今天看Linux多线程，用pthread_create()创建了一个线程，然后在主线程里用pthread_join()等待其退出，但是发现一个奇怪的现象，在pthread_join()函数传递的第二个参数非空的时候，主线程里保存的创建的线程的id被改变了。第二个参数为NULL时就不会。

代码如下：

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

void *thread_join_test(void *args)
{
    printf("in thread_join_test thread id: %lu\n", pthread_self());
}

int main(void)
{
    pthread_t thread_id;
    int statu;

    pthread_create(&thread_id, NULL, (void *)thread_join_test, NULL);
    printf("in main before join thread id: %lu\n", thread_id);
    pthread_join(thread_id, (void *)&statu);
//    pthread_join(thread_id, NULL);
    printf("in main after join thread id: %lu\n", thread_id);

    sleep(3);

    return 0;
}

pthread_join()参数第二个为NULL输出：

in main before join thread id: 140221570873088
in thread_join_test thread id: 140221570873088
in main after join thread id: 140221570873088

pthread_join()参数第二个为(void *)&statu输出：

in main before join thread id: 140593447913216
in thread_join_test thread id: 140593447913216
in main after join thread id: 140591459467264

为什么会不一样？ 这里的thread_id是pthread_t类型变量也就是个unsigned long int类型的局部变量，做形参传到pthread_join()里怎么会变？