Below what I've been able to find about alarm(): [1] Oldest alarm() code I can find is in PWB1, dated July 1977: http://minnie.tuhs.org/cgi-bin/utree.pl?file=PWB1/sys/sys/os/sys4.c http://minnie.tuhs.org/cgi-bin/utree.pl?file=PWB1/sys/sys/os/clock.c Either alarm existed in V5 and V6, and was removed from distributions (which seems unlikely to me), or is was added after V6 was released, perhaps soon after. In the latter case the 'nfs' code that we have must be later than 1974 (even though the man page is dated that way). It could be from the 2nd half of 1975. [2] Interestingly, the idea to implement sleep() in terms of alarm() seems to originate in UoI network unix: http://minnie.tuhs.org/cgi-bin/utree.pl?file=SRI-NOSC/ken/sys2.c -- sslep() This occurs in Oct 1977. In V7 this idea is taken to user space and sleep() is no longer a system call. [3] The UoI code has an instance of alarm() being used to break out of a potentially stalled network call, so that usage seems to have established itself early on.