You must declare functions before use, but of course, when you define a function, you declare it at the same time. You can both use an old style to declare a function arguments list:
or the ANSI C syntax:
You cannot define a function with a variable number of arguments (ellipsis) , although some predefined functions use this facility (e.g. printf()). Classically, by default, the function return value type and its parameters type are int.
You cannot redefine predefined functions, and when you
redefine
a user function or variable displays a warning to avoid surprises.
This
default protection can be changed with the debugmode() function
(see §6.6.4 page 
).