C++ Concurrent Programming: How to use atomic classes and memory barriers?

C Concurrent programming: Use atomic classes and memory barriers to ensure concurrency safety

In a multi-threaded environment, concurrent programming deals with shared resources Common techniques. However, if appropriate measures are not taken, concurrent access can cause data races and memory visibility issues. To solve these problems, C provides atomic classes and memory barriers.

Atomic Class

Atomic class is a special class that encapsulates a basic type, ensuring that access to its instances is atomic even in a multi-threaded environment sex. This avoids data races when reading and writing shared variables.

Memory Barrier

A memory barrier is a special instruction used to force a program between different threads. They ensure that all memory accesses performed before the barrier are visible after the barrier. Four types of memory barriers are provided in C:

• memory_order_acquire: Disallows out-of-order access and ensures that all writes before the barrier are visible to all threads.
• memory_order_release: Disables out-of-order access and ensures that all reads after the barrier acquire all previous writes.
• memory_order_acq_rel: Combines the functions of memory_order_acquire and memory_order_release.
• memory_order_seq_cst: The strictest barrier to ensure all program order.

Practical Case

Consider the following example where two threads share a counter:

// 原子计数器
std::atomic<int> counter;

void thread1() {
    // ...
    counter.fetch_add(1, std::memory_order_release);
    // ...
}

void thread2() {
    // ...
    int value = counter.load(std::memory_order_acquire);
    // ...
}

In thread1 , the fetch_add operation uses the memory_order_release barrier to ensure that writes to the counter are visible in all threads. In thread2, the load operation uses the memory_order_acquire barrier to ensure that all previous pairs of counter## are acquired before reading the counter # writing. This eliminates data races and memory visibility issues.

Note

Memory barriers may reduce performance. Therefore, use them only when necessary. Additionally, always use

std::memory_order_seq_cst which guarantees the highest memory visibility, but it also has the highest performance overhead.

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