Mail Archives: cygwin/2005/06/30/13:42:21
Dave Korn schrieb:
> ----Original Message----
>>From: Gerrit P. Haase
>>Sent: 30 June 2005 12:10
>>>>static int hack30_pray(ax, items, func)
>>>>int ax;
>>>>int items;
>>>>void *func;
>>>>{
>>>> return 0;
>>>>}
>>>>int main () {
>>>> int ax, items;
>>>> void * symref;
>>>> float num;
>>>> num = ((*((float (*)()) hack30_pray))(ax,items,symref)); return 0;
>>>>}
>
>>S.th. wrong with your testcase?
>
> Absolutely. There are two *VERY* bad things about that bit of code:
>
> 1) The function call is through a pointer-to-function-returning float, but
> the function itself returns a void *. This is likely to screw up the x87 FP
> stack when the caller pops a return value that the callee didn't push.
If it's really coming from the SP(0), which gcc obviously assumes
(hard-floats) vs. some other cpu's/clib's return floats on eax
(soft-floats) or just emulates the SP.
I'll write an extra function for the float case. Only gcc dislikes that.
> 2) Also, the function call is through a pointer-to-function-taking-stdargs,
> but the function itself is not a stdargs function. Given the wrong calling
> conventions, this is liable to lead to both caller *and* callee cleaning the
> args off the stack.....
Calling conventions should be determined at run-time who will pop the
stack. Both DLL types should be able to be loaded. So the hack src
should duplicate itself for both ways.
E.g. WinAPI (pascal = stdcall convention) will fail with the current
hack30 library.
> A name like 'hack*_pray' suggests that this is an ugly hack that the
> author was praying was going to work..... I am obliged to agree!
Yes, that's the hairy hack30 trick from unknown old sources, which is
used for several FFI's as last resort. ;-)
That's why I didn't want to fix perl Win32::API callbacks with that mess.
BTW: I fixed C::Dynalib now at least so we do have another working perl
FFI with callbacks again.
--
Reini Urban
http://xarch.tu-graz.ac.at/home/rurban/
http://phpwiki.org/
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