linux 条件变量与线程池

条件变量Condition Variables

概述

1. 条件变量提供了另外一种线程同步的方式。如果没有条件变量，程序需要使用线程连续轮询（可能在临界区critical section内）方式检查条件是否满足。由于线程连续忙于轮询检查，这会非常消耗资源，而条件变量是一种实现同样目标不需要轮询的方式。

2. 条件变量总是和互斥锁相结合使用。

3. 条件变量使用示例结构：

Main Thread Declare and initialize global data/variables which require synchronization (such as "count") Declare and initialize a condition variable object Declare and initialize an associated mutex Create threads A and B to do work
Thread A Do work up to the point where a certain condition must occur (such as "count" must reach a specified value) Lock associated mutex and check value of a global variable Call pthread_cond_wait() to perform a blocking wait for signal from Thread-B. Note that a call to pthread_cond_wait() automatically and atomically unlocks the associated mutex variable so that it can be used by Thread-B. When signalled, wake up. Mutex is automatically and atomically locked. Explicitly unlock mutex Continue	Thread B Do work Lock associated mutex Change the value of the global variable that Thread-A is waiting upon. Check value of the global Thread-A wait variable. If it fulfills the desired condition, signal Thread-A. Unlock mutex. Continue
Main Thread Join / Continue

创建和销毁条件变量

pthread_cond_init (condition,attr)

pthread_cond_destroy (condition)

pthread_condattr_init (attr)

pthread_condattr_destroy (attr)

条件变量必须声明为pthread_cond_t，并且使用之前必须初始化。有两种方式初始化条件变量：
1）静态初始化：pthread_cond_t myconvar = PTHREAD_COND_INITIALIZER;

2）动态初始化： pthread_cond_init()。条件变量的id号通过条件变量参数返回于调用线程，这种方式允许设置条件变量的属性。然而，只有一种条件变量属性process-shared，这允许其他进程的线程可见该条件变量。如果使用条件变量属性，那么必须是pthread_condattr_t 类型（为了接受默认值可以指定为NULL）。需要注意的是，并非所有实现提供process-shared属性。

信号等待与信号通知

pthread_cond_wait (condition,mutex):阻塞调用线程直到特定的条件触发。当互斥量被锁住时该函数应当被调用；当它等待时它将自动释放互斥锁。接收到信号通知和线程被唤醒后，互斥量将自动地被线程锁住。当线程完成任务时，需要手动解锁互斥量。

pthread_cond_signal (condition)用于唤醒另外一个等待条件变量的线程。互斥量被锁住之后才可调用pthread_cond_signal并且按序解锁用于pthread_cond_wait完成。

pthread_cond_broadcast (condition)如果多于一个线程处于阻塞等待状态，那么应当使用pthread_cond_broadcast而不是pthread_cond_signal。

建议使用while循环而不是if，这样可以检查一些潜在的问题，例如：如果若干线程在等待同一个唤醒信号，它们将轮询捕获互斥量，它们中的任何一个可以修改条件；由于程序bug，线程接收到错误信号；线程库允许不违反标准的前提下虚假的唤醒一个等待线程。

使用这些函数时，必须正确地加锁解锁互斥变量。

调用pthread_cond_wait前锁定互斥量失败可能导致线程阻塞失败；

调用pthread_cond_signal后解锁互斥量失败可能不允许匹配的pthread_cond_wait完成（即阻塞掉）。

实际上pthread_cond_wait的返回不仅仅是pthread_cond_signal和pthread_cond_broadcast导致的，还会有一些假唤醒，也就是spurious wakeup。

pthread_cond_wait的通常使用方法：

pthread_mutex_lock();

while(condition_is_false)

    pthread_cond_wait();

pthread_mutex_unlock();

为什么在pthread_cond_wait()前要加一个while循环来判断条件是否为假呢？

APUE中写道:

传递给pthread_cond_wait的互斥量对条件进行保护，调用者把锁住的互斥量传给函数。函数把调用线程放到等待条件的线程列表上，然后对互斥量解锁，这两个操作是原子操作。

线程释放互斥量，等待其他线程发给该条件变量的信号（唤醒一个等待者）或广播该条件变量（唤醒所有等待者）。当等待条件变量时，互斥量必须始终为释放的，这样其他线程才有机会锁住互斥量，修改条件变量。当线程从条件变量等待中醒来时，它重新继续锁住互斥量，对临界资源进行处理。

