thread pool

什么时候需要创建线程池呢？简单的说，如果一个应用需要频繁的创建和销毁线程，而任务执行的时间又非常短，这样线程创建和销毁的带来的开销就不容忽视，这时也是线程池该出场的机会了。如果线程创建和销毁时间相比任务执行时间可以忽略不计，则没有必要使用线程池了。

   下面是Linux系统下用C语言创建的一个线程池。线程池会维护一个任务链表(每个CThread_worker结构就是一个任务)。
   pool_init()函数预先创建好max_thread_num个线程，每个线程执thread_routine ()函数。该函数中

    while (pool->cur_queue_size == 0)
    {
           pthread_cond_wait (&(pool->queue_ready),&(pool->queue_lock));
    }

表示如果任务链表中没有任务，则该线程出于阻塞等待状态。否则从队列中取出任务并执行。
   
   pool_add_worker()函数向线程池的任务链表中加入一个任务，加入后通过调用pthread_cond_signal (&(pool->queue_ready))唤醒一个出于阻塞状态的线程(如果有的话)。
   
   pool_destroy ()函数用于销毁线程池，线程池任务链表中的任务不会再被执行，但是正在运行的线程会一直把任务运行完后再退出。

    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <sys/types.h>
    #include <pthread.h>
    #include <assert.h>

    /*
    *线程池里所有运行和等待的任务都是一个CThread_worker
    *由于所有任务都在链表里，所以是一个链表结构
    */
    typedef struct worker
    {
        /*回调函数，任务运行时会调用此函数，注意也可声明成其它形式*/
        void *(*process) (void *arg);
        void *arg;/*回调函数的参数*/
        struct worker *next;

    } CThread_worker;


    /*线程池结构*/
    typedef struct
    {
         pthread_mutex_t queue_lock;
         pthread_cond_t queue_ready;

        /*链表结构，线程池中所有等待任务*/
         CThread_worker *queue_head;

        /*是否销毁线程池*/
        int shutdown;
         pthread_t *threadid;
        /*线程池中允许的活动线程数目*/
        int max_thread_num;
        /*当前等待队列的任务数目*/
        int cur_queue_size;

    } CThread_pool;


    int pool_add_worker (void *(*process) (void *arg), void *arg);
    void *thread_routine (void *arg);


    static CThread_pool *pool = NULL;
    void
    pool_init (int max_thread_num)
    {
         pool = (CThread_pool *) malloc (sizeof (CThread_pool));

         pthread_mutex_init (&(pool->queue_lock), NULL);
         pthread_cond_init (&(pool->queue_ready), NULL);

         pool->queue_head = NULL;

         pool->max_thread_num = max_thread_num;
         pool->cur_queue_size = 0;

         pool->shutdown = 0;

         pool->threadid =
             (pthread_t *) malloc (max_thread_num * sizeof (pthread_t));
        int i = 0;
        for (i = 0; i < max_thread_num; i++)
         {
             pthread_create (&(pool->threadid[i]), NULL, thread_routine,
                     NULL);
         }
    }


    /*向线程池中加入任务*/
    int
    pool_add_worker (void *(*process) (void *arg), void *arg)
    {
        /*构造一个新任务*/
         CThread_worker *newworker =
             (CThread_worker *) malloc (sizeof (CThread_worker));
         newworker->process = process;
         newworker->arg = arg;
         newworker->next = NULL;/*别忘置空*/

         pthread_mutex_lock (&(pool->queue_lock));
        /*将任务加入到等待队列中*/
         CThread_worker *member = pool->queue_head;
        if (member != NULL)
         {
            while (member->next != NULL)
                 member = member->next;
             member->next = newworker;
         }
        else
         {
             pool->queue_head = newworker;
         }

         assert (pool->queue_head != NULL);

         pool->cur_queue_size++;
         pthread_mutex_unlock (&(pool->queue_lock));
        /*好了，等待队列中有任务了，唤醒一个等待线程；
         注意如果所有线程都在忙碌，这句没有任何作用*/
         pthread_cond_signal (&(pool->queue_ready));
        return 0;
    }


    /*销毁线程池，等待队列中的任务不会再被执行，但是正在运行的线程会一直
    把任务运行完后再退出*/
    int
    pool_destroy ()
    {
        if (pool->shutdown)
            return -1;/*防止两次调用*/
         pool->shutdown = 1;

