Sig_Val value: This is an enumerated type which contains the value of the signal. This value can be one of SIG_HIGH, SIG_LOW, SIG_X or SIG_NULL. The latter signal value represents an invalid signal.
ckt_time t: This data member represents the time at which the signal occurred. The ckt_time type is defined to be of type long in this particular implementation. Two constants, CKT_TIME_INIT and CKT_TIME_NULL represent an initial time and an invalid time respectively.