How to slice a large Vec<i32> as &[u8]? - rust

Question

I don't know how to convert a Vec<i32> into a &[u8] slice.
fn main() {
let v: Vec<i32> = vec![1; 100_000_000];
let v_bytes: &[u8] = /* ... */;
}
I want to write a large Vec<i32> to a file so I can read it back at a future time.

Answer 1

You can use std::slice::from_raw_parts:
let v_bytes: &[u8] = unsafe {
std::slice::from_raw_parts(
v.as_ptr() as *const u8,
v.len() * std::mem::size_of::<i32>(),
)
};
Following the comments on this answer, you should wrap this code in a function and have the return value borrow the input, so that you use the borrow checker as far as possible:
fn as_u8_slice(v: &[i32]) -> &[u8] {
unsafe {
std::slice::from_raw_parts(
v.as_ptr() as *const u8,
v.len() * std::mem::size_of::<i32>(),
)
}
}

Answer 2

Since Rust 1.30, the best solution is to use slice::align_to:
fn main() {
let v: Vec<i32> = vec![1; 8];
let (head, body, tail) = unsafe { v.align_to::<u8>() };
assert!(head.is_empty());
assert!(tail.is_empty());
println!("{:#x?}", body);
}
This properly handles the cases where the alignment of the first type and the second type do not match. In this example, I ensure that the alignment of the i32 is greater than that of the u8 via the assert! statements.
I took #swizards answer and ran with it a bit to get the other side of the coin - reading the vector back in:
use std::fs::File;
use std::io::{Read, Write};
use std::{mem, slice};
fn as_u8_slice(v: &[i32]) -> &[u8] {
let element_size = mem::size_of::<i32>();
unsafe { slice::from_raw_parts(v.as_ptr() as *const u8, v.len() * element_size) }
}
fn from_u8(v: Vec<u8>) -> Vec<i32> {
let data = v.as_ptr();
let len = v.len();
let capacity = v.capacity();
let element_size = mem::size_of::<i32>();
// Make sure we have a proper amount of capacity (may be overkill)
assert_eq!(capacity % element_size, 0);
// Make sure we are going to read a full chunk of stuff
assert_eq!(len % element_size, 0);
unsafe {
// Don't allow the current vector to be dropped
// (which would invalidate the memory)
mem::forget(v);
Vec::from_raw_parts(
data as *mut i32,
len / element_size,
capacity / element_size,
)
}
}
fn do_write(filename: &str, v: &[i32]) {
let mut f = File::create(filename).unwrap();
f.write_all(as_u8_slice(v)).unwrap();
}
fn do_read(filename: &str) -> Vec<i32> {
let mut f = File::open(filename).unwrap();
let mut bytes = Vec::new();
f.read_to_end(&mut bytes).unwrap();
from_u8(bytes)
}
fn main() {
let v = vec![42; 10];
do_write("vector.dump", &v);
let v2 = do_read("vector.dump");
assert_eq!(v, v2);
println!("{:?}", v2)
}