anchor/tests/declare-program/programs/external/src/lib.rs

use anchor_lang::prelude::*;

declare_id!("Externa111111111111111111111111111111111111");

#[program]
pub mod external {
    use super::*;

    pub fn init(_ctx: Context<Init>) -> Result<()> {
        Ok(())
    }

    pub fn update(ctx: Context<Update>, value: u32) -> Result<()> {
        ctx.accounts.my_account.field = value;
        Ok(())
    }
}

#[derive(Accounts)]
pub struct Init<'info> {
    #[account(mut)]
    pub authority: Signer<'info>,
    #[account(
        init,
        payer = authority,
        space = 8 + 4,
        seeds = [authority.key.as_ref()],
        bump
    )]
    pub my_account: Account<'info, MyAccount>,
    pub system_program: Program<'info, System>,
}

#[derive(Accounts)]
pub struct Update<'info> {
    pub authority: Signer<'info>,
    #[account(mut, seeds = [authority.key.as_ref()], bump)]
    pub my_account: Account<'info, MyAccount>,
}

#[account]
pub struct MyAccount {
    pub field: u32,
}