How Does `std::atomic` Ensure Atomic Operations in C Concurrency?

Understanding the Concept of std::atomic

Introduction

Concurrency in programming involves multiple threads executing simultaneously. To ensure data integrity and prevent race conditions, certain operations must be atomic, meaning they occur without interruption or interference from other threads. This is where the std::atomic<> comes into play.

Atomic at Which Level?

An atomic operation is one where the entire sequence of steps is indivisible. In C , std::atomic<> provides this guarantee. However, it's important to clarify that:

• Single operations on atomic objects are atomic: Assigning, loading, or storing values in an atomic object is guaranteed to be atomic.
• Compound operations are not necessarily atomic: Operations like "a = a 12" are not atomic because they involve multiple atomic operations (load, add, and store) in sequence.

Understanding Overloaded Operators and Atomic Operations

• Using = for atomic operations: Overloaded arithmetic operators like = allow for atomic operations. For example, "value = 5" is atomic.
• explicit atomic operations: Non-atomic operations can be made atomic using explicit forms like "value.fetch_add(5, std::memory_order_relaxed)".

Examining the Example

In the example "a = a 12", it is not one atomic operation. It involves:

1. Atomically loading the value of a (a.load()).
2. Performing addition outside of atomic context.
3. Atomically storing the result back in a (a.store()).

This is why using = is the preferred approach for atomic operations.

Conclusion

std::atomic<> encapsulates operations that are atomic across different threads. It provides precise control over synchronization and ordering constraints, allowing programmers to explicitly define the behavior of their code. This is crucial for establishing well-defined communication and data sharing among threads in concurrent systems. However, it's important to understand that while basic operations on atomic objects are atomic, compound operations may not be, unless overloaded operators or explicit atomic functions are used.

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