A particular implementation of printf used that technique. There probably weren't a whole lot of different implementations at the time, but it's not part of the

A particular implementation of printf used that technique. There probably weren't a whole lot of different implementations at the time, but it's not part of the C specification. <varargs.h> was a pre-Standard header that provided a consistent interface for accessing variable arguments. It was superseded by <stdarg.h> in the 1989 ANSI C standard. gcc in particular dropped support for <varargs.h> a number of years ago.