C++多线程

c++多线程

多线程其实非常简单

多线程是多任务处理的一种特殊形式，多任务处理允许让电脑同时运行两个或两个以上的程序。一般情况下，两种类型的多任务处理：基于进程和基于线程。

创建线程

#include <pthread.h>
pthread_create (thread, attr, start_routine, arg) 
//创建一个 POSIX 线程
/*
thread 指向线程标识符指针
attr 一个不透明的属性对象，可以被用来设置线程属性。您可以指定线程属性对象，也可以使用默认值 NULL
start_routine 线程运行函数起始地址，一旦线程被创建就会执行
arg 运行函数的参数。它必须通过把引用作为指针强制转换为 void 类型进行传递。如果没有传递参数，则使用 NULL
*/

终止线程

#include <pthread.h>
pthread_exit (status) 
//终止一个 POSIX 线程
/*
在这里，pthread_exit 用于显式地退出一个线程。通常情况下，pthread_exit() 函数是在线程完成工作后无需继续存在时被调用。
 
如果 main() 是在它所创建的线程之前结束，并通过 pthread_exit() 退出，那么其他线程将继续执行。否则，它们将在 main() 结束时自动被终止。
*/

实例

#include <iostream>
#include <cstdlib>
#include <pthread.h>
using namespace std;
#define NUM_THREADS     5
void *PrintHello(void *threadid)
{  
   int tid = *((int*)threadid);
   cout << "线程ID, " << tid << endl;
   pthread_exit(NULL);
}
int main ()
{
   pthread_t threads[NUM_THREADS];
   int indexes[NUM_THREADS];
   int rc;
   int i;
   for( i=0; i < NUM_THREADS; i++ ){      
      cout << " 创建线程, " << i << endl;
      indexes[i] = i; 
      rc = pthread_create(&threads[i], NULL,  PrintHello, (void *)&(indexes[i]));
      if (rc){
         cout << "无法创建线程," << rc << endl;
         exit(-1);
      }
   }
   pthread_exit(NULL);
}

向线程传递参数

#include <iostream>
#include <cstdlib>
#include <pthread.h>
using namespace std;
#define NUM_THREADS 5
struct thread_data{
   int  thread_id;
   char *message;
};
void *PrintHello(void *threadarg)
{
   struct thread_data *my_data;
   my_data = (struct thread_data *) threadarg;
   cout << "Thread ID : " << my_data->thread_id ;
   cout << " Message : " << my_data->message << endl;
   pthread_exit(NULL);
}
 int main ()
{
   pthread_t threads[NUM_THREADS];
   struct thread_data td[NUM_THREADS];
   int rc;
   int i;
   for( i=0; i < NUM_THREADS; i++ ){
      cout <<"main() : creating thread, " << i << endl;
      td[i].thread_id = i;
      td[i].message = (char*)"This is message";
      rc = pthread_create(&threads[i], NULL,
                          PrintHello, (void *)&td[i]);
      if (rc){
         cout << "Error:unable to create thread," << rc << endl;
         exit(-1);
      }
   }
   pthread_exit(NULL);
}

这个实例演示了如何通过结构传递多个参数。

连接和分离线程

//我们可以使用以下两个函数来连接或分离线程
pthread_join (threadid, status)
pthread_detach (threadid)

这个实例演示了如何使用 pthread_join() 函数来等待线程的完成。

#include <iostream>
#include <cstdlib>
#include <pthread.h>
#include <unistd.h>
using namespace std;
#define NUM_THREADS 5
void *wait(void *t)
{
   int i;
   long tid;
   tid = (long)t;
   sleep(1);
   cout << "Sleeping in thread " << endl;
   cout << "Thread with id : " << tid << "  ...exiting " << endl;
   pthread_exit(NULL);
}
int main ()
{
   int rc;
   int i;
   pthread_t threads[NUM_THREADS];
   pthread_attr_t attr;
   void *status;
   pthread_attr_init(&attr);
   pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
    for( i=0; i < NUM_THREADS; i++ ){
      cout << "main() : creating thread, " << i << endl;
      rc = pthread_create(&threads[i], NULL, wait, (void *)&i );
      if (rc){
         cout << "Error:unable to create thread," << rc << endl;
         exit(-1);
      }
   }
   pthread_attr_destroy(&attr);
   for( i=0; i < NUM_THREADS; i++ ){
      rc = pthread_join(threads[i], &status);
      if (rc){
         cout << "Error:unable to join," << rc << endl;
         exit(-1);
      }
      cout << "Main: completed thread id :" << i ;
      cout << "  exiting with status :" << status << endl;
   }
 
   cout << "Main: program exiting." << endl;
   pthread_exit(NULL);
}

c++11后的标准的线程库

#include <iostream>
 
#include <thread>
 
std::thread::id main_thread_id = std::this_thread::get_id();
 
void hello()  
{
    std::cout << "Hello Concurrent World\n";
    if (main_thread_id == std::this_thread::get_id())
        std::cout << "This is the main thread.\n";
    else
        std::cout << "This is not the main thread.\n";
}
 
void pause_thread(int n) {
    std::this_thread::sleep_for(std::chrono::seconds(n));
    std::cout << "pause of " << n << " seconds ended\n";
}
 
int main() {
    std::thread t(hello);
    std::cout << t.hardware_concurrency() << std::endl;
    std::cout << "native_handle " << t.native_handle() << std::endl;
    t.join();
    std::thread a(hello);
    a.detach();
    std::thread threads[5];
 
    std::cout << "Spawning 5 threads...\n";
    for (int i = 0; i < 5; ++i)
        threads[i] = std::thread(pause_thread, i + 1);
    std::cout << "完成生成线程，现在正等待加入:\n";
    for (auto &thread : threads)
        thread.join();
    std::cout << "All threads joined!\n";
}