#include <thread.h> int cond_reltimedwait(cond_t *cv, mutex_t *mp, timestruct_t reltime);
Use cond_reltimedwait(3THR) as you would use cond_timedwait(), except that cond_reltimedwait() takes a relative time interval value in its third argument rather than an absolute time of day value. (For POSIX threads see, pthread_cond_reltimedwait_np(3THR).
cond_reltimedwait() always returns with the mutex locked and owned by the calling thread even when returning an error. The cond_reltimedwait() function blocks until the condition is signaled or until the time interval specified by the last argument has elapsed.