Linux多线程编程入门笔记

最基础，进程同时创建5个线程，各自调用同一个函数

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#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;  
    return NULL;
} //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 
    return 0; 
}

输入命令：g++ pthread_chap1.cpp -o pthread_chap1 -lpthread

注意:

  • 此为c++程序，故用g++来编译生成可执行文件，并且要调用处理多线程操作相关的静态链接库文件pthread。
  • -lpthread 编译选项到位置可任意，如g++ -o pthread_chap1 pthread_chap1.cpp -lpthread
  • ​

测试结果：

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jack@jack:~/coding/muti_thread$ ./pthread_chap1 
hello...hello...  
hello...  
hello...  
  
hello... 


jack@jack:~/coding/muti_thread$ ./pthread_chap1 
hello...hello...hello...  
hello...  
  
hello...

可知，两次运行的结果会有差别，这不是多线程的特点吧？这显然没有同步？还有待进一步探索…
多线程的运行是混乱的，混乱就是正常？

线程调用到函数在一个类中，那必须将该函数声明为静态函数函数

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。

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#include <iostream>  
#include <pthread.h>  
  
using namespace std;  
  
#define NUM_THREADS 5  
  
class Hello  
{  
public:  
    static void* say_hello( void* args )  
    {  
        cout << "hello..." << endl;  
        return NULL;
    }  
};  
  
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 );  
    return 0;
}

测试结果：

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jack@jack:~/coding/muti_thread$ ./pthread_chap2
hello...hello...hello...  
hello...  
hello... 

jack@jack:~/coding/muti_thread$ ./pthread_chap2
hello...
hello...
hello...  
hello...  
hello...

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：

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#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; 
    return NULL; 
} //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 ); //传入到参数必须强转为void*类型，即无类型指针，&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 
    return 0; 
}

测试结果：

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jack@jack:~/coding/muti_thread$ ./pthread_chap2 
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？
这是因为多线程到缘故，主进程还没开始对i赋值，线程已经开始跑了…?
修改代码如下：

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#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;  
    return NULL;
} //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 

    return 0; 
}

测试结果：

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jack@jack:~/coding/muti_thread$ ./pthread_chap3 
hello in thread hello in thread hello in thread hello in thread hello in thread 30124

这是正常的吗？感觉还是有问题…待续

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。

线程创建时属性参数的设置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。

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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;
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#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 ); 
    return NULL;  
}  
  
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;  
        }  
    }  
    return 0;
}

测试结果：

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jack@jack:~/coding/muti_thread$ ./pthread_chap4  
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

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。

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#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 ); 
    return NULL;  
}  
  
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  
    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  
    return 0;
}

测试结果:

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jack@jack:~/coding/muti_thread$ ./pthread_chap5 
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？

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。

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#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;  
    }      
    return NULL;
}  
  
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;  
    }   
    return NULL;
}  
  
  
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  
    return 0;
}

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。

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jack@jack:~/coding/muti_thread$ ./pthread_chap6  
[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。