Function entrypoint 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn entrypoint(input: *mut u8) -> u64

This program expects one account and writes instruction data into the account’s data. The first 8 bytes of the instruction data contain the little-endian offset into the account data. The rest of the instruction data is written into the account data starting at that offset.

This program uses the raw Solana runtime’s entrypoint which takes a pointer to serialized input parameters. For more information about the format of the serialized input parameters see solana_sdk::entrypoint::deserialize

Safety