pthread_cond_wait作用？

main线城里：
pthread_mutex_lock(&mutex);
data[count++] = i;
if (count == N) pthread_cond_signal(&cond);
pthread_mutex_unlock(&mutex);
这块代码没看懂，主线城不释放lock，那么thread_function里的pthread_mutex_lock必然会堵塞，
当主线程释放锁后，子线城才会执行if (count < N)这句代码， 主线程都释放锁了，子线城才会执行，
问一下pthread_cond_wait(&cond, mutext)有什么用呢？ main都释放锁了，让你去访问全局变量data数组和
count变量。

#include <stdio.h>
        #include <stdlib.h>
        #include <unistd.h>
        #include <pthread.h>
        #define N 6
        /*初始化互斥锁*/
        pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
        /*初始化条件变量*/
        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;
        }