Prints formatted output based on an argument list.

    This function is the same as the printf 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
    va_start macro (and possibly with subsequent va_arg calls) from
    <stdarg.h>.

    Format

      #include  <stdio.h>

      int vprintf  (const char *format, va_list ap);