I should also emphasize the difference: pointer_from_objref returns a
jl_value_t*, which includes the type tag found at the beginning of all
(well, most) Julia value instances. With "Ptr{T}" and "&v" in ccall, you
get a pointer to the memory after the type tag, which is C-compatible
(provided isbits(T) == true)
On Sun, Jan 18, 2015 at 10:12 AM, Isaiah Norton <isaiah.nor...@gmail.com>
wrote:
> - if you want to pass a pointer to the *complete* Julia type, including
> the type tag, use pointer_from_objref. From the manual:
>
> Julia values v can be converted to jl_value_t*pointers, as Ptr{Void}, by
>> calling pointer_from_objref(v).
>
>
> - if you want to pass a pointer to a C-compatible struct, tell ccall that
> is what you want, and use the "&" symbol before the variable. From the
> manual:
>
> struct T* (where T represents an appropriately defined bits type)Ptr{T} (call
>> using &variable_name in the parameter list)
>
> Here is an example:
>
> https://gist.github.com/ihnorton/1c1338a22fb3d143fa7f
>
> Really the best way to work through these things is to write small
> examples where you can control everything, and see what happens. Trying to
> figure it out on the go in a large system with many moving pieces is a
> recipe for frustration.
>
> On Sun, Jan 18, 2015 at 9:28 AM, J Luis <jmfl...@gmail.com> wrote:
>
>> Thank you very much Jiahao. That's indeed the result I want
>>
>> ... but sorry, I'm still confused. Why doesn't the other way work too? At
>> the end it also gets composite type with the same members but with a
>> whatever in their values.
>>
>> Also, I have here a case where it worked with unsafe_load() but crashes
>> if I use unsafe_pointer_to_objref() (note that 'image' is a pointer to a
>> composite type)
>>
>> https://github.com/joa-quim/IUP.jl/blob/master/examples/im_view_.jl#L106
>>
>> I read and reread the docs but find confusing what is a *memory address
>> of a Julia object* is that apparently is different to the pointer
>> obtained with convert()
>>
>>
>> domingo, 18 de Janeiro de 2015 às 06:17:11 UTC, Jiahao Chen escreveu:
>>
>>> I don't think the conversion does what you want. Instead you may be
>>> looking for this:
>>>
>>> julia> type foo a::Int; b::Int end
>>>
>>> julia> tp=foo(2,3)
>>> foo(2,3)
>>>
>>> julia> p=pointer_from_objref(tp)
>>> Ptr{Void} @0x00007fa70872e188
>>>
>>> julia> pt=unsafe_pointer_to_objref(p)
>>> foo(2,3)
>>>
>>> Thanks,
>>>
>>> Jiahao Chen
>>> Staff Research Scientist
>>> MIT Computer Science and Artificial Intelligence Laboratory
>>>
>>
>
