solana/programs/bpf/rust/external_spend/src/lib.rs

//! @brief Example Rust-based BPF program that moves a lamport from one account to another

extern crate solana_sdk;
use solana_sdk::entrypoint;
use solana_sdk::entrypoint::*;
use solana_sdk::pubkey::Pubkey;

entrypoint!(process_instruction);
fn process_instruction(_program_id: &Pubkey, ka: &mut [SolKeyedAccount], _data: &[u8]) -> u32 {
    // account 0 is the mint and not owned by this program, any debit of its lamports
    // should result in a failed program execution.  Test to ensure that this debit
    // is seen by the runtime and fails as expected
    *ka[0].lamports -= 1;
    SUCCESS
}