linux --> 多线程编程

多线程编程

一、最基础，进程同时创建5个线程，各自调用同一个函数

#include <iostream>
#include <pthread.h> //多线程相关操作头文件，可移植众多平台

using namespace std;

#define NUM_THREADS 5 //线程数

void* say_hello( void* args )
{
    cout << "hello..." << endl;
} //函数返回的是函数指针，便于后面作为参数

int main()
{
    pthread_t tids[NUM_THREADS]; //线程id
    for( int i = 0; i < NUM_THREADS; ++i )
    {
        int ret = pthread_create( &tids[i], NULL, say_hello, NULL ); //参数：创建的线程id，线程参数，线程运行函数的起始地址，运行函数的参数
        if( ret != 0 ) //创建线程成功返回0
        {
            cout << "pthread_create error:error_code=" << ret << endl;
        }
    }
    pthread_exit( NULL ); //等待各个线程退出后，进程才结束，否则进程强制结束，线程处于未终止的状态
}

输入命令：

g++ -o thread_test thread_test.cpp -lpthread

两次测试结果：

wq@wq-desktop$ ./thread_test
hello...hello...
hello...
hello...

hello...

和

wq@wq-desktop$ ./thread_test
hello...hello...hello...

hello...
hello...

两次运行的结果会有差别，多线程的运行是混乱的，混乱就是正常？

二、调用类中的函数，必须将该函数声明为静态函数

因为静态成员函数属于静态全局区，线程可以共享这个区域，故可以各自调用。

#include <iostream>
#include <pthread.h>

using namespace std;

#define NUM_THREADS 5

class Hello
{
public:
    static void* say_hello( void* args )
    {
        cout << "hello..." << endl;
    }
};

int main()
{
    pthread_t tids[NUM_THREADS];
    for( int i = 0; i < NUM_THREADS; ++i )
    {
        int ret = pthread_create( &tids[i], NULL, Hello::say_hello, NULL );
        if( ret != 0 )
        {
            cout << "pthread_create error:error_code" << ret << endl;
        }
    }
    pthread_exit( NULL );
}

测试结果：

wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_2
hello...
hello...
hello...
hello...
hello...

和

wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_2
hello...hello...hello...


hello...
hello...

三、如何在线程调用函数时传入参数呢？

先看下面修改的代码，传入线程编号作为参数：

#include <iostream>
#include <pthread.h> //多线程相关操作头文件，可移植众多平台

using namespace std;

#define NUM_THREADS 5 //线程数

void* say_hello( void* args )
{
    int i = *( (int*)args ); //对传入的参数进行强制类型转换，由无类型指针转变为整形指针，再用*读取其指向到内容
    cout << "hello in " << i <<  endl;
} //函数返回的是函数指针，便于后面作为参数

int main()
{
    pthread_t tids[NUM_THREADS]; //线程id
    cout << "hello in main.." << endl;
    for( int i = 0; i < NUM_THREADS; ++i )
    {
        int ret = pthread_create( &tids[i], NULL, say_hello, (void*)&i ); //参数必须强转为void*类型，&i表示取i的地址，即指向i的指针
        cout << "Current pthread id = " << tids[i] << endl; //用tids数组打印创建的进程id信息
        if( ret != 0 ) //创建线程成功返回0
        {
            cout << "pthread_create error:error_code=" << ret << endl;
        }
    }
    pthread_exit( NULL ); //等待各个线程退出后，进程才结束，否则进程强制结束，线程处于未终止的状态
}

测试结果：

wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_3
hello in main..
Current pthread id = 3078458224
Current pthread id = 3070065520
hello in hello in 2
1
Current pthread id = hello in 2
3061672816
Current pthread id = 3053280112
hello in 4
Current pthread id = hello in 4
3044887408

显然不是想要的结果，调用顺序很乱，这是为什么呢？

修改代码如下：

#include <iostream>
#include <pthread.h> //多线程相关操作头文件，可移植众多平台

using namespace std;

