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volatile int a = 5,;
volatile int b = 10;

/* Lock to enable threads to access a and b safely */
pthread_mutex_t global_lock = PTHREAD_MUTEX_INITIALIZER;

void* worker_thread(void* dummy) {
  int i,;
  int c;
  int result;

  if ((result = pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS,&i)) != 0) {
    /* handle error */
  }

  while (1) {
    if ((result = pthread_mutex_lock(&global_lock)) != 0) {
      /* handle error */
    }
    c = b;
    b = a;
    a = c;
    if ((result = pthread_mutex_unlock(&global_lock)) != 0) {
      /* handle error */
    }
  }
  return NULL;
}


int main(void) {
  int result;
  pthread_t worker;

  if ((result = pthread_create( &worker, NULL, worker_thread, NULL)) != 0) {
    /* handle error */
  }

  /* .. Do stuff...meanwhile worker thread runs for some time */

  /* since we don't know when the character is read in, the program could continue at any time */
  if ((result = pthread_cancel(worker)) != 0) {
    /* handle error */
  }
  /* pthread_join waits for the thread to finish up before continuing */
  if ((result = pthread_join(worker, 0)) != 0) {
    /* handle error */
  }

  if ((result = pthread_mutex_lock(&global_lock)) != 0) {
    /* handle error */
  }
  printf("a: %i | b: %i", a, b);
  if ((result = pthread_mutex_unlock(&global_lock)) != 0) {
    /* handle error */
  }

  return 0;
}

void release_global_lock(void* dummy) {
  int result;
  if ((result = pthread_mutex_unlock(&global_lock)) != 0) {
    /* handle error */
  }
}

void* worker_thread(void* dummy) {
  int i,;
  int c;
  int result;

  if ((result = pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS,&i)) != 0) {
    /* handle error */
  }

  while (1) {
    if ((result = pthread_mutex_lock(&global_lock)) != 0) {
      /* handle error */
    }
    pthread_cleanup_push( release_global_lock, NULL);
    c = b;
    b = a;
    a = c;
    pthread_cleanup_pop(1);
  }
  return NULL;
}