什么是线程池：
　　　　首先，顾名思义，就是把一堆开辟好的线程放在一个池子里统一管理，就是一个线程池。

　　其次，为什么要用线程池，难道来一个请求给它申请一个线程，请求处理完了释放线程不行么？也行，但是如果创建线程和销毁线程的时间比线程处理请求的时间长，而且请求很多的情况下，我们的CPU资源都浪费在了创建和销毁线程上了，所以这种方法的效率比较低，于是，我们可以将若干已经创建完成的线程放在一起统一管理，如果来了一个请求，我们从线程池中取出一个线程来处理，处理完了放回池内等待下一个任务，线程池的好处是避免了繁琐的创建和结束线程的时间，有效的利用了CPU资源。

　　按照我的理解，线程池的作用和双缓冲的作用类似，可以完成任务处理的“鱼贯”动作。

　　最后，如何才能创建一个线程池的模型呢，一般需要以下三个参与者：

　　　　1、线程池结构，它负责管理多个线程并提供任务队列的接口

　　　　2、工作线程，它们负责处理任务

　　　　3、任务队列，存放待处理的任务

　　有了三个参与者，下一个问题就是怎么使线程池安全有序的工作，可以使用POSIX中的信号量、互斥锁和条件变量等同步手段。有了这些认识，我们就可以创建自己的线程池

 

 代码示例如下：

#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <assert.h>
#include <stdio.h>
#include <string.h>
#include <signal.h>
#include <errno.h>
#include "threadpool.h"

#define DEFAULT_TIME 10                 /*10s检测一次*/
#define MIN_WAIT_TASK_NUM 10            /*如果queue_size > MIN_WAIT_TASK_NUM 添加新的线程到线程池*/ 
#define DEFAULT_THREAD_VARY 10          /*每次创建和销毁线程的个数*/
#define true 1
#define false 0

typedef struct
{
    void *(*function)(void *);          /*函数指针，回调函数*/
    void *arg;                          /*上面函数的参数*/
} threadpool_task_t;                    /*任务结构体*/

struct threadpool_t
{
    pthread_mutex_t lock;               /*用于锁住当前这个结构体体taskpoll*/    
    pthread_mutex_t thread_counter;     /*记录忙状态线程个数*/
    pthread_cond_t queue_not_full;      /*当任务队列满时，添加任 务的线程阻塞，等待此条件变量*/
    pthread_cond_t queue_not_empty;     /*任务队列里不为空时，通知等待任务的线程*/
    pthread_t *threads;                 /*保存工作线程tid的数组*/
    pthread_t adjust_tid;               /*管理线程tid*/
    threadpool_task_t *task_queue;      /*任务队列*/
    int min_thr_num;                    /*线程组内默认最小线程数*/
    int max_thr_num;                    /*线程组内默认最大线程数*/
    int live_thr_num;                   /*当前存活线程个数*/
    int busy_thr_num;                   /*忙状态线程个数*/
    int wait_exit_thr_num;              /*要销毁的线程个数*/
    int queue_front;                    /*队头索引下标*/
    int queue_rear;                     /*队未索引下标*/
    int queue_size;                     /*队中元素个数*/
    int queue_max_size;                 /*队列中最大容纳个数*/
    int shutdown;                       /*线程池使用状态，true或false*/
};

/**
 * @function void *threadpool_thread(void *threadpool)
 * @desc the worker thread
 * @param threadpool the pool which own the thread
 */
void *threadpool_thread(void *threadpool);
/**
 * @function void *adjust_thread(void *threadpool);
 * @desc manager thread
 * @param threadpool the threadpool
 */
void *adjust_thread(void *threadpool);
/**
 * check a thread is alive
 */
int is_thread_alive(pthread_t tid);

int threadpool_free(threadpool_t *pool);

threadpool_t *threadpool_create(int min_thr_num, int max_thr_num, int queue_max_size)
{
    int i;
    threadpool_t *pool = NULL;
    do{
        if((pool = (threadpool_t *)malloc(sizeof(threadpool_t))) == NULL)
        {
            printf("malloc threadpool fail");
            break;          /*跳出do while*/
        }

        pool->min_thr_num = min_thr_num;
        pool->max_thr_num = max_thr_num;
        pool->busy_thr_num = 0;
        pool->live_thr_num = min_thr_num;
        pool->queue_size = 0;
        pool->queue_max_size = queue_max_size;
        pool->queue_front = 0;
        pool->queue_rear = 0;
        pool->shutdown = false;

        pool->threads = (pthread_t *)malloc(sizeof(pthread_t)*max_thr_num);
        if (pool->threads == NULL)
        {
            printf("malloc threads fail");
            break;
        }
        memset(pool->threads, 0, sizeof(pool->threads));

        pool->task_queue = (threadpool_task_t *)malloc(sizeof(threadpool_task_t)*queue_max_size);
        if (pool->task_queue == NULL)
        {
            printf("malloc task_queue fail");
            break;
        }

        if (pthread_mutex_init(&(pool->lock), NULL) != 0
                || pthread_mutex_init(&(pool->thread_counter), NULL) != 0
                || pthread_cond_init(&(pool->queue_not_empty), NULL) != 0
                || pthread_cond_init(&(pool->queue_not_full), NULL) != 0)
        {
            printf("init the lock or cond fail");
            break;
        }

