file - 为什么 BufReader::read 并不总是完全填满给定的缓冲区？

Question

我希望这段代码在每个循环中读取 3 个字节而不打印，但是文件的每 8000 个字节左右，它只读取两个字节:

use std::error::Error;
use std::fs::File;
use std::io::prelude::*;
use std::io::BufReader;
use std::io::BufWriter;
use std::io::SeekFrom;
use std::path::Path;

fn main() -> std::io::Result<()> {
    let sequence: [u8; 3] = [1, 2, 3];
    let file_path = Path::new("./test_file");
    //fill a file with 1,2,3,1,2,3...
    {
        let mut output_file: std::io::BufWriter<std::fs::File>;
        output_file = BufWriter::new(File::create(file_path)?);
        for _i in 0..100000 {
            match output_file.write(&sequence) {
                Err(why) => panic!("could not write {}", Error::description(&why)),
                Ok(_) => {}
            }
        }
    }
    //read the file 3 bytes at a time
    {
        let mut input_file: std::io::BufReader<std::fs::File>;
        input_file = BufReader::new(File::open(file_path)?);
        for i in 0..100000 {
            let mut raw = [0; 3];
            let result = match input_file.read(&mut raw) {
                Err(why) => panic!("could not read {}", Error::description(&why)),
                Ok(x) => x,
            };
            // print if something other than 3 bytes were read
            if result != 3 {
                println!(
                    "file pos {}, data read {}, buffer = [{},{},{}]",
                    i * 3,
                    result,
                    raw[0],
                    raw[1],
                    raw[2]
                );
            }
        }
    }
    Ok(())
}

在 Mac 上使用 rustc problem.rs 编译并使用 ./problem 运行。

输出:

file pos 8190, data read 2, buffer = [1,2,0]
file pos 16383, data read 2, buffer = [3,1,0]
file pos 24576, data read 2, buffer = [2,3,0]
file pos 32769, data read 2, buffer = [1,2,0]
file pos 40962, data read 2, buffer = [3,1,0]
file pos 49155, data read 2, buffer = [2,3,0]
file pos 57348, data read 2, buffer = [1,2,0]
file pos 65541, data read 2, buffer = [3,1,0]
...

这似乎表明与内部 8192 大小的缓冲区有关。

为什么我每次得不到3个字节？我一次读取 5 个字节得到类似的结果。

Answer 1

默认缓冲区大小为 8KB。查看docs for BufReader::new .

8192 字节不能被 3 整除，因此您会在每个缓冲区的末尾得到几个尾随字节。

您可以使用 with_capacity constructor 将缓冲区大小设置为 8KB 以外的值.