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To: Cheng Jin <jincheng@ca.ibm.com>
Subject: Re: Incorrect data detected in the nested float struct with
 x86/libffi on Linux/64bit
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Date: Wed, 09 Jun 2021 16:48:37 -0700
From: "Kaz Kylheku (libffi)" <382-725-6798@kylheku.com>
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On 2021-06-09 09:41, Cheng Jin via Libffi-discuss wrote:
> Hi,

Hi Cheng Jin,

A small matter on a tangent.

>=20
> 	  cls_struct_fields1[0] =3D &ffi_type_float;
> 	  cls_struct_fields1[1] =3D &cls_struct_type2;
> 	  cls_struct_fields1[2] =3D NULL;

Indeed, the array describing the element types of a structure must be
null terminated, so this is required.

> 	  cls_struct_type1.size =3D 0;
> 	  cls_struct_type1.alignment =3D 0;
> 	  cls_struct_type1.type =3D FFI_TYPE_STRUCT;
> 	  cls_struct_type1.elements =3D cls_struct_fields1;

Note that here, the size of the object is not indicated. That's why
the extra null pointer is required to infer the size.

>=20
> 	  dbl_arg_types[0] =3D &ffi_type_float;
> 	  dbl_arg_types[1] =3D &cls_struct_type1;
> 	  dbl_arg_types[2] =3D NULL;

But, for the argument array of a ffi_cif, no null termination is=20
required.

> 	 ffi_prep_cif(&cif_temp, FFI_DEFAULT_ABI, 2, retType, dbl_arg_types);

No requirement for a null terminator is documented or appears in
examples. The 2 argument indicates the number of elements.

There is no harm in it; just pointing it out.

When I looked at your code, I went scrambling through documentation
to see if I was missing a required null terminator. :)

Cheers ...