Rust by Example

Raw Pointers

Raw pointers * and references &T function similarly, but references are always safe because they are guaranteed to point to valid data due to the borrow checker. Dereferencing a raw pointer can only be done through an unsafe block.

fn main() {

let raw_p: *const u32 = &10;

unsafe {

assert!(*raw_p == 10);

}

}

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Calling Unsafe Functions

Some functions can be declared as unsafe, meaning it is the programmer's responsibility to ensure correctness instead of the compiler's. One example of this is std::slice::from_raw_parts which will create a slice given a pointer to the first element and a length.

use std::slice;

fn main() {

let some_vector = vec![1, 2, 3, 4];

let pointer = some_vector.as_ptr();

let length = some_vector.len();

unsafe {

let my_slice: &[u32] = slice::from_raw_parts(pointer, length);

assert_eq!(some_vector.as_slice(), my_slice);

}

}

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For slice::from_raw_parts, one of the assumptions which must be upheld is that the pointer passed in points to valid memory and that the memory pointed to is of the correct type. If these invariants aren't upheld then the program's behaviour is undefined and there is no knowing what will happen.