CODEWITHSUNDEEP
pointersrustlanguage-lawyer
orlp
orlp

Reputation: 117771

Is comparing pointers across allocations well-defined in Rust?

I have a pointer buf: *const T pointing to the start of an allocation of n elements of T, and I define the following check:

let in_alloc = buf <= ptr && ptr < unsafe { buf.add(n) };

Is it guaranteed that in_alloc is true for any ptr that lies in the allocation of buf, and false in any other case? We may assume that ptr is a valid pointer to a T object (so not misaligned/null/dangling), however it may or may not be from the same allocation as buf. Finally we may assume T is not zero-sized.

Upvotes: 12

Views: 833

Answers (3)

GManNickG
GManNickG

Reputation: 504083

Answering the title, comparing any two pointers is well-defined since pointers implement Ord.

With pointers being totally ordered, the body of the question follows easily from this. You have a set of n distinct pointers, starting at buf + 0 and ending at buf + (n - 1). If ptr is less than buf it can't be equal to any of them. If ptr is greater than buf + (n - 1) it also cannot be equal to them. If ptr is one of them, both expressions evaluate to true.

You can somewhat sidestep the issue and use Range instead:

let end = unsafe { buf.add(n) };
let in_alloc = (buf..end).contains(ptr);

This is often used, for example, to check if a slice contains a pointer.

Upvotes: 8

AnttiP
AnttiP

Reputation: 316

Yes, this code works as you would except. Comparing pointers that point to different allocations should behave as expected as implied by the documentation of std::ptr::eq.

Upvotes: 0

Diego Veralli
Diego Veralli

Reputation: 1052

According to the official documentation, it is valid to produce a raw pointer one byte past the allocation of the object the pointer points to (so +1). Obviously you cannot dereference the pointer, but it can be used for comparisons, eg. for bounds checking in a loop.

Beyond that, it's undefined behaviour, so you're not guaranteed anything at all. In your case adding an arbitrary offset to the pointer would not be a good idea for this reason.

So, to answer more specifically, it will return the value you expect as long as buf.add(n) points to an address that is at most 1 byte past the allocation of the object buf points to.

See https://doc.rust-lang.org/1.59.0/std/primitive.pointer.html#method.offset for more details.

Upvotes: 1

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