procedure Foo (var x);
like in Borland Pascal. In GNU Pascal, you can also use
procedure Foo (var x: Void);
procedure Foo (a: Integer; ...);
However, GPC does not (yet) provide a portable mechanism to access the additional arguments.
procedure Foo (a: array of Integer);
are implemented. However, Standard Pascal conformant array parameters are usually a cleaner mechanism to pass arrays of variable size.
procedure DrawGraph (function f (x: Real): Real);