/*
 * 编写程序完成如下功能：
 * (1)有一int型全局变量g_Flag初始值为0
 * (2)在主线程中创建线程1，打印“this is thread1”，并将g_Flag设置为1
 * (3)在主线程中创建线程2，打印“this is thread2”，并将g_Flag设置为2
 * (4)线程1需要在线程2退出后才能退出
 * (5)主线程在检测到g_Flag从1变为2，或者2变为1的时候退出
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>

int g_Flag = 0;
static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

// 线程1在线程2退出后退出
void print_change1(void *arg) {
    printf("this is thread1\n");
    
    // 进入临界区，设置g_Flag
    pthread_mutex_lock(&mutex);
    if(g_Flag == 2) {
        pthread_cond_signal(&cond);
    }
    g_Flag = 1;
    pthread_mutex_unlock(&mutex);

    // 等待线程2结束
    pthread_join(*(pthread_t *)arg, NULL);
    printf("线程1等到线程2结束了!\n");

    //printf("thread1 exit\n");
    pthread_exit(0);
}

void print_change2(void *arg) {
    printf("this is thread2\n");

    // 进入临界区，设置g_Flag
    pthread_mutex_lock(&mutex);
    if(g_Flag == 1) {
        pthread_cond_signal(&cond);
    }
    g_Flag = 2;
    pthread_mutex_unlock(&mutex);

    printf("thread2 exit\n");
    pthread_exit(0);
}

int main() {
    int ret;
    pthread_t t1, t2;

    // 创建线程
    ret = pthread_create(&t2, NULL, (void *)print_change2, NULL);
    ret = pthread_create(&t1, NULL, (void *)print_change1, &t2);

    // 等待条件变量，然后退出
    pthread_mutex_lock(&mutex);
    pthread_cond_wait(&cond, &mutex);
    pthread_mutex_unlock(&mutex);
    printf("main exit\n");
    exit(0);
}

为什么“线程1等到线程2结束了!”会打印两次？求解！