atomicthreadfence

Atomic_thread_fence, often referred to as a thread fence, is a memory synchronization primitive available in the C11/C++11 memory model. It provides a memory barrier that affects the ordering of memory operations across threads without performing any atomic load or store itself. The function is declared as atomic_thread_fence(memory_order order) in C11 (via stdatomic.h) or std::atomic_thread_fence(memory_order order) in C++11 (via <atomic>).

The single parameter, memory_order order, selects the strength of the barrier. Supported values include memory_order_relaxed, memory_order_acquire, memory_order_release, memory_order_acq_rel, and memory_order_seq_cst. An acquire fence prevents memory reads and writes after the fence from being reordered before it. A release fence prevents memory writes before the fence from being reordered after it. An acquire-release fence combines both capabilities. A memory_order_seq_cst fence participates in the global sequentially consistent order with other seq_cst operations. A relaxed fence provides minimal synchronization guarantees and is primarily concerned with local compiler barriers rather than cross-thread ordering.

Typical use cases involve establishing happens-before relationships between threads without performing atomic operations themselves. For example, after updating a shared data structure with release semantics, a subsequent fence can help ensure that other threads observing a synchronization variable with an acquire or seq_cst operation see the updated state in a well-defined order.

Support for atomic_thread_fence is widespread across modern compilers and platforms that implement C11/C++11 atomics. It is a fundamental tool for low-level lock-free programming and careful memory ordering without introducing additional atomic operations. See also memory_order and std::atomic_thread_fence.

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