Implementing A "Future"

Lets implement a "future" that stores an 'int' value, without the feature that allows waiting on several "futures" simultaneously. First, the data structure of the "future":

/* data for a future object. */
struct int_future {
  int value_is_set; /* '1' if value is set, '0' otherwise. */
  int value;
  pthread_mutex_t mutex;
  pthread_cond_t cond;
};