线程池原理并用C语言实现

• 线程池的目的是为了减少线程创建、销毁所带来的代价，当有非常多的任务需要独立的线程去做时，可以使用线程池，从线程池中获取线程来处理一个个任务。

      本文就来实现一个线程池。实现环境为Linux操作系统，采用C语言实现pthread线程的线程池。

  线程分配设计：

      线程池中的每个子线程都是等价的。我们用线程信号量来控制子线程和任务的分配问题。设置一个信号量来表示任务队列中的任务资源。每个子线程都会处于死循环中，每轮循环首先等待一个任务资源信号量，当等到之后，互斥地从任务队列中摘取一个任务结点，任务结点中记录着该任务所要执行的函数指针及其参数。之后子线程开始执行该任务。执行完之后释放一个信号量并进入下一轮循环。当没有信号量小于1时，子线程将会阻塞。

      因此一个任务由哪一个线程来执行，这要看哪个线程能够获取到对应的信号量资源。

  具体实现：

      任务队列由双向链表构造，每个节点包含一个任务的函数指针和参数指针。

  一般一个简单的线程池有下列组件：

  1. 线程池管理器（ThreadPoolManager）:用于创建并管理线程池
  2. 工作线程（WorkThread）: 线程池中线程
  3. 任务接口（Task）:每个任务必须实现的接口，以供工作线程调度任务的执行。
  4. 任务队列:用于存放没有处理的任务。提供一种缓冲机制。

  
  

  代码如下：

  //threadpool.h

  /**********************************************
    * @author Jacky Lau
    * @date   10/26/2014
    *
    *
    * **********************************************/
  
  #include
  #include
  
  
  /*========================STRUCTS==========================*/
  
  typedef void *(*function_t)(void *arg);
  
  /* jobs */
  typedef struct thread_job_t{
      function_t pf;      /**< function pointer        */
      void *arg;          /**< function's pointer      */
      struct thread_job_t *next; /**< pointer to next job     */
      struct thread_job_t*prev; /**< pointer to previous job */
  }thread_job_t;
  
  
  /* threadpool */
  typedef struct threadpool_t{
      pthread_t *threads; /**< pointer to thread's ID */
      int num_threads;    /**< number of threads      */
  
      thread_job_t *head; /**< pointer to head of workqueue */
      thread_job_t *tail; /**< pointer to tail of workqueue */
      int num_jobs;       /**< number of jobs in the workqueue*/
      sem_t *queue_sem;
  
  }threadpool_t;
  
  /*==========================FUNCTION=================================*/
  
  /*-------------------------threadpool specific------------------------------- */
  
  /**
   * @brief initialize threadpool
   * Allocates memory for the threadpool, workqueue,semphore and fixes pointer
   * in job queue
   * @param num_threads to be used
   * @return pointer to threadpool_t struct on success,
   *         NULL on error
   */
  threadpool_t *threadpool_init(int num_threads);
  
  
  /**
   * @brief destroy threadpool
   * if there are so many jobs and can't handle all of them, free the job_queue
   * if there are so many threads, wake all of them in case of zoombie
   * @param threadpool
   */
  void threadpool_destroy(threadpool_t *threadpool);
  
  /*-------------------------work specific--------------------------------- */
  
  /**
   * @brief do the jobs, all of the threads are blocked when they first created
   * @param threadpool_in
   */
  void threadpool_thread_do(threadpool_t *threadpool_in);
  
  /**
   * @brief add the job into the job_queue, it will wake the blocked threads by sem_post
   * @param threadpool
   * @param pointer to the job function
   * @param argument's of function
   * @return 0 on success,other on failure
   */
  int threadpool_add_work(threadpool_t *threadpool, function_t pf, void *arg);
  
  
  /*-------------------------queue specific--------------------------------- */
  
  /**
   * @brief add job into the queue
   * @param threadpool
   * @param pointer to the struct of thread_job_t
   * @return 0 on success,
   *         other on failure
   */
  int threadpool_job_enqueue(threadpool_t *threadpool, thread_job_t *job);
  
  /**
   * @brief remove the first data unit of queue
   * @param threadpool
   * @return 0 on success
   *         other on failure
   */
  thread_job_t *threadpool_job_dequeue(threadpool_t *threadpool);
  
  /**
   * @brief make the job_queue empty
   * @param threadpool
   * @return 0 on success
   *         other on failure
   */
  int threadpool_job_queue_empty(threadpool_t *threadpool);

  
  //threadpool.c

  #include"threadpool.h"
  #include
  #include
  #include
  #include
  
  pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
  typedef int bool;
  #define true  1
  #define false 0
  bool thread_islive = true;
  
  #define CHECK_ERROR(a)                            \
  if((a))                                           \
  {                                                 \
      perror("Error at line\n\t" #a "\nSystem Msg");\
      exit(EXIT_FAILURE);                           \
  }
  
  /*==================FUNCTIONS========================*/
  
  threadpool_t *threadpool_init(int num_threads)
  {
      if(num_threads < 1)
          num_threads =1;
      /* malloc the struct threadpool*/
      threadpool_t *threadpool = (threadpool_t *) malloc(sizeof(threadpool_t));
  
      /*malloc the threads' ID */
      threadpool->threads = (pthread_t *)malloc(sizeof(pthread_t) * num_threads);
      threadpool->num_threads = num_threads;
  
      /*initialize the workqueue*/
      threadpool->head = NULL;
      threadpool->tail = NULL;
      threadpool->num_jobs = 0;
  
