2006/10/13, Luís Oliveira luismbo@gmail.com:
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?
I have no longer access to this machine, but my interpreter of lisp at some point broke because I was using converting (char*) pointers to (short *) pointers, where "short" was the right size of the bytecodes and "char" was the pointer to the buffer. Now it turns out that I was not careful enough and the pointers in the buffer were not aligned on 32-bit boundary and reading the short cause a signal. There are other examples like this.
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.
These lisps play a trick which is to copy all arguments onto the stack, no matter what the function signature is. I am still learning PPC and AMD64 ABIs to understand how to implement a dynamic FFI on those platforms, but those solutions seem to be very fragile.
If ECL can do something special about varargs, we can go ahead and use that!
It is not that it can do something special, it is rather that such information is required. See the post by James Bielman.
Juanjo