12.线程同步与互斥

liu****2025-11-11 12:27

一.上集回顾

建议先学上篇博客,再向下学习,上篇博客的链接如下:

https://blog.csdn.net/weixin_60668256/article/details/154572336?fromshare=blogdetail&sharetype=blogdetail&sharerId=154572336&sharerefer=PC&sharesource=weixin_60668256&sharefrom=from_link

二.线程互斥

1.快速见一见,没有加保护,会有什么问题

cpp 复制代码 
#include <iostream>
#include "Thread.hpp"
#include <vector>


using namespace ThreadModule;

#define NUM 4

int ticketnum = 10000;//临界资源

void Ticket()
{
    while(true)
    {
        if(ticketnum > 0)
        {
            usleep(1000);
            //1.抢票
            printf("get a new ticket, id: %d\n",ticketnum--);            
        }
        else
        {
            break;
        }
    }
}

int main()
{
    //1.构建线程对象
    std::vector<Thread> threads;
    for(int i = 0;i < NUM;i++)
    {
        threads.emplace_back(Ticket);
    }

    //2.启动线程
    for(auto& thread:threads)
    {
        thread.Start();
    }

    //3.等待线程
    for(auto& thread:threads)
    {
        thread.Join();
    }
    return 0;
}

代码为啥会出现这个问题呢?

ticketnum--这一条指令,就会被转换成为三条汇编

所以,这个ticketnum--不是原子性的(ticketnum > 0 也不是原子的)

要么是一行汇编,要么是多行代码,但是加锁保护

2.解决方案

所有对资源的保护,都是对临界区代码的保护,因为资源是通过代码访问的!

3.接口介绍

a.锁的初始化

b.加锁

4.改进售票系统

a.加默认锁

cpp 复制代码 
#include <iostream>
#include "Thread.hpp"
#include <vector>


using namespace ThreadModule;

#define NUM 4

int ticketnum = 10000;//临界资源
//锁本身是全局的,那么锁也是共享资源
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

void Ticket()
{
    while(true)
    {
        pthread_mutex_lock(&lock);//加锁
        if(ticketnum > 0)
        {
            usleep(1000);
            //1.抢票
            printf("get a new ticket, id: %d\n",ticketnum--);
            pthread_mutex_unlock(&lock);//解锁           
        }
        else
        {
            pthread_mutex_unlock(&lock);
            break;
        }
    }
}

int main()
{
    //1.构建线程对象
    std::vector<Thread> threads;
    for(int i = 0;i < NUM;i++)
    {
        threads.emplace_back(Ticket);
    }

    //2.启动线程
    for(auto& thread:threads)
    {
        thread.Start();
    }

    //3.等待线程
    for(auto& thread:threads)
    {
        thread.Join();
    }
    return 0;
}

但是锁本身就是临界资源,那么谁保证锁的安全呢?

加锁和解锁被设计成为原子的(本身就是原子的)

如何看待锁呢?

二元信号量就是锁! 加锁的本质就是对资源展开预定! 整体使用资源!

如果申请锁的时候,锁被别人已经拿走了,怎么办?

其他线程要进行阻塞等待

线程在访问临界区代码的时候,可不可以切换?

可以切换! 我被切换的时候,别人没办法进入,我是抱着锁,被切换的!!(串行,这就是效率低的原因)

b.局部锁

cpp 复制代码 
#include <iostream>
#include "Thread.hpp"
#include <vector>


using namespace ThreadModule;

#define NUM 4

int ticketnum = 10000;//临界资源

class ThreadData
{
public:
    std::string name;
    pthread_mutex_t* lock_ptr;
};

void Ticket(ThreadData& td)
{
    while(true)
    {
        pthread_mutex_lock(td.lock_ptr);//加锁
        if(ticketnum > 0)
        {
            usleep(1000);
            //1.抢票
            printf("get a new ticket, who get it: %s, id: %d\n",td.name.c_str(),ticketnum--);
            pthread_mutex_unlock(td.lock_ptr);//解锁           
        }
        else
        {
            pthread_mutex_unlock(td.lock_ptr);
            break;
        }
    }
}

int main()
{
    pthread_mutex_t lock;
    pthread_mutex_init(&lock,nullptr);

