【操作系统】死锁C++代码示意

/// @author zhaolu
/// @date 2020/04/16
#include <iostream>
#include <atomic> // std::atomic_flag
#include <thread> // std::thread std::this_thread
#include <chrono> // std::chrono::seconds

std::atomic_flag print = ATOMIC_FLAG_INIT;

void fun(int id, std::atomic_flag *flag_1, std::atomic_flag *flag_2) {
    while(print.test_and_set());
    std::cout << "thread " << id << " request resources " << flag_1 << std::endl;
    print.clear();

    while (flag_1->test_and_set())
        ;

    while(print.test_and_set());
    std::cout << "thread " << id << " get resources " << flag_1 <<  std::endl;
    print.clear();

    std::this_thread::sleep_for(std::chrono::seconds(1));

    while(print.test_and_set());
    std::cout << "thread " << id << " request resources " << flag_2 << std::endl;
    print.clear();

    while(flag_2->test_and_set())
        ;
    
    while(print.test_and_set());
    std::cout << "thread " << id << " get resources " << flag_2 <<  std::endl;
    print.clear();

    // do something
    
    flag_1->clear();

    while(print.test_and_set());
    std::cout << "thread " << id << " release resources " << flag_1 <<  std::endl;
    print.clear();

    flag_2->clear();

    while(print.test_and_set());
    std::cout << "thread " << id << " release resources " << flag_2 <<  std::endl;
    print.clear();
}

int main() {
    std::atomic_flag flag_1 = ATOMIC_FLAG_INIT;
    std::atomic_flag flag_2 = ATOMIC_FLAG_INIT;
    std::thread thread_1(fun, 1, &flag_1, &flag_2);
    std::thread thread_2(fun, 2, &flag_2, &flag_1);
    thread_1.join();
    thread_2.join();
    return 0;
}

flag_1 => resource_1
flag_2 => resource_2

输出结果为：

:./main
thread 1 request resources 0x7ffeef93e998
thread 1 get resources 0x7ffeef93e998
thread 2 request resources 0x7ffeef93e990
thread 2 get resources 0x7ffeef93e990
thread 1 request resources 0x7ffeef93e990
thread 2 request resources 0x7ffeef93e998

无反应。。

死锁的条件

• 互斥条件：一个资源每次只能被一个进程使用，即在一段时间内某资源仅为一个进程所占有。此时若有其他进程请求该资源，则请求进程只能等待。

• 请求与保持条件：进程已经保持了至少一个资源，但又提出了新的资源请求，而该资源 已被其他进程占有，此时请求进程被阻塞，但对自己已获得的资源保持不放。

• 不可剥夺条件:进程所获得的资源在未使用完毕之前，不能被其他进程强行夺走，即只能 由获得该资源的进程自己来释放（只能是主动释放)。

• 循环等待条件: 若干进程间形成首尾相接循环等待资源的关系