Failed to fill whole buffer with rust read_exact

Question

I have a little code snippet where I'm trying to write struct to a file and then read it. I have seen other similar posts where the asker has forgotten to zero initialise the buffer they are trying to read into. I have made sure not to do this, but I still am getting the error that 'failed to fill whole buffer' error when using read_exact, even though my buffer size and the size of the file I'm trying to read are the same.

Here is the code:

use std::fs::{File, OpenOptions};
use std::io::prelude::*;
use bincode::*;
use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize, PartialEq, Debug)]
pub struct Node {

    pub name: String,

    end_ptr: u32 // number of bytes away the next node is
}

impl Node {

    pub fn to_string(&self) -> String {

        return self.name.clone();
    }
}

fn main() {

    let node = Node { name: String::from("node_1"), end_ptr: 0 };

    let node_as_buf = bincode::serialize(&node).unwrap();

    let len_of_bytes_serialised: usize = node_as_buf.len();

    let mut file = OpenOptions::new()
        .read(true)
        .write(true)
        .create(true)
        .truncate(true)
        .open("test.txt")
        .unwrap();

    match file.write_all(&node_as_buf) {

        Ok(result) => {println!("Write success")},
        Err(err) => {println!("{}", &err)}
    }

    println!("{}", file.metadata().unwrap().len()); //  this and

    println!("{}", len_of_bytes_serialised); //  this are the same size

    let mut buffer = vec![0; len_of_bytes_serialised];

    match file.read_exact(&mut buffer[..]) {

        Ok(result) => println!("Read success"),
        Err(err) => println!("{}", &err) // prints 'failed to fill whole buffer'
    }
    let read_node: Node = bincode::deserialize(&buffer[..]).unwrap();
}

score 1 · Accepted Answer · answered Jun 23 '22 at 06:41

Because you use the same stream for reading and writing, the cursor is moved to the end of the file and any reading will attempt to read there, which will of course fail. You can observe that if you'll print file.stream_position().

You need to rewind() before reading.

Failed to fill whole buffer with rust read_exact

1 Answers1