    //1.构建线程对象
    std::vector<Thread<ThreadData>> threads;
    for(int i = 0;i < NUM;i++)
    {
        ThreadData* td = new ThreadData();
        td->lock_ptr = &lock;
        threads.emplace_back(Ticket,*td);
        td->name = threads.back().Name();
    }

    //2.启动线程
    for(auto& thread:threads)
    {
        thread.Start();
    }

    //3.等待线程
    for(auto& thread:threads)
    {
        thread.Join();
    }

    pthread_mutex_destroy(&lock);
    return 0;
}
cpp 复制代码 
#include <iostream>
#include "Thread.hpp"
#include <vector>


using namespace ThreadModule;

#define NUM 4

int ticketnum = 10000;//临界资源

class ThreadData
{
public:
    std::string name;
    pthread_mutex_t* lock_ptr;
};

void Ticket(ThreadData& td)
{
    while(true)
    {
        pthread_mutex_lock(td.lock_ptr);//加锁
        if(ticketnum > 0)
        {
            usleep(1000);
            //1.抢票
            printf("get a new ticket, who get it: %s, id: %d\n",td.name.c_str(),ticketnum--);
            pthread_mutex_unlock(td.lock_ptr);//解锁
            //入库模拟操作
            usleep(50);       
        }
        else
        {
            pthread_mutex_unlock(td.lock_ptr);
            break;
        }
    }
}

int main()
{
    pthread_mutex_t lock;
    pthread_mutex_init(&lock,nullptr);

    //1.构建线程对象
    std::vector<Thread<ThreadData>> threads;
    for(int i = 0;i < NUM;i++)
    {
        ThreadData* td = new ThreadData();
        td->lock_ptr = &lock;
        threads.emplace_back(Ticket,*td);
        td->name = threads.back().Name();
    }

    //2.启动线程
    for(auto& thread:threads)
    {
        thread.Start();
    }

    //3.等待线程
    for(auto& thread:threads)
    {
        thread.Join();
    }

    pthread_mutex_destroy(&lock);
    return 0;
}

三.锁的如何实现的?

所以,这个1就是我们的锁,这里就是exchange指令通过1条汇编,就完成了加锁的操作

四.封装锁

1.C++锁的使用

cpp 复制代码 
#include <iostream>
#include <mutex>
#include <vector>


using namespace ThreadModule;

#define NUM 4

int ticketnum = 10000;//临界资源

std::mutex gmutex;
class ThreadData
{
public:
    std::string name;
    pthread_mutex_t* lock_ptr;
};

void Ticket(ThreadData& td)
{
    while(true)
    {
        gmutex.lock();
        if(ticketnum > 0)
        {
            usleep(1000);
            //1.抢票
            printf("get a new ticket, who get it: %s, id: %d\n",td.name.c_str(),ticketnum--);
            gmutex.unlock();
            //入库模拟操作
            usleep(50);       
        }
        else
        {
            gmutex.unlock();
            break;
        }
    }
}

int main()
{
    pthread_mutex_t lock;
    pthread_mutex_init(&lock,nullptr);

    //1.构建线程对象
    std::vector<Thread<ThreadData>> threads;
    for(int i = 0;i < NUM;i++)
    {
        ThreadData* td = new ThreadData();
        td->lock_ptr = &lock;
        threads.emplace_back(Ticket,*td);
        td->name = threads.back().Name();
    }

