大多数的网络服务器，包括web服务器都具有一个特点，就是单位时间内必须处理数目巨大的连接请求，但是处理时间却是比较短的。在传统的多线程服务器模型中是这样实现的：一旦有个请求到达，就创建一个新的线程，由该线程执行任务，任务执行完毕之后，线程就退出。这就是"即时创建，即时销毁"的策略。尽管与创建进程相比，创建线程的时间已经大大的缩短，但是如果提交给线程的任务是执行时间较短，而且执行次数非常频繁，那么服务器就将处于一个不停的创建线程和销毁线程的状态。这笔开销是不可忽略的，尤其是线程执行的时间非常非常短的情况。
线程池就是为了解决上述问题的，它的实现原理是这样的：在应用程序启动之后，就马上创建一定数量的线程，放入空闲的队列中。这些线程都是处于阻塞状态，这些线程只占一点内存，不占用cpu。当任务到来后，线程池将选择一个空闲的线程，将任务传入此线程中运行。当所有的线程都处在处理任务的时候，线程池将自动创建一定的数量的新线程，用于处理更多的任务。执行任务完成之后线程并不退出，而是继续在线程池中等待下一次任务。当大部分线程处于阻塞状态时，线程池将自动销毁一部分的线程，回收系统资源。
下面是一个简单线程池的实现，这个线程池的代码是我参考网上的一个例子实现的，由于找不到出处了，就没办法注明参考自哪里了。它的方案是这样的：程序启动之前，初始化线程池，启动线程池中的线程，由于还没有任务到来，线程池中的所有线程都处在阻塞状态，当一有任务到达就从线程池中取出一个空闲线程处理，如果所有的线程都处于工作状态，就添加到队列，进行排队。如果队列中的任务个数大于队列的所能容纳的最大数量，那就不能添加任务到队列中，只能等待队列不满才能添加任务到队列中。
主要由两个文件组成一个threadpool.h头文件和一个threadpool.c源文件组成。源码中已有重要的注释，就不加以分析了。
threadpool.h文件：
struct job { void* (*callback_function)(void *arg); //线程回调函数 void *arg; //回调函数参数 struct job *next; }; struct threadpool { int thread_num; //线程池中开启线程的个数 int queue_max_num; //队列中最大job的个数 struct job *head; //指向job的头指针 struct job *tail; //指向job的尾指针 pthread_t *pthreads; //线程池中所有线程的pthread_t pthread_mutex_t mutex; //互斥信号量 pthread_cond_t queue_empty; //队列为空的条件变量 pthread_cond_t queue_not_empty; //队列不为空的条件变量 pthread_cond_t queue_not_full; //队列不为满的条件变量 int queue_cur_num; //队列当前的job个数 int queue_close; //队列是否已经关闭 int pool_close; //线程池是否已经关闭 }; //================================================================================================ //函数名： threadpool_init //函数描述： 初始化线程池 //输入： [in] thread_num 线程池开启的线程个数 // [in] queue_max_num 队列的最大job个数 //输出： 无 //返回： 成功：线程池地址 失败：null //================================================================================================ struct threadpool* threadpool_init(int thread_num, int queue_max_num); //================================================================================================ //函数名： threadpool_add_job //函数描述： 向线程池中添加任务 //输入： [in] pool 线程池地址 // [in] callback_function 回调函数 // [in] arg 回调函数参数 //输出： 无 //返回： 成功：0 失败：-1 //================================================================================================ int threadpool_add_job(struct threadpool *pool, void* (*callback_function)(void *arg), void *arg); //================================================================================================ //函数名： threadpool_destroy //函数描述： 销毁线程池 //输入： [in] pool 线程池地址 //输出： 无 //返回： 成功：0 失败：-1 //================================================================================================ int threadpool_destroy(struct threadpool *pool); //================================================================================================ //函数名： threadpool_function //函数描述： 线程池中线程函数 //输入： [in] arg 线程池地址 //输出： 无 //返回： 无 //================================================================================================ void* threadpool_function(void* arg);
threadpool.c文件：
#include "threadpool.h" struct threadpool* threadpool_init(int thread_num, int queue_max_num) { struct threadpool *pool = null; do { pool = malloc(sizeof(struct threadpool)); if (null == pool) { printf("failed to malloc threadpool!\n"); break; } pool->thread_num = thread_num; pool->queue_max_num = queue_max_num; pool->queue_cur_num = 0; pool->head = null; pool->tail = null; if (pthread_mutex_init(&(pool->mutex), null)) { printf("failed to init mutex!\n"); break; } if (pthread_cond_init(&(pool->queue_empty), null)) { printf("failed to init queue_empty!\n"); break; } if (pthread_cond_init(&(pool->queue_not_empty), null)) { printf("failed to init queue_not_empty!\n"); break; } if (pthread_cond_init(&(pool->queue_not_full), null)) { printf("failed to init queue_not_full!\n"); break; } pool->pthreads = malloc(sizeof(pthread_t) * thread_num); if (null == pool->pthreads) { printf("failed to malloc pthreads!\n"); break; } pool->queue_close = 0; pool->pool_close = 0; int i; for (i = 0; i < pool->thread_num; i) { pthread_create(&(pool->pthreads[i]), null, threadpool_function, (void *)pool); } return pool; } while (0); return null; } int threadpool_add_job(struct threadpool* pool, void* (*callback_function)(void *arg), void *arg) { assert(pool != null); assert(callback_function != null); assert(arg != null); pthread_mutex_lock(&(pool->mutex)); while ((pool->queue_cur_num == pool->queue_max_num) && !