pthread_exit() pthread_join() - where is the retval kept?

From: David Harel (hareldvd_at_nonexisting.hamakor.org.il)
Date: Thu 06 Jan 2005 - 18:17:21 IST

Hi,

I hope you can help me. I was looking for the right answer everywhere on
the net.

When I do pthread_exit(retval); and I do pthread_join(&retval); at the
"calling" thread, is the stack frame of the exiting thread kept until
the caller/joiner receives retval? (like in processes, wait() clears
zombie)?
Still, retval is a pointer, i guess to somewhere in the terminated
thread space (stack frame) but after pthread_join() the terminated
thread space can be reclaimed. What if another thread reclaims that
space and reuse it. Does that mean that retval will be invalid? and if
so. How can I save retval safely?

attached a small example that seem to show the retval issue. In the
example I intentionally start the threads synchronously so I hope the
next thread will reclaim memory of the terminated one. I save the return
values in an array. As I suspected, the reference in the array becomes
invalid and eventually I get segment violation. If I do anything wrong
here, please correct me.

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

#define NUM_THREADS 8
void *PrintHello(void *threadid)
{
   int *id_ptr, taskid;
   id_ptr = (int *) threadid;
   taskid = *id_ptr;
   printf("Thread %d says hello\n", taskid);
   pthread_exit(& taskid);
}
int main()
{
  pthread_t threads[NUM_THREADS];
  int rc, i, t[NUM_THREADS];
  int * retval[NUM_THREADS];
    for(i=0; i < NUM_THREADS; i++)
    {
       t[i] = i;
       rc = pthread_create(&threads[i], NULL, PrintHello,
            (void *) & t[i]);
       pthread_join(threads[i], (void *) &retval[i]);
    }
    for(i=0; i < NUM_THREADS; i++)
    {
       printf("thread returned value %d\n", * retval[i]);
    }
    sleep(1);
    exit(0);
} 

-- 
Thanks.
David Harel,
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