main.rs

use borsh::{BorshDeserialize, BorshSerialize};
use solana_program::{
   account_info::{next_account_info, AccountInfo},
   entrypoint,
   program_error::ProgramError,
   pubkey::Pubkey,
};

#[derive(BorshSerialize, BorshDeserialize, Debug)]
pub struct GreetingAccount {
   pub counter: u32,
}

#[cfg(not(feature = "exclude_entrypoint"))]
entrypoint!(process_instruction);

pub fn process_instruction(
   program_id: &Pubkey,
   accounts: &[AccountInfo],
   instruction_data: &[u8],
) -> entrypoint::ProgramResult {
   let accounts_iter = &mut accounts.iter();
   let account = next_account_info(accounts_iter)?;

   if account.owner != program_id {
       return Err(ProgramError::IncorrectProgramId);
   }
   let mut greeting = GreetingAccount::try_from_slice(&account.data.borrow())?;
   greeting.counter += 1;
   greeting.serialize(&mut &mut account.data.borrow_mut()[..])?;
   Ok(())
}