cast_slice on a memory-mapped file fails on Windows but succeeds on Linux

[sebcrozet]

I have this test which creates an empty temporary file, memory-maps it, and cast it to a &[f64]:

#[test]
fn empty_mmap_cast_slice() {
    let f = tempfile::tempfile().unwrap();
    let mmap = unsafe { memmap2::Mmap::map(&f) }.unwrap();
    let s: &[f64] = cast_slice(&mmap);
    assert!(s.is_empty());
}

While it works fine on Linux, it fails on Windows with the following error:

thread 'array_mmap::empty_mmap_cast_slice' panicked at 'cast_slice>TargetAlignmentGreaterAndInputNotAligned', C:\Users\devel\.cargo\registry\src\github.com-1ecc6299db9ec823\bytemuck-1.12.3\src\internal.rs:32:3

Is that the expected behavior?

[Lokathor]

Well it's expected if the buffer isn't aligned for an f64.

I've got no idea what the minimum alignment of mmap is on Windows vs Linux, but perhaps Linux guarantees a higher alignment minimum.

[bonsairobo]

We could try looking at the raw pointer that we get from the mmap and see if it has the expected alignment.

[Lokathor]

yeah, and try_cast_slice is available if there's any risk of insufficient alignment.

[sebcrozet]

It looks like mmap on an empty file is giving an address equal to 0x01:

Mmap { ptr: 0x1, len: 0 }

Let’s close this then, that’s not an issue in bytemuck. Thanks!