Call write syscall on x86-64 Linux in Rust

syscall.rs

use std::arch::asm;

fn main() {
    let str = b"Hello world\n";
    unsafe {
        let res = syswrite(str.as_ptr() as u64, str.len() as u64);
        println!("res: {}", res);
    }
}

unsafe fn syswrite(strptr: u64, strlen: u64) -> i64 {
    let res: i64;
    asm!(
        "syscall",
        in("rax") 1,
        in("rdi") 1,
        in("rsi") strptr,
        in("rdx") strlen,
        lateout("rax") res
    );
    res
}