pthread_self返回不同的值

Question

我正在使用C学习线程，并找到了以下“Hello World”程序。当我使用pthread_join（）在线程内和线程外打印pthread_value（）的值时，两个线程返回的值完全不同。

/******************************************************************************
* FILE: hello.c
* DESCRIPTION:
*   A "hello world" Pthreads program.  Demonstrates thread creation and
*   termination.
* AUTHOR: Blaise Barney
* LAST REVISED: 08/09/11
******************************************************************************/
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#define NUM_THREADS     5

void *PrintHello(void *threadid)
{
   long tid;
   tid = (long)threadid;
   printf("Hello World! It's me, thread #%ld!, %ld
", tid, pthread_self());
   long a=  pthread_self();
   //pthread_exit(NULL);
        long *p = &a;
   return p;
}

int main(int argc, char *argv[])
{
   pthread_t threads[NUM_THREADS];
   int rc;
   void *status;
   long t;
   FILE* file = fopen("test.txt", "r");
   for(t=0;t<NUM_THREADS;t++){
     printf("In main: creating thread %ld
", t);
     rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
     if (rc){
       printf("ERROR; return code from pthread_create() is %d
", rc);
       exit(-1);
       }
     }
   for(t=0; t<NUM_THREADS;t++){
     pthread_join(threads[t], &status);
     printf("%ld
",(long)status);

        }
   /* Last thing that main() should do */
   pthread_exit(NULL);
}