        /* 启动min_thr_num个work thread */
        for (i = 0; i < min_thr_num; i++)
        {
            pthread_create(&(pool->threads[i]), NULL, threadpool_thread, (void *)pool);
            printf("start thread 0x%x...\n", (unsigned int)pool->threads[i]);
        }
        pthread_create(&(pool->adjust_tid), NULL, adjust_thread, (void *)pool);
        return pool;

    }while(0);
    
    threadpool_free(pool);      /*前面代码调用失败时，释放poll存储空间*/
    return NULL;
}

int threadpool_add(threadpool_t *pool, void*(*function)(void *arg), void *arg)
{
    pthread_mutex_lock(&(pool->lock));

    while ((pool->queue_size == pool->queue_max_size) && (!pool->shutdown))
    {
        pthread_cond_wait(&(pool->queue_not_full), &(pool->lock));
    }
    if (pool->shutdown)
    {
        pthread_mutex_unlock(&(pool->lock));
    }

    /*添加任务到任务队列里*/
    if (pool->task_queue[pool->queue_rear].arg != NULL)
    {
        free(pool->task_queue[pool->queue_rear].arg);
        pool->task_queue[pool->queue_rear].arg = NULL;
    }
    pool->task_queue[pool->queue_rear].function = function;
    pool->task_queue[pool->queue_rear].arg = arg;
    pool->queue_rear = (pool->queue_rear + 1)%pool->queue_max_size;
    pool->queue_size++;

    /*任务队列不为空，唤醒等待处理任务的线程*/
    pthread_cond_signal(&(pool->queue_not_empty));
    pthread_mutex_unlock(&(pool->lock));

    return 0;
}

void *threadpool_thread(void *threadpool)
{
    threadpool_t *pool = (threadpool_t *)threadpool;
    threadpool_task_t task;
    while(true)
    {
        /* Lock must be taken to wait on conditional variable */
        /*刚创建出线程，等待任务队列里有任务，否则阻塞等待任务队列里有任务后再唤醒接收任务*/
        pthread_mutex_lock(&(pool->lock));

        while ((pool->queue_size == 0) && (!pool->shutdown))
        {
            printf("thread 0x%x is waiting\n", (unsigned int)pthread_self());
            pthread_cond_wait(&(pool->queue_not_empty), &(pool->lock));
            /*清除指定数目的空闲线程，如果要结束的线程个数大于0，结束线程*/
            if (pool->wait_exit_thr_num > 0)
            {
                pool->wait_exit_thr_num--;
                /*如果线程池里线程个数大于最小值时可以结束当前线程*/
                if (pool->live_thr_num > pool->min_thr_num)
                {
                    printf("thread 0x%x is exiting\n", (unsigned int)pthread_self());
                    pool->live_thr_num--;
                    pthread_mutex_unlock(&(pool->lock));
                    pthread_exit(NULL);
                }
            }
        }

        /*如果指定了true，要关闭线程池里的每个线程，自行退出处理*/
        if (pool->shutdown)
        {
            pthread_mutex_unlock(&(pool->lock));
            printf("thread 0x%x is exiting\n", (unsigned int)pthread_self());
            pthread_exit(NULL);
        }
        /*从任务队列里获得任务*/
        task.function = pool->task_queue[pool->queue_front].function;
        task.arg = pool->task_queue[pool->queue_front].arg;
        pool->queue_front = (pool->queue_front + 1)%pool->queue_max_size;
        pool->queue_size--;

        /*通知可以有新的任务添加进来*/
        pthread_cond_broadcast(&(pool->queue_not_full));

        pthread_mutex_unlock(&(pool->lock));

        /*执行任务*/ 
        printf("thread 0x%x start working\n", (unsigned int)pthread_self());
        pthread_mutex_lock(&(pool->thread_counter));
        pool->busy_thr_num++;                                       /*忙状态线程数加1*/
        pthread_mutex_unlock(&(pool->thread_counter));
        (*(task.function))(task.arg);                               /*执行回调函数任务*/
        //task.function(task.arg);                               /*执行回调函数任务*/

