


Which C and C Types Exhibit Naturally Atomic Behavior on a 64-Bit x86-64 System?
Dec 14, 2024 am 07:40 AMWhich 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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