main.rs

use std::fs::File;
use std::io::prelude::*;

const BYTES_PER_LINE: usize = 16;

fn main() {
    let mut f= File::open("/tmp/file.txt").unwrap();
    let mut pos = 0;
    let mut buffer = [0; BYTES_PER_LINE];

    while let Ok(_) = f.read_exact(&mut buffer) {
        print!("[0x{:08x}] ", pos);
        for byte in &buffer {
            match *byte {
                0x00 => print!(".  "),
                0xff => print!("XX "),
                _ => print!("{:02x}", byte),
            }
        }
        println!("");
        pos += BYTES_PER_LINE;
    }
}

output.md

Actual output

$ cargo build
   Compiling foo v0.1.0 (/tmp/foo)
    Finished dev [unoptimized + debuginfo] target(s) in 0.64s
$ echo -n "0123456789abcdef" > /tmp/file.txt                
$ ./target/debug/foo
[0x00000000] 30 31 32 33 34 35 36 37 38 39 61 62 63 64 65 66 
$ echo -n "0123456789abcdef0123456789abcdef" > /tmp/file.txt    
$ ./target/debug/foo                                        
[0x00000000] 30 31 32 33 34 35 36 37 38 39 61 62 63 64 65 66 
[0x00000010] 30 31 32 33 34 35 36 37 38 39 61 62 63 64 65 66 
$ echo -n "0123456789abcdef0123456789abcdef0123" > /tmp/file.txt
$ ./target/debug/foo
[0x00000000] 30 31 32 33 34 35 36 37 38 39 61 62 63 64 65 66 
[0x00000010] 30 31 32 33 34 35 36 37 38 39 61 62 63 64 65 66

Expected output (for the last call)

$ echo -n "0123456789abcdef0123456789abcdef0123" > /tmp/file.txt
$ ./target/debug/foo
[0x00000000] 30 31 32 33 34 35 36 37 38 39 61 62 63 64 65 66 
[0x00000010] 30 31 32 33 34 35 36 37 38 39 61 62 63 64 65 66
[0x00000020] 30 31 32 33