Collections

Eventually you'll want to use dynamic data structures (AKA collections) in your program. std provides a set of common collections: Vec, String, HashMap, etc. All the collections implemented in std use a global dynamic memory allocator (AKA the heap).

As core is, by definition, free of memory allocations these implementations are not available there, but they can be found in the alloc crate that's shipped with the compiler.

If you need collections, a heap allocated implementation is not your only option. You can also use fixed capacity collections; one such implementation can be found in the heapless crate.

In this section, we'll explore and compare these two implementations.

Using alloc

The alloc crate is shipped with the standard Rust distribution. To import the crate you can directly use it without declaring it as a dependency in your Cargo.toml file.

#![feature(alloc)]

extern crate alloc;

use alloc::vec::Vec;

To be able to use any collection you'll first need use the global_allocator attribute to declare the global allocator your program will use. It's required that the allocator you select implements the GlobalAlloc trait.

For completeness and to keep this section as self-contained as possible we'll implement a simple bump pointer allocator and use that as the global allocator. However, we strongly suggest you use a battle tested allocator from crates.io in your program instead of this allocator.

// Bump pointer allocator implementation

use core::alloc::{GlobalAlloc, Layout};
use core::cell::UnsafeCell;
use core::ptr;

use cortex_m::interrupt;

// Bump pointer allocator for *single* core systems
struct BumpPointerAlloc {
    head: UnsafeCell<usize>,
    end: usize,
}

unsafe impl Sync for BumpPointerAlloc {}

unsafe impl GlobalAlloc for BumpPointerAlloc {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        // `interrupt::free` is a critical section that makes our allocator safe
        // to use from within interrupts
        interrupt::free(|_| {
            let head = self.head.get();
            let size = layout.size();
            let align = layout.align();
            let align_mask = !(align - 1);

            // move start up to the next alignment boundary
            let start = (*head + align - 1) & align_mask;

            if start + size > self.end {
                // a null pointer signal an Out Of Memory condition
                ptr::null_mut()
            } else {
                *head = start + size;
                start as *mut u8
            }
        })
    }

    unsafe fn dealloc(&self, _: *mut u8, _: Layout) {
        // this allocator never deallocates memory
    }
}

// Declaration of the global memory allocator
// NOTE the user must ensure that the memory region `[0x2000_0100, 0x2000_0200]`
// is not used by other parts of the program
#[global_allocator]
static HEAP: BumpPointerAlloc = BumpPointerAlloc {
    head: UnsafeCell::new(0x2000_0100),
    end: 0x2000_0200,
};

Apart from selecting a global allocator the user will also have to define how Out Of Memory (OOM) errors are handled using the unstable alloc_error_handler attribute.

#![feature(alloc_error_handler)]

use cortex_m::asm;

#[alloc_error_handler]
fn on_oom(_layout: Layout) -> ! {
    asm::bkpt();

    loop {}
}

Once all that is in place, the user can finally use the collections in alloc.

#[entry]
fn main() -> ! {
    let mut xs = Vec::new();

    xs.push(42);
    assert!(xs.pop(), Some(42));

    loop {
        // ..
    }
}

If you have used the collections in the std crate then these will be familiar as they are exact same implementation.

Using heapless

heapless requires no setup as its collections don't depend on a global memory allocator. Just use its collections and proceed to instantiate them:

// heapless version: v0.4.x
use heapless::Vec;
use heapless::consts::*;

#[entry]
fn main() -> ! {
    let mut xs: Vec<_, U8> = Vec::new();

    xs.push(42).unwrap();
    assert_eq!(xs.pop(), Some(42));
    loop {}
}

You'll note two differences between these collections and the ones in alloc.

First, you have to declare upfront the capacity of the collection. heapless collections never reallocate and have fixed capacities; this capacity is part of the type signature of the collection. In this case we have declared that xs has a capacity of 8 elements that is the vector can, at most, hold 8 elements. This is indicated by the U8 (see typenum) in the type signature.

Second, the push method, and many other methods, return a Result. Since the heapless collections have fixed capacity all operations that insert elements into the collection can potentially fail. The API reflects this problem by returning a Result indicating whether the operation succeeded or not. In contrast, alloc collections will reallocate themselves on the heap to increase their capacity.

As of version v0.4.x all heapless collections store all their elements inline. This means that an operation like let x = heapless::Vec::new(); will allocate the collection on the stack, but it's also possible to allocate the collection on a static variable, or even on the heap (Box<Vec<_, _>>).

Trade-offs

Keep these in mind when choosing between heap allocated, relocatable collections and fixed capacity collections.

Out Of Memory and error handling

With heap allocations Out Of Memory is always a possibility and can occur in any place where a collection may need to grow: for example, all alloc::Vec.push invocations can potentially generate an OOM condition. Thus some operations can implicitly fail. Some alloc collections expose try_reserve methods that let you check for potential OOM conditions when growing the collection but you need be proactive about using them.

If you exclusively use heapless collections and you don't use a memory allocator for anything else then an OOM condition is impossible. Instead, you'll have to deal with collections running out of capacity on a case by case basis. That is you'll have deal with all the Results returned by methods like Vec.push.

OOM failures can be harder to debug than say unwrap-ing on all Results returned by heapless::Vec.push because the observed location of failure may not match with the location of the cause of the problem. For example, even vec.reserve(1) can trigger an OOM if the allocator is nearly exhausted because some other collection was leaking memory (memory leaks are possible in safe Rust).

Memory usage

Reasoning about memory usage of heap allocated collections is hard because the capacity of long lived collections can change at runtime. Some operations may implicitly reallocate the collection increasing its memory usage, and some collections expose methods like shrink_to_fit that can potentially reduce the memory used by the collection -- ultimately, it's up to the allocator to decide whether to actually shrink the memory allocation or not. Additionally, the allocator may have to deal with memory fragmentation which can increase the apparent memory usage.

On the other hand if you exclusively use fixed capacity collections, store most of them in static variables and set a maximum size for the call stack then the linker will detect if you try to use more memory than what's physically available.

Furthermore, fixed capacity collections allocated on the stack will be reported by -Z emit-stack-sizes flag which means that tools that analyze stack usage (like stack-sizes) will include them in their analysis.

However, fixed capacity collections can not be shrunk which can result in lower load factors (the ratio between the size of the collection and its capacity) than what relocatable collections can achieve.

Worst Case Execution Time (WCET)

If you are building time sensitive applications or hard real time applications then you care, maybe a lot, about the worst case execution time of the different parts of your program.

The alloc collections can reallocate so the WCET of operations that may grow the collection will also include the time it takes to reallocate the collection, which itself depends on the runtime capacity of the collection. This makes it hard to determine the WCET of, for example, the alloc::Vec.push operation as it depends on both the allocator being used and its runtime capacity.

On the other hand fixed capacity collections never reallocate so all operations have a predictable execution time. For example, heapless::Vec.push executes in constant time.

Ease of use

alloc requires setting up a global allocator whereas heapless does not. However, heapless requires you to pick the capacity of each collection that you instantiate.

The alloc API will be familiar to virtually every Rust developer. The heapless API tries to closely mimic the alloc API but it will never be exactly the same due to its explicit error handling -- some developers may feel the explicit error handling is excessive or too cumbersome.