Do C/C Standards Guarantee Atomic Operations for Any Data Type on ARM and x86-64 Architectures?

ARM and x86-64: No Guaranteed Atomic Types

Contrary to the initial assumption, the C/C language standards provide no guarantees of atomic operations for any specific data type, even on 64-bit computers.

Atomic Access via Signals vs. Threads

It's crucial to distinguish between two notions of atomicity:

• Atomic with respect to signals: Ensures signal safety, allowing correct reads for the updated value after a signal interrupt within the same thread.
• Atomic with respect to threads: Ensures consistency of data across multiple threads, preventing corruption during concurrent access.

GCC Implementations on ARM and x86-64

While modern CPUs like ARMv8 and x86-64 may guarantee atomic access for certain operations, these guarantees are not reflected at the language level. GCC and other compilers can optimize code in ways that violate these hardware-level atomicities, as demonstrated by the following example:

volatile uint32_t x;

uint32_t foo(void) {
    return (x >> 8) & 0xffff;
}

Even though x is a 32-bit variable, GCC compiles foo as two separate 16-bit loads, which could result in a non-atomic read if x is modified concurrently.

Reliance on std::atomic or _Atomic

The only way to ensure atomic operations in C and C is to explicitly use the std::atomic (C ) or _Atomic (C) types, which provide the necessary synchronization mechanisms to guarantee thread-safe access.

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