条件变量的作用是发信号，而不是互斥。

wait前检查

对于多线程程序，不能够用常规串行的思路来思考它们，因为它们是完全异步的，会出现很多临界情况。比如：pthread_cond_signal的时间早于pthread_cond_wait的时间，这样pthread_cond_wait就会一直等下去，漏掉了之前的条件变化。

对于这种情况，解决的方法是在锁住互斥量之后和等待条件变量之前，检查条件变量是否已经发生变化。

if(condition_is_false)

    pthread_cond_wait();

这样在等待条件变量前检查一下条件变量的值，如果条件变量已经发生了变化，那么就没有必要进行等待了，可以直接进行处理。这种方法在并发系统中比较常见

1.等待函数里面要传入一个互斥量，这个互斥量会在这个函数调用时会发生如下变化：函数刚刚被调用时，会把这个互斥量解锁，然后让调用线程阻塞，解锁后其他线程才有机会获得这个锁。当某个线程调用通知函数时，这个函数收到通知后，又把互斥量加锁，然后继续向下操作临界区。可见这个设计是非常合理的！

2.条件变量的等待函数用while循环包围。原因：如果有多个线程都在等待这个条件变量关联的互斥量，当条件变量收到通知，它下一步就是要锁住这个互斥量，但在这个极小的时间差里面，其他线程抢先获取了这互斥量并进入临界区把某个状态改变了。此时这个条件变量应该继续判断别人刚刚抢先修改的状态，即继续执行while的判断。还有一个原因时防止虚假通知，收到虚假通知后，只要while里面的条件为真，就继续休眠.

参考资料：https://computing.llnl.gov/tutorials/pthreads/#ConVarSignal

http://www.cnblogs.com/leaven/archive/2010/06/03/1750973.html

https://www.cnblogs.com/yuuyuu/p/5140875.html

linux下C 线程池的原理讲解和代码实现（能自行伸缩扩展线程数）

Linux C++线程池框架

Linux的多任务编程-线程池

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define NUM_THREADS  3
#define TCOUNT 10
#define COUNT_LIMIT 12

int     count = 0;
int     thread_ids[3] = {0,1,2};
pthread_mutex_t count_mutex;
pthread_cond_t count_threshold_cv;

void *inc_count(void *t)
{
    int i;
    long my_id = (long)t;

    for (i=0; i<TCOUNT; i++) {
        pthread_mutex_lock(&count_mutex);
        count++;

        /*
     Check the value of count and signal waiting thread when condition is
     reached.  Note that this occurs while mutex is locked.
     */
        if (count == COUNT_LIMIT) {
            pthread_cond_signal(&count_threshold_cv);
            printf("inc_count(): thread %ld, count = %d  Threshold reached.
",
                   my_id, count);
        }
        printf("inc_count(): thread %ld, count = %d, unlocking mutex
",
               my_id, count);
        pthread_mutex_unlock(&count_mutex);

        /* Do some "work" so threads can alternate on mutex lock */
        sleep(1);
    }
    pthread_exit(NULL);
}

void *watch_count(void *t)
{
    long my_id = (long)t;

    printf("Starting watch_count(): thread %ld
", my_id);

    /*
   Lock mutex and wait for signal.  Note that the pthread_cond_wait
   routine will automatically and atomically unlock mutex while it waits.
   Also, note that if COUNT_LIMIT is reached before this routine is run by
   the waiting thread, the loop will be skipped to prevent pthread_cond_wait
   from never returning.
   */
    pthread_mutex_lock(&count_mutex);
    while (count<COUNT_LIMIT) {
        pthread_cond_wait(&count_threshold_cv, &count_mutex);
        printf("watch_count(): thread %ld Condition signal received.
", my_id);
    }
    count += 125;
    printf("watch_count(): thread %ld count now = %d.
", my_id, count);
    pthread_mutex_unlock(&count_mutex);
    pthread_exit(NULL);
}

int main (int argc, char *argv[])
{
    int i, rc;
    long t1=1, t2=2, t3=3;
    pthread_t threads[3];
    pthread_attr_t attr;

    /* Initialize mutex and condition variable objects */
    pthread_mutex_init(&count_mutex, NULL);
    pthread_cond_init (&count_threshold_cv, NULL);

    /* For portability, explicitly create threads in a joinable state */
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

    pthread_create(&threads[0], &attr, watch_count, (void *)t1);
    pthread_create(&threads[1], &attr, inc_count, (void *)t2);
    pthread_create(&threads[2], &attr, inc_count, (void *)t3);

    /* Wait for all threads to complete */
    for (i=0; i<NUM_THREADS; i++) {
        pthread_join(threads[i], NULL);
    }
    printf ("Main(): Waited on %d  threads. Done.
", NUM_THREADS);

