CODEWITHSUNDEEP
rust
Poperton
Poperton

Reputation: 1796

How to create *mut u8 from Rust string?

I'm new to Rust. I want to create a mutable u8 buffer.

I tried

let mut str = "hello";
let r = str as *mut u8;

but I get

error[E0606]: casting `&str` as `*mut u8` is invalid

I want to know how but also why. What is the type of "hello"? How does casting works on Rust?

Upvotes: 3

Views: 3280

Answers (2)

L. F.
L. F.

Reputation: 20649

There is a method str::as_mut_ptr that does exactly this.

let mut str = "hello";
let r = str.as_mut_ptr(); // r: *mut u8

However, you are responsible for upholding the safety invariants of str if you proceed to use r. Quoting from the documentation (emphasis mine):

Converts a mutable string slice to a raw pointer.

As string slices are a slice of bytes, the raw pointer points to a u8. This pointer will be pointing to the first byte of the string slice.

It is your responsibility to make sure that the string slice only gets modified in a way that it remains valid UTF-8.

Upvotes: 0

Ikechukwu Eze
Ikechukwu Eze

Reputation: 3171

First: str doesn't need to be mutable because you are not mutating it. Second: "hello" is of type &str (string slice). Third: You can convert &str to &[u8] by calling as_bytes() on it, which is the buffer you needed. Appending mut before the variable name makes it mutable.

   let str = "hello";
   let mut r = str.as_bytes();
   println!("{:?}", r);

Upvotes: 3

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