embassy-rs/embassy · error

DataLength > 64

Error message

DataLength > 64

What it means

`DataLength::new` panics when an FDCAN frame is given a payload length greater than 64 bytes. The CAN-FD protocol allows at most 64 data bytes, so lengths above 64 are rejected as invalid rather than truncated.

Solutions

1. Slice the payload into chunks of at most 64 bytes and send multiple frames.
2. Validate `len <= 64` before calling DataLength::new with FrameFormat::Fdcan.
3. Clamp with `len.min(64)` if truncation is acceptable.

Example fix

// before
let dl = DataLength::new(buffer.len() as u8, FrameFormat::Fdcan);
// after
assert!(buffer.len() <= 64, "FD frame payload exceeds 64 bytes");
let dl = DataLength::new(buffer.len() as u8, FrameFormat::Fdcan);

fn valid_fd_len(len: u8) -> bool { len <= 64 }

When it happens

Trigger: Calling `DataLength::new(len, FrameFormat::Fdcan)` with `len > 64`; typically while constructing a transmit frame whose source buffer exceeds 64 bytes.

Common situations: Passing a whole application buffer instead of slicing it into CAN-sized chunks; confused protocol limits (thinking FD allows 128 bytes); chunking loop with wrong stride.

Understand the failure class

Background: "value must be between 0 and 1" / "out of range" / "must not be negative" errors: fixing range-validation failures across open-source libraries — this error's family across 42 libraries.

Related errors

• DataLength > 8
• DLC > 15
• DLC > 15
• Passphrase is too short or too long
• Boot prepare error

AI-assisted analysis of embassy-rs/embassy@463a07b963 (2026-09-10). Data as JSON: /api/errors/9f168c8ebf05b4d1. Report an issue: GitHub.

Appendix: source

Thrown at embassy-stm32/src/can/fd/message_ram/enums.rs:23

#[derive(Clone, Copy, Debug, PartialEq)]
pub enum DataLength {
    Classic(u8),
    Fdcan(u8),
}
impl DataLength {
    /// Creates a DataLength type
    ///
    /// Uses the byte length and Type of frame as input
    pub fn new(len: u8, ff: FrameFormat) -> DataLength {
        match ff {
            FrameFormat::Classic => match len {
                0..=8 => DataLength::Classic(len),
                _ => panic!("DataLength > 8"),
            },
            FrameFormat::Fdcan => match len {
                0..=64 => DataLength::Fdcan(len),
                _ => panic!("DataLength > 64"),
            },
        }
    }
    /// Specialised function to create classic frames
    pub fn new_classic(len: u8) -> DataLength {
        Self::new(len, FrameFormat::Classic)
    }
    /// Specialised function to create FDCAN frames
    pub fn new_fdcan(len: u8) -> DataLength {
        Self::new(len, FrameFormat::Fdcan)
    }

    /// returns the length in bytes
    pub fn len(&self) -> u8 {
        match self {
            DataLength::Classic(l) | DataLength::Fdcan(l) => *l,
        }
    }

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