File System Operations

read_to_end()

Read::read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> reads all remaining bytes from an input stream until End-of-File (EOF), appends them directly onto buf, and returns the total number of bytes appended. For whole-file reading by path, std::fs::read(path) -> io::Result<Vec<u8>> wraps File::open(path) + read_to_end into a single call.

Comparing read_to_end and read_to_string

Feature reader.read_to_end(&mut vec) / fs::read(path) reader.read_to_string(&mut s) / fs::read_to_string(path)
Target buffer type Vec<u8> String
Accepts arbitrary binary bytes (0x00..=0xFF)? Yes (never fails with InvalidData on byte values) No (returns Err(ErrorKind::InvalidData) on non-UTF-8 bytes)
Appends to existing buffer? Yes (read_to_end appends to vec) Yes (read_to_string appends to s)
Behavior on mid-read I/O error Partial bytes read before the error remain appended in vec Rolls back partial bytes so s remains valid UTF-8

Bounding Memory Growth with .take(max_bytes) Before read_to_end

Because read_to_end keeps reading and growing the Vec<u8> until EOF, calling read_to_end on an unbounded pseudo-file (such as /dev/zero or /dev/urandom) or an untrusted network stream will allocate until the process runs out of memory!

Chaining .take(max_bytes + 1) before .read_to_end(&mut buf) caps the maximum read size and lets you detect whether the file exceeded max_bytes if buf.len() > max_bytes:

use std::fs::{self, File};
use std::io::{self, Read};
use std::path::Path;

fn read_binary_bounded(path: &Path, max_bytes: usize) -> io::Result<Vec<u8>> {
    let file = File::open(path)?;
    let mut buf = Vec::new();

    // Read at most `max_bytes + 1` bytes so we can detect oversized files without OOM:
    let bytes_read = file
        .take((max_bytes as u64) + 1)
        .read_to_end(&mut buf)?;

    if bytes_read > max_bytes {
        return Err(io::Error::new(
            io::ErrorKind::FileTooLarge,
            format!("file exceeds maximum allowed size of {max_bytes} bytes"),
        ));
    }

    Ok(buf)
}

fn main() -> io::Result<()> {
    let path = std::env::temp_dir().join("rust_ch50_binary.bin");
    fs::write(&path, [0x00_u8, 0xFF, 0x42, 0x7F])?;

    let ok_bytes = read_binary_bounded(&path, 16)?;
    let too_large = read_binary_bounded(&path, 2);

    fs::remove_file(&path)?;
    println!("ok_bytes (max 16 B): {:02x?}", ok_bytes);
    println!("too_large (max 2 B): {:?}", too_large.unwrap_err().kind());
    Ok(())
}

ok_bytes (max 16 B): [00, ff, 42, 7f]
too_large (max 2 B): FileTooLarge