#define NUM_THREADS 5 //线程数

void* say_hello( void* args )
{
    cout << "hello in thread " << *( (int *)args ) <<  endl;
} //函数返回的是函数指针，便于后面作为参数

int main()
{
    pthread_t tids[NUM_THREADS]; //线程id
    int indexes[NUM_THREADS]; //用来保存i的值避免被修改

    for( int i = 0; i < NUM_THREADS; ++i )
    {
        indexes[i] = i;
        int ret = pthread_create( &tids[i], NULL, say_hello, (void*)&(indexes[i]) );
        if( ret != 0 ) //创建线程成功返回0
        {
            cout << "pthread_create error:error_code=" << ret << endl;
        }
    }
    for( int i = 0; i < NUM_THREADS; ++i )
        pthread_join( tids[i], NULL ); //pthread_join用来等待一个线程的结束，是一个线程阻塞的函数
}

测试结果：

wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_3
hello in thread hello in thread hello in thread hello in thread hello in thread 30124

还是有问题.，代码中如果没有pthread_join主线程会很快结束从而使整个进程结束，从而使创建的线程没有机会开始执行就结束了。加入pthread_join后，主线程会一直等待直到等待的线程结束自己才结束，使创建的线程有机会执行。

四、属性参数的设置pthread_attr_t及join功能的使用

线程的属性由结构体pthread_attr_t进行管理。

typedef struct
{
    int detachstate;      //  线程的分离状态
    int schedpolicy;      // 线程调度策略
    struct sched_param schedparam;  // 线程的调度参数
    int inheritsched;     //线程的继承性 
    int scope;            //线程的作用域 
    size_t guardsize;     //线程栈末尾的警戒缓冲区大小 
    int stackaddr_set; 
    void * stackaddr;     // 线程栈的位置 
    size_t stacksize;     // 线程栈的大小
}pthread_attr_t;

测试用例：

#include <iostream>
#include <pthread.h>

using namespace std;

#define NUM_THREADS 5

void* say_hello( void* args )
{
    cout << "hello in thread " << *(( int * )args) << endl;
    int status = 10 + *(( int * )args); //线程退出时添加退出的信息，status供主程序提取该线程的结束信息
    pthread_exit( ( void* )status ); 
}

int main()
{
    pthread_t tids[NUM_THREADS];
    int indexes[NUM_THREADS];
    
    pthread_attr_t attr; //线程属性结构体，创建线程时加入的参数
    pthread_attr_init( &attr ); //初始化
    pthread_attr_setdetachstate( &attr, PTHREAD_CREATE_JOINABLE ); //是设置你想要指定线程属性参数，这个参数表明这个线程是可以join连接的，join功能表示主程序可以等线程结束后再去做某事，实现了主程序和线程同步功能
    for( int i = 0; i < NUM_THREADS; ++i )
    {
        indexes[i] = i;
        int ret = pthread_create( &tids[i], &attr, say_hello, ( void* )&( indexes[i] ) );
        if( ret != 0 )
        {
        　　cout << "pthread_create error:error_code=" << ret << endl;
    　　 }
    } 
    pthread_attr_destroy( &attr ); //释放内存 
    void *status;
    for( int i = 0; i < NUM_THREADS; ++i )
    {
    　　int ret = pthread_join( tids[i], &status ); //主程序join每个线程后取得每个线程的退出信息status
    　　if( ret != 0 )
    　　{
     　　   cout << "pthread_join error:error_code=" << ret << endl;
    　　}
    　　else
    　　{
    　　    cout << "pthread_join get status:" << (long)status << endl;
    　　}
    }
}

测试结果：

wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_4
hello in thread hello in thread hello in thread hello in thread 0hello in thread 321

4
pthread_join get status:10
pthread_join get status:11
pthread_join get status:12
pthread_join get status:13
pthread_join get status:14

