pointer_from_objref is mentioned as a parenthetical, and actually I'm not
sure if it needs to be mentioned at all in the manual because this usage is
covered by specifying a type of `Any` to ccall. I agree it could be better
documented in the stdlib docs.
On Sun, Jan 18, 2015 at 11:36 AM, Milan Bouchet-Valat <nalimi...@club.fr>
wrote:
> Le dimanche 18 janvier 2015 à 10:15 -0500, Isaiah Norton a écrit :
> > 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)
> I think this deserves to be in the manual, and in relevant documentation
> for functions in stdlib. ?pointer_from_objref and ?pointer do not really
> allow grasping the difference between the two functions.
>
>
> Regards
>
> > 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
> >
> >
> >
> >
>
>
