Re: small return types

[Bruno Haible <bruno at clisp dot org>]

Hi Kaz,

Changing the subject, since we're talking about
https://github.com/libffi/libffi/issues/361
https://github.com/libffi/libffi/issues/362
https://github.com/libffi/libffi/issues/368
https://github.com/libffi/libffi/issues/369

> I suspect that some of your ffi_call bug reports on 64 bits may be 
> invalid.
> 
> The return value on 64 bits, of small types, requires special treatment
> due to known quirk/design flaw in the API. It was originally not 
> documented,
> and then it was just documented as is. The way your code is doing it
> is how it *should* be, but isn't.

The 3.2.1 documentation says:

 -- Function: ffi_status ffi_prep_cif (ffi_cif *CIF, ffi_abi ABI,
          unsigned int NARGS, ffi_type *RTYPE, ffi_type **ARGTYPES)
     This initializes CIF according to the given parameters.
     ...
     RTYPE is a pointer to an 'ffi_type' structure that describes the
     return type of the function.  *Note Types::.

Types:
'Libffi' provides a number of built-in type descriptors that can be used
to describe argument and return types:
    ...

The new documentation says:

  +That is, in most cases, @var{ret} points to an object of exactly the
  +size of the type specified when @var{cif} was constructed.  However,
  +integral types narrower than the system register size are widened.

Which is not useful, because the point of using a library such as libffi
is to NOT NEED TO KNOW about the ABI, about the width of system registers
etc.

> This situation bears some resemblance to promotion in the C language!

But this resemblance is not a justification for libffi's behaviour,
because
  1) 'char'. 'unsigned char' etc. are considered as valid return types
     of functions (and different from 'int') since ANSI C, 1989/1990.
  2) C does not do promotion from 32-bit integer types to 64-bit integer
     types.

> Naively written code will appear to work fine on little endian 64 bit,

No, the bug https://github.com/libffi/libffi/issues/368 also affect
some little-endian platforms.

Bruno