Crate rel_ptr

rel-ptr

rel-ptr a library for relative pointers, which can be used to create moveable self-referential types. This library was inspired by Johnathan Blow’s work on Jai, where he added relative pointers as a primitive into Jai.

A relative pointer is a pointer that uses an offset and it’s current location to calculate where it points to.

Safety

See the RelPtr type docs for safety information

Features

no_std

This crate is no-std compatible, simply add the feature no_std to move into no_std mode.

nightly

with nightly you get the ability to use trait objects with relative pointers

Example

take the memory segment below

[.., 0x3a, 0x10, 0x02, 0xe4, 0x2b ..]

where 0x3a has the address 0xff304050 (32-bit system) then 0x2b has the address 0xff304054.

if we have a 1-byte relative pointer (RelPtr<_, i8>) at the address 0xff304052, then that relative pointer points to 0x2b as well, this is because its address 0xff304052, plus its offset, 0x02 points to 0x2b.

There are three interesting things about this

1. it only took 1 byte to point to another value,
2. a relative pointer cannot access all memory, only memory near it
3. if both the relative pointer and the pointee move together, then the relative pointer will not be invalidated

The third point is what makes moveable self-referential structs possible

The type RelPtr<T, I> is a relative pointer. T is what it points to, and I is what it uses to store its offset. In practice you can ignore I, which is defaulted to isize, because that will cover all of your cases for using relative pointers. But if you want to optimize the size of the pointer, you can use any type that implements Delta. Some types from std that do so are: i8, i16, i32, i64, i128, and isize. Note that the trade off is that as you decrease the size of the offset, you decrease the range to which you can point to. isize will cover at least half of addressable memory, so it should work unless you do something really crazy. For self-referential structs use a type whose max value is atleast as big as your struct. i.e. std::mem::size_of::<T>() <= I::max_value().

Note on usized types: these are harder to get working

Self Referential Type Example

struct SelfRef {
    value: (String, u32),
    ptr: RelPtr<String, i8>
}

impl SelfRef {
    pub fn new(s: String, i: u32) -> Self {
        let mut this = Self {
            value: (s, i),
            ptr: RelPtr::null()
        };

        this.ptr.set(&mut this.value.0).unwrap();

        this
    }

    pub fn fst(&self) -> &str {
        unsafe { self.ptr.as_ref_unchecked() }
    }

    pub fn snd(&self) -> u32 {
        self.value.1
    }
}

let s = SelfRef::new("Hello World".into(), 10);

assert_eq!(s.fst(), "Hello World");
assert_eq!(s.snd(), 10);

let s = Box::new(s); // force a move, note: relative pointers even work on the heap

assert_eq!(s.fst(), "Hello World");
assert_eq!(s.snd(), 10);

This example is contrived, and only useful as an example. In this example, we can see a few important parts to safe moveable self-referential types, lets walk through them.

First, the definition of SelfRef, it contains a value and a relative pointer, the relative pointer that will point into the tuple inside of SelfRef.value to the String. There are no lifetimes involved because they would either make SelfRef immovable, or they could not be resolved correctly.

We see a pattern inside of SelfRef::new, first create the object, and use the sentinel RelPtr::null() and immediately afterwards assigning it a value using RelPtr::set and unwraping the result. This unwrapping is get quick feedback on whether or not the pointer was set, if it wasn’t set then we can increase the size of the offset and resolve that.

Once the pointer is set, moving the struct is still safe because it is using a relative pointer, so it doesn’t matter where it is, only it’s offset from its pointee. In SelfRef::fst we use RelPtr::as_ref_unchecked because it is impossible to invalidate the pointer. It is impossible because we cannot set the relative pointer directly, and we cannot change the offsets of the fields of SelfRef after the relative pointer is set.

Structs

• IntegerDeltaError
  If an integer’s range is too small to store an offset, then this error is generated
• RelPtr
  This represents a relative pointers

Traits

• Delta
  Delta trait generalizes differences in memory locations to types like i8 and i16
• MetaData
  A trait to abstract over the sizedness of types, and to access metadata about a type
• Nullable
  A index which can contain null

Type Aliases

• Ptr
  A nullable pointer, using NonNull