Stack overflow with heap buffer?

Question

I have the following code to read from a file:

let mut buf: Box<[u8]> = Box::new([0; 1024 * 1024]);
while let Ok(n) = f.read(&mut buf) {
    if n > 0 {
        resp.send_data(&buf[0..n]);
    } else {
        break;
    }
}

But it causes:

fatal runtime error: stack overflow

I am on OS X 10.11 with Rust 1.12.0.

What's the default size of your stack? Because `Box::new([0; 1024 * 1024])` will allocate 1MB on the stack prior to moving it to the box. — Matthieu M., Oct 14 '16 at 06:46
@MatthieuM. Ugh. Is there a way to allocate directly on the heap? — ElefEnt, Oct 14 '16 at 06:52
@ElefEnt: It's in the works ([placement_new_protocol](https://github.com/rust-lang/rust/issues/27779)). In the mean time, I advise creating a `Vec` and then converting it to a `Box` via [`into_boxed_slice`](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.into_boxed_slice). — Matthieu M., Oct 14 '16 at 06:56
For this particular case, using Vec or Box doesn't matter, so you could also just use Vec and not convert. — bluss, Oct 14 '16 at 09:14

score 5 · Accepted Answer · edited May 23 '17 at 12:08

5

As Matthieu said, Box::new([0; 1024 * 1024]) will currently overflow the stack due to initial stack allocation. If you are using Rust Nightly, the box_syntax feature will allow it to run without issues:

#![feature(box_syntax)]

fn main() {
    let mut buf: Box<[u8]> = box [0; 1024 * 1024]; // note box instead of Box::new()

    println!("{}", buf[0]);
}

You can find additional information about the difference between box and Box::new() in the following question: What the difference is between using the box keyword and Box::new?.

edited May 23 '17 at 12:08

Community

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answered Oct 14 '16 at 07:46

ljedrz

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Will it really overflow the stack? I mean, it's only 1MB, which is not THAT much. I know some OSes have tight limits by default, but I don't know if OS X is one of them. – Matthieu M. Oct 14 '16 at 08:37
Well, that is the error that is reported; perhaps it is bogus, but I'm not sure how to investigate it further. – ljedrz Oct 14 '16 at 08:47

Stack overflow with heap buffer?

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