Byte swapping and bindat

Byte swapping and bindat

[Po Lu]

I was looking into replacing some of the very complicated Motif binary
message parsing code in x-dnd.el with bindat, but there doesn't seem to
be a way to make bindat do byte-swapping automatically, which makes it a
lot less attractive.

Is there a way to specify a bindat type with fields whose endianess
depends on an external parameter (or another field of the type?)

Defining each type twice for both byte orders seems very ugly.

Re: Byte swapping and bindat

[Stefan Monnier via Users list for the GNU Emacs text editor]

> Is there a way to specify a bindat type with fields whose endianess
> depends on an external parameter (or another field of the type?)

I'm so glad you asked:

    (let* ((threshold 32)
           (type (bindat-type (kind   uint 8)
                              (length sint 32 (> kind threshold)))))
      (list (bindat-unpack type "\x10\x00\x00\x01\x01")
            (bindat-unpack type "\x80\x00\x00\x01\x01")))
    =>
    (((kind . 16)  (length . 257))
     ((kind . 128) (length . 16842752)))

That's one of the benefits of the new Bindat :-)


        Stefan

Re: Byte swapping and bindat

[Eli Zaretskii]

> Date: Fri, 10 Jun 2022 09:32:31 -0400
> From:  Stefan Monnier via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org>
> 
>     (let* ((threshold 32)
>            (type (bindat-type (kind   uint 8)
>                               (length sint 32 (> kind threshold)))))
>       (list (bindat-unpack type "\x10\x00\x00\x01\x01")
>             (bindat-unpack type "\x80\x00\x00\x01\x01")))
>     =>
>     (((kind . 16)  (length . 257))
>      ((kind . 128) (length . 16842752)))

It would be nice to have sint documented in the ELisp manual...

Re: Byte swapping and bindat

[Stefan Monnier via Users list for the GNU Emacs text editor]

Stefan Monnier via Users list for the GNU Emacs text editor [2022-06-10 09:32:31] wrote:
>> Is there a way to specify a bindat type with fields whose endianess
>> depends on an external parameter (or another field of the type?)
>
> I'm so glad you asked:
>
>     (let* ((threshold 32)
>            (type (bindat-type (kind   uint 8)
>                               (length sint 32 (> kind threshold)))))
>       (list (bindat-unpack type "\x10\x00\x00\x01\x01")
>             (bindat-unpack type "\x80\x00\x00\x01\x01")))
>     =>
>     (((kind . 16)  (length . 257))
>      ((kind . 128) (length . 16842752)))
>
> That's one of the benefits of the new Bindat :-)

Oh, and if you want to do that for `uint` (which currently doesn't take
a "reverse" argument (patch welcome)), you can do:

    (let* ((threshold 32)
           (type (bindat-type (kind   uint 8)
                              (length if (> kind threshold)
                                         (uintr 32) (uint 32)))))
      (list (bindat-unpack type "\x10\x00\x00\x01\x01")
            (bindat-unpack type "\x80\x00\x00\x01\x01")))
    =>
    (((kind . 16)  (length . 257))
     ((kind . 128) (length . 16842752)))


-- Stefan

Re: Byte swapping and bindat

[Po Lu]

Stefan Monnier via Users list for the GNU Emacs text editor
<help-gnu-emacs@gnu.org> writes:

>> Is there a way to specify a bindat type with fields whose endianess
>> depends on an external parameter (or another field of the type?)
>
> I'm so glad you asked:
>
>     (let* ((threshold 32)
>            (type (bindat-type (kind   uint 8)
>                               (length sint 32 (> kind threshold)))))
>       (list (bindat-unpack type "\x10\x00\x00\x01\x01")
>             (bindat-unpack type "\x80\x00\x00\x01\x01")))
>     =>
>     (((kind . 16)  (length . 257))
>      ((kind . 128) (length . 16842752)))
>
> That's one of the benefits of the new Bindat :-)

Thanks.  Where is that documented?

(And what about an unsigned integer? Is it just `uint', which according
to the doc is a big-endian number?)

Re: Byte swapping and bindat

[Stefan Monnier]

>> That's one of the benefits of the new Bindat :-)
> Thanks.  Where is that documented?

Not sure what "that" refers to.
If you mean "sint" it's documented in `C-h f bindate-type RET`.

If you mean the fact that the `r` argument can be an arbitrary ELisp
expression, it's not explicitly documented, because it holds for
basically every part of `bindate-type` specs.

> (And what about an unsigned integer? Is it just `uint', which according
> to the doc is a big-endian number?)

I assume you've now seen my other post which answers this question.


        Stefan

Re: Byte swapping and bindat

[Po Lu]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> Not sure what "that" refers to.
> If you mean "sint" it's documented in `C-h f bindate-type RET`.

Hmm, I must've missed that.

> I assume you've now seen my other post which answers this question.

Yes, thanks.