On 10/12/06, Juan Jose Garcia-Ripoll jjgarcia@users.sourceforge.net wrote:
First of all, every time you call a function with foreign-funcall you have to supply the types of the arguments. The aliasing of all pointer types to :pointer means that type information is lost. Not all pointer type are equivalent, because some types have alignment restrictions.
Can you give me an example?
Supplying the argument types is extremely tricky in some cases and might not be enough type information. For example, int f1(int i, ...) is not the same function as int f2(int i, int j) even though when we invoke f1(1, 2) the (2) is coerced to type int.
On all platforms I have access too they seem to behave the same since we manage to call varargs functions and the various Lisps don't know anything about their varargs-ness. I hear that might not always be true, so see below.
The information about "..." is taken cared of by DEFCFUN, but not by FOREIGN-FUNCALL. That may seem useless but really, it is not the same thing to call f1 and f2, because the C compiler does produce different assembly code. For instance, using AMD64, in the first case it has to push the arguments on the stack, while in the second case everything goes into registers Similar things happen with a PPC backend.
See my comment in src/functions.lisp about foreign-funcall-varargs:
;;; ATM, the only difference between this macro and FOREIGN-FUNCALL is that ;;; it does argument promotion for that variadic argument. This could be useful ;;; to call an hypothetical %foreign-funcall-varargs on some hypothetical lisp ;;; on an hypothetical platform that has different calling conventions for ;;; varargs functions. :-)
If ECL can do something special about varargs, we can go ahead and use that!