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MIT License
Copyright (c) 2023 Djordje Nedic
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

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# lockfree
![CMake](https://github.com/DNedic/lockfree/actions/workflows/.github/workflows/cmake.yml/badge.svg)
`lockfree` is a collection of lock-free data structures written in standard C++11 and suitable for all platforms - from deeply embedded to HPC.
## What are lock-free data structures?
Lock-free data structures are data structures that are thread and interrupt safe for concurrent use without having to use mutual exclusion mechanisms. They are most useful for inter process communication, and often scale much better than lock-based structures with the number of operations and threads.
## Why use `lockfree`
* Written in standard C++11, compatible with all platforms supporting it
* All data structures are thread and interrupt safe in their respective usecases
* No dynamic allocation
* Optimized for high performance
* MIT Licensed
* Additional APIs for newer C++ versions
## What data structures are available?
### Single-producer single-consumer data structures
* [Queue](docs/spsc/queue.md) - Best for single element operations, extremely fast, simple API consisting of only 2 methods.
* [Ring Buffer](docs/spsc/ring_buf.md) - A more general data structure with the ability to handle multiple elements at a time, uses standard library copies making it very fast for bulk operations.
* [Bipartite Buffer](docs/spsc/bipartite_buf.md) - A variation of the ring buffer with the ability to always provide linear space in the buffer, enables in-buffer processing.
* [Priority Queue](docs/spsc/priority_queue.md) - A Variation of the queue with the ability to provide different priorities for elements, very useful for things like signals, events and communication packets.
These data structures are more performant and should generally be used whenever there is only one thread/interrupt pushing data and another one retrieving it.
### Multi-producer multi-consumer data structures
* [Queue](docs/mpmc/queue.md) - Best for single element operations, extremely fast, simple API consisting of only 2 methods.
* [Priority Queue](docs/mpmc/priority_queue.md) - A Variation of the queue with the ability to provide different priorities for elements, very useful for things like signals, events and communication packets.
These data structures are more general, supporting multiple producers and consumers at the same time, however they have storage and performance overhead compared to single producer single consumer data structures. They also require atomic instructions which can be missing from some low-end microcontrollers.
## How to get
There are three main ways to get the library:
* Using CMake [FetchContent()](https://cmake.org/cmake/help/latest/module/FetchContent.html)
* As a [git submodule](https://git-scm.com/book/en/v2/Git-Tools-Submodules)
* By downloading a release from GitHub
## Configuration
`lockfree` uses cacheline alignment for indexes to avoid the [False Sharing](https://en.wikipedia.org/wiki/False_sharing) phenomenon by default, avoiding the performance loss of cacheline invalidation on cache coherent systems. This aligns the indexes to ```LOCKFREE_CACHELINE_LENGTH```, ```64``` by default.
On embedded systems, ```LOCKFREE_CACHE_COHERENT``` should almost always be set as ```false``` to avoid wasting memory.
Additionally, some systems have a non-typical cacheline length (for instance the apple M1/M2 CPUs have a cacheline length of 128 bytes), and ```LOCKFREE_CACHELINE_LENGTH``` should be set accordingly in those cases.
## Known limitations
All of the data structures in `lockfree` are only meant to be used for [trivial](https://en.cppreference.com/w/cpp/language/classes#Trivial_class) types.
## FAQ
### Why would I use this over locking data structures on a hosted machine?
The biggest reason you would want to use a lock-free data structure on hosted environments would be avoiding issues surrounding locking such as deadlocks, priority inversion and nondeterministic access latency. When used properly, lock-free data structures can also improve performance in some scenarios.
Additionally, `lockfree` provides a way to build applications and libraries that can be compiled to work on both POSIX and non-POSIX environments without `#ifdef`s or polymorphism.
### Why use this over RTOS-provided IPC mechanisms on an embedded system?
While locking usually isn't expensive on embedded systems such as microcontrollers, there is a wide variety of RTOS-es and no standardized API for locking. The fact that multiple architectures are present from 8051 to RISC-V means that interrupt management methods are not standardized either.
`lockfree` provides a way to build portable embedded code with a negligible performance cost as opposed to locking, code using `lockfree` can be compiled to run on any embedded platform supporting C++11. Additionally, the code can easily be tested on a host machine without the need for mocking.
### What advantages does using C++ over C provide for the library?
* Type safety, as data structures are type and size templated
* Much simpler and less error-prone instantiation
* Higher performance due to compile-time known size and header-only implementation
* Encapsulation, the data buffer is a class member instead of being passed by a pointer
### What is the formal classification of the data structures in `lockfree`?
All structures in `lockfree` are **bounded**, **array-based** and **lock-free**, spsc data structures are also **waitfree** and **termination safe**.
## Theory and references
For more insight into lock-free programming, take a look at:
* This [brilliant talk series](https://youtu.be/c1gO9aB9nbs) from Herb Sutter
* [Live Lock-Free or Deadlock](https://youtu.be/lVBvHbJsg5Y) talk series from Fedor Pikus
* Dmitry Vyukov's excellent [blog](https://www.1024cores.net/home/lock-free-algorithms/introduction)

