定义
什么是线程池？简单点说，线程池就是有一堆已经创建好了的线程，初始它们都处于空闲等待状态，当有新的任务需要处理的时候，就从这个池子里面取一个空闲等待的线程来处理该任务，当处理完成了就再次把该线程放回池中，以供后面的任务使用。当池子里的线程全都处理忙碌状态时，线程池中没有可用的空闲等待线程，此时，根据需要选择创建一个新的线程并置入池中，或者通知任务线程池忙，稍后再试。
为什么要用线程池？
我们说，线程的创建和销毁比之进程的创建和销毁是轻量级的，但是当我们的任务需要大量进行大量线程的创建和销毁操作时，这个消耗就会变成的相当大。比如，当你设计一个压力性能测试框架的时候，需要连续产生大量的并发操作，这个是时候，线程池就可以很好的帮上你的忙。线程池的好处就在于线程复用，一个任务处理完成后，当前线程可以直接处理下一个任务，而不是销毁后再创建，非常适用于连续产生大量并发任务的场合。
线程池工作原理
线程池的任务就在于负责这些线程的创建，销毁和任务处理参数传递、唤醒和等待。
1. 创建若干线程，置入线程池
2. 任务达到时，从线程池取空闲线程
3. 取得了空闲线程，立即进行任务处理
4. 否则新建一个线程，并置入线程池，执行3
5. 如果创建失败或者线程池已满，根据设计策略选择返回错误或将任务置入处理队列，等待处理
6. 销毁线程池
Thread_pool.h
#ifndef _THREAD_POOL_H_
#define _THREAD_POOL_H_
#include <stdio.h>
#include <stdbool.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <strings.h>
#include <errno.h>
#include <pthread.h>
#define MAX_WAITING_TASKS 1000 //最大等待数
#define MAX_ACTIVE_THREADS 20 //最大运行线程数
struct task //单向链表的任务节点
{
void *(*task)(void *arg); //想要运行的函数 void * 可以返回任何格式的值
void *arg; //函数的参数
struct task *next; //指向下一个的节点的指针
};
typedef struct thread_pool//线程池结构体
{
pthread_mutex_t lock;//互斥锁
pthread_cond_t cond;//线程间同步，一般和pthread_mutex_t一起使用，以防止出现逻辑错误
//即如果单独使用条件变量，某些情况下（条件变量前后出现对共享变量的读写）会出现问题
struct task *task_list;//初始化任务链表节点
pthread_t *tids;//初始化线程
unsigned waiting_tasks;//等待任务数量
unsigned active_threads; //正在执行线程数
bool shutdown; //进程结束全体，任务符号
}thread_pool;
bool
init_pool(thread_pool *pool,
unsigned int threads_number); //初始化池
bool
add_task(thread_pool *pool,//添加任务
void *(*task)(void *arg),
void *arg);
int
add_thread(thread_pool *pool,//增加线程
unsigned int additional_threads_number);
int
remove_thread(thread_pool *pool, //去除线程
unsigned int removing_threads_number);
bool destroy_pool(thread_pool *pool); //销毁线程池
void *routine(void *arg);
#endif
#include "thread_pool.h"
void handler(void *arg) //预防 线程在运行过程中 被remove掉而导致的死锁
pthread_mutex_unlock((pthread_mutex_t *)arg); 解锁
}
void *routine(void *arg)//线程的任务
{
thread_pool *pool = (thread_pool *)arg; //初始化线程池节点
struct task *p; //初始化任何节点
while(1)
{
pthread_cleanup_push(handler, (void *)&pool->lock);
/*
pthread_cleanup_push
work();
pthread_cleanup_pop
这两个是一对
他们防止死锁，因为若在运行work 的过程中，线程被摧毁了，他们就会调用 handle，然后解锁
*/
pthread_mutex_lock(&pool->lock); //锁定
while(pool->waiting_tasks == 0 && !pool->shutdown) //如果任务为空就进入条件变量 休眠模式
{
pthread_cond_wait(&pool->cond, &pool->lock);
}
if(pool->waiting_tasks == 0 && pool->shutdown == true) //如果任务为空，任务全部完成，则推出线程
{ pthread_mutex_unlock(&pool->lock);pthread_exit(NULL);}
p = pool->task_list->next;//把节点插入单向链表
pool->task_list->next = p->next;
pool->waiting_tasks--;
pthread_mutex_unlock(&pool->lock);//解锁pthread_cleanup_pop(0); //解除预防死锁功能
pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);//进入不接受 任何删除线程的模式
(p->task)(p->arg);//运行任务
pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);//回到可接受模式
free(p);//释放节点
}
pthread_exit(NULL);//退出线程
}
bool init_pool(thread_pool *pool, unsigned int threads_number) //初始化线程池
{
pthread_mutex_init(&pool->lock, NULL);
pthread_cond_init(&pool->cond, NULL);
pool->shutdown = false;
pool->task_list = malloc(sizeof(struct task));
pool->tids = malloc(sizeof(pthread_t) * MAX_ACTIVE_THREADS);
if(pool->task_list == NULL || pool->tids == NULL)
{
perror("allocate memory error");
return false;
}
pool->task_list->next = NULL;
pool->waiting_tasks = 0;
pool->active_threads = threads_number;
int i;
for(i=0; i<pool->active_threads; i++)
{
