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

[Tim Caswell <tim@...>]

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
>
>