This is a demonstration that threadlocal storage can be written to by other threads.

threadlocals_are_a_lie.c

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

__thread int very_local = 5;

/* this function is run by the second thread */
void *do_other_thing(void *very_local_pointer)
{
    printf("in thread 2, very_local is %d\n", very_local);
    *(int*) very_local_pointer = 10;
    return NULL;
}

int main()
{
    printf("very_local is %d\n", very_local);
    very_local = 2;
    printf("very_local is %d\n", very_local);

    pthread_t other_thread;

    // I'm omitting error handling here, which is bad of me.
    pthread_create(&other_thread, NULL, do_other_thing, &very_local);
    pthread_join(other_thread, NULL);

    printf("very_local is %d\n", very_local);

    return 0;
}