pthread_exit的返回值没有被pthread_join取到

pthread_join的man说，它会拿到pthread_exit的返回值:
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if retval is not NULL, then pthread_join() copies the exit status of the target thread (i.e.,  the  value  that  the  target  thread  supplied  to
       pthread_exit(3)) into the location pointed to by *retval.  If the target thread was canceled, then PTHREAD_CANCELED is placed in *retval.
--------------------

于是我写了个小程序来验证:

C/C++ code?#include <assert.h> #include <pthread.h> #include <unistd.h> #include <iostream> using namespace std; void* fun(void*) {     cout << "fun" << endl;     int i=20;     pthread_exit(&i); } int main() {     pthread_t pt;     cout << "main" << endl;     int ret=pthread_create(&pt,NULL,fun,NULL);     assert(ret==0);     int *pi;//pthread return?     sleep(1);     cout << "join" << endl;     pthread_join(pt,(void**)&pi); // get from pthread_exit     cout << "i=" << *pi << endl;     return 0; }

运行结果是:
main
fun
join
i=32591

我期待的结果是pthread_create创建的子线程线程当中，整形数i=20返回给了主线程的pthread_exit。但是和我预想的不太一样。
这是为什么呢?

不要返回线程函数的本地堆栈变量。把i=20声明为全局变量就行了。