uthread_state_t is a enum type that holds the current state of each thread.

typedef enum
{
        UT_NO_STATE,            /* invalid thread state */
        UT_ON_CPU,              /* thread is running */
        UT_RUNNABLE,            /* thread is runnable */
        UT_WAIT,                /* thread is blocked */
        UT_ZOMBIE,              /* zombie threads eat your brains! */
        UT_NUM_THREAD_STATES
} uthread_state_t;