What is the most efficient way to read a large fil

问题:

What is the most efficient general purpose way of reading "large" files (which may be text or binary), without going into unsafe territory? I was surprised how few relevant results there were when I did a web search for "rust read large file in chunks".

For example, one of my use cases is to calculate an MD5 checksum for a file using rust-crypto (the Md5 module allows you to add &[u8] chunks iteratively).

Here is what I have, which seems to perform slightly better than some other methods like read_to_end:

use std::{
    fs::File,
    io::{self, BufRead, BufReader},
};

fn main() -> io::Result<()> {
    const CAP: usize = 1024 * 128;
    let file = File::open("my.file")?;
    let mut reader = BufReader::with_capacity(CAP, file);

    loop {
        let length = {
            let buffer = reader.fill_buf()?;
            // do stuff with buffer here
            buffer.len()
        };
        if length == 0 {
            break;
        }
        reader.consume(length);
    }

    Ok(())
}

回答1:

I don't think you can write code more efficient than that. fill_buf on a BufReader over a File is basically just a straight call to read(2).

That said, BufReader isn't really a useful abstraction when you use it like that; it would probably be less awkward to just call file.read(&mut buf) directly.