Read file bytes

For highly optimized file reading in Rust, we often need to act upon the bytes in a file directly. Rust provides ways to load and loop over the bytes in a file.

To add file IO in Rust, we need to include the "std::io" and related modules. We also use the Result type in functions that read files.

Example

This program reads in a file and stores its data in a Vector of bytes (like a byte array). This is efficient: it does not waste any memory for the file representation in memory.

Start We call "File::open" to open the file on the disk. We create BufReader, and a Vec (to store the data once read).

Then We call read_to_end and pass the vector as a mutable reference argument. Then we use "for" to loop over the vector bytes.

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

fn main() -> io::Result<()> {
    let f = File::open("/Users/sam/file.txt")?;
    let mut reader = BufReader::new(f);
    let mut buffer = Vec::new();

    // Read file into vector.
    reader.read_to_end(&mut buffer)?;

    // Read.
    for value in buffer {
        println!("BYTE: {}", value);
    }
    Ok(())
}BYTE: 97
BYTE: 98
BYTE: 99

Output notes

The file in the example happens to contain 3 letters, the characters "abc." The ASCII values 97, 98 and 99 are those 3 letters.

Reading in byte arrays in Rust is efficient and wastes very little memory or time. For Rust programs where every millisecond matters, this approach is effective.