Hi Kevin,
I had already tried something similar to what you propose, but neither
version works. Hence my confusion. println in either case just prints
garbage. It does compile though. I was hoping to be able to hide some of
this C callback complexity from the api user. It's beginning to look like
that would be difficult to do. I haven't really had the chance to think
through what a macro might provide though. In any case, I think the C
version ends up being much more straight forward. I'm going to stop
bothering you guys with this. There are many other areas in Julia to
explore ;) Thanks for your help and patience.
Thanks,
-G
On Sunday, August 10, 2014 3:02:16 PM UTC-5, Kevin Squire wrote:
>
> More specifically, you can at least do the following:
>
> function my_callback(_cbdata::Ptr{Void})
> cbdata = reinterpret(Ptr{Ptr{Uint8}}, _cbdata) # equivalent to casting
> arr = pointer_to_array(cbdata, 4) # arr of length 4
> println(bytestring(arr[1]))
> nothing
> end
>
> cbfunc = cfunction(my_callback, Void, (Ptr{Void},))
>
> Although I think the previous version might work (did you try it?)--Julia
> can't tell a Ptr{Void} from a Ptr{Ptr{Uint8}} when called from C, and the C
> routine just trusts that you're passing a function which takes a void
> pointer--it also can't check the type of the Julia function.
>
> Cheers,
> Kevin
>
>
> On Sun, Aug 10, 2014 at 12:47 PM, Stefan Karpinski <[email protected]
> <javascript:>> wrote:
>
>> This is absolutely possible. You can take any pointer and turn it into an
>> array. The only concern is garbage collection and making sure that Julia
>> knows whether it should free the memory or not.
>>
>> On Aug 10, 2014, at 3:31 PM, Gerry Weaver <[email protected]
>> <javascript:>> wrote:
>>
>> Hi Kevin,
>>
>> It probably would, but in this case the C function takes a void pointer
>> argument and passes that to the Julia callback function. Unfortunately, I
>> can't change the type of the argument. I was hoping that there was a way to
>> convert that void pointer back to an array, but it isn't looking like that
>> is possible. I guess the answer is to pass a blob of text with some type of
>> delimiter to pass multiple values. That would be significantly slower
>> though.
>>
>> Thanks,
>> -G
>>
>> On Sunday, August 10, 2014 10:56:23 AM UTC-5, Kevin Squire wrote:
>>>
>>> Hi Gerry,
>>>
>>> I think I see whats going on. If the pointer that is being passed to
>>>> pointer_to_array is void, then you get a array of void pointers. In my
>>>> case
>>>> the original array was an array of strings, so...
>>>>
>>>> arr = pointer_to_array(cbdata, 4, true)
>>>> println(bytestring(convert(Ptr{Uint8}, val[1])))
>>>>
>>>
>>> Was the original array of strings allocated by Julia, or in your C
>>> function?
>>>
>>> I think your solution is close. It would be slightly cleaner if you
>>> declared you function to take a Ptr{Ptr{Uint8}}:
>>>
>>> function my_callback(cbdata::Ptr{Ptr{Uint8}})
>>> arr = pointer_to_array(cbdata, 4) # arr of length 4
>>> println(bytestring(arr[1]))
>>> nothing
>>> end
>>>
>>> cbfunc = cfunction(my_callback, Void, (Ptr{Ptr{Uint8}},))
>>>
>>> Does this work?
>>>
>>> BTW, have you seen Steven Johnson's blog post on Passing Julia Callback
>>> Functions to C <http://julialang.org/blog/2013/05/callback/>? I've
>>> found it quite useful.
>>>
>>> Cheers!
>>> Kevin
>>>
>>>
>>>
>>>
>>>> Is there a better way to do this?
>>>>
>>>> Thanks,
>>>> -G
>>>>
>>>>
>>>> On Sunday, August 10, 2014 4:23:29 AM UTC-5, Gerry Weaver wrote:
>>>>>
>>>>> Hi Kevin,
>>>>>
>>>>> I found the pointer_to_array function, but I'm having some trouble
>>>>> figuring how to call it correctly. I assume it has something to do with
>>>>> the
>>>>> pointer being a void pointer. I've been searching for something that
>>>>> might
>>>>> give me a clue how to do this, but I haven't found anything very useful
>>>>> so
>>>>> far.
>>>>>
>>>>> Thanks,
>>>>> -G
>>>>>
>>>>> On Sunday, August 10, 2014 4:06:52 AM UTC-5, Kevin Squire wrote:
>>>>>>
>>>>>> Hi Gerry,
>>>>>>
>>>>>> "pointer_to_array" is probably what you're looking for. See
>>>>>> http://julia.readthedocs.org/en/latest/manual/calling-c
>>>>>> -and-fortran-code/#accessing-data-through-a-pointer for more
>>>>>> information.
>>>>>>
>>>>>> Cheers,
>>>>>> Kevin
>>>>>>
>>>>>>
>>>>>> On Sun, Aug 10, 2014 at 1:17 AM, Gerry Weaver <[email protected]>
>>>>>> wrote:
>>>>>>
>>>>>>> I forgot to mention that I'm trying to accomplish this with arrays
>>>>>>> right now. I can pass a Julia array to C and access it there, but I
>>>>>>> can't
>>>>>>> figure out how to convert it back to a Julia array from a C void
>>>>>>> pointer
>>>>>>> when calling back to Julia from C.
>>>>>>>
>>>>>>>
>>>>>>> On Sunday, August 10, 2014 1:57:18 AM UTC-5, Gerry Weaver wrote:
>>>>>>>>
>>>>>>>> Hello All,
>>>>>>>>
>>>>>>>> I have a C function called mycfunc that takes a callback function
>>>>>>>> pointer (cbfunc) and a void pointer (cbdata) as arguments. mycfunc
>>>>>>>> calls
>>>>>>>> the callback function (cbfunc) passing the void pointer (cbdata) as
>>>>>>>> its
>>>>>>>> argument. I'm trying to figure out how to pass a Julia type as the
>>>>>>>> void
>>>>>>>> pointer argument and convert it back to the Julia type in the Julia
>>>>>>>> function passed as the callback function argument. I can do this with
>>>>>>>> a
>>>>>>>> simple string, but I'm having some difficulty in figuring out how to
>>>>>>>> do it
>>>>>>>> with arrays and other types. Any advice on this would be much
>>>>>>>> appreciated.
>>>>>>>>
>>>>>>>>
>>>>>>>> typedef int (* mycallback)(void *cbdata);
>>>>>>>>
>>>>>>>> void mycfunc(mycallback cbfunc, void *cbdata)
>>>>>>>> {
>>>>>>>> cbfunc(cbdata);
>>>>>>>> }
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> -G
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>
>>>
>
