I've looked through various packages, but can't seem to find it.

I tried defining

     export @symbol("ioctl") fn ioctl(fd : io::file, request:i64, ...) int;

but it appears that it is more complicated than that.

A better signature would presumably be something like

     @symbol("ioctl") fn ioctl(fd : io::file, request:u64, ...) (void | 
error);

although it doesn't help, of course.

Ah, never mind, I see that it's in rt::ioctl.

In fact, ioctl is discussed here:

https://harelang.org/blog/2022-05-14-generating-ioctls/