要个效率高的方法
需附上全部源代码

这个比较全面了！
#include 
#include 
#include 

pthread_mutex_t mutex;
pthread_cond_t  cond;

void * child1(void *arg)
{
        pthread_cleanup_push(pthread_mutex_unlock,&mutex);  
        while(1){
                printf("thread 1 get running n");
        printf("thread 1 pthread_mutex_lock returns %dn",
pthread_mutex_lock(&mutex));
        pthread_cond_wait(&cond,&mutex);
                    printf("thread 1 condition appliedn");
        pthread_mutex_unlock(&mutex);
                    sleep(5);
    }
        pthread_cleanup_pop(0);    
}

void *child2(void *arg)
{
        while(1){
                sleep(3);               
                printf("thread 2 get running.n");
        printf("thread 2 pthread_mutex_lock returns %dn",
        pthread_mutex_lock(&mutex));
        pthread_cond_wait(&cond,&mutex);
        printf("thread 2 condition appliedn");
        pthread_mutex_unlock(&mutex);
        sleep(1);
        }
}

int main(void)
{
        int tid1,tid2;

        printf("hello, condition variable testn");
        pthread_mutex_init(&mutex,NULL);
        pthread_cond_init(&cond,NULL);
        pthread_create(&tid1,NULL,child1,NULL);
        pthread_create(&tid2,NULL,child2,NULL);
        do{
        sleep(2);                   /* comment 4 */
                pthread_cancel(tid1);       /* comment 5 */
                sleep(2);                   /* comment 6 */
        pthread_cond_signal(&cond);
    }while(1);  
        sleep(100);
        pthread_exit(0);
}

有关于各种锁的效率的文章
http://www-128.ibm.com/developerworks/cn/linux/sdk/rt/part5/index.html

用POSIX 互斥锁比信号灯效率快20倍左右。
如果对全局变量的读、写操作差不多的话可以使用POSIX 互斥锁，可能回牺牲系统效率10%左右。
如果读操作比写操作要多的话可以使用读写锁。
互斥锁: pthread_mutex_lock   pthread_mutex_unlock
读写锁: pthread_rwlock_rdlock   pthread_rwlock_wrlock  pthread_rwlock_unlock

访问全部变量之前加锁互斥就可以了，用信号量没有必要啊