Prints formatted output based on an argument list.

    This function is the same as the sprintf 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 va_
    start (and possibly with subsequent va_arg calls).

    Format

      #include  <stdio.h>

      int vsprintf  (char *str, const char *format, va_list ap);