Comments (5)
this would mainly occur when using unsafe code to turn the bytes buffer into a slice or something else
Ideally, such transformations would keep the lifetimes intact, e.g starting from a &'a Mmap
going through a &'a [u8]
should yield a &'a [T]
which could also not outlive the Mmap
itself.
from memmap2-rs.
Yes, Mmap
calls munmap(2)
on Drop
meaning that the memory mapping becomes invalid.
It should also not be possible to access it using safe code because it is access via impl Deref for Mmap
which means that the borrow cannot continue after the Mmap
is dropped?
from memmap2-rs.
Okay thanks! And yeah, this would mainly occur when using unsafe code to turn the bytes buffer into a slice or something else
from memmap2-rs.
I'm investigating whether I can avoid causing undefined behavior by creating a Vec<T>
from an mmap'd file. Currently looking at something like:
struct MemoryMappedVec<T> {
data: Option<Vec<T>>,
#[allow(unused)]
mmap: memmap2::Mmap,
}
impl<T> MemoryMappedVec<T> {
pub unsafe fn new(path: &str, length: usize) -> Self {
let file = std::fs::File::open(path).unwrap();
let mmap: memmap2::Mmap = unsafe { memmap2::Mmap::map(&file).unwrap() };
let bytes: &[u8] = &mmap;
let ptr = bytes.as_ptr() as *mut T;
assert!(bytes.len() < (isize::MAX as usize));
assert_eq!(bytes.len(), length * std::mem::size_of::<T>());
assert_eq!(ptr.align_offset(std::mem::align_of::<T>()), 0);
let data = unsafe { Vec::from_raw_parts(ptr, length, length) };
Self {
data: Some(data),
mmap,
}
}
}
impl<T> Drop for MemoryMappedVec<T> {
fn drop(&mut self) {
if let Some(data) = std::mem::take(&mut self.data) {
// Avoid dropping the Vec, since it wasn't actually allocated in the first place.
std::mem::forget(data);
}
}
}
I know this is insanely unsafe, especially the ownership of Vec stuff. I'm wondering in what situations I can get away with this.
from memmap2-rs.
I am not sure why you would want to do use a Vec<T>
if you can never hand it out anyway (because it cannot be dropped, which should also be ingrained into the type using ManuallyDrop
).
Why not just implement Deref<Target=[T]>
for your struct which could contain just the Mmap
as turning the &[u8]
into a &[T]
should not cost anything as long as the size, alignment, etc. is checked upfront?
from memmap2-rs.
Related Issues (20)
- Migrate to `safer_owning_ref` HOT 1
- why does memmap2::Mmap::map need to be unsafe? HOT 4
- Implement AsRawFd, IntoRawFd, AsRawHandle, IntoRawHandle and Into<Stdio> for MMap HOT 2
- Composite memory maps currently impossible HOT 7
- mmap'd anon pages fail in io_uring IOSQE_BUFFER_SELECT usage HOT 2
- advise_writes_unsafely_to_part_of_map test fails on powerpc64le-unknown-linux-gnu HOT 2
- error[E0433]: failed to resolve: could not find `Advice` in `memmap2` HOT 1
- PermissionDenied, message: "Access is denied." at remove file after read on Github Action WIndows HOT 2
- Doesn't compile for freebsd HOT 3
- v0.9.2 build failed for android target. HOT 3
- Support for `MS_INVALIDATE`? HOT 8
- Mac osx support HOT 1
- Support `mremap(2)` HOT 1
- Mapping beyond 4GB offset if broken on 32 bit glibc (Linux) HOT 3
- Support multiple advice HOT 2
- Contributing a migration from rustix to libc HOT 9
- MSRV and edition HOT 5
- Merging changes with memmapix HOT 24
- memmap2 `advise` is unsound HOT 8
- Interest is supporting mbind HOT 10
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from memmap2-rs.