updwtmp() appends the utmp structure ut to the wtmp file.
logwtmp() constructs an utmp structure using line", "name", "host, current time and current process id. Then it calls updwtmp() to append the structure to the utmp file.
Both functions are available under glibc2, but not under libc5. However, logwtmp used to occur in the old libbsd. These days, the logwtmp function is included in libutil. (Hence youll need to add -lutil to your compiler commandline to get it.)