libgcj/9078: libffi: problems with uint8 on powerpc

[Matthias Klose]

Ok, closing the Debian report. Leave the gnats entry to Jeff.

Prof. Etienne M. Gagnon writes:
> On Sun, Mar 16, 2003 at 05:15:47PM +0100, Matthias Klose wrote:
> > hmm, you didn't open this report. Sure I can close it?
> 
> Yes, I am sure.  The bug was opened by of the developers of the
> SableVM project, and he forwarded to me the information in the Debian
> BTS and in the upstream BTS.  Based on that information, I went back
> into the SableVM source code and fixed the bug.  It was definitely not
> a libffi bug (yet one has to very carfully read the libffi
> documentation to notice the inconsistency in the treatment of function
> arguments and return values).


Jeff Sturm writes:
> On Sat, 28 Dec 2002, Matthias Klose wrote:
> > ffi_type_uint8 and other arguments shorter than one word are
> > not passed in correctly to the function called by ffi_call.
> 
> For an ffi_type_uint8, ffi_call expects the corresponding value
> pointer to be a (unsigned char *).  So this is correct usage:
> 
>     int b = 24;
>     args[0] = &ffi_type_uint;
>     values[0] = &b;
> 
> Also correct would be:
> 
>     unsigned char b = 24;
>     args[0] = &ffi_type_uint8;
>     values[0] = &b;
> 
> > Also, returns values are not passed correctly.  For example, instead of
> > finding the returned byte value where the pointer points to, it is found
> > at an offset of 3 from the pointer.
> 
> Return values are handled a little differently than arguments.
> libffi/README says:
> 
>     RVALUE is a pointer to a chunk of memory that is to hold the
>         result of the function call. Currently, it must be
>         at least one word in size (except for the n32 version
>         under Irix 6.x, which must be a pointer to an 8 byte
>         aligned value (a long long). It must also be at least
>         word aligned (depending on the return type, and the
>         system's alignment requirements). If RTYPE is
>         &ffi_type_void, this is ignored. If RVALUE is NULL,
>         the return value is discarded.
> 
> So this cannot work:
> 
>     unsigned char result;
>     if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
>   		   &ffi_type_uint8, args) != FFI_OK) {
> 
> You could use "unsigned int result" on a 32-bit target, or use ffi_arg
> which is typedef'ed to work correctly on 32 or 64-bit targets:
> 
>     ffi_arg result;
>     if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
>                    &ffi_type_uint8, args) != FFI_OK) {
> 
> With those changes your example should be portable to any target supported
> by libffi.
> 
> Jeff
> 
> 
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