作者：曾宏安,讲师。

我们在编写多线程程序时经常需要在线程之间实现通信，常见的机制有信号量和互斥锁。这里再向大家介绍一种用于实现线程间同步的机制——条件变量。

条件变量可以使线程睡眠等待直到某个条件满足为止。条件变量基本使用操作有两种：一、当判断条件不满足时，某些线程睡眠在相应的条件变量上；二、某些线程改变了条件，唤醒睡眠在条件变量上的其他线程。

为了在判断或是改变条件时防止竞争，条件变量通常和互斥锁结合在一起使用。条件变量类型为pthread_cond_t，互斥锁类型为pthread_mutex_t。

条件变量的创建和注销

int 　pthread_cond_init(pthread_cond_t *restrict cond,pthread_condattr_t *restrict cond_attr); // 创建

        cond : 指向要初始化的条件变量的指针         cond_attr : 条件变量的初始化属性         或         pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

int pthread_cond_destroy(pthread_cond_t *cond); // 注销

条件变量的等待和激活

int pthread_cond_wait(pthread_cond_t *restrict cond,

        pthread_mutex_t *restrict mutex);

该函数使当前线程释放互斥锁mutex并睡眠在条件变量cond上，正确返回时当前线程获得互斥锁mutex

int pthread_cond_timewait(pthread_cond_t *restrict cond, pthread_mutex_t *restrict mutex,const struct timespec *restrict abstime);

该函数功能同pthread_cond_wait，但睡眠时间不超过abstime，正确返回时当前线程获得互斥锁mutex

int pthread_cond_signal(pthread_cond_t *cond);

        该函数唤醒等待在条件变量cond上的某个线程

int pthread_cond_broadcast(pthread_cond_t *cond);

        该函数唤醒等待在条件变量cond上的所有线程

下面我们看一个例子，了解一下条件变量的用法

#include <stdio.h>

        #include <stdlib.h>         #include <unistd.h>         #include <pthread.h>         #define N 6                  pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;                  pthread_cond_t cond = PTHREAD_COND_INITIALIZER;         void *thread_function(void *);         int count = 0;         int data[N];         int main()         {                 pthread_t thread_a;                 int i;

        if (pthread_create(&thread_a, NULL, thread_function, NULL) < 0)

                {                         perror(“fail to pthread_create”);                         exit(-1);                 }                 while ( 1 )                 {                         printf(“please input number : “);                         scanf(“%d”, &i);                         if (i = = 0) break;                         pthread_mutex_lock(&mutex);                         data[count++] = i;                         if (count == N) pthread_cond_signal(&cond);                         pthread_mutex_unlock(&mutex);                         usleep(100000);                 }                 pthread_mutex_destroy(&mutex);                 pthread_cond_destroy(&cond);

        return 0;

        }         void *thread_function(void *arg)         {                 while ( 1 )                 {                         int i, sum;                         pthread_mutex_lock(&mutex);                         if (count < N)                         {                                 pthread_cond_wait(&cond, &mutex);                         }                         for (i=0,sum=0; i<N; i++) sum += data[i];                         printf(“average = %d\n”, sum/N);                         count = 0;                         pthread_mutex_unlock(&mutex);                 } // end while                 return NULL;         }

在上面的代码中，主线程从键盘读取整数。每读入6个数后，唤醒另外一个线程统计并打印平均数。输入0退出程序。