Module spsc

Fixed capacity Single Producer Single Consumer (SPSC) queue

Implementation based on https://www.codeproject.com/Articles/43510/Lock-Free-Single-Producer-Single-Consumer-Circular

Portability

This module requires CAS atomic instructions which are not available on all architectures (e.g. ARMv6-M (thumbv6m-none-eabi) and MSP430 (msp430-none-elf)). These atomics can be emulated however with portable-atomic, which is enabled with the cas feature and is enabled by default for thumbv6m-none-eabi and riscv32 targets.

Examples

• Queue can be used as a plain queue

use heapless::spsc::Queue;

let mut rb: Queue<u8, 4> = Queue::new();

assert!(rb.enqueue(0).is_ok());
assert!(rb.enqueue(1).is_ok());
assert!(rb.enqueue(2).is_ok());
assert!(rb.enqueue(3).is_err()); // full

assert_eq!(rb.dequeue(), Some(0));

• Queue can be split and then be used in Single Producer Single Consumer mode.

“no alloc” applications can create a &'static mut reference to a Queue – using a static variable – and then split it: this consumes the static reference. The resulting Consumer and Producer can then be moved into different execution contexts (threads, interrupt handlers, etc.)

use heapless::spsc::{Producer, Queue};

enum Event { A, B }

fn main() {
    let queue: &'static mut Queue<Event, 4> = {
        static mut Q: Queue<Event, 4> = Queue::new();
        unsafe { &mut Q }
    };

    let (producer, mut consumer) = queue.split();

    // `producer` can be moved into `interrupt_handler` using a static mutex or the mechanism
    // provided by the concurrency framework you are using (e.g. a resource in RTIC)

    loop {
        match consumer.dequeue() {
            Some(Event::A) => { /* .. */ },
            Some(Event::B) => { /* .. */ },
            None => { /* sleep */ },
        }
    }
}

// this is a different execution context that can preempt `main`
fn interrupt_handler(producer: &mut Producer<'static, Event, 4>) {

    // ..

    if condition {
        producer.enqueue(Event::A).ok().unwrap();
    } else {
        producer.enqueue(Event::B).ok().unwrap();
    }

    // ..
}

Benchmarks

Measured on a ARM Cortex-M3 core running at 8 MHz and with zero Flash wait cycles

-C opt-level	3
Consumer<u8>::dequeue	15
Queue<u8>::dequeue	12
Producer<u8>::enqueue	16
Queue<u8>::enqueue	14

• All execution times are in clock cycles. 1 clock cycle = 125 ns.
• Execution time is dependent of mem::size_of::<T>(). Both operations include one memcpy(T) in their successful path.
• The optimization level is indicated in the first row.
• The numbers reported correspond to the successful path (i.e. Some is returned by dequeue and Ok is returned by enqueue).

Structs

• Consumer
  A queue “consumer”; it can dequeue items from the queue NOTE the consumer semantically owns the head pointer of the queue
• Iter
  An iterator over the items of a queue
• IterMut
  A mutable iterator over the items of a queue
• Producer
  A queue “producer”; it can enqueue items into the queue NOTE the producer semantically owns the tail pointer of the queue
• Queue
  A statically allocated single producer single consumer queue with a capacity of N - 1 elements