Space & Rent

Solana accounts must be rent-exempt. Anchor init uses your space attribute to allocate bytes including the 8-byte discriminator.

Recipe

#[account]
#[derive(InitSpace)]
pub struct User {
    pub authority: Pubkey,
    pub points: u64,
}
 
// space = 8 + User::INIT_SPACE
#[account(init, payer = payer, space = 8 + User::INIT_SPACE)]
pub user: Account<'info, User>,

When to reach for this: You add fields to on-chain state or create new account types.

Working Example

#[account]
#[derive(InitSpace)]
pub struct Config {
    pub admin: Pubkey,
    #[max_len(64)]
    pub name: String,
    pub fee_bps: u16,
}
 
#[account(init, payer = payer, space = 8 + Config::INIT_SPACE)]
pub config: Account<'info, Config>,

What this demonstrates:

• InitSpace derive calculates size at compile time
• 8-byte discriminator is mandatory for Anchor accounts
• Payer supplies lamports for rent exemption
• max_len on strings/vecs bounds allocation

Deep Dive

Sizing Rules

• Fixed fields: sum of Borsh sizes.
• String / Vec: need max_len with InitSpace.
• Zero-copy accounts use different sizing (no Borsh on chain).

Rent

Rent-exempt minimum depends on allocated size. Use Rent::get()?.minimum_balance(space) in tests to assert funding.

Gotchas

• Forgetting discriminator in space - Account too small; serialize panics. Fix: Always add 8 to INIT_SPACE.
• Unbounded String - No max_len; size unknown. Fix: Use #[max_len(n)] with InitSpace.
• Undersized Vec - Grows beyond allocation. Fix: Preallocate max length or use realloc.
• Wrong INIT_SPACE after field add - Upgrade breaks new inits. Fix: Bump version and migrate.
• Closing without reclaiming rent - Users lose SOL. Fix: Use close = recipient.

Alternatives

FAQs

Related

• realloc in Anchor - grow accounts
• Space Calculation - serialization sizing
• init & init_if_needed - allocation

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.