A read-modify-write operation reads an atomic object, applies a modification, and writes the result as one indivisible operation. Unlike a separate load followed by a store, it can synchronize on its read side with one thread and on its write side with others. section 19.1
C identifies several atomic operations as read-modify-write operations:
- atomic_exchange and atomic_compare_exchange_weak for _Atomic objects. section 19.1
- Compound assignments and equivalent interfaces, including increment and decrement operators for arithmetic _Atomic objects. section 19.1
- atomic_flag_test_and_set for atomic_flag. section 19.1
Selecting acquire-release semantics
An atomic counter can be incremented with atomic_fetch_add_explicit, passing memory_order_acq_rel as its consistency model. The explicit interface makes the synchronization choice visible instead of relying on the default sequentially consistent form. section 19.4
_Atomic(unsigned) at = 67;
...
if (atomic_fetch_add_explicit(&at, 1, memory_order_acq_rel)) {
...
}
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Inspect the complete counter fragment: it declares an atomic unsigned object and applies atomic_fetch_add_explicit with memory_order_acq_rel. section 19.4
