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## Chapter 1. Boost.Atomic

### Helge Bahmann

### Andrey Semashev

Copyright © 2011 Helge Bahmann

Copyright © 2012 Tim Blechmann

Copyright © 2013, 2017, 2018, 2020-2022 Andrey Semashev

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

## Introduction

### Presenting Boost.Atomic

Boost.Atomic is a library that provides `atomic` data types and operations on these data types, as well as memory ordering constraints required for coordinating multiple threads through atomic variables. It implements the interface as defined by the C++11 standard, but makes this feature available for platforms lacking system/compiler support for this particular C++11 feature.

Users of this library should already be familiar with concurrency in general, as well as elementary concepts such as "mutual exclusion".

The implementation makes use of processor-specific instructions where possible (via inline assembler, platform libraries or compiler intrinsics), and falls back to "emulating" atomic operations through locking.

### Purpose

Operations on "ordinary" variables are not guaranteed to be atomic. This means that with `int n=0` initially, two threads concurrently executing

    void function()
    {
      n ++;
    }

might result in `n==1` instead of 2: Each thread will read the old value into a processor register, increment it and write the result back. Both threads may therefore write `1`, unaware that the other thread is doing likewise.

Declaring `atomic<int> n=0` instead, the same operation on this variable will always result in `n==2` as each operation on this variable is atomic: This means that each operation behaves as if it were strictly sequentialized with respect to the other.

Atomic variables are useful for two purposes:

- as a means for coordinating multiple threads via custom coordination protocols
- as faster alternatives to "locked" access to simple variables

Take a look at the examples section for common patterns.