Re: Making FFI callbacks call free() automatically when it's collected

  • From: Johnson Lin <arch.jslin@xxxxxxxxx>
  • To: luajit@xxxxxxxxxxxxx
  • Date: Fri, 20 Jul 2012 11:39:03 +0800

2012/7/20 Robert G. Jakabosky <bobby@xxxxxxxxxxxxxxx>:
> On Thursday 19, Johnson Lin wrote:
>> 2012/7/19 Mike Pall <mike-1207@xxxxxxxxxx>:
> If the C callback has a related userdata value (void *) then you can create
> one FFI callback instance for each C callback prototype and pass a callback id
> as the userdata value.  The id would be used to lookup the Lua callback
> function in a table.  Since the FFI callback slots are a limited resource this
> would help decrease the number of slots needed.
> local button_callbacks = {}
> local function wrapped_button_cb(event, ud)
>   local id = tonumber(ffi.cast("uintptr_t", ud))
>   local cb = button_callbacks[id]
>   if cb then return cb(event, id) end
> end
> local button_cb ="ButtonCB", wrapped_button_cb)
> local function wrap_button_cb(func)
>   local id = #button_callbacks+1
>   local ud = ffi.cast("uintptr_t", id)
>   button_callbacks[id] = func
>   return button_cb, ud
> end
> For UI callbacks it might be possible to do the callback lookup via a widget
> id or the widget's cdata value.
> --
> Robert G. Jakabosky

Yes, thanks for the suggestion. I now just use:

local function cdata_addr(cd) return tonumber(ffi.cast("int", cd)) end

to produce the address' value, and use that as the key internally, to
avoid the problem of cdata objects not suitable as the key directly.


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