    //2.启动线程
    for(auto& thread:threads)
    {
        thread.Start();
    }

    //3.等待线程
    for(auto& thread:threads)
    {
        thread.Join();
    }

    pthread_mutex_destroy(&lock);
    return 0;
}

2.封装mutex

a.基本的初始化和销毁

cpp 复制代码 
#pragma once
#include <iostream>
#include <pthread.h>

namespace LockModule
{
    class Mutex
    {
    public:
        Mutex()
        {
            int n = ::pthread_mutex_init(&_lock,nullptr);
            (void)n;
        }
        ~Mutex()
        {
            int n = ::pthread_mutex_destroy(&_lock);
            (void)n;
        }
    private:
        pthread_mutex_t _lock;
    };
};

b.加锁和解锁

cpp 复制代码 
#pragma once
#include <iostream>
#include <pthread.h>

namespace LockModule
{
    class Mutex
    {
    public:
        Mutex()
        {
            int n = ::pthread_mutex_init(&_lock,nullptr);
            (void)n;
        }
        void Lock()
        {
            int n = ::pthread_mutex_lock(&_lock);
            (void)n;
        }
        void Unlock()
        {
            int n = ::pthread_mutex_unlock(&_lock);
            (void)n;
        }
        ~Mutex()
        {
            int n = ::pthread_mutex_destroy(&_lock);
            (void)n;
        }
    private:
        pthread_mutex_t _lock;
    };
};

c.禁用拷贝和赋值

cpp 复制代码 
#pragma once
#include <iostream>
#include <pthread.h>

namespace LockModule
{
    class Mutex
    {
    public:
        Mutex(const Mutex&) = delete;
        const Mutex& operator=(const Mutex&) = delete;
        Mutex()
        {
            int n = ::pthread_mutex_init(&_lock,nullptr);
            (void)n;
        }
        void Lock()
        {
            int n = ::pthread_mutex_lock(&_lock);
            (void)n;
        }
        void Unlock()
        {
            int n = ::pthread_mutex_unlock(&_lock);
            (void)n;
        }
        ~Mutex()
        {
            int n = ::pthread_mutex_destroy(&_lock);
            (void)n;
        }
    private:
        pthread_mutex_t _lock;
    };
};

d.LockGuard类的设计

cpp 复制代码 
#pragma once
#include <iostream>
#include <pthread.h>

namespace LockModule
{
    class Mutex
    {
    public:
        Mutex(const Mutex&) = delete;
        const Mutex& operator=(const Mutex&) = delete;
        Mutex()
        {
            int n = ::pthread_mutex_init(&_lock,nullptr);
            (void)n;
        }
        void Lock()
        {
            int n = ::pthread_mutex_lock(&_lock);
            (void)n;
        }
        void Unlock()
        {
            int n = ::pthread_mutex_unlock(&_lock);
            (void)n;
        }
        ~Mutex()
        {
            int n = ::pthread_mutex_destroy(&_lock);
            (void)n;
        }
    private:
        pthread_mutex_t _lock;
    };
    class LockGuard
    {
    public:
        LockGuard(Mutex& mtx):_mtx(mtx)
        {
            _mtx.Lock();
        }
        ~LockGuard()
        {
            _mtx.Unlock();
        }
    private:
        Mutex& _mtx;
    };
};

e.锁的使用

原来的代码:

cpp 复制代码 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include <sched.h>

int ticket = 1000;
pthread_mutex_t mutex;

void *route(void *arg)
{
    char *id = (char*)arg;
    while (1) {
        pthread_mutex_lock(&mutex);
        if (ticket > 0) {
            usleep(1000);
            printf("%s sells ticket:%d\n", id, ticket);
            ticket--;
            pthread_mutex_unlock(&mutex);
        } else {
            pthread_mutex_unlock(&mutex);
            break;
        }
    }
    return nullptr;
}

int main(void)
{
    pthread_t t1, t2, t3, t4;

    pthread_mutex_init(&mutex, NULL);

    pthread_create(&t1, NULL, route, (void*)"thread_1");
    pthread_create(&t2, NULL, route, (void*)"thread_2");
    pthread_create(&t3, NULL, route, (void*)"thread_3");
    pthread_create(&t4, NULL, route, (void*)"thread_4");

    pthread_join(t1, NULL);
    pthread_join(t2, NULL);
    pthread_join(t3, NULL);
    pthread_join(t4, NULL);
    pthread_mutex_destroy(&mutex);

