What is the role of memory barriers in C++ multi-threaded programming?

In C++ multi-threaded programming, the role of memory barriers is to ensure the consistency of data between threads. It prevents data races by forcing threads to execute in the expected order. C++ provides memory barriers such as sequential consistency barriers, acquire/release barriers, and consume/relaxed barriers. By adding memory barriers to your code, you can prevent data races and ensure correct data consistency between threads.

The role of memory barrier in C++ multi-threaded programming

In C++ multi-threaded programming, the memory barrier is a special piece of code Sequence, designed to ensure data consistency between threads. When multiple threads access shared data at the same time, data races may occur, leading to data corruption or program crashes. Memory barriers prevent data races by forcing threads to execute in the expected order.

Types of memory barriers

C++ provides multiple types of memory barriers:

• Sequential consistency barrier ( std::memory_order_seq_cst): Ensure that all threads see the data in memory in the same order.
• acquire/release barrier (std::memory_order_acquire/release): The acquire barrier ensures that all data has been loaded at execution time, while the release barrier ensures that all data has been stored after execution.
• Other barriers (std::memory_order_consume/relaxed): The consume barrier is used to prohibit optimization, and the relaxed barrier is not guaranteed and is mainly used for debugging purposes.

Practical case

Consider the following sample code:

int shared_value = 0;

void thread1() {
  // 从共享变量中读取
  int value = shared_value;
  
  // 使用该值进行计算
  value += 1;

  // 将更新后的值写入共享变量
  shared_value = value;
}

void thread2() {
  // 将新的值写入共享变量
  shared_value = 10;
}

If thread 1 and thread 2 execute concurrently, it is possible to read and write data will overlap, causing thread 1 to see the old value of the shared variable before writing the new value. To prevent this, you can add memory barriers to your code.

After adding the sequential consistency barrier, the code will:

void thread1() {
  // 从共享变量中读取
  int value = shared_value;
  
  // 使用该值进行计算
  value += 1;

  // 强制线程按顺序执行
  std::atomic_thread_fence(std::memory_order_seq_cst);

  // 将更新后的值写入共享变量
  shared_value = value;
}

void thread2() {
  // 强制线程按顺序执行
  std::atomic_thread_fence(std::memory_order_seq_cst);

  // 将新的值写入共享变量
  shared_value = 10;
}

The sequential consistency barrier ensures that thread 1 loads the latest value of the shared variable before updating the shared variable, and that thread 2 is writing to the shared variable Before waiting for thread 1 to complete the calculation. This prevents data races and ensures correct data consistency between threads.

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