What is an Atomic Object in std::atomic?

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What is Atomic Object in std::atomic<>?

The std::atomic<> template provides a type that can be concurrently operated on by multiple threads without raising undefined behavior.

Atomicity of std::atomic<>

Each instantiation of std::atomic<> represents an atomic type that allows for simultaneous operations by different threads on its instances. Unlike regular C objects, atomic objects ensure exclusive access to their data, preventing data races.

Atomic Operations

Operations performed on atomic objects are atomic in nature. This means that:

• Changes to an atomic object's state are guaranteed to be visible to all threads.
• Accesses to atomic objects may establish inter-thread synchronization and order non-atomic memory accesses.

Memory Order and std::atomic<>

std::atomic<> provides precise control over synchronization and memory order. It supports various memory order options, including:

• std::memory_order_relaxed: No synchronization or ordering guarantees.
• std::memory_order_release: Ensures that writes are visible to other threads acquiring the same atomic variable.
• std::memory_order_acquire: Ensures that loads are visible after a release operation on the same atomic variable in other threads.
• std::memory_order_seq_cst: Provides sequential consistency (total global ordering) between all atomic operations.

Example: Arithmetic Operations

While individual arithmetic operators like = and are atomic operations, decomposing them into separate load, add, and store operations may not be atomic. For example:

a = a + 12;

This operation will involve the following steps:

1. Load a's value.
2. Add 12 to it.
3. Store the result back to a.

Steps 2 and 3 are not atomic, so there is a potential race condition where another thread could modify a between the add and the store.

Alternatives to Basic Arithmetic Operations

For typical use cases involving arithmetic operations, it's recommended to use the equivalent overloaded operators defined in std::atomic<>. These operators perform the entire operation atomically, ensuring sequential consistency.

Additional Resources on Atomicity and Memory Model

• C atomics and memory ordering
• Comparison: Lockless programming with atomics in C 11 vs. mutex and RW-locks
• C 11 introduced a standardized memory model. What does it mean? And how is it going to affect C programming?
• Concurrency in C 11

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