# From type to ptr and back

**URL:** <https://discourse.julialang.org/t/from-type-to-ptr-and-back/888>\
**Category:** General Usage\
**Created:** [December 12, 2016, 1:03pm UTC](https://discourse.julialang.org/t/from-type-to-ptr-and-back/888 "2016-12-12T13:03:42Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![rveltz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rveltz/32/2707_2.png) [@rveltz](https://discourse.julialang.org/u/rveltz)\
**Post date:** [December 12, 2016, 1:03pm UTC](https://discourse.julialang.org/t/from-type-to-ptr-and-back/888/1 "2016-12-12T13:03:42Z")

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Dear User,

I am calling a C library that ask for a **void \* data** in a struct. Hence, I pass to this struct my julia variable `a` of Type **Data** using the void pointer `b = pointer_from_objref(a)`.

However, later on, I want to modify the content of the field data of the C-struct.

Is there a way to get access to the content of the variable `a` using only `b::Ptr{Void}`?

Thank you for your help,

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**Author:** ![bicycle1885](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bicycle1885/32/107_2.png) [@bicycle1885](https://discourse.julialang.org/u/bicycle1885)\
**Post date:** [December 12, 2016, 1:47pm UTC](https://discourse.julialang.org/t/from-type-to-ptr-and-back/888/2 "2016-12-12T13:47:04Z")

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You can use `unsafe_pointer_to_objref` to restore a Julia object from a pointer, but you should keep in mind that this operation is “unsafe”, meaning it may cause a segfault if you access an invalid pointer. EzXML.jl uses this operation here: [https://github.com/bicycle1885/EzXML.jl/blob/1c935a3fe11bfb0c0a707d8a213ecf5b4520c3af/src/node.jl#L296-L299](https://github.com/bicycle1885/EzXML.jl/blob/1c935a3fe11bfb0c0a707d8a213ecf5b4520c3af/src/node.jl#