Function write_output 

pub unsafe extern "C" fn write_output(
    start: *const u8,
    length: u32,
    offset: u32,
) -> u32

Write to the return value of the contract. The parameters are

• start the pointer to the location in memory where the data resides
• length the size of data (in bytes)
• offset where in the return value to write the data

The return value indicates how many bytes were written.