There are no default argument promotions on arguments that have type decimal when the called function does not include a prototype. If the expression for the called function has a type that includes a prototype, the behavior is as documented in ANSI, with the exception of prototype with an ellipsis (...). If the prototype ends with an ellipsis (...), default argument promotions are not performed on arguments with decimal types.
A function may change the values of its parameters, but these changes cannot affect the values of the arguments. However, it is possible to pass a pointer to a decimal object, and the function may change the value of the decimal object to which it points.