    /* Clean up and exit */
    pthread_attr_destroy(&attr);
    pthread_mutex_destroy(&count_mutex);
    pthread_cond_destroy(&count_threshold_cv);
    pthread_exit(NULL);

}

翻译资料：https://computing.llnl.gov/tutorials/pthreads/#ConVarSignal

线程池

上面之所以会谈到条件变量，有两个原因，其一线程池的实现需要条件变量方面的知识，其二因为它的实现牵涉到一些细节，理解条件变量有一定的困难，如果不理解它与互斥锁结合使用的实现原理，也就无法正确使用条件变量。

#ifndef THREADPOOL_H
#define THREADPOOL_H
/*
 *线程池包括：n个执行任务的线程，一个任务队列，一个管理线程
1、预先启动一些线程，线程负责执行任务队列中的任务，当队列空时，线程挂起。
2、调用的时候，直接往任务队列添加任务，并发信号通知线程队列非空。
3、管理线程负责监控任务队列和系统中的线程状态，当任务队列为空，线程数目多且很多处于空闲的时候，便通知一些线程退出以节约系统资源；当任务队列排队任务多且线程都在忙，便负责再多启动一些线程来执行任务，以确保任务执行效率。
 *
 */
#include <pthread.h>

typedef struct threadpool_task_t
{
    void *(*function)(void *);
    void *arg;
} threadpool_task_t;

typedef struct threadpool_t
{
    pthread_mutex_t lock;// mutex for the taskpool
    pthread_mutex_t thread_counter;//mutex for count the busy thread
    pthread_cond_t queue_not_full;
    pthread_cond_t queue_not_empty;//任务队列非空的信号
    pthread_t *threads;//执行任务的线程
    pthread_t adjust_tid;//负责管理线程数目的线程
    threadpool_task_t *task_queue;//任务队列
    int min_thr_num;
    int max_thr_num;
    int live_thr_num;
    int busy_thr_num;
    int wait_exit_thr_num;
    int queue_front;
    int queue_rear;
    int queue_size;
    int queue_max_size;
    bool shutdown;
}threadpool_t;

threadpool_t *threadpool_create(int min_thr_num, int max_thr_num, int queue_max_size);

int threadpool_add(threadpool_t *pool, void*(*function)(void *arg), void *arg);

/**
 * @function void *threadpool_thread(void *threadpool)
 * @desc the worker thread
 * @param threadpool the pool which own the thread
 */

void *threadpool_thread(void *threadpool);
/**
 * @function void *adjust_thread(void *threadpool);
 * @desc manager thread
 * @param threadpool the threadpool
 */

void *adjust_thread(void *threadpool);
/**
 * check a thread is alive
 */

bool is_thread_alive(pthread_t tid);

int threadpool_destroy(threadpool_t *pool);

int threadpool_free(threadpool_t *pool);

int threadpool_all_threadnum(threadpool_t *pool);

int threadpool_busy_threadnum(threadpool_t *pool);

#endif // THREADPOOL_H

#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <assert.h>
#include <stdio.h>
#include <string.h>
#include <signal.h>
#include <errno.h>
#include <stdbool.h>
#include "threadpool.h"
#define DEFAULT_TIME 10 // 领导定时检查队列、线程状态的时间间隔
#define MIN_WAIT_TASK_NUM 10 // 队列中等待的任务数>这个值，便会增加线程
#define DEFAULT_THREAD_VARY 10 //每次线程加减的数目


