Re: FFI questions

[Taylan Kammer <taylan.kammer@gmail.com>]

On 15.05.2020 15:47, Jan Synacek wrote:
> Hello,
> 
> Currently I have something like this:
> 
> (define libxcb (dynamic-link "libxcb"))
> 
> ...
> 
> (define c-change-window-attributes
>    (pointer->procedure void
>       (dynamic-func "xcb_change_window_attributes" libxcb)
>       (list '* uint32 uint32 '*)))
> 
> (define (change-window-attributes conn win mask vals)
>     (c-change-window-attributes conn win mask (bytevector->pointer vals)))
> 
> The last argument to xcb_change_window_attributes is 'const void *' and I
> need to pass a u32 vector to it. As it is right now, it segfaults when I
> try passing #u32(something) to 'change-window-attributes'. Is it possible
> to make it accept a u32 vector using just Scheme or do I have to work
> around it on the C level?

I think the reason it segfaults might be that the #u32() object gets 
garbage collected soon after you call bytevector->pointer on it.  See 
that you put it in a variable, and reference that variable at some point 
after xcb_change_windor_attributes should be done with the vector, to 
make sure that this is not the problem.

Otherwise I'm not sure why it should segfault.  A #u32() bytevector 
really should be backed by a contiguous array of uint32_t values, just 
like what you'll get when you define a uint32_t[] in C.

> My second question is about FFI and structs. Is it possible to access C
> struct members from Scheme by name? If not, how do I generally approach the
> problem? I've checked how guile-xcb does it and it seems to be building
> hash tables with field names as keys. But that basically requires me to
> "redefine" all the C structs that I would be interested in at the Scheme
> level.

These might be useful:

https://github.com/TaylanUB/scheme-bytestructures

https://www.nongnu.org/nyacc/ffi-help.html

I maintain bytestructures, am happy to answer questions.  (If I don't 
respond on the mailing list, feel free to mail me directly or open an 
issue on GitHub; mails with guile-user@gnu.org and guile-devel@gnu.org 
in the To: or Cc: fields land in a folder that I don't always check.)

The FFI Helper is by Matt Wette who also hangs around on this mailing 
list I believe.


- Taylan