Calculating Account Size

We’ve talked fair game about how the size of Solana accounts need to be known at creation time in order to calculate the minimum amount of lamports required to make the account rent exempt.

In the last project, you calculated the space using:

let account_space = std::mem::size_of::<CounterAccount>();

std::mem::size_of::<T>() tells you how many bytes the struct occupies in RAM on the host that compiles the program, padding included. Borsh, by contrast, writes a packed stream with no alignment padding. The two numbers match only when every field already aligns naturally, for example, the one-field CounterAccount.

In your new structs the compiler slips in padding after odd-sized fields so that the next u64 or Pubkey starts on an eight-byte boundary. std::mem::size_of::<Poll>() therefore comes back larger than the Borsh payload you will actually store. If you rely on that value you either waste rent by over-allocating or, worse, risk an under-allocation if the compiler’s padding strategy ever changes.

The safe approach is to define an explicit constant that adds up the exact serialized bytes, i.e discriminator plus every field length, and use that constant whenever you create the account.

For example, to calculate the size of the Poll account:

impl Poll {
    pub const SIZE: usize = {
        use std::mem::size_of;
        size_of::<[u8; 8]>() +      // discriminator
        size_of::<Pubkey>() +       // poll_creator
        size_of::<u8>() +           // desc_len
        MAX_DESC_LEN +              // poll_description
        size_of::<u64>() +          // start_slot
        size_of::<u64>() +          // end_slot
        size_of::<u8>() +           // poll_stage (PollStage as u8)
        size_of::<u8>() // options_count
    };

    pub fn assert_valid(&self) -> Result<(), ProgramError> {
        if self.discriminator != POLL_DISCRIMINATOR {
            return Err(ProgramError::InvalidAccountData);
        }

        Ok(())
    }
}

Implement the size constant for the rest of your accounts. You now have a reliable SIZE constant for every account; the next step is to create the data itself in a single, fool-proof call.