//创建线程池
threadpool_t *threadpool_create(int min_thr_num, int max_thr_num, int queue_max_size)
{
    threadpool_t *pool = NULL;
    do{
        if((pool = (threadpool_t *)malloc(sizeof(threadpool_t))) == NULL)
        {
            printf("malloc threadpool fail");
            break;
        }
        pool->min_thr_num = min_thr_num;
        pool->max_thr_num = max_thr_num;
        pool->busy_thr_num = 0;
        pool->live_thr_num = min_thr_num;
        pool->queue_size = 0;
        pool->queue_max_size = queue_max_size;
        pool->queue_front = 0;
        pool->queue_rear = 0;
        pool->shutdown = false;
        pool->threads = (pthread_t *)malloc(sizeof(pthread_t)*max_thr_num);
        if (pool->threads == NULL)
        {
            printf("malloc threads fail");
            break;
        }
        memset(pool->threads, 0, sizeof(pool->threads));
        pool->task_queue = (threadpool_task_t *)malloc(sizeof(threadpool_task_t)*queue_max_size);
        if (pool->task_queue == NULL)
        {
            printf("malloc task_queue fail");
            break;
        }
        if (pthread_mutex_init(&(pool->lock), NULL) != 0
                || pthread_mutex_init(&(pool->thread_counter), NULL) != 0
                || pthread_cond_init(&(pool->queue_not_empty), NULL) != 0
                || pthread_cond_init(&(pool->queue_not_full), NULL) != 0)
        {
            printf("init the lock or cond fail");
            break;
        }
        /**
 * start work thread min_thr_num
 */
        for (int i = 0; i < min_thr_num; i++)
        {
            //启动任务线程
            pthread_create(&(pool->threads[i]), NULL, threadpool_thread, (void *)pool);
            printf("start thread 0x%x...
", pool->threads[i]);
        }
        //启动管理线程
        pthread_create(&(pool->adjust_tid), NULL, adjust_thread, (void *)pool);
        return pool;
    }while(0);
    threadpool_free(pool);
    return NULL;
}

//把任务添加到队列中
int threadpool_add(threadpool_t *pool, void*(*function)(void *arg), void *arg)
{
    assert(pool != NULL);
    assert(function != NULL);
    assert(arg != NULL);
    pthread_mutex_lock(&(pool->lock));
    //队列满的时候，等待
    while ((pool->queue_size == pool->queue_max_size) && (!pool->shutdown))
    {
        //queue full wait
        pthread_cond_wait(&(pool->queue_not_full), &(pool->lock));
    }
    if (pool->shutdown)
    {
        pthread_mutex_unlock(&(pool->lock));
    }
    //如下是添加任务到队列，使用循环队列
    if (pool->task_queue[pool->queue_rear].arg != NULL)
    {
        free(pool->task_queue[pool->queue_rear].arg);
        pool->task_queue[pool->queue_rear].arg = NULL;
    }
    pool->task_queue[pool->queue_rear].function = function;
    pool->task_queue[pool->queue_rear].arg = arg;
    pool->queue_rear = (pool->queue_rear + 1)%pool->queue_max_size;
    pool->queue_size++;
    //每次加完任务，发个信号给线程
    //若没有线程处于等待状态，此句则无效，但不影响
    pthread_cond_signal(&(pool->queue_not_empty));
    pthread_mutex_unlock(&(pool->lock));
    return 0;
}

//线程执行任务
void *threadpool_thread(void *threadpool)
{
    threadpool_t *pool = (threadpool_t *)threadpool;
    threadpool_task_t task;
    while(true)
    {
        /* Lock must be taken to wait on conditional variable */
        pthread_mutex_lock(&(pool->lock));
        //任务队列为空的时候，等待
        while ((pool->queue_size == 0) && (!pool->shutdown))
        {
            printf("thread 0x%x is waiting
", pthread_self());
            pthread_cond_wait(&(pool->queue_not_empty), &(pool->lock));
            //被唤醒后，判断是否是要退出的线程
            if (pool->wait_exit_thr_num > 0)
            {
                pool->wait_exit_thr_num--;
                if (pool->live_thr_num > pool->min_thr_num)
                {
                    printf("thread 0x%x is exiting
", pthread_self());
                    pool->live_thr_num--;
                    pthread_mutex_unlock(&(pool->lock));
                    pthread_exit(NULL);
                }
            }
        }
        if (pool->shutdown)
        {
            pthread_mutex_unlock(&(pool->lock));
            printf("thread 0x%x is exiting
", pthread_self());
            pthread_exit(NULL);
        }
        //get a task from queue
        task.function = pool->task_queue[pool->queue_front].function;
        task.arg = pool->task_queue[pool->queue_front].arg;
        pool->queue_front = (pool->queue_front + 1)%pool->queue_max_size;
        pool->queue_size--;
        //now queue must be not full
        pthread_cond_broadcast(&(pool->queue_not_full));
        pthread_mutex_unlock(&(pool->lock));
        // Get to work
        printf("thread 0x%x start working
", pthread_self());
        pthread_mutex_lock(&(pool->thread_counter));
        pool->busy_thr_num++;
        pthread_mutex_unlock(&(pool->thread_counter));
        (*(task.function))(task.arg);
        // task run over
        printf("thread 0x%x end working
", pthread_self());
        pthread_mutex_lock(&(pool->thread_counter));
        pool->busy_thr_num--;
        pthread_mutex_unlock(&(pool->thread_counter));
    }
    pthread_exit(NULL);
    return (NULL);
}

