for a large performance improvement.

Structs

Decoder

Enums

CobsError

Errors that can occur while decoding.

DecodeStatus

Constants

ZERO

The termination byte used by corncobs. Yes, it’s a bit silly to have this as a constant – but the implementation is careful to use this named constant whenever it is talking about the termination byte, for clarity.

Functions

decode_buf

Decodes input from bytes into output starting at index 0. Returns the number of bytes used in output.

decode_in_place

Decodes an encoded message, in-place. This is useful when you’re short on memory. Since the decoded form of a COBS frame is always shorter than the encoded form, bytes is guaranteed to be long enough.

encode_buf

Encodes the message bytes into the buffer output. Returns the number of bytes used in output, which also happens to be the index of the first zero byte.

encode_iter

Encodes bytes into COBS form, yielding individual encoded bytes through an iterator.

max_encoded_len

Returns the largest possible encoded size for an input message of raw_len bytes, considering overhead.