stdatomic

Stdatomic, as implemented in the C standard library header <stdatomic.h>, provides a portable interface for atomic operations on shared data in multi-threaded programs. It defines atomic types and a set of operations that guarantee atomicity and allow explicit memory ordering. The goal is to enable lock-free synchronization where supported by the platform, or to provide a well-defined fallback otherwise.

Its core concept is the atomic type, declared with the _Atomic qualifier or by the type aliases provided by the header (for example, atomic_bool, atomic_int, atomic_long, atomic_size_t, etc.). There is also a special atomic_type called atomic_flag designed for simple test-and-set use. Atomic objects can be initialized like regular objects, and some implementations provide atomic_init to initialize after declaration.

Operations include atomic_load and atomic_store for reading and writing, atomic_exchange to replace a value, and atomic_compare_exchange_weak and atomic_compare_exchange_strong for compare-and-swap semantics. Arithmetic and bitwise updates are provided by atomic_fetch_add, atomic_fetch_sub, atomic_fetch_or, atomic_fetch_xor, and atomic_fetch_and. Each operation can specify a memory_order parameter.

Memory ordering values include memory_order_relaxed, memory_order_acquire, memory_order_release, memory_order_acq_rel, and memory_order_seq_cst. Seq_cst is the strongest and provides a single total order; relaxed offers no synchronization guarantees beyond atomicity. The explicit order parameters allow fine-grained control over visibility and ordering between threads.

Portability and performance vary by platform. While many types are lock-free on common architectures, some operations may coerce to locks under certain conditions. Stdatomic.h is part of C11 and is complemented by atomic_thread_fence and atomic_signal_fence for ordering operations without data access.

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