How to read a specific number of bytes from a stream?

Question

I have a struct with a BufStream where T: Read+Write. The BufStream can be a TcpStream and I\'d like to read n b

Answer 1

Since Rust 1.6, Read::read_exact can be used to do this. If bytes_to_read is the number of bytes you need to read, possibly determined at runtime, and reader is the stream to read from:

let mut buf = vec![0u8; bytes_to_read];
reader.read_exact(&mut buf)?;

The part that wasn't clear to me from the read_exact documentation was that the target buffer can be a dynamically-allocated Vec.

Thanks to the Rust Gitter community for pointing me to this solution.

Answer 2

It sounds like you want Read::take and Read::read_to_end:

use std::{
    io::{prelude::*, BufReader},
    str,
};

fn read_n<R>(reader: R, bytes_to_read: u64) -> Vec<u8>
where
    R: Read,
{
    let mut buf = vec![];
    let mut chunk = reader.take(bytes_to_read);
    // Do appropriate error handling for your situation
    // Maybe it's OK if you didn't read enough bytes?
    let n = chunk.read_to_end(&mut buf).expect("Didn't read enough");
    assert_eq!(bytes_to_read as usize, n);
    buf
}

fn main() {
    let input_data = b"hello world";
    let mut reader = BufReader::new(&input_data[..]);

    let first = read_n(&mut reader, 5);
    let _ = read_n(&mut reader, 1);
    let second = read_n(&mut reader, 5);

    println!(
        "{:?}, {:?}",
        str::from_utf8(&first),
        str::from_utf8(&second)
    );
}