        /*任务结束处理*/ 
        printf("thread 0x%x end working\n", (unsigned int)pthread_self());
        pthread_mutex_lock(&(pool->thread_counter));
        pool->busy_thr_num--;                                       /*忙状态数减1*/
        pthread_mutex_unlock(&(pool->thread_counter));
    }

    pthread_exit(NULL);
    //return (NULL);
}


void *adjust_thread(void *threadpool)
{
    int i;
    threadpool_t *pool = (threadpool_t *)threadpool;
    while (!pool->shutdown)
    {
        sleep(DEFAULT_TIME);                                    /*延时10秒*/
        pthread_mutex_lock(&(pool->lock));
        int queue_size = pool->queue_size;
        int live_thr_num = pool->live_thr_num;
        pthread_mutex_unlock(&(pool->lock));

        pthread_mutex_lock(&(pool->thread_counter));
        int busy_thr_num = pool->busy_thr_num;
        pthread_mutex_unlock(&(pool->thread_counter));

        /*任务数大于最小线程池个数并且存活的线程数少于最大线程个数时，创建新线程*/
        if (queue_size >= MIN_WAIT_TASK_NUM && live_thr_num < pool->max_thr_num)
        {
            pthread_mutex_lock(&(pool->lock));
            int add = 0;
            /*一次增加DEFAULT_THREAD个线程*/
            for (i = 0; i < pool->max_thr_num && add < DEFAULT_THREAD_VARY && pool->live_thr_num < pool->max_thr_num; i++)
            {
                if (pool->threads[i] == 0 || !is_thread_alive(pool->threads[i]))
                {
                    pthread_create(&(pool->threads[i]), NULL, threadpool_thread, (void *)pool);
                    add++;
                    pool->live_thr_num++;
                }
            }
            pthread_mutex_unlock(&(pool->lock));
        }

        /*销毁多余的空闲线程*/
        if ((busy_thr_num * 2) < live_thr_num
                && live_thr_num > pool->min_thr_num)
        {
            /*一次销毁DEFAULT_THREAD个线程*/
            pthread_mutex_lock(&(pool->lock));
            pool->wait_exit_thr_num = DEFAULT_THREAD_VARY;
            pthread_mutex_unlock(&(pool->lock));

            for (i = 0; i < DEFAULT_THREAD_VARY; i++)
            {
                /*通知处在空闲状态的线程*/
                pthread_cond_signal(&(pool->queue_not_empty));
            }
        }
    }
}

int threadpool_destroy(threadpool_t *pool)
{
    int i;
    if (pool == NULL)
    {
        return -1;
    }

    pool->shutdown = true;
    /*先销毁管理线程*/
    pthread_join(pool->adjust_tid, NULL);
    for (i = 0; i < pool->live_thr_num; i++)
    {
        /*通知所有的空闲线程*/
        pthread_cond_broadcast(&(pool->queue_not_empty));
        pthread_join(pool->threads[i], NULL);
    }
    threadpool_free(pool);
    return 0;
}

int threadpool_free(threadpool_t *pool)
{
    if (pool == NULL)
    {
        return -1;
    }
    if (pool->task_queue)
    {
        free(pool->task_queue);
    }
    if (pool->threads)
    {
        free(pool->threads);
        pthread_mutex_lock(&(pool->lock));
        pthread_mutex_destroy(&(pool->lock));
        pthread_mutex_lock(&(pool->thread_counter));
        pthread_mutex_destroy(&(pool->thread_counter));
        pthread_cond_destroy(&(pool->queue_not_empty));
        pthread_cond_destroy(&(pool->queue_not_full));
    }
    free(pool);
    pool = NULL;
    return 0;
}

int threadpool_all_threadnum(threadpool_t *pool)
{
    int all_threadnum = -1;
    pthread_mutex_lock(&(pool->lock));
    all_threadnum = pool->live_thr_num;
    pthread_mutex_unlock(&(pool->lock));
    return all_threadnum;
}

int threadpool_busy_threadnum(threadpool_t *pool)
{
    int busy_threadnum = -1;
    pthread_mutex_lock(&(pool->thread_counter));
    busy_threadnum = pool->busy_thr_num;
    pthread_mutex_unlock(&(pool->thread_counter));
    return busy_threadnum;
}

int is_thread_alive(pthread_t tid)
{
    int kill_rc = pthread_kill(tid, 0);
    if (kill_rc == ESRCH)
    {
        return false;
    }
    return true;
}

/*测试使用，作成库时请注释掉下面代码*/ 
#if 1
void *process(void *arg)
{
    printf("thread 0x%x working on task %d\n ",(unsigned int)pthread_self(),*(int *)arg);
    sleep(1);
    printf("task %d is end\n",*(int *)arg);
    return NULL;
}
int main(void)
{
    threadpool_t *thp = threadpool_create(3,100,100);    /*线程池里最小3个线程，最大100个，队列最大值12*/
    printf("pool inited");

    int *num = (int *)malloc(sizeof(int)*20);
    //int num[20];
    int i;
    for (i=0;i<10;i++)
    {
        num[i]=i;
        printf("add task %d\n",i);
        threadpool_add(thp,process,(void*)&num[i]);
    }
    sleep(10);
    threadpool_destroy(thp);
}
#endif