    return 0;
}

换我们自己的锁:

cpp 复制代码 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include "Mutex.hpp"


using namespace LockModule;
int ticket = 1000;
Mutex mtx;

void *route(void *arg)
{
    char *id = (char*)arg;
    while (1) {
        mtx.Lock();
        if (ticket > 0) {
            usleep(1000);
            printf("%s sells ticket:%d\n", id, ticket);
            ticket--;
            mtx.Unlock();
        } else {
            mtx.Unlock();
            break;
        }
    }
    return nullptr;
}

int main(void)
{
    pthread_t t1, t2, t3, t4;

    pthread_create(&t1, NULL, route, (void*)"thread_1");
    pthread_create(&t2, NULL, route, (void*)"thread_2");
    pthread_create(&t3, NULL, route, (void*)"thread_3");
    pthread_create(&t4, NULL, route, (void*)"thread_4");

    pthread_join(t1, NULL);
    pthread_join(t2, NULL);
    pthread_join(t3, NULL);
    pthread_join(t4, NULL);

    return 0;
}

但是这还是得我们自己进行加锁和解锁,还是太麻烦了,我们这里采用RAII的加锁方式

cpp 复制代码 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include "Mutex.hpp"


using namespace LockModule;
int ticket = 1000;
Mutex mtx;

void *route(void *arg)
{
    char *id = (char*)arg;
    while (1) {
        {
            LockGuard lockguard(mtx);
            if (ticket > 0) {
                usleep(1000);
                printf("%s sells ticket:%d\n", id, ticket);
                ticket--;
            } else {
                break;
            }
        }
    }
    return nullptr;
}

int main(void)
{
    pthread_t t1, t2, t3, t4;

    pthread_create(&t1, NULL, route, (void*)"thread_1");
    pthread_create(&t2, NULL, route, (void*)"thread_2");
    pthread_create(&t3, NULL, route, (void*)"thread_3");
    pthread_create(&t4, NULL, route, (void*)"thread_4");

    pthread_join(t1, NULL);
    pthread_join(t2, NULL);
    pthread_join(t3, NULL);
    pthread_join(t4, NULL);

    return 0;
}

五.线程同步

1.条件变量接口

signal是唤醒一个线程,broadcast是唤醒全部线程

2.理解条件变量

敲一下铃铛就相当于是唤醒一个人,broadcast就是唤醒所有的线程

3.写一下demo代码

cpp 复制代码 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include <sched.h>

int ticket = 0;
pthread_mutex_t mutex;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

void *route(void *arg)
{
    char *id = (char*)arg;
    while (1) {
        pthread_mutex_lock(&mutex);
        if (ticket > 0) {
            usleep(1000);
            printf("%s sells ticket:%d\n", id, ticket);
            ticket--;
            pthread_mutex_unlock(&mutex);
        } else {
            printf("%s wait on cond!\n",id);
            pthread_cond_wait(&cond,&mutex);//醒来的时候,会重新申请锁!!
            printf("%s 被叫醒了\n",id);
        }
        pthread_mutex_unlock(&mutex);
    }
    return nullptr;
}

int main(void)
{
    pthread_t t1, t2, t3, t4;

    pthread_mutex_init(&mutex, NULL);

    pthread_create(&t1, NULL, route, (void*)"thread_1");
    pthread_create(&t2, NULL, route, (void*)"thread_2");
    pthread_create(&t3, NULL, route, (void*)"thread_3");
    pthread_create(&t4, NULL, route, (void*)"thread_4");
    int cnt = 10;
    while(true)
    {
        sleep(5);
        ticket += cnt;
        printf("主线程放票咯! ticket: %d\n",ticket);
        pthread_cond_signal(&cond);
    }

    pthread_join(t1, NULL);
    pthread_join(t2, NULL);
    pthread_join(t3, NULL);
    pthread_join(t4, NULL);
    pthread_mutex_destroy(&mutex);

    return 0;
}
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