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C ++ Multithreading

Multithreading is a specialized form of multi-tasking, multi-tasking allows let the computer run two or more programs. In general, two types of multitasking: process-based and thread-based.

  • Process-based multitasking program is executed concurrently.
  • Processing thread-based multitasking is the concurrent execution of the same program fragment.

Multithreaded program contains two or more parts that can run simultaneously. Such a program for each part is called a thread, each thread defines a separate path of execution.

C ++ does not include any built-in support for multi-threaded applications. Instead, it is totally dependent on the operating system to provide this functionality.

This tutorial assumes that you are using the Linux operating system, we want to use POSIX write multithreaded C ++ program. API POSIX Threads or Pthreads can be provided on multiple types of Unix POSIX systems available, such as FreeBSD, NetBSD, GNU / Linux, Mac OS X and Solaris.

Create a thread

The following program, we can use it to create a POSIX threads:

#include <pthread.h>
pthread_create (thread, attr, start_routine, arg) 

Here, pthread_create creates a new thread and make it executable.The following is a description of the parameters:

参数 描述
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。

When a thread is created successful, the function returns 0, if the return value of 0 indicates that failed to create thread.

Terminate the thread

Use the following procedure, we can use it to terminate a POSIX threads:

#include <pthread.h>
pthread_exit (status) 

Here, pthread_exit used to explicitly exit a thread.Under normal circumstances, pthread_exit () function is called when the job done without having to continue to exist after the thread.

If the main () before the end of the thread that it created, and exits through pthread_exit (), then the other threads will continue to execute. Otherwise, they will be in the main () at the end is automatically terminated.

Examples

The following simple example code uses pthread_create () function creates five threads, each output "Hello w3big!":

#include <iostream>
// 必须的头文件是
#include <pthread.h>

using namespace std;

#define NUM_THREADS 5

// 线程的运行函数
void* say_hello(void* args)
{
    cout << "Hello w3big!" << endl;
}

int main()
{
    // 定义线程的 id 变量,多个变量使用数组
    pthread_t tids[NUM_THREADS];
    for(int i = 0; i < NUM_THREADS; ++i)
    {
        //参数依次是:创建的线程id,线程参数,调用的函数,传入的函数参数
        int ret = pthread_create(&tids[i], NULL, say_hello, NULL);
        if (ret != 0)
        {
           cout << "pthread_create error: error_code=" << ret << endl;
        }
    }
    //等各个线程退出后,进程才结束,否则进程强制结束了,线程可能还没反应过来;
    pthread_exit(NULL);
}

Use -lpthread library compiled the following program:

$ g++ test.cpp -lpthread -o test.o

Now, the implementation of the program will produce the following results:

$ ./test.o
Hello w3big!
Hello w3big!
Hello w3big!
Hello w3big!
Hello w3big!

The following simple example code uses pthread_create () function creates five threads, and receive incoming parameters. Each thread prints a "Hello w3big!" Message, and outputs the received parameters, and then call pthread_exit () terminates the thread.

//文件名:test.cpp

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

using namespace std;

#define NUM_THREADS     5

void *PrintHello(void *threadid)
{  
   // 对传入的参数进行强制类型转换,由无类型指针变为整形数指针,然后再读取
   int tid = *((int*)threadid);
   cout << "Hello w3big! 线程 ID, " << tid << endl;
   pthread_exit(NULL);
}

int main ()
{
   pthread_t threads[NUM_THREADS];
   int indexes[NUM_THREADS];// 用数组来保存i的值
   int rc;
   int i;
   for( i=0; i < NUM_THREADS; i++ ){      
      cout << "main() : 创建线程, " << i << endl;
      indexes[i] = i; //先保存i的值
      // 传入的时候必须强制转换为void* 类型,即无类型指针        
      rc = pthread_create(&threads[i], NULL, 
                          PrintHello, (void *)&(indexes[i]));
      if (rc){
         cout << "Error:无法创建线程," << rc << endl;
         exit(-1);
      }
   }
   pthread_exit(NULL);
}

Now compile and execute the program will produce the following results:

$ g++ test.cpp -lpthread -o test.o
$ ./test.o
main() : 创建线程, 0
main() : 创建线程, 1
main() : 创建线程, 2
main() : 创建线程, 3
main() : 创建线程, 4
Hello w3big! 线程 ID, 4
Hello w3big! 线程 ID, 3
Hello w3big! 线程 ID, 2
Hello w3big! 线程 ID, 1
Hello w3big! 线程 ID, 0

Pass parameters to a Thread

This example demonstrates how to pass multiple parameters structure. You can pass any type of data in the thread callback, because it points to the void, as shown in the following examples:

#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 = "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);
}

When the above code is compiled and executed, it produces the following results:

$ g++ -Wno-write-strings test.cpp -lpthread -o test.o
$ ./test.o
main() : creating thread, 0
main() : creating thread, 1
main() : creating thread, 2
main() : creating thread, 3
main() : creating thread, 4
Thread ID : 3 Message : This is message
Thread ID : 2 Message : This is message
Thread ID : 0 Message : This is message
Thread ID : 1 Message : This is message
Thread ID : 4 Message : This is message

Connecting and separating thread

We can use the following two functions to connect or separate threads:

pthread_join (threadid, status) 
pthread_detach (threadid) 

pthread_join () subroutine hinder the calling program until the specified threadid thread terminates. When you create a thread, it's an attribute defines whether it is connected (joinable) or detachable (detached). Define only create a thread that can be connected can be connected. If a thread is created is defined as separable, it can never be connected.

This example demonstrates how to use pthread_join () function to wait for the completion thread.

#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;

   // 初始化并设置线程为可连接的(joinable)
   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);
      }
   }

   // 删除属性,并等待其他线程
   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);
}

When the above code is compiled and executed, it produces the following results:

main() : creating thread, 0
main() : creating thread, 1
main() : creating thread, 2
main() : creating thread, 3
main() : creating thread, 4
Sleeping in thread 
Thread with id : 4  ...exiting 
Sleeping in thread 
Thread with id : 3  ...exiting 
Sleeping in thread 
Thread with id : 2  ...exiting 
Sleeping in thread 
Thread with id : 1  ...exiting 
Sleeping in thread 
Thread with id : 0  ...exiting 
Main: completed thread id :0  exiting with status :0
Main: completed thread id :1  exiting with status :0
Main: completed thread id :2  exiting with status :0
Main: completed thread id :3  exiting with status :0
Main: completed thread id :4  exiting with status :0
Main: program exiting.