#include <utmp.h> void logwtmp(const char *line, const char *name, const char *host);
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 occurs in the old libbsd.