五、互斥锁的实现
互斥锁是实现线程同步的一种机制，只要在临界区前后对资源加锁就能阻塞其他进程的访问。

#include <iostream>
#include <pthread.h>

using namespace std;

#define NUM_THREADS 5

int sum = 0; //定义全局变量，让所有线程同时写，这样就需要锁机制
pthread_mutex_t sum_mutex; //互斥锁

void* say_hello( void* args )
{
    cout << "hello in thread " << *(( int * )args) << endl;
    pthread_mutex_lock( &sum_mutex ); //先加锁，再修改sum的值，锁被占用就阻塞，直到拿到锁再修改sum;
    cout << "before sum is " << sum << " in thread " << *( ( int* )args ) << endl;
    sum += *( ( int* )args );
    cout << "after sum is " << sum << " in thread " << *( ( int* )args ) << endl;
    pthread_mutex_unlock( &sum_mutex ); //释放锁，供其他线程使用
    pthread_exit( 0 ); 
}

int main()
{
    pthread_t tids[NUM_THREADS];
    int indexes[NUM_THREADS];
    
    pthread_attr_t attr; //线程属性结构体，创建线程时加入的参数
    pthread_attr_init( &attr ); //初始化
    pthread_attr_setdetachstate( &attr, PTHREAD_CREATE_JOINABLE ); //设置线程属性参数，这个参数表明这个线程是可以join连接的，join功能表示主程序可以等线程结束后再去做某事，实现了主程序和线程同步功能
    pthread_mutex_init( &sum_mutex, NULL ); //对锁进行初始化    

    for( int i = 0; i < NUM_THREADS; ++i )
    {
        indexes[i] = i;
        int ret = pthread_create( &tids[i], &attr, say_hello, ( void* )&( indexes[i] ) ); //5个进程同时去修改sum
        if( ret != 0 )
        {
        cout << "pthread_create error:error_code=" << ret << endl;
    　　}
    } 
    pthread_attr_destroy( &attr ); //释放内存 
    void *status;
    for( int i = 0; i < NUM_THREADS; ++i )
    {
    　　int ret = pthread_join( tids[i], &status ); //主程序join每个线程后取得每个线程的退出信息status
    　　if( ret != 0 )
    　　{
    　　    cout << "pthread_join error:error_code=" << ret << endl;
    　　}
    }
    cout << "finally sum is " << sum << endl;
    pthread_mutex_destroy( &sum_mutex ); //注销锁
}

测试结果：

wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_5
hello in thread hello in thread hello in thread 410
before sum is hello in thread 0 in thread 4
after sum is 4 in thread 4hello in thread 

2
3
before sum is 4 in thread 1
after sum is 5 in thread 1
before sum is 5 in thread 0
after sum is 5 in thread 0
before sum is 5 in thread 2
after sum is 7 in thread 2
before sum is 7 in thread 3
after sum is 10 in thread 3
finally sum is 10

可知，sum的访问和修改顺序是正常的，这就达到了多线程的目的了，但是线程的运行顺序是混乱的，混乱就是正常？

六、信号量的实现

信号量是线程同步的另一种实现机制，信号量的操作有signal和wait，本例子采用条件信号变量pthread_cond_t tasks_cond;信号量的实现也要给予锁机制。

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

using namespace std;

