Are C Integer Reads and Writes Always Atomic?

Are C Reads and Writes of an int Atomic?

The question pertains to the atomicity of integer read and write operations in C . Atomicity ensures that the operation completes as a single indivisible unit, ensuring that no interruptions occur during the process.

The answer to this question hinges on the architecture of the system. On processors such as IA32, aligned writes to ints are typically atomic. However, unaligned writes may not be atomic depending on the caching system employed. If the memory resides within a single cache line, atomicity is maintained; otherwise, it is not.

Additionally, the width of the bus between the CPU and RAM can influence atomicity. In the case of older processors like the 8086 with a 16-bit bus, aligned 16-bit writes were atomic, while the same operation was not guaranteed on the 8088 with an 8-bit bus.

For C/C programs, it's crucial to declare shared values as volatile. This prevents the optimizer from assuming that the variable will not be modified within a thread, ensuring that the actual value is always read.

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