线程同步-条件变量

线程同步-条件变量

1.条件变量
下面是等待断言x == y为真时所用的正确非忙等策略
1.锁定互斥量
2.测试条件x == y
3.如果为真，解除互斥量的锁定，并退出循环
4。如果为假， 将线程挂起，并解除对互斥量的锁定

创建和销毁条件变量

＃include <pthread.h>
int pthread_cond_init(pthread_cond_t *restrict cond,
                   const pthread_cond_attr_t *restrict attr );
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

int pthread_cond_destroy(pthread_cond_t *cond);
等待并通知条件

int pthread_cond_timedwait(pthread_cond_tcond, pthread_mutex_t mutex, const struct timespec abstime);

int pthread_cond_wait(pthread_cond_tcond, pthread_mutex_t mutex);

int pthread_cond_broadcast(pthread_cond_t *cond);
int pthread_cond_signal(pthread_cond_t *cond);

下面是使用条件变量时要遵守的规则：
1.在测试断言之前获得互斥量
2.因为返回可能是由某些不相关的事件或无法使断言成真的pthread_cond_signal引起的，所以要在pthread_cond_wait返回之前重新对断言进行判断
3.在修改断言出现的任意变量前获得互斥量
4.仅仅在较短的时间内持有互斥量
5.显示的或者隐式的（pthread_cond_wait)释放互斥量

一个线程安全的路障实现

＃include <errno.h>
#include <pthread.h>

static pthread_cond_t bcond = PTHREAD_COND_INITIALIZER;
static pthread_mutex_t bmutex = PTHREAD_MUTEX_INITIALIZER;
static int count = 0;
static int limit = 0;

int initbarrier(int n) /* initialize the barrier

                                      to be size n */

{

int error;
if(error = pthread_mutex_lock(&bmutex))
  return error;
if(limit != 0) // barrier can only be initialized once

{
  pthread_mutex_unlock(&bmutex);
  return EINVAL;
}

  limit = n;
  return pthread_mutex_unlock(&bmutex);
}


int waitbarrier(void)

{

   int berror = 0;

   int error;

   if(error = pthread_mutex_lock(&bmutex));

        return error;

   if(limit <= 0) //make sure barrier initialized


   {

      pthread_mutex_unlock(&bmutex);

      return EINVAL;

   }

   count ++;

   while((count < limit) && !berror)

     berror = pthread_cond_wait(&bcond, &bmutex);

  if(!berror)

   berror = pthread_cond_broadcast(&bcond); //wakeup everyone

  error = pthread_mutex_unlock(&bmutex);

  if(berror)

   return berror;

  return error;


}