Read an arbitrary number of bytes from type implementing Read

Question

I have something that is Read; currently it's a File. I want to read a number of bytes from it that is only known at runtime (length prefix in a binary data structure).

So I tried this:

let mut vec = Vec::with_capacity(length);
let count = file.read(vec.as_mut_slice()).unwrap();

but count is zero because vec.as_mut_slice().len() is zero as well.

[0u8;length] of course doesn't work because the size must be known at compile time.

I wanted to do

let mut vec = Vec::with_capacity(length);
let count = file.take(length).read_to_end(vec).unwrap();

but take's receiver parameter is a T and I only have &mut T (and I'm not really sure why it's needed anyway).

I guess I can replace File with BufReader and dance around with fill_buf and consume which sounds complicated enough but I still wonder: Have I overlooked something?

Answer 1

Like the Iterator adaptors, the IO adaptors take self by value to be as efficient as possible. Also like the Iterator adaptors, a mutable reference to a Read is also a Read.

To solve your problem, you just need Read::by_ref:

use std::io::Read;
use std::fs::File;

fn main() {
    let mut file = File::open("/etc/hosts").unwrap();
    let length = 5;

    let mut vec = Vec::with_capacity(length);
    file.by_ref().take(length as u64).read_to_end(&mut vec).unwrap();

    let mut the_rest = Vec::new();
    file.read_to_end(&mut the_rest).unwrap();
}