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]> 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]> 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
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>
>>
