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...权协议,转载请附上原文出处链接及本声明。 原文链接:https://blog.csdn.net/hitwengqi/article/details/8015646
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_ 3hello 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