Difference between &[u8;3] and *const u8?

I'm wondering what's the difference between
let a = b"123\0";
let b = b"123\0".as_ptr()
Originally I thought one is a slice (a) which includes both length and pointer and one is just a pointer (b).
But in practice when I pass both options to an unsafe c function that prints a null terminated string they both work the same. Also running std::mem::size_of::<&[u8;3]>() and std::mem::size_of::<*const u8>() both return 8, so they both only contain a pointer and not a size. How can this be?

The only difference is in what the compiler allows you to do with the value: access to &[u8;3] (from Rust code) will produce a compile error if the backing data has been deallocated and a runtime panic if indexed outside the range 0..3.

The *const u8 bypasses all of these checks, so misuse can potentially read uninitialized or sensitive data, potentially causing undesired behavior like security leaks or program crashes.


So is it true to say that rust slices are compile time creatures, and that a slice variable's "length" is fixed?

Not quite. &[u8;3] is a reference to an array, and Rust arrays are always fixed-length (known at compile-time).

&[u8] is a reference to a slice; it carries runtime length information alongside the pointer to its contents. This lets it be different lengths during different runs of the same program.


std::mem::size_of::<&[u8]>() really returns 16.
Thanks that explains it.
Although I'm still a bit surprised a reference to something can have different sizes. What I mean is that a reference to a slice has a different size than a reference to an array or just a plain unsafe pointer.

Within your program, all accesses to the array will be checked at compile-time, i.e. there's no need to keep the information of the length at runtime.

Yes, that’s right. The other case when the reference can be a “fat pointer” is when it is a reference to a trait object, in that case rather than a length, the second half of the fat pointer is a second pointer to a vtable for looking up methods.



This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.