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Re: FFI questions
From: |
Taylan Kammer |
Subject: |
Re: FFI questions |
Date: |
Fri, 15 May 2020 22:09:22 +0200 |
User-agent: |
Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 |
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 address@hidden and address@hidden
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
- FFI questions, Jan Synacek, 2020/05/15
- Re: FFI questions,
Taylan Kammer <=