* Help: correct way to handle struct return values.
@ 2017-05-28 3:14 Kaz Kylheku
2017-05-28 4:13 ` Kaz Kylheku
0 siblings, 1 reply; 2+ messages in thread
From: Kaz Kylheku @ 2017-05-28 3:14 UTC (permalink / raw)
To: libffi-discuss
Hi all,
In the current texinfo doc, the callback stub does this to
return a value of type int:
/* Acts like puts with the file given at time of enclosure. */
void puts_binding(ffi_cif *cif, void *ret, void* args[],
void *stream)
{
*(ffi_arg *)ret = fputs(*(char **)args[0], (FILE *)stream);
}
Can someone show what the code would look like for a function
that returns the following type, and work everywhere: all
supported platforms, big or little endian:
struct little { char a, b };
Obvously, this can't be used:
*(ffi_arg *)ret = fun_returning_struct_little(...);
After everything, I'm not confident that this is correct, either:
*(struct little *)ret = fun_returning_struct_little(...);
Also, can someone show how to extract "struct little" in after
a ffi down call? If the type is int, we can do this:
ffi_arg rc_buf;
int rc;
ffi_call(&cif, puts, &rc_buf, values);
rc = rc_buf;
But what if rc is of type "little struct"? Again, we can't just use a
cast.
Lastly, how does any of this change for struct bigger_struct { long x,
y, z; }?
Thanks.
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Help: correct way to handle struct return values.
2017-05-28 3:14 Help: correct way to handle struct return values Kaz Kylheku
@ 2017-05-28 4:13 ` Kaz Kylheku
0 siblings, 0 replies; 2+ messages in thread
From: Kaz Kylheku @ 2017-05-28 4:13 UTC (permalink / raw)
To: libffi-discuss
On 27.05.2017 20:14, Kaz Kylheku wrote:
> Hi all,
>
> In the current texinfo doc, the callback stub does this to
> return a value of type int:
>
> /* Acts like puts with the file given at time of enclosure. */
> void puts_binding(ffi_cif *cif, void *ret, void* args[],
> void *stream)
> {
> *(ffi_arg *)ret = fputs(*(char **)args[0], (FILE *)stream);
> }
>
> Can someone show what the code would look like for a function
> that returns the following type, and work everywhere: all
> supported platforms, big or little endian:
>
> struct little { char a, b };
This corresponds to libffi.call/struct5.c
Everything I'm seeing in the test suite seems like structs are handled
in the straighforward way in both ffi_call and closures. But
in this small struct test case there is this comment:
/* This is a hack to get a properly aligned result buffer */
test_structure_5 *ts5_result =
(test_structure_5 *) malloc (sizeof(test_structure_5));
What are the alignment requirements here; must the small
structure have the same alignment as ffi_arg?
And why so, if its size is smaller? If this has to be,
say, 8 bytes aligned, doesn't it mean something will be writing
an 8 byte data unit into it? In that case, shouldn't
the malloc request be padded up to 8 so there is no overrun?
I am not concerned about the alignment because I'm using alloca
for the return value buffer given to ffi_call.
I am worried though about the size. alloca(2) could give me an
8 byte aligned pointer which is only two byte away from the next
object on the stack; if something writes 8 bytes there, that is
very bad.
^ permalink raw reply [flat|nested] 2+ messages in thread
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