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T1和销毁时间T3相比任务执行时间T2可以忽略不计，则没有必要使用线程池了。反之如果T1+T3>T2，那就很有必要使用线程池。
- 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，则该线程处于阻塞等待状态。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。
下面贴出完整代码
#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;
}
//非常重要的任务接口函数，各子线程统一调用这个函数，而这个函数内部检查调用任务队列中的实际任务函数指针。
void * thread_routine (void *arg)
{
printf ("starting thread 0x%x/n", 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", 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", pthread_self ());
pthread_exit (NULL);
}
printf ("thread 0x%x is starting to work/n", 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", 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); //这句可能出问题，偷懒写法。
/*销毁线程池*/
pool_destroy ();
free (workingnum);
return 0;
}
将上述所有代码放入threadpool.c文件中，
在Linux输入编译命令
$ gcc -o threadpool threadpool.c -lpthread
以下是运行结果
starting thread 0xb7df6b90
thread 0xb7df6b90 is waiting
starting thread 0xb75f5b90
thread 0xb75f5b90 is waiting
starting thread 0xb6df4b90
thread 0xb6df4b90 is waiting
thread 0xb7df6b90 is starting to work
threadid is 0xb7df6b90, working on task 0
thread 0xb75f5b90 is starting to work
threadid is 0xb75f5b90, working on task 1
thread 0xb6df4b90 is starting to work
threadid is 0xb6df4b90, working on task 2
thread 0xb7df6b90 is starting to work
threadid is 0xb7df6b90, working on task 3
thread 0xb75f5b90 is starting to work
threadid is 0xb75f5b90, working on task 4
thread 0xb6df4b90 is starting to work
threadid is 0xb6df4b90, working on task 5
thread 0xb7df6b90 is starting to work
threadid is 0xb7df6b90, working on task 6
thread 0xb75f5b90 is starting to work
threadid is 0xb75f5b90, working on task 7
thread 0xb6df4b90 is starting to work
threadid is 0xb6df4b90, working on task 8
thread 0xb7df6b90 is starting to work
threadid is 0xb7df6b90, working on task 9
thread 0xb75f5b90 is waiting
thread 0xb6df4b90 is waiting
thread 0xb7df6b90 is waiting
thread 0xb75f5b90 will exit
thread 0xb6df4b90 will exit
thread 0xb7df6b90 will exit