#define BOUNDARY 5

int tasks = 10;
pthread_mutex_t tasks_mutex; //互斥锁
pthread_cond_t tasks_cond; //条件信号变量，处理两个线程间的条件关系，当task>5，hello2处理，反之hello1处理，直到task减为0

void* say_hello2( void* args )
{
    pthread_t pid = pthread_self(); //获取当前线程id
    cout << "[" << pid << "] hello in thread " <<  *( ( int* )args ) << endl;
    
    bool is_signaled = false; //sign
    while(1)
    {
    pthread_mutex_lock( &tasks_mutex ); //加锁
    if( tasks > BOUNDARY )
    {
        cout << "[" << pid << "] take task: " << tasks << " in thread " << *( (int*)args ) << endl;
         --tasks; //modify
    }
    else if( !is_signaled )
    {
        cout << "[" << pid << "] pthread_cond_signal in thread " << *( ( int* )args ) << endl;
        pthread_cond_signal( &tasks_cond ); //signal:向hello1发送信号，表明已经>5
        is_signaled = true; //表明信号已发送，退出此线程
    }
    pthread_mutex_unlock( &tasks_mutex ); //解锁
    if( tasks == 0 )
        break;
    }    
}

void* say_hello1( void* args )
{
    pthread_t pid = pthread_self(); //获取当前线程id
    cout << "[" << pid << "] hello in thread " <<  *( ( int* )args ) << endl;

    while(1)
    {
        pthread_mutex_lock( &tasks_mutex ); //加锁
        if( tasks > BOUNDARY )
        {
        　　cout << "[" << pid << "] pthread_cond_signal in thread " << *( ( int* )args ) << endl;
        　　pthread_cond_wait( &tasks_cond, &tasks_mutex ); //wait:等待信号量生效，接收到信号，向hello2发出信号，跳出wait,执行后续 
        }
        else
        {
        　　cout << "[" << pid << "] take task: " << tasks << " in thread " << *( (int*)args ) << endl;
            --tasks;
   　　 }
        pthread_mutex_unlock( &tasks_mutex ); //解锁
        if( tasks == 0 )
            break;
    } 
}


int main()
{
    pthread_attr_t attr; //线程属性结构体，创建线程时加入的参数
    pthread_attr_init( &attr ); //初始化
    pthread_attr_setdetachstate( &attr, PTHREAD_CREATE_JOINABLE ); //是设置你想要指定线程属性参数，这个参数表明这个线程是可以join连接的，join功能表示主程序可以等线程结束后再去做某事，实现了主程序和线程同步功能
    pthread_cond_init( &tasks_cond, NULL ); //初始化条件信号量
    pthread_mutex_init( &tasks_mutex, NULL ); //初始化互斥量
    pthread_t tid1, tid2; //保存两个线程id
    int index1 = 1;
    int ret = pthread_create( &tid1, &attr, say_hello1, ( void* )&index1 );
    if( ret != 0 )
    {
        cout << "pthread_create error:error_code=" << ret << endl;
    }
    int index2 = 2;
    ret = pthread_create( &tid2, &attr, say_hello2, ( void* )&index2 );
    if( ret != 0 )
    {
        cout << "pthread_create error:error_code=" << ret << endl;
    }
    pthread_join( tid1, NULL ); //连接两个线程
    pthread_join( tid2, NULL ); 

    pthread_attr_destroy( &attr ); //释放内存 
    pthread_mutex_destroy( &tasks_mutex ); //注销锁
    pthread_cond_destroy( &tasks_cond ); //正常退出
}

测试结果：先在线程2中执行say_hello2，再跳转到线程1中执行say_hello1，直到tasks减到0为止。

wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_6
[3069823856] hello in thread 2
[3078216560] hello in thread 1[3069823856] take task: 10 in thread 2

[3069823856] take task: 9 in thread 2
[3069823856] take task: 8 in thread 2
[3069823856] take task: 7 in thread 2
[3069823856] take task: 6 in thread 2
[3069823856] pthread_cond_signal in thread 2
[3078216560] take task: 5 in thread 1
[3078216560] take task: 4 in thread 1
[3078216560] take task: 3 in thread 1
[3078216560] take task: 2 in thread 1
[3078216560] take task: 1 in thread 1

到此，对多线程编程有了一个初步的了解，当然还有其他实现线程同步的机制，有待进一步探索。

参考：http://blog.csdn.net/hitwengqi/article/details/8015646