Re: LuaJIT FFI callbacks (was Re: Calling lua functions from C when using luajit ffi.)

[CrazyButcher <crazybutcher@...>]

brilliant work, thanks Mike

2011/11/16 Robert G. Jakabosky <bobby@sharedrealm.com>:
> On Tuesday 15, Tim Caswell wrote:
>> I should mention that this particular function doesn't need to call
>> into lua, so if it's not supported, how would I implement the callback
>> in C?
>
> You can create a C function that will be used as the callback, then export it
> so Lua can pass it to uv_read_start().
>
> in C:
> uv_buf_t lua_ev_alloc(uv_handle_t* handle, size_t suggested_size) {
>  uv_buf_t buf;
>  buf.base = malloc(suggested_size);
>  buf.len = suggested_size;
>  return buf;
> }
>
> in Lua:
> FFI.cdef[[
> uv_buf_t lua_ev_alloc(uv_handle_t* handle, size_t suggested_size);
> ]]
>
> function stream_prototype:read_start(on_read)
>  uv_assert(C.uv_read_start(FFI.cast("uv_stream_t*", self),
>    C.lua_ev_alloc, on_read))
> end
>
>>
>> On Tue, Nov 15, 2011 at 9:47 AM, Tim Caswell <tim@creationix.com> wrote:
>> > Well, it seems this works for most callbacks, but not all.  In the
>> > docs I see it says it doesn't work with "functions with pass-by-value
>> > aggregate argument or result types".  I'm not sure what that means.
>> >
>> > My function signature is:
>> >
>> >    typedef uv_buf_t (*uv_alloc_cb)(uv_handle_t* handle, size_t
>> > suggested_size);
>> >
>> > When it tries to convert a lua function to this I get:
>> >
>> >    /home/tim/luvit/examples/uvffi/uv.lua:46: bad argument #2 to
>> > 'uv_read_start' (cannot convert 'function' to 'struct 2967 (*)()')
>> >
>> > Am I doing something wrong, or is this kind of function not supported?
>> >
>> > -Tim Caswell
>> >
>> > The header file I'm feeding to luajit is
>> > https://github.com/creationix/luvit/blob/master/examples/uvffi/ffi_uv.h
>> > my lua script is
>> > https://github.com/creationix/luvit/blob/master/examples/uvffi/uv.lua
>> >
>> > On Tue, Nov 15, 2011 at 3:12 AM, Duncan Cross <duncan.cross@gmail.com>
> wrote:
>> >> On Tue, Nov 15, 2011 at 8:25 AM, Duncan Cross <duncan.cross@gmail.com>
> wrote:
>> >>> On Tue, Nov 15, 2011 at 5:15 AM, James McKaskill <james@foobar.co.nz>
> wrote:
>> >>>> On Nov 14, 2011, at 19:30 , Duncan Cross wrote:
>> >>>>> This is fantastic news! Thanks so much for your work, and thanks too
>> >>>>> to the anonymous sponsor.
>> >>>>>
>> >>>>> One thing - I'm not certain this is related to the addition of
>> >>>>> callbacks, but I was just experimenting with Win32 calls and found
>> >>>>> that GetLastError() seems to return the wrong value (127,
>> >>>>> ERROR_PROC_NOT_FOUND) the first time it's called. This is a minimal
>> >>>>> test case:
>> >>>>>
>> >>>>>  local ffi = require("ffi")
>> >>>>>
>> >>>>>  ffi.cdef [[
>> >>>>>  uint32_t GetLastError();
>> >>>>>  void SetLastError(uint32_t code);
>> >>>>>  ]]
>> >>>>>
>> >>>>>  ffi.C.SetLastError(0)
>> >>>>>  print(ffi.C.GetLastError()) -- 127
>> >>>>>  ffi.C.SetLastError(0)
>> >>>>>  print(ffi.C.GetLastError()) -- 0
>> >>>>>
>> >>>>> (I'm explicitly setting the error here, but I discovered it because
>> >>>>> it was hiding a parameter error while I was experimenting with
>> >>>>> trying to call RegisterClassExA().)
>> >>>>>
>> >>>>> -Duncan
>> >>>>
>> >>>> I would not suggest trying to get errors using Get/SetLastError as
>> >>>> they will be invalidated by the vm. If you really want the error use
>> >>>> ffi.errno().
>> >>>>
>> >>>> -- James
>> >>>
>> >>> It was specifically addressed before that it should work, and is not
>> >>> the same thing as errno:
>> >>>
>> >>> http://lua-users.org/lists/lua-l/2011-05/msg00219.html
>> >>>
>> >>> "I've fixed this in git HEAD. LuaJIT now preserves errno across hooks,
>> >>> memory allocations, invocations of the JIT compiler and other internal
>> >>> VM activity. Ditto for GetLastError() on Windows.
>> >>>
>> >>> (...)
>> >>>
>> >>> On Windows you need to declare and call GetLastError() yourself (note:
>> >>> this does _not_ hold the same value as errno)."
>> >>>
>> >>> -Duncan
>> >>
>> >> It's also mentioned at the end of the documentation for ffi.errno():
>> >>
>> >> http://luajit.org/ext_ffi_api.html#ffi_errno
>> >>
>> >> -Duncan
>
>
> --
> Robert G. Jakabosky
>
>