On Fri, May 15, 2020 at 10:09 PM Taylan Kammer <taylan.kam...@gmail.com> wrote: > > 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.
This works, thanks! (define (change-window-attributes conn win mask vals) (define b (list->u32vector vals)) (c-xcb-change-window-attributes conn win mask (bytevector->pointer b))) > > 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-de...@gnu.org > in the To: or Cc: fields land in a folder that I don't always check.) Very interesting, I'll take a look. Thank you, -- Jan Synacek Software Engineer, Red Hat