Fetchandadd

Fetch-and-add (FAA) is an atomic read-modify-write operation that returns the value currently stored in a variable and then adds a specified delta to that variable, all in a single, indivisible step. The operation is designed to be atomic with respect to other concurrent accesses to the same variable, so no interleaving can observe a partially updated value.

Typical forms include getting the old value while adding delta (getAndAdd(delta)) or adding delta and then returning the new value (addAndGet(delta)). Languages and libraries expose these as atomic fetch_add or getAndAdd operations on atomic types. The exact memory ordering is configurable in most implementations; by default many provide sequential consistency, but weaker orders may be chosen for performance.

Hardware and language support: Many CPUs provide an atomic fetch-and-add instruction or can implement it with a compare-and-swap loop. Languages offer atomic types or built-in operations such as std::atomic::fetch_add in C++, or Java's AtomicInteger/getAndAdd. The operation is commonly used to implement counters, task schedulers, or index generation without locks.

Limitations and considerations: FAA modifies only one variable atomically and does not by itself synchronize across multiple variables. It is subject to overflow wrap-around for integral types. Memory ordering is important to avoid surprising visibility effects; in some cases weaker orders are sufficient, but they require careful reasoning. For complex synchronization, FAA may be combined with additional primitives (like barriers or other atomic operations) to guarantee broader correctness.