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

[Tim Caswell <tim@...>]

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?

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
>>
>>
>