Are Integer Reads and Writes Atomic in Multithreaded C ?

Atomic Access to Multi-Byte Integers in C Threads: A Nuanced Answer

In multi-threaded applications, coordinating access to shared data can be challenging. Consider the question of whether reads and writes of an integer in C are atomic.

The Issue

Imagine two threads: one updating and one reading an integer value. Typically, the order of reads and writes is unimportant for this statistic value. However, concerns arise about potential interruptions during write operations.

For example, consider a value initialized to 0x0000FFFF. If it's incremented to 0x00010000, is there a risk of an intermediate state of 0x0001FFFF that could cause issues?

The Answer: Architectural Considerations

The answer lies in the architecture of the system. On an IA32 processor, aligned memory writes are atomic. However, unaligned writes may behave differently based on the caching setup.

• Single L1 Cache Line: If the memory falls within the same L1 cache line, write operations are atomic.
• Split L1 Cache Line: If the memory is spread across multiple cache lines, there's a risk of interrupted writes.

The bus width between the CPU and RAM also affects atomic writes. For instance, a 16-bit aligned write on an 8086 processor was atomic, while the same write on an 8088 was not due to its 8-bit bus.

Volatile Variable Declaration

In C , it's crucial to declare shared variables as volatile. This prevents the compiler from optimizing them to assume they're never updated, ensuring correct behavior in multithreaded scenarios.

Conclusion

Determining the atomicity of integer reads and writes in C threads requires a careful consideration of the target architecture and caching mechanisms. In case of unaligned writes or systems with potential cache line splitting, synchronization mechanisms may be necessary to guarantee data integrity.

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