一个Linux下C线程池的实现-优快云博客

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。

下面是完整代码：

#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;

	/// 线程ID 要在创建线程的时候传递给pthread_create 函数，这个需要是堆内存。
	/// 要在整个线程运行期间保持有效,并且不要修改,否则运行时会出现莫名其妙错误很难发现！！！ 
	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*/
        printf("add a word to thread_pool\n");
 
        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));

        ///对于一个可join线程（非分离态） 所占内存仅当有线程对其执行pthread_join 才会释放。为此所有线程必须处于选择以下2种方式  
        ///1）创建的时候设置为DETACHED状态       2）运行完之后使用pthread_join 来回收
	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%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，注意在函数pool_add_worker调用过程中首先是cur_queue_size 加1，                  然后再pthread_cond_signal                */

		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);
}

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);

	/*销毁线程池*/
	pool_destroy ();
	free (workingnum);
	return 0;
}

注意上面代码中同步条件 pthread_cond_t 使用需要配合mutex使用

pthread_mutex_lock (&(pool->queue_lock));
pthread_cond_wait (&(pool->queue_ready), &(pool->queue_lock));
pthread_mutex_unlock (&(pool->queue_lock));

使用pthread_cond_wait 需要在 lock/unlock 之间，以防止在进入wait状态前有signal。需要先lock, 一旦进入wait状态，会释放 mutex的lock。而一旦有收到signal信号就会自动重新获到mutex的lock。而且cond的lock是原子操作。

在需要的地方进行 pthread_cond_signal (&(pool->queue_ready)); 之前的wait 位置就可以执行，达到多个线程同步。

编译代码

$ gcc -o threadpool threadpool.c -lpthread

运行结果如下所示：

$ ./threadpool 
starting thread 0x59413700
thread 0x59413700 is waiting
starting thread 0x58411700
thread 0x58411700 is waiting
starting thread 0x58c12700
thread 0x58c12700 is waiting
add a word to thread_pool
add a word to thread_pool
thread 0x59413700 is starting to work
threadid is 0x59413700, working on task 0
add a word to thread_pool
thread 0x58411700 is starting to work
threadid is 0x58411700, working on task 1
thread 0x58c12700 is starting to work
threadid is 0x58c12700, working on task 2
add a word to thread_pool
add a word to thread_pool
add a word to thread_pool
add a word to thread_pool
add a word to thread_pool
add a word to thread_pool
add a word to thread_pool
thread 0x59413700 is starting to work
threadid is 0x59413700, working on task 3
thread 0x58411700 is starting to work
threadid is 0x58411700, working on task 4
thread 0x58c12700 is starting to work
threadid is 0x58c12700, working on task 5
thread 0x59413700 is starting to work
threadid is 0x59413700, working on task 6
thread 0x58c12700 is starting to work
threadid is 0x58c12700, working on task 7
thread 0x58411700 is starting to work
threadid is 0x58411700, working on task 8
thread 0x59413700 is starting to work
threadid is 0x59413700, working on task 9
thread 0x58c12700 is waiting
thread 0x58411700 is waiting
thread 0x59413700 is waiting
thread 0x58c12700 will exit
thread 0x58411700 will exit
thread 0x59413700 will exit

原文链接：

http://hi.baidu.com/boahegcrmdghots/item/f3ca1a3c2d47fcc52e8ec2e1

参考：

http://blog.youkuaiyun.com/zclongembedded/article/details/7337729