A buggy parallel swap program

parallel_swap.c

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

static volatile char* person1;
static volatile char* person2;
static const int LOOPS = 1e4;

void* mythread(void* arg) {
  printf("%s: begin\n", (char*)arg);
  int i;
  for (i=0; i<LOOPS; i++) {
    // swap                                                                                      
    volatile char* tmp = person1;
    person1 = person2;
    person2 = tmp;
  }
  printf("%s: done\n", (char*)arg);
  return NULL;
}

int main(int argc, char* argv[]) {
  pthread_t p1, p2;
  person1 = "Jack";
  person2 = "Jill";
  printf("main: begin (%s, %s)\n", person1, person2);
  pthread_create(&p1, NULL, mythread, "A");
  pthread_create(&p2, NULL, mythread, "B");

  // wait for threads to finish                                                                  
  pthread_join(p1, NULL);
  pthread_join(p2, NULL);
  printf("main: end (%s, %s)\n", person1, person2);
}