Reading & Writing Account Data

Account data lives in a runtime-managed byte buffer. Programs borrow immutably or mutably, parse into types, mutate, and write back with Borsh or zero-copy updates.

Recipe

let mut data = account.try_borrow_mut_data()?;
let state = State::deserialize(&mut &data[..])?;

When to reach for this:

• Updating counters, config, or user state.
• Reading SPL token account fields (via spl-token layout).
• Migrating account layouts in upgrade instructions.

Working Example

pub fn increment(account: &AccountInfo) -> ProgramResult {
    if account.data_len() < State::LEN { return Err(ProgramError::InvalidAccountData); }
    let mut data = account.try_borrow_mut_data()?;
    let mut state = State::deserialize(&mut &data.as_ref()).map_err(|_| ProgramError::InvalidAccountData)?;
    state.count = state.count.checked_add(1).ok_or(ProgramError::InvalidArgument)?;
    state.serialize(&mut &mut data.as_mut()).map_err(|_| ProgramError::AccountDataTooSmall)?;
    Ok(())
}

What this demonstrates:

• Length guard before deserialize.
• try_borrow_mut_data for writes.
• Map serialize errors to AccountDataTooSmall.

Deep Dive

Borrow Rules

• One mutable borrow OR many immutable.
• Drop borrows before CPI on same account.

Partial Updates

Zero-copy allows field mutation without full serialize.

Rust Notes

// Prefer &mut [u8] subslices for discriminator + body.

Gotchas

• Borrow across CPI - Runtime error/panic. Fix: End borrow before invoke.
• Stale layout - Deserialize old struct on new program. Fix: Version byte + migration.
• Token account confusion - Treat token layout separately from custom State. Fix: Use spl-token pack/unpack.
• Resize without realloc - Serialize fails if account too small. Fix: CPI realloc instruction first.
• Unaligned zero-copy - UB on bad casts. Fix: Check size and alignment.

Alternatives

FAQs

Related

• Borsh Structs - Layouts
• realloc & Growing Accounts - Resize
• Account Discriminators - Typing

Stack versions: This page was written for Agave 4.1.1, Solana CLI 3.0.10, Anchor 0.32.1, anchor-lang 0.32.1, Rust 1.91.1, @solana/kit 7.0.0, Surfpool 0.12.0, and LiteSVM 0.6.x.