        /*唤醒所有等待线程，线程池要销毁了*/
         pthread_cond_broadcast (&(pool->queue_ready));

        /*阻塞等待线程退出，否则就成僵尸了*/
        int i;
        for (i = 0; i < pool->max_thread_num; i++)
             pthread_join (pool->threadid[i], NULL);
         free (pool->threadid);

        /*销毁等待队列*/
         CThread_worker *head = NULL;
        while (pool->queue_head != NULL)
         {
             head = pool->queue_head;
             pool->queue_head = pool->queue_head->next;
             free (head);
         }
        /*条件变量和互斥量也别忘了销毁*/
         pthread_mutex_destroy(&(pool->queue_lock));
         pthread_cond_destroy(&(pool->queue_ready));
        
         free (pool);
        /*销毁后指针置空是个好习惯*/
         pool=NULL;
        return 0;
    }


    void *
    thread_routine (void *arg)
    {
         printf ("starting thread 0x%d\n", (int)pthread_self());
        while (1)
         {
             pthread_mutex_lock (&(pool->queue_lock));
            /*如果等待队列为0并且不销毁线程池，则处于阻塞状态; 注意
             pthread_cond_wait是一个原子操作，等待前会解锁，唤醒后会加锁*/
            while (pool->cur_queue_size == 0 && !pool->shutdown)
             {
                 printf ("thread 0x%x is waiting\n", (int)pthread_self ());
                 pthread_cond_wait (&(pool->queue_ready), &(pool->queue_lock));
             }

            /*线程池要销毁了*/
            if (pool->shutdown)
             {
                /*遇到break,continue,return等跳转语句，千万不要忘记先解锁*/
                 pthread_mutex_unlock (&(pool->queue_lock));
                 printf ("thread 0x%x will exit\n", (int)pthread_self());
                 pthread_exit (NULL);
             }

             printf ("thread 0x%x is starting to work\n", (int)pthread_self());

            /*assert是调试的好帮手*/
             assert (pool->cur_queue_size != 0);
             assert (pool->queue_head != NULL);
            
            /*等待队列长度减去1，并取出链表中的头元素*/
             pool->cur_queue_size--;
             CThread_worker *worker = pool->queue_head;
             pool->queue_head = worker->next;
             pthread_mutex_unlock (&(pool->queue_lock));

            /*调用回调函数，执行任务*/
             (*(worker->process)) (worker->arg);
             free (worker);
             worker = NULL;
         }
        /*这一句应该是不可达的*/
         pthread_exit (NULL);
    }

   // 下面是测试代码

    void *
    myprocess (void *arg)
    {
         printf ("threadid is 0x%x, working on task %d\n", (int)pthread_self (),*(int *) arg);
         sleep (1);/*休息一秒，延长任务的执行时间*/
        return NULL;
    }

    int
    main (int argc, char **argv)
    {
         pool_init (3);/*线程池中最多三个活动线程*/
        
        /*连续向池中投入10个任务*/
        int *workingnum = (int *) malloc (sizeof (int) * 10);
        int i;
        for (i = 0; i < 10; i++)
         {
             workingnum[i] = i;
             pool_add_worker (myprocess, &workingnum[i]);
         }
        /*等待所有任务完成*/
         sleep (5);
        /*销毁线程池*/
         pool_destroy ();

         free (workingnum);
        return 0;
    }

 

starting thread 0x-1871501568
thread 0x90732700 is starting to work
threadid is 0x90732700, working on task 0
starting thread 0x-1854716160
thread 0x91734700 is starting to work
threadid is 0x91734700, working on task 1
starting thread 0x-1863108864
thread 0x90f33700 is starting to work
threadid is 0x90f33700, working on task 2
thread 0x90732700 is starting to work
threadid is 0x90732700, working on task 3
thread 0x91734700 is starting to work
threadid is 0x91734700, working on task 4
thread 0x90f33700 is starting to work
threadid is 0x90f33700, working on task 5
thread 0x90732700 is starting to work
threadid is 0x90732700, working on task 6
thread 0x91734700 is starting to work
threadid is 0x91734700, working on task 7
thread 0x90f33700 is starting to work
threadid is 0x90f33700, working on task 8
thread 0x90732700 is starting to work
threadid is 0x90732700, working on task 9
thread 0x91734700 is waiting
thread 0x90f33700 is waiting
thread 0x90732700 is waiting
thread 0x91734700 will exit
thread 0x90f33700 will exit
thread 0x90732700 will exit