The varargs.h header file contains definitions for handling variable argument lists.
This header is kept for historical reasons. It was part of the Legacy Feature in Single UNIX Specification, Version 2, but has been withdrawn and is not supported as part of Single UNIX Specification, Version 3. New applications should use the <stdarg.h> header to support variable argument list functionality compatible with IEEE Std 1003.1-2001.
Applications conforming to Single UNIX Specification, Version 3 must not include the header file.