/sys$common/syshlp/helplib.hlb CC, Run-time functions, vfprintf  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Prints formatted output based on an argument list.  This function
  is the same as the fprintf function except that instead of being
  called with a variable number of arguments, it is called with an
  argument list that has been initialized by the macro va_start (and
  possibly subsequent va_arg calls).

  Syntax:

       #include <stdio.h>

       int vfprintf (FILE *file_ptr, const char *format,
                     va_list arg);