<三>线程间互斥-mutex互斥锁和lock_guard
多线程程序
竞态条件:多线程程序执行的结果是一致的,不会随着CPU对线程不同的调用顺序而产生不同的运行结果.
解决？:互斥锁 mutex
经典的卖票问题,三个线程卖100张票
代码1
#include <iostream>
#include <thread>
#include <list>
#include <mutex>
int ticketCount = 100;
std::mutex mtx;//互斥锁
void sellTicket(int window) {
while (ticketCount > 0) {
mtx.lock();
std::cout << "窗口" << window << "销售" << ticketCount << std::endl;
ticketCount--;
mtx.unlock();
std::this_thread::sleep_for(std::chrono::milliseconds(50));
}
}//end
int main() {
std::list<std::thread> tlist;
for (int i = 0; i < 3; i++) {
tlist.push_back(std::thread(sellTicket,i));
}
for (std::thread & t : tlist) {
t.join();
}
system("pause");
return 0;
}
上面代码的问题...
while (ticketCount > 0) {
mtx.lock();
std::cout << "窗口" << window << "销售" << ticketCount << std::endl;
ticketCount--;
mtx.unlock();
std::this_thread::sleep_for(std::chrono::milliseconds(50));
}
如果ticketCount =1 ，当前有一个线程A while (ticketCount > 0)为true,线程A还没执行ticketCount--完成时,cpu交给了线程B
线程B while (ticketCount > 0)也为true,进入 循环体内,造成了买0号票,改进如下
代码2
#include <iostream>
#include <thread>
#include <list>
#include <mutex>
int ticketCount = 100;
std::mutex mtx;//互斥锁
void sellTicket(int window) {
while (ticketCount > 0) {
mtx.lock();
if(ticketCount >0){
std::cout << "窗口" << window << "销售" << ticketCount << std::endl;
ticketCount--;
}
mtx.unlock();
std::this_thread::sleep_for(std::chrono::milliseconds(50));
}
}//end
int main() {
std::list<std::thread> tlist;
for (int i = 0; i < 3; i++) {
tlist.push_back(std::thread(sellTicket,i));
}
for (std::thread & t : tlist) {
t.join();
}
system("pause");
return 0;
}
代码2还有些问题!! 如下
mtx.lock();
代码
代码
代码
代码
.....
mtx.unlock();
如果在代码lock()和unlock()之间 非常返回,导致mtx没有正常unlock(),那么出现死锁问题 =》智能指针 lock_gurad unique_lock
看lock_gurad的代码描述,其实就是对mutex封装,关闭了拷贝构造和赋值函数
#include <iostream>
#include <thread>
#include <list>
#include <mutex>
int ticketCount = 100;
std::mutex mtx;//互斥锁
void sellTicket(int window) {
while (ticketCount > 0) {
{
std::lock_guard<std::mutex> lock(mtx);
if(ticketCount >0){
std::cout << "窗口" << window << "销售" << ticketCount << std::endl;
ticketCount--;
}
std::this_thread::sleep_for(std::chrono::milliseconds(50));
}
}
}//end
int main() {
std::list<std::thread> tlist;
for (int i = 0; i < 3; i++) {
tlist.push_back(std::thread(sellTicket,i));
}
for (std::thread & t : tlist) {
t.join();
}
system("pause");
return 0;
}
上面的图片中我们知道lock_gurad 的拷贝构造函数被关闭了,所以当我们遇到函数调用需要拷贝构造lock_guard的时候,就有障碍了，这个时候可以用unique_lock
unique_lock 转移指针,支持带右值得拷贝赋值,支持参数传递拷贝构造的,他的左值的拷贝构造也是被关闭了 看下图
