Re: libgcj/9078: libffi: problems with uint8 on powerpc

[Jeff Sturm <jsturm@one-point.com>]

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