Arguments and Accounts

This tutorial covers the basics of creating and mutating accounts using Anchor. It's recommended to read Tutorial 0 first, as this tutorial will build on top of it.

Clone the Repo

To get started, clone the repo.

git clone https://github.com/project-serum/anchor

Change directories to the example (opens new window).

cd anchor/examples/tutorial/basic-1

And install any additional JavaScript dependencies:

yarn install

Defining a Program

We define our program as follows

use anchor_lang::prelude::*;
declare_id!("Fg6PaFpoGXkYsidMpWTK6W2BeZ7FEfcYkg476zPFsLnS");
#[program]
mod basic_1 {
    use super::*;
    pub fn initialize(ctx: Context<Initialize>, data: u64) -> Result<()> {
        let my_account = &mut ctx.accounts.my_account;
        my_account.data = data;
        Ok(())
    }
    pub fn update(ctx: Context<Update>, data: u64) -> Result<()> {
        let my_account = &mut ctx.accounts.my_account;
        my_account.data = data;
        Ok(())
    }
}
#[derive(Accounts)]
pub struct Initialize<'info> {
    #[account(init, payer = user, space = 8 + 8)]
    pub my_account: Account<'info, MyAccount>,
    #[account(mut)]
    pub user: Signer<'info>,
    pub system_program: Program<'info, System>,
}
#[derive(Accounts)]
pub struct Update<'info> {
    #[account(mut)]
    pub my_account: Account<'info, MyAccount>,
}
#[account]
pub struct MyAccount {
    pub data: u64,
}

Some new syntax elements are introduced here.

initialize instruction

First, let's start with the initialize instruction. Notice the data argument passed into the program. This argument and any other valid Rust types can be passed to the instruction to define inputs to the program.

Additionally, notice how we take a mutable reference to my_account and assign the data to it. This leads us to the Initialize struct, deriving Accounts. There are two things to notice about Initialize.

1. The my_account field is of type Account<'info, MyAccount> and the deserialized data structure is MyAccount.
2. The my_account field is marked with the init attribute. This will create a new account owned by the current program, zero initialized. When using init, one must also provide payer, which will fund the account creation, space, which defines how large the account should be, and the system_program, which is required by the runtime for creating the account.

DETAILS

All accounts created with Anchor are laid out as follows: 8-byte-discriminator || borsh serialized data. The 8-byte-discriminator is created from the first 8 bytes of the Sha256 hash of the account's type--using the example above, sha256("account:MyAccount")[..8]. The account: is a fixed prefix.

Importantly, this allows a program to know for certain an account is indeed of a given type. Without it, a program would be vulnerable to account injection attacks, where a malicious user specifies an account of an unexpected type, causing the program to do unexpected things.

On account creation, this 8-byte discriminator doesn't exist, since the account storage is zeroed. The first time an Anchor program mutates an account, this discriminator is prepended to the account storage array and all subsequent accesses to the account (not decorated with #[account(init)]) will check for this discriminator.

update instruction

Similarly, the Update accounts struct is marked with the #[account(mut)] attribute. Marking an account as mut persists any changes made upon exiting the program.

Here we've covered the basics of how to interact with accounts. In a later tutorial, we'll delve more deeply into deriving Accounts, but for now, just know you must mark an account init when using it for the first time and mut for persisting changes.

Creating and Initializing Accounts

We can interact with the program as follows.

// The program to execute.
const program = anchor.workspace.Basic1;
// The Account to create.
const myAccount = anchor.web3.Keypair.generate();
// Create the new account and initialize it with the program.
await program.rpc.initialize(new anchor.BN(1234), {
  accounts: {
    myAccount: myAccount.publicKey,
    user: provider.wallet.publicKey,
    systemProgram: SystemProgram.programId,
  },
  signers: [myAccount],
});

The last element passed into the method is common amongst all dynamically generated methods on the rpc namespace, containing several options for a transaction. Here, we specify the accounts field, an object of all the addresses the transaction needs to touch, and the signers array of all Signer objects needed to sign the transaction. Because myAccount is being created, the Solana runtime requires it to sign the transaction.

DETAILS

If you've developed on Solana before, you might notice two things 1) the ordering of the accounts doesn't matter and 2) the isWritable and isSigner options are not specified on the account anywhere. In both cases, the framework takes care of these details for you, by reading the IDL.

As before, we can run the example tests.

anchor test

Next Steps

We've covered all the basics of developing applications using Anchor. However, we've left out one important aspect to ensure the security of our programs--validating input and access control. We'll cover that next.