Multi-thread synchronization problems and solutions in C++

Multi-thread synchronization problems and solutions in C

Multi-thread programming is a way to improve program performance and efficiency, but it also brings a series of synchronization issues. In multi-threaded programming, multiple threads may access and modify shared data resources at the same time, which may lead to data race conditions, deadlocks, starvation and other problems. To avoid these problems, we need to use synchronization mechanisms to ensure cooperation and mutually exclusive access between threads.

In C, we can use a variety of synchronization mechanisms to solve synchronization problems between threads, including mutexes, condition variables, and atomic operations. Below we will discuss common synchronization problems and give corresponding solutions and code examples.

1. Competition conditions
A competition condition means that multiple threads access shared resources at the same time. Due to the uncertainty of the access sequence, the execution result of the program is uncertain. To avoid race conditions, we need to use mutex locks to protect shared resources and ensure that only one thread can access and modify shared resources.

The following is a code example that uses a mutex lock to solve the problem of race conditions:

#include <iostream>
#include <thread>
#include <mutex>

std::mutex mtx;
int counter = 0;

void increment() {
    std::lock_guard<std::mutex> lock(mtx);
    counter++;
}

int main() {
    std::thread t1(increment);
    std::thread t2(increment);

    t1.join();
    t2.join();

    std::cout << "Counter: " << counter << std::endl;

    return 0;
}

In the above code, we use std::mutex to create the mutex lock mtx, and then in the increment function Use std::lock_guard<:mutex> to lock the mutex to ensure that only one thread can perform the counter operation. This ensures that the result of counter is correctly defined.

2. Deadlock
Deadlock means that two or more threads are waiting for each other to release resources, causing the program to be unable to continue execution. In order to avoid deadlock, we can use RAII (resource acquisition is initialization) technology and avoid multi-lock waiting and other methods.

The following is an example to avoid deadlock:

#include <iostream>
#include <thread>
#include <mutex>

std::mutex mtx1, mtx2;

void thread1() {
    std::unique_lock<std::mutex> lock1(mtx1);
    std::this_thread::sleep_for(std::chrono::milliseconds(10)); // 延迟10毫秒，让线程2有足够时间锁住mtx2
    std::unique_lock<std::mutex> lock2(mtx2);
    
    // 访问共享资源
    std::cout << "Thread 1" << std::endl;
}

void thread2() {
    std::unique_lock<std::mutex> lock2(mtx2);
    std::this_thread::sleep_for(std::chrono::milliseconds(10)); // 延迟10毫秒，让线程1有足够时间锁住mtx1
    std::unique_lock<std::mutex> lock1(mtx1);
    
    // 访问共享资源
    std::cout << "Thread 2" << std::endl;
}

int main() {
    std::thread t1(thread1);
    std::thread t2(thread2);

    t1.join();
    t2.join();

    return 0;
}

In the above code, we use std::unique_lock<:mutex> instead of std::lock_guard<:mutex which allows manual control of lock acquisition and release. deadlock is avoided by locking one mutex then another in each thread.>

3. Hungry
Hunger refers to a situation where a thread cannot obtain the required resources for some reason and cannot continue execution. In order to avoid starvation, we can use lock priority, fair scheduling and other mechanisms to ensure that threads obtain resources fairly.

The following is a code example that uses the priority of the mutex lock to solve the starvation problem:

#include <iostream>
#include <thread>
#include <mutex>

std::mutex mtx;
int counter = 0;

void increment() {
    std::unique_lock<std::mutex> lock(mtx, std::defer_lock);
    while (true) {
        lock.lock(); // 获取互斥锁
        counter++;
        lock.unlock(); // 释放互斥锁
    }
}

void decrement() {
    std::unique_lock<std::mutex> lock(mtx, std::defer_lock);
    while (true) {
        lock.lock(); // 获取互斥锁
        counter--;
        lock.unlock(); // 释放互斥锁
    }
}

int main() {
    std::thread t1(increment);
    std::thread t2(decrement);

    t1.join();
    t2.join();

    return 0;
}

In the above code, we use the std::defer_lock parameter to delay the acquisition of the mutex lock, Then manually call lock.lock() to obtain the mutex when needed. This ensures that threads acquire the mutex fairly and avoids starvation problems.

Summary:
Multi-thread synchronization problem is one of the important challenges in multi-thread programming. Reasonable selection and use of synchronization mechanism is the key to solving these problems. In C, we can use mutex locks, condition variables and atomic operations to achieve synchronization and cooperation between threads. By properly designing and writing multi-threaded programs, we can effectively solve multi-thread synchronization problems and improve program performance and reliability.

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