output:
changed from thread: 99
thread: 0x7f9c3e91e6fc
thread: 0x7f9c3e91f73c
thread: 0x7f9c3e91f73c
10


#include <sys/wait.h>
#include <stdio.h>

int __thread lala;

static pthread_cond_t wc = PTHREAD_COND_INITIALIZER;
static pthread_mutex_t wt = PTHREAD_MUTEX_INITIALIZER;

void *thread(void *n)
{
    setgid(99);
    pthread_cond_wait(&wc, &wt);
    printf ("thread: %p\n", &lala);

}

int main()
{
    pid_t pid;
    lala = 10;

    pthread_t t;
    pthread_create(&t, NULL, &thread, NULL);

    sleep (10);

    printf ("changed from thread: %lu\n", getgid());
    pthread_cond_broadcast(&wc);
    pthread_join(t, NULL);

    if ((pid = fork()) > 0) {
        printf ("thread: %p\n", &lala);
        wait(NULL);
    } else {
        printf ("thread: %p\n", &lala);
        printf ("%d\n", lala);
    }
}