c++多线程编程

一直对多线程编程这一块很陌生，决定花一点时间整理一下。

os:ubuntu 10.04  c++

1.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 muti_thread_test_1 muti_thread_test_1.cpp -lpthread

注意：

1）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。

2）-lpthread 编译选项到位置可任意，如g++ -lpthread -o muti_thread_test_1 muti_thread_test_1.cpp

3）注意gcc和g++的区别，转到此文：点击打开链接

测试结果：

 wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_1
 hello...hello...
 hello...
 hello...

 hello...

 wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_1
 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？

2.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。

 #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...

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


 hello...
 hello...

3.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 ); //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
 }

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？

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：

 #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

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。

4.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。

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;

 #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

5.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 ); //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
 }

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？

6.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。