diff --git a/src/ch19-01-unsafe-rust.md b/src/ch19-01-unsafe-rust.md index fad896b55..d2b29264a 100644 --- a/src/ch19-01-unsafe-rust.md +++ b/src/ch19-01-unsafe-rust.md @@ -310,16 +310,16 @@ mutable slice to `i32` values, `as_mut_ptr` returns a raw pointer with the type We keep the assertion that the `mid` index is within the slice. Then we get to the unsafe code: the `slice::from_raw_parts_mut` function takes a raw pointer and a length, and it creates a slice. We use this function to create a slice -that starts from `ptr` and is `mid` items long. Then we call the `offset` +that starts from `ptr` and is `mid` items long. Then we call the `add` method on `ptr` with `mid` as an argument to get a raw pointer that starts at `mid`, and we create a slice using that pointer and the remaining number of items after `mid` as the length. The function `slice::from_raw_parts_mut` is unsafe because it takes a raw -pointer and must trust that this pointer is valid. The `offset` method on raw +pointer and must trust that this pointer is valid. The `add` method on raw pointers is also unsafe, because it must trust that the offset location is also a valid pointer. Therefore, we had to put an `unsafe` block around our calls to -`slice::from_raw_parts_mut` and `offset` so we could call them. By looking at +`slice::from_raw_parts_mut` and `add` so we could call them. By looking at the code and by adding the assertion that `mid` must be less than or equal to `len`, we can tell that all the raw pointers used within the `unsafe` block will be valid pointers to data within the slice. This is an acceptable and