Which C and C Types Exhibit Naturally Atomic Behavior on a 64-Bit x86-64 System?

Which C and C Types Are Naturally Atomic on a 64-Bit Computer?

Unlike language standards, GNU C and GNU C compilers treat atomicity based on specific architectures. While C11 and C 11 introduce _Atomic types and std::atomic<> types respectively, this article focuses solely on naturally atomic reads and writes, excluding atomic increment, decrement, or compound assignment.

Atomicity Guarantees on 64-Bit Linux Computers

On a 64-bit Linux computer with an x86-64 processor, the following types have naturally atomic reads and writes:

• int
• _Atomic_word

However, it's important to note that even these types are not definitively automatically atomic according to the language standards.

Understanding Atomicity

There are two main senses of "atomic":

• Atomic with respect to signals: Ensures that signal handlers invoked during different instruction execution will only see the old or new value of a volatile sig_atomic_t variable.
• Atomic with respect to threads: Guarantees that concurrent access to an object will result in a correct value being seen by all threads. To achieve this, _Atomic or std::atomic types are necessary.

Compiler Optimizations and Atomicity

Just because a type is naturally atomic at the hardware level does not mean that the compiler will always use atomic instructions to access it. Optimizations may lead to non-atomic access, even for data types that are known to be atomic on the target hardware.

For example, a load from a 32-bit integer on x86 is atomic, but a compiler may use a 16-bit partial load or store that is not guaranteed to be atomic.

Conclusion

In summary, there are no types in C or C that are definitively automatically atomic on a 64-bit computer. To ensure atomic access, it is crucial to use _Atomic or std::atomic types or rely on documentation to verify atomic guarantees for specific architectures and compilers.

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