//管理线程
void *adjust_thread(void *threadpool)
{
    threadpool_t *pool = (threadpool_t *)threadpool;
    while (!pool->shutdown)
    {
        sleep(DEFAULT_TIME);
        pthread_mutex_lock(&(pool->lock));
        int queue_size = pool->queue_size;
        int live_thr_num = pool->live_thr_num;
        pthread_mutex_unlock(&(pool->lock));
        pthread_mutex_lock(&(pool->thread_counter));
        int busy_thr_num = pool->busy_thr_num;
        pthread_mutex_unlock(&(pool->thread_counter));
        //任务多线程少，增加线程
        if (queue_size >= MIN_WAIT_TASK_NUM
                && live_thr_num < pool->max_thr_num)
        {
            //need add thread
            pthread_mutex_lock(&(pool->lock));
            int add = 0;
            for (int i = 0; i < pool->max_thr_num && add < DEFAULT_THREAD_VARY
                 && pool->live_thr_num < pool->max_thr_num; i++)
            {
                if (pool->threads[i] == 0 || !is_thread_alive(pool->threads[i]))
                {
                    pthread_create(&(pool->threads[i]), NULL, threadpool_thread, (void *)pool);
                    add++;
                    pool->live_thr_num++;
                }
            }
            pthread_mutex_unlock(&(pool->lock));
        }
        //任务少线程多，减少线程
        if ((busy_thr_num * 2) < live_thr_num
                && live_thr_num > pool->min_thr_num)
        {
            //need del thread
            pthread_mutex_lock(&(pool->lock));
            pool->wait_exit_thr_num = DEFAULT_THREAD_VARY;
            pthread_mutex_unlock(&(pool->lock));
            //wake up thread to exit
            for (int i = 0; i < DEFAULT_THREAD_VARY; i++)
            {
                pthread_cond_signal(&(pool->queue_not_empty));
            }
        }
    }
    return NULL;
}

int threadpool_destroy(threadpool_t *pool)
{
    if (pool == NULL)
    {
        return -1;
    }
    pool->shutdown = true;
    //adjust_tid exit first
    pthread_join(pool->adjust_tid, NULL);
    // wake up the waiting thread
    pthread_cond_broadcast(&(pool->queue_not_empty));
    for (int i = 0; i < pool->min_thr_num; i++)
    {
        pthread_join(pool->threads[i], NULL);
    }
    threadpool_free(pool);
    return 0;
}

int threadpool_free(threadpool_t *pool)
{
    if (pool == NULL)
    {
        return -1;
    }
    if (pool->task_queue)
    {
        free(pool->task_queue);
    }
    if (pool->threads)
    {
        free(pool->threads);
        pthread_mutex_lock(&(pool->lock));
        pthread_mutex_destroy(&(pool->lock));
        pthread_mutex_lock(&(pool->thread_counter));
        pthread_mutex_destroy(&(pool->thread_counter));
        pthread_cond_destroy(&(pool->queue_not_empty));
        pthread_cond_destroy(&(pool->queue_not_full));
    }
    free(pool);
    pool = NULL;
    return 0;
}

int threadpool_all_threadnum(threadpool_t *pool)
{
    int all_threadnum = -1;
    pthread_mutex_lock(&(pool->lock));
    all_threadnum = pool->live_thr_num;
    pthread_mutex_unlock(&(pool->lock));
    return all_threadnum;
}

int threadpool_busy_threadnum(threadpool_t *pool)
{
    int busy_threadnum = -1;
    pthread_mutex_lock(&(pool->thread_counter));
    busy_threadnum = pool->busy_thr_num;
    pthread_mutex_unlock(&(pool->thread_counter));
    return busy_threadnum;
}

bool is_thread_alive(pthread_t tid)
{
    int kill_rc = pthread_kill(tid, 0);
    if (kill_rc == ESRCH)
    {
        return false;
    }
    return true;
}

//for test
void *process(void *arg)
{
    printf("thread 0x%x working on task %d
 ",pthread_self(),*(int *)arg);
    sleep(1);
    printf("task %d is end
",*(int *)arg);
    return NULL;
}

int main()
{
    threadpool_t *thp = threadpool_create(3,100,12);
    printf("pool inited");

    int *num = (int *)malloc(sizeof(int)*20);
    for (int i=0;i<10;i++)
    {
        num[i]=i;
        printf("add task %d
",i);
        threadpool_add(thp,process,(void*)&num[i]);
    }
    sleep(10);
    threadpool_destroy(thp);
    return 0;
}