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| layout | title | description | category | tags | |
|---|---|---|---|---|---|
| post | Understanding Heap Allocations in Rust | An introduction to the Rust memory model |
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There's an alchemy of distilling complex technical topics into articles and videos
that change the way programmers see the tools they interact with on a regular basis.
I knew what a linker was, but there's a staggering complexity to get from
from main() to an executable.
Rust programmers use the Box
type all the time, but there's a rich history of the Rust language itself wrapped up in
how special it is.
In a similar vein, I want you to look at code and understand memory; the complex choreography of processor, operating system, and program that frees you to focus on functionality far beyond frivolous book-keeping. The Rust compiler relieves a great deal of the cognitive burden associated with memory management, but let's make time to explore what's going on under the hood.
Let's learn a bit about allocating memory in Rust.
Table of Contents
This post is intended as both guide and reference material; we'll work to establish an understanding of the different memory types Rust makes use of, then summarize each section for easy citation in the future. To that end, a table of contents is provided to assist in easy navigation:
- Foreword
- Non-Heap Memory Types
- Piling On - the Heap in Rust
- Compiler Optimizations Make Everything Complicated
- Summary: When Does Rust Allocate?
- Appendix and Further Reading
Foreword
There's a simple way to guarantee you never need to know the content of this article:
- Only write
#![no_std]crates - Never use
unsafe - Never use
#![feature(alloc)]
For some uses of Rust, typically embedded devices, these constraints make sense. They're working with very limited memory, and the program binary size itself may affect the memory available! There's no operating system able to manage the heap, but that's not an issue because your program is likely the only one running. The embedonomicon is ever in mind, and you just might interact with extra peripherals by reading and writing to exact memory addresses.
Most Rust programs find these requirements overly burdensome though. C++ developers
would struggle without access to std::vector,
and Rust developers would struggle without std::vec.
But in this scenario, std::vec is actually part of the
alloc crate, and thus off-limits.
Or how would you use trait objects? Rust's monomorphization still works, but there's no
Box<dyn Trait>
available to use for dynamic dispatch.
Given a target audience of "basically every Rust developer," let's talk about
some of the details you don't normally have to worry about. This article will focus
on "safe" Rust only; unsafe mode allows you to make use of platform-specific
allocation APIs (think [libc] and [winapi] implementations of [malloc]) that
we'll ignore for the time being. We'll also assume a "debug" build of libraries
and applications (what you get with cargo run and cargo test) and address
"release" mode at the end (cargo run --release and cargo test --release).
Finally, a caveat: while the details are unlikely to change, the Rust docs include a warning worth repeating here:
Rust does not currently have a rigorously and formally defined memory model.
- the Rust docs
Distinguishing regions of memory
Rust and the Stack
Example: Why doesn't Vec::new() go to the allocator?
Questions:
- What is the "Push" instruction? Why do we like the stack?
- How does Rust allocate arguments to the function?
- How does Rust allocate variables created in the function but never returned?
- How does Rust allocate variables created in the function and returned?
- How do Option<> or Result<> affect structs?
- How are arrays allocated?
- Legal to pass an array as an argument?
Rust and the Heap
Example: How to trigger a heap allocation
Questions:
- Where do collection types allocate memory?
- Does a Box<> always allocate heap?
- Passing Box vs. genericizing/monomorphization
- Other pointer types? Do Rc<>/Arc<> force heap allocation?
Understanding compiler optimizations
Example: Compiler stripping out allocations of Box<>, Vec::push()
[libc]: CRATES.IO LINK [winapi]: CRATES.IO LINK [malloc]: MANPAGE LINK