(pool->queue_close || pool->pool_close)) { pthread_cond_wait(&(pool->queue_not_full), &(pool->mutex)); //队列满的时候就等待 } if (pool->queue_close || pool->pool_close) //队列关闭或者线程池关闭就退出 { pthread_mutex_unlock(&(pool->mutex)); return -1; } struct job *pjob =(struct job*) malloc(sizeof(struct job)); if (null == pjob) { pthread_mutex_unlock(&(pool->mutex)); return -1; } pjob->callback_function = callback_function; pjob->arg = arg; pjob->next = null; if (pool->head == null) { pool->head = pool->tail = pjob; pthread_cond_broadcast(&(pool->queue_not_empty)); //队列空的时候，有任务来时就通知线程池中的线程：队列非空 } else { pool->tail->next = pjob; pool->tail = pjob; } pool->queue_cur_num ; pthread_mutex_unlock(&(pool->mutex)); return 0; } void* threadpool_function(void* arg) { struct threadpool *pool = (struct threadpool*)arg; struct job *pjob = null; while (1) //死循环 { pthread_mutex_lock(&(pool->mutex)); while ((pool->queue_cur_num == 0) && !pool->pool_close) //队列为空时，就等待队列非空 { pthread_cond_wait(&(pool->queue_not_empty), &(pool->mutex)); } if (pool->pool_close) //线程池关闭，线程就退出 { pthread_mutex_unlock(&(pool->mutex)); pthread_exit(null); } pool->queue_cur_num--; pjob = pool->head; if (pool->queue_cur_num == 0) { pool->head = pool->tail = null; } else { pool->head = pjob->next; } if (pool->queue_cur_num == 0) { pthread_cond_signal(&(pool->queue_empty)); //队列为空，就可以通知threadpool_destroy函数，销毁线程函数 } if (pool->queue_cur_num == pool->queue_max_num - 1) { pthread_cond_broadcast(&(pool->queue_not_full)); //队列非满，就可以通知threadpool_add_job函数，添加新任务 } pthread_mutex_unlock(&(pool->mutex)); (*(pjob->callback_function))(pjob->arg); //线程真正要做的工作，回调函数的调用 free(pjob); pjob = null; } } int threadpool_destroy(struct threadpool *pool) { assert(pool != null); pthread_mutex_lock(&(pool->mutex)); if (pool->queue_close || pool->pool_close) //线程池已经退出了，就直接返回 { pthread_mutex_unlock(&(pool->mutex)); return -1; } pool->queue_close = 1; //置队列关闭标志 while (pool->queue_cur_num != 0) { pthread_cond_wait(&(pool->queue_empty), &(pool->mutex)); //等待队列为空 } pool->pool_close = 1; //置线程池关闭标志 pthread_mutex_unlock(&(pool->mutex)); pthread_cond_broadcast(&(pool->queue_not_empty)); //唤醒线程池中正在阻塞的线程 pthread_cond_broadcast(&(pool->queue_not_full)); //唤醒添加任务的threadpool_add_job函数 int i; for (i = 0; i < pool->thread_num; i) { pthread_join(pool->pthreads[i], null); //等待线程池的所有线程执行完毕 } pthread_mutex_destroy(&(pool->mutex)); //清理资源 pthread_cond_destroy(&(pool->queue_empty)); pthread_cond_destroy(&(pool->queue_not_empty)); pthread_cond_destroy(&(pool->queue_not_full)); free(pool->pthreads); struct job *p; while (pool->head != null) { p = pool->head; pool->head = p->next; free(p); } free(pool); return 0; }
测试文件main.c文件：
#include "threadpool.h" void* work(void* arg) { char *p = (char*) arg; printf("threadpool callback fuction : %s.\n", p); sleep(1); } int main(void) { struct threadpool *pool = threadpool_init(10, 20); threadpool_add_job(pool, work, "1"); threadpool_add_job(pool, work, "2"); threadpool_add_job(pool, work, "3"); threadpool_add_job(pool, work, "4"); threadpool_add_job(pool, work, "5"); threadpool_add_job(pool, work, "6"); threadpool_add_job(pool, work, "7"); threadpool_add_job(pool, work, "8"); threadpool_add_job(pool, work, "9"); threadpool_add_job(pool, work, "10"); threadpool_add_job(pool, work, "11"); threadpool_add_job(pool, work, "12"); threadpool_add_job(pool, work, "13"); threadpool_add_job(pool, work, "14"); threadpool_add_job(pool, work, "15"); threadpool_add_job(pool, work, "16"); threadpool_add_job(pool, work, "17"); threadpool_add_job(pool, work, "18"); threadpool_add_job(pool, work, "19"); threadpool_add_job(pool, work, "20"); threadpool_add_job(pool, work, "21"); threadpool_add_job(pool, work, "22"); threadpool_add_job(pool, work, "23"); threadpool_add_job(pool, work, "24"); threadpool_add_job(pool, work, "25"); threadpool_add_job(pool, work, "26"); threadpool_add_job(pool, work, "27"); threadpool_add_job(pool, work, "28"); threadpool_add_job(pool, work, "29"); threadpool_add_job(pool, work, "30"); threadpool_add_job(pool, work, "31"); threadpool_add_job(pool, work, "32"); threadpool_add_job(pool, work, "33"); threadpool_add_job(pool, work, "34"); threadpool_add_job(pool, work, "35"); threadpool_add_job(pool, work, "36"); threadpool_add_job(pool, work, "37"); threadpool_add_job(pool, work, "38"); threadpool_add_job(pool, work, "39"); threadpool_add_job(pool, work, "40"); sleep(5); threadpool_destroy(pool); return 0; }
用gcc编译，运行就可以看到效果，1到40个回调函数分别被执行。