What is Layout?

Question

I want to reimplement some stdlib smart pointers in rust (mainly Box) to learn it better, i come from a C background which has the simple malloc and free functions, but rust's alloc and dealloc need some Layout. What is it? It's not really explained in the docs.

Answer 1

As its documentation says, Layout describes a block of memory that is to be allocated or deallocated. In addition to memory size, it specifies alignment and optionally the trailing padding of the block.

C's malloc isn't told what alignment to use, so it has to assume worst-case alignment, possibly wasting memory. And even in C one sometimes needs memory of non-standard alignment, for which APIs outside of standard C had to be used until C11.

One final difference between Rust's and C's allocator interface is that Rust requires the layout to be passed to dealloc as well as to alloc - equivalent to C's free requiring the size that was passed to malloc. At first this sounds like a disadvantage because the user of the API must track the allocated size in order to be able to deallocate. But it turns out not to be an issue in practice because:

• when allocating a single value such as Box<T> or array Box<[T; n]>, the size is determined at compile-time and thus known when Box::drop is invoked.

• when allocating a dynamic array, such as Vec<T>, the capacity of the vector is tracked by the vector itself and thus also available in Vec::drop.

• when boxing a slice, such as Box<[T]>, the slice cannot be resized and its capacity is equal to its length and again known to Box::drop.

So it turns out that it is the C-style allocation API that results in storing redundant size information. Passing the size to dealloc eliminates the redundancy, providing opportunity to save space, especially for small blocks where the size information is a non-negligible percentage of the memory used.

When comparing Rust's allocation interface to that of C, keep in mind that in Rust raw allocation is much more of a specialized tool, used only to implement safe abstractions such as Box, Rc or Vec. Unlike C, where a programmer is routinely expected to invoke malloc and free, most Rust programmers never need to invoke the global allocator directly.