Rust pointer iteration issue

I try to iterate pointer from an allocated array.

Consider this example :

let layout = Layout::array::<u32>(5).unwrap();

let mut ptr = alloc(layout) as *mut u32;

let base = ptr;

ptr = ptr.add(0);
*ptr = 10;
ptr = ptr.add(1);
*ptr = 100;
ptr = ptr.add(2);
*ptr = 200;
ptr = ptr.add(3);
*ptr = 300;
ptr = ptr.add(4);
*ptr = 400;


let a = *base.add(0);
let b = *base.add(1);
let c = *base.add(2);
let d = *base.add(3);
let e = *base.add(4);

result is :

a : 10
b : 100
c : 0
d : 200
e : 0

What did I mistake, or is it a bug?

Thanks

>Solution :

You’re setting the following values:

• base.add(0) via ptr.add(0)
• base.add(1) via ptr.add(1)
• base.add(3) via ptr.add(2) (on the already modified pointer)
• and so on:

ptr = ptr.add(0); // total offset =  0
*ptr = 10;
ptr = ptr.add(1); // total offset =  1
*ptr = 100;
ptr = ptr.add(2); // total offset =  3 <--- whoops!
*ptr = 200;
ptr = ptr.add(3); // total offset =  6 <--- undefined behavior!
*ptr = 300;
ptr = ptr.add(4); // total offset = 10 <--- further undefined behavior
*ptr = 400;

You either need to keep the original pointer or only use ptr.add(1) to advance to the next location.