为什么是线程2先退出，然后才是线程1返回？

Question

#include "apue.h"

#include <pthread.h>

void *

thr_fn1(void *arg)

{

printf("thread 1 returning\n");

return((void *)1);

}

void *

thr_fn2(void *arg)

{

printf("thread 2 exiting\n");

pthread_exit((void *)2);

}

int

main(void)

{

int err;

pthread_t tid1, tid2;

void *tret;

err = pthread_create(&tid1, NULL, thr_fn1, NULL);

if (err != 0)

err_exit(err, "can't create thread 1");

err = pthread_create(&tid2, NULL, thr_fn2, NULL);

if (err != 0)

err_exit(err, "can't create thread 2");

err = pthread_join(tid1, &tret);

if (err != 0)

err_exit(err, "can't join with thread 1");

printf("thread 1 exit code %ld\n", (long)tret);

err = pthread_join(tid2, &tret);

if (err != 0)

err_exit(err, "can't join with thread 2");

printf("thread 2 exit code %ld\n", (long)tret);

exit(0);

}

But in the code ,there are:

        err = pthread_create(&tid1, NULL, thr_fn1, NULL);

if (err != 0)

err_exit(err, "can't create thread 1");

err = pthread_create(&tid2, NULL, thr_fn2, NULL);

if (err != 0)

err_exit(err, "can't create thread 2");