      /*initialize the semaphore*/
      threadpool->queue_sem = (sem_t *)malloc(sizeof(sem_t));
      sem_init(threadpool->queue_sem, 0, 0);
  
      /*make the threads in pool*/
      int i;
      for(i = 0; i < num_threads; i++)
      {
          pthread_create(&threadpool->threads[i], NULL, (void *)threadpool_thread_do, (void *)threadpool);
      }
      return threadpool;
  }
  
  /*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*/
  
  void threadpool_destroy(threadpool_t *threadpool)
  {
      thread_islive = false;
      int i;
      /*because create threads more than jobs, the threads will be zoombie*, so wake all of them*/
      for(i = 0; i < threadpool->num_threads; i++)
      {
          CHECK_ERROR(sem_post(threadpool->queue_sem));
      }
      CHECK_ERROR(sem_destroy(threadpool->queue_sem));
  
      /*barrier for synchronization*/
      for(i = 0; i < threadpool->num_threads; i++)
          pthread_join(threadpool->threads[i], NULL);
  
      /*dealloc job queue*/
      /*if the jobs is so many and can't handle them, it will cause mem leak, so empty the queue*/
      threadpool_job_queue_empty(threadpool);
      /*dealloc threads*/
      free(threadpool->threads);
      /*dealloc semphore*/
      free(threadpool->queue_sem);
      /*dealloc threadpool*/
      free(threadpool);
  
  }
  
  int threadpool_job_queue_empty(threadpool_t *threadpool)
  {
      thread_job_t *prev = threadpool->head;
      thread_job_t *p = prev;
      while(threadpool->num_jobs)
      {
          p = p->next;
          free(prev);
          prev = p;
          threadpool->num_jobs--;
      }
      threadpool->head = NULL;
      threadpool->tail = NULL;
      return 0;
  }
  
  void threadpool_thread_do(threadpool_t *threadpool_in)
  {
      assert(threadpool_in);
      threadpool_t *threadpool = threadpool_in;
  
      if(sem_wait(threadpool->queue_sem))
      {
          perror("thread waiting for semaphore");
          exit(EXIT_FAILURE);
      }
  
      while(thread_islive)
      {
          pthread_mutex_lock(&mutex);     /* LOCK */
  
          thread_job_t *job= threadpool_job_dequeue(threadpool);
  
          pthread_mutex_unlock(&mutex);    /* UNLOCK */
  
          if(job)
          {
              function_t pf = job->pf;
              void *arg = job->arg;
              pf(arg);
              free(job);
          }
      }
  }
  
  thread_job_t *threadpool_job_dequeue(threadpool_t *threadpool)
  {
      assert(threadpool);
  
      thread_job_t *job = threadpool->head;
      switch (threadpool->num_jobs) {
      case 0:
          job = NULL;
          break;
      case 1:
          threadpool->head = NULL;
          threadpool->tail = NULL;
          threadpool->num_jobs--;
          break;
      default:
          threadpool->head = job->next;
          job->next = NULL;
          threadpool->head->prev = NULL;
          threadpool->num_jobs--;
          break;
      }
      return job;
  }
  
  int threadpool_add_work(threadpool_t *threadpool, function_t pf, void *arg)
  {
      assert(threadpool);
      thread_job_t *job = (thread_job_t *)malloc(sizeof(thread_job_t));
      job->pf = pf;
      job->arg = arg;
      job->next = NULL;
      job->prev = NULL;
  
      pthread_mutex_lock(&mutex);          /* LOCK */
  
      CHECK_ERROR( threadpool_job_enqueue(threadpool, job) );
  
      pthread_mutex_unlock(&mutex);         /* UNLOCK */
      return 0;
  }
  
  int threadpool_job_enqueue(threadpool_t *threadpool, thread_job_t *job)
  {
      assert(threadpool);
      assert(job);
      switch(threadpool->num_jobs)
      {
      case 0:
          threadpool->head = job;
          threadpool->tail = job;
          threadpool->num_jobs++;
          break;
      default:
          threadpool->tail->next = job;
          job->prev = threadpool->tail;
          threadpool->tail = job;
          threadpool->num_jobs++;
      }
  
      sem_post(threadpool->queue_sem);
      return 0;
  }

  
  //main.c

  /********************************************
   *@author    JackyLau
   *@date      10/26/2014
   *
   *
   * ******************************************/
  
  #include 
  #include 
  #include"threadpool.h"
  
  void task1()
  {
      printf("thread %d running Task 1\n", (int)pthread_self());
      int i;
      /*for delay*/
      int sum = 0;
      for(i = 0; i < 100000; i++)
      {
          sum += i;
      }
  }
  
  void task2(int id)
  {
      printf("%d thread %d running Task 2\n", id, (int)pthread_self());
      /*for delay*/
      int i;
      int sum = 0;
      for(i = 0; i < 100000; i++)
      {
          sum += i;
      }
  }
  
  int main(void)
  {
      threadpool_t *threadpool = threadpool_init(10);
      int i;
      for(i = 0; i < 1000; i++)
      {
          threadpool_add_work(threadpool, (void *)task1, NULL);
          threadpool_add_work(threadpool, (void *)task2, (void *)i);
      }
  
      sleep(5);/*so they can handle all of the jobs, or just a bit of them*/
      threadpool_destroy(threadpool);
  
      //printf("Hello World!\n");
      return 0;
  }
  

  运行结果: