Memory consistency is a crucial concept in programming, especially in concurrent systems where multiple threads access shared data simultaneously. C++ provides memory consistency properties to define the order and visibility of memory operations in multithreaded programs. Understanding these properties is essential for writing correct and efficient concurrent code.

Sequential Consistency

Sequential consistency is the default memory consistency model in C++. It guarantees that the result of any execution of a program will be the same as if the operations by all threads were executed serially in some order, preserving the program order for each individual thread.

In simpler terms, sequential consistency ensures that each thread’s operations are executed in the order they appear in the code, and the results of those operations are visible to all threads in the same order.

Acquire-Release Consistency

Acquire-Release consistency provides a weaker form of memory consistency compared to sequential consistency. It is typically used in scenarios where the programmer needs to explicitly define the ordering of memory operations between threads.

In Acquire-Release consistency, certain synchronization operations are used to enforce memory ordering. An “acquire” operation ensures all the memory operations preceding it are visible to the current thread before any subsequent memory operations. Conversely, a “release” operation ensures that all the memory operations following it are visible to other threads after performing the release.

Example Code

To illustrate how memory consistency properties work in C++, consider the following example:

#include <atomic>
#include <thread>
#include <iostream>

std::atomic<int> x;
int y;

void threadFunction()
{
    y = 42;
    x.store(1, std::memory_order_release);
}

int main()
{
    x.store(0, std::memory_order_release);
    std::thread t(threadFunction);
    t.join();

    int val = x.load(std::memory_order_acquire);
    std::cout << "Value of x: " << val << std::endl;
    std::cout << "Value of y: " << y << std::endl;

    return 0;
}

In this code, we have two variables x and y, where x is an atomic variable and y is a regular integer. The threadFunction is executed in a separate thread and modifies both variables.

By using the std::memory_order_acquire and std::memory_order_release memory ordering directives, we enforce the acquire-release consistency. The x.store and x.load operations use these directives to define the memory ordering semantics.

When the code is executed, we can expect the value of x to be 1 and y to be 42, ensuring proper synchronization and memory visibility between threads.

#memoryconsistency #c++