C++ 多线程编程示例

2022-08-20

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创建线程

下面的程序，我们可以用它来创建一个 POSIX 线程：

1 2	#include <pthread.h> pthread_create (thread, attr, start_routine, arg)

使用 -lpthread 库编译下面的程序：

1	$ g++ test.cpp -lpthread -o test.o

条件变量

pthread_cond_wait unlocks the mutex just before it sleeps，but then it reaquires the mutex (which may require waiting) when it is signalled, before it wakes up.

示例 1

创建两个线程，其中一个输出1-52，另外一个输出A-Z。

输出格式要求：12A 34B 56C 78D …

#include <iostream>
#include <cstdlib>
#include <pthread.h>
#include <unistd.h>
 
using namespace std;

#define NUM_THREADS 5
pthread_mutex_t mutex;  //互斥锁
pthread_cond_t cond;    //条件变量
bool flag = true;

void *PrintNum(void *arg)
{
    for(int i=1;i<=52;i=i+2)
    {   
        pthread_mutex_lock(&mutex);
        while(!flag)
        {
            //函数用于等待目标条件变量。mutex参数是用于保护条件变量的互斥锁，以确保pthread_cond_wait操作的原子性。
            pthread_cond_wait(&cond,&mutex);
        }
        cout<<i<<i+1;
        flag = false;
        // 函数用于唤醒一个等待目标条件变量的线程。至于哪个线程将被唤醒，则取决于线程的优先级和调度策略。
        pthread_cond_signal(&cond);
        pthread_mutex_unlock(&mutex);
    }
    pthread_exit(NULL);//结束线程
}

void *PrintChar(void *arg)
{
    for(int i=0;i<26;i++)
    {
        pthread_mutex_lock(&mutex);
        while(flag)
        {
            pthread_cond_wait(&cond,&mutex);
        }
        printf("%c\n",i+'A');
        flag = true;
        pthread_cond_signal(&cond);
        pthread_mutex_unlock(&mutex);
    }
    pthread_exit(NULL);//结束线程
}

int main()
{
    pthread_mutex_init(&mutex,NULL); //初始化互斥锁
    pthread_cond_init(&cond,NULL); //初始化条件变量
    pthread_t thread[NUM_THREADS];
    int indexs[NUM_THREADS];
    
    int ret1 = pthread_create(&thread[0],NULL,PrintNum,NULL);
    if(ret1)
    {
        cout<<"cread thread 1 error "<<ret1<<endl;
        exit(-1);
    }
    int ret2 = pthread_create(&thread[1],NULL,PrintChar,NULL);
    if(ret2)
    {
        cout<<"cread thread 2 error "<<ret1<<endl;
        exit(-1);
    }

    pthread_exit(NULL);//结束线程
    return 0;
}

示例 2 死锁

#include <pthread.h>
#include <unistd.h>
#include <stdio.h>

int a = 0;
int b = 0;
pthread_mutex_t mutex_a;
pthread_mutex_t mutex_b;

void* another( void* arg )
{
    pthread_mutex_lock( &mutex_b );
    printf( "in child thread, got mutex b, waiting for mutex a\n" );
    sleep( 5 );
    ++b;
    pthread_mutex_lock( &mutex_a );
    b += a++;
    pthread_mutex_unlock( &mutex_a );
    pthread_mutex_unlock( &mutex_b );
    pthread_exit( NULL );
}

int main()
{
    pthread_t id;
    
    pthread_mutex_init( &mutex_a, NULL );
    pthread_mutex_init( &mutex_b, NULL );
    pthread_create( &id, NULL, another, NULL );

    pthread_mutex_lock( &mutex_a );
    printf( "in parent thread, got mutex a, waiting for mutex b\n" );
    sleep( 5 );
    ++a;
    pthread_mutex_lock( &mutex_b );
    a += b++;
    pthread_mutex_unlock( &mutex_b );
    pthread_mutex_unlock( &mutex_a );

    pthread_join( id, NULL );
    pthread_mutex_destroy( &mutex_a );
    pthread_mutex_destroy( &mutex_b );
    return 0;
}

输出：

in parent thread, got mutex a, waiting for mutex b
in child thread, got mutex b, waiting for mutex a

关键字:

noexcept explicit
std::ref C++11 的 std::ref 用法 | 拾荒志 (murphypei.github.io)
std::bind
std::forward
auto&