Does Volatile Truly Solve Multithreading Synchronization Problems?

The Inherent Limitations of Volatile in Multithreaded Programming

In the realm of multithreaded programming, synchronizing access to shared data is crucial. One approach involves utilizing the volatile keyword to ensure the preservation of variables' values. However, a recent discussion has raised concerns regarding the efficacy of volatile in such contexts.

What is Volatile?

Volatile declares variables that should not be optimized out of a program. This ensures that the compiler doesn't cache the value in a register but rather fetches it from memory every time it's accessed. It was initially intended for use with hardware registers or I/O operations but has often been employed in multithreaded programming situations.

The Challenges of Volatile in Multithreading

Unfortunately, volatile has limitations when used in multithreaded contexts. While it guarantees that reads and writes to volatile variables happen immediately and in the correct order relative to other volatile accesses, it doesn't prevent reordering of non-volatile memory accesses around volatile ones.

Consider a global variable foo declared as volatile, shared by multiple threads. One thread sets foo atomically, while another reads it. The volatile declaration ensures that the write and read operations are not optimized out. However, the compiler may still reorder other memory operations, such as loading non-volatile variables, relative to the volatile operations.

The Solution: Memory Barriers and Atomic Variables

To prevent reordering, memory barriers are required. They instruct the compiler and CPU to ensure that no memory access is reordered across the barrier. Placing a memory barrier after a write to a volatile variable prevents the reordering of subsequent non-volatile reads relative to the volatile write.

However, since memory barriers also guarantee that all pending reads and writes are executed, they essentially provide the same functionality as volatile. Therefore, the volatile keyword becomes unnecessary when using memory barriers.

Modern Alternatives in C : Atomic Variables

In C 11, atomic variables (std::atomic) provide a comprehensive solution for these issues. They combine the guarantees of both volatile and memory barriers, eliminating the need for explicit volatile declarations and ensuring correct synchronization of shared data in multithreaded programs.

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