memoryorderseqcst

Memory order memory_order_seq_cst, commonly referred to as sequentially consistent order, is the strongest memory ordering defined for atomic operations in C++11 and later. It requires that all operations specified with this order participate in a single, global modification order that is consistent with the program order observed by each thread, effectively creating a universal history of seq_cst operations across the program.

Semantics and guarantees: An atomic operation performed with memory_order_seq_cst participates in a global total order of such operations. This order must be consistent with the happens-before relationships established by the code, and every thread must observe updates to seq_cst operations in a way that respects that global sequence. As a result, seq_cst provides strong correctness guarantees about visibility and ordering of atomic updates, while still ensuring atomicity of the individual operations.

Performance and usage considerations: memory_order_seq_cst is stronger than memory_order_relaxed, acquire, release, and acq_rel. Because it may require additional fences to maintain a single global order, it can incur more overhead on some platforms. In practice, compilers and hardware may optimize away some costs when the surrounding code makes the sequencing constraints provable, but seq_cst remains the default and most conservative option for atomic operations.

Default and scope: In C++, the default memory order for atomic operations is memory_order_seq_cst if no other order is specified. It applies only to atomic operations; non-atomic accesses remain subject to data races if shared without proper synchronization.

Limitations: memory_order_seq_cst does not eliminate races involving non-atomic data, nor does it guarantee ordering with non-atomic operations. It also cannot replace higher-level synchronization primitives when complex inter-thread coordination is required.