if(pthread_create(&((pool->tids)[i]), NULL,routine, (void *)pool) != 0)
{
perror("create threads error");
return false;
}
}
return true;
}
bool add_task(thread_pool *pool,void *(*task)(void *arg), void *arg)
{
struct task *new_task = malloc(sizeof(struct task));
if(new_task == NULL)
{
perror("allocate memory error");
return false;
}
new_task->task = task;
new_task->arg = arg;
new_task->next = NULL;
pthread_mutex_lock(&pool->lock);
if(pool->waiting_tasks >= MAX_WAITING_TASKS)
{
pthread_mutex_unlock(&pool->lock);
fprintf(stderr, "too many tasks.\n");
free(new_task);
return false;
}
struct task *tmp = pool->task_list;
while(tmp->next != NULL)
tmp = tmp->next;
tmp->next = new_task;
pool->waiting_tasks++;
pthread_mutex_unlock(&pool->lock);
pthread_cond_signal(&pool->cond);
return true;
}
int add_thread(thread_pool *pool, unsigned additional_threads)
{
if(additional_threads == 0)
return 0;
unsigned total_threads = pool->active_threads + additional_threads;
int i, actual_increment = 0;
for(i = pool->active_threads;
i < total_threads && i < MAX_ACTIVE_THREADS;i++)
{
if(pthread_create(&((pool->tids)[i]),NULL, routine, (void *)pool) != 0)
{
perror("add threads error");
if(actual_increment == 0)
return -1;
break;
}actual_increment++;
}pool->active_threads += actual_increment;
return actual_increment;
}
int remove_thread(thread_pool *pool, unsigned int removing_threads)
{
if(removing_threads == 0)return pool->active_threads;
int remain_threads = pool->active_threads - removing_threads;
remain_threads = remain_threads>0 ? remain_threads:1;
int i;
for(i=pool->active_threads-1; i>remain_threads-1; i--)
{
errno = pthread_cancel(pool->tids[i]);
if(errno != 0)
break;
}
if(i == pool->active_threads-1)
return -1;
else{
pool->active_threads = i+1;
return i+1;
}
}
bool destroy_pool(thread_pool *pool)
{
pool->shutdown = true;
pthread_cond_broadcast(&pool->cond);
int i;
for(i=0; i<pool->active_threads; i++)
{
errno = pthread_join(pool->tids[i], NULL);
if(errno != 0)
{
printf("join tids[%d] error: %s\n",i, strerror(errno));
}
else
printf("[%u] is joined\n", (unsigned)pool->tids[i]);
}
free(pool->task_list);
free(pool->tids);
free(pool);
return true;
}
main.c
#include "thread_pool.h"
void *mytask(void *arg)
{
int n = (int)arg;
printf("[%u][%s] ==> job will be done in %d sec...\n",
(unsigned)pthread_self(), __FUNCTION__, n);
sleep(n);
printf("[%u][%s] ==> job done!\n",
(unsigned)pthread_self(), __FUNCTION__);
return NULL;
}
void *count_time(void *arg)
{
int i = 0;
while(1)
{
sleep(1);
printf("sec: %d\n", ++i);
}
}
int main(void)
{
pthread_t a;
pthread_create(&a, NULL, count_time, NULL);
// 1, initialize the pool
thread_pool *pool = malloc(sizeof(thread_pool));
init_pool(pool, 2);
// 2, throw tasks
printf("throwing 3 tasks...\n");
add_task(pool, mytask, (void *)(rand()%10));
add_task(pool, mytask, (void *)(rand()%10));
add_task(pool, mytask, (void *)(rand()%10));
// 3, check active threads number
printf("current thread number: %d\n",
remove_thread(pool, 0));
sleep(9);
// 4, throw tasks
printf("throwing another 2 tasks...\n");
add_task(pool, mytask, (void *)(rand()%10));
add_task(pool, mytask, (void *)(rand()%10));
// 5, add threads
add_thread(pool, 2);
sleep(5);
// 6, remove threads
printf("remove 3 threads from the pool, "
"current thread number: %d\n",
remove_thread(pool, 3));
// 7, destroy the pool
destroy_pool(pool);
return 0;
}