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/**************************************************************
* @file queue.hpp
* @brief A queue implementation written in standard c++11
* suitable for both low-end microcontrollers all the way
* to HPC machines. Lock-free for single consumer single
* producer scenarios.
**************************************************************/
/**************************************************************
* Copyright (c) 2023 Djordje Nedic
*
* Permission is hereby granted, free of charge, to any person
* obtaining a copy of this software and associated
* documentation files (the "Software"), to deal in the Software
* without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense,
* and/or sell copies of the Software, and to permit persons to
* whom the Software is furnished to do so, subject to the
* following conditions:
*
* The above copyright notice and this permission notice shall
* be included in all copies or substantial portions of the
* Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
* KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
* WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
* PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
* COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
* OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*
* This file is part of lockfree
*
* Author: Djordje Nedic <nedic.djordje2@gmail.com>
* Version: v2.0.8
**************************************************************/
/************************** INCLUDE ***************************/
#ifndef LOCKFREE_QUEUE_HPP
#define LOCKFREE_QUEUE_HPP
#include <atomic>
#include <cstddef>
#include <type_traits>
#if __cplusplus >= 201703L || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
#include <optional>
#endif
namespace lockfree {
namespace spsc {
/*************************** TYPES ****************************/
template <typename T, size_t size> class Queue {
static_assert(std::is_trivial<T>::value, "The type T must be trivial");
static_assert(size > 2, "Buffer size must be bigger than 2");
/********************** PUBLIC METHODS ************************/
public:
Queue();
/**
* @brief Adds an element into the queue.
* Should only be called from the producer thread.
* @param[in] element
* @retval Operation success
*/
bool Push(const T &element);
/**
* @brief Removes an element from the queue.
* Should only be called from the consumer thread.
* @param[out] element
* @retval Operation success
*/
bool Pop(T &element);
#if __cplusplus >= 201703L || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
/**
* @brief Removes an element from the queue.
* Should only be called from the consumer thread.
* @retval Either the element or nothing
*/
std::optional<T> PopOptional();
#endif
/********************** PRIVATE MEMBERS ***********************/
private:
T _data[size]; /**< Data array */
#if LOCKFREE_CACHE_COHERENT
alignas(LOCKFREE_CACHELINE_LENGTH) std::atomic_size_t _r; /**< Read index */
alignas(
LOCKFREE_CACHELINE_LENGTH) std::atomic_size_t _w; /**< Write index */
#else
std::atomic_size_t _r; /**< Read index */
std::atomic_size_t _w; /**< Write index */
#endif
};
} /* namespace spsc */
} /* namespace lockfree */
/************************** INCLUDE ***************************/
/* Include the implementation */
#include "queue_impl.hpp"
#endif /* LOCKFREE_QUEUE_HPP */

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/**************************************************************
* @file queue_impl.hpp
* @brief A queue implementation written in standard c++11
* suitable for both low-end microcontrollers all the way
* to HPC machines. Lock-free for single consumer single
* producer scenarios.
**************************************************************/
/**************************************************************
* Copyright (c) 2023 Djordje Nedic
*
* Permission is hereby granted, free of charge, to any person
* obtaining a copy of this software and associated
* documentation files (the "Software"), to deal in the Software
* without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense,
* and/or sell copies of the Software, and to permit persons to
* whom the Software is furnished to do so, subject to the
* following conditions:
*
* The above copyright notice and this permission notice shall
* be included in all copies or substantial portions of the
* Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
* KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
* WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
* PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
* COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
* OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*
* This file is part of lockfree
*
* Author: Djordje Nedic <nedic.djordje2@gmail.com>
* Version: v2.0.8
**************************************************************/
namespace lockfree {
namespace spsc {
/********************** PUBLIC METHODS ************************/
template <typename T, size_t size> Queue<T, size>::Queue() : _r(0U), _w(0U) {}
template <typename T, size_t size> bool Queue<T, size>::Push(const T &element) {
/*
The full check needs to be performed using the next write index not to
miss the case when the read index wrapped and write index is at the end
*/
const size_t w = _w.load(std::memory_order_relaxed);
size_t w_next = w + 1;
if (w_next == size) {
w_next = 0U;
}
/* Full check */
const size_t r = _r.load(std::memory_order_acquire);
if (w_next == r) {
return false;
}
/* Place the element */
_data[w] = element;
/* Store the next write index */
_w.store(w_next, std::memory_order_release);
return true;
}
template <typename T, size_t size> bool Queue<T, size>::Pop(T &element) {
/* Preload indexes with adequate memory ordering */
size_t r = _r.load(std::memory_order_relaxed);
const size_t w = _w.load(std::memory_order_acquire);
/* Empty check */
if (r == w) {
return false;
}
/* Remove the element */
element = _data[r];
/* Increment the read index */
r++;
if (r == size) {
r = 0U;
}
/* Store the read index */
_r.store(r, std::memory_order_release);
return true;
}
/********************* std::optional API **********************/
#if __cplusplus >= 201703L || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
template <typename T, size_t size>
std::optional<T> Queue<T, size>::PopOptional() {
T element;
bool result = Pop(element);
if (result) {
return element;
} else {
return {};
}
}
#endif
} /* namespace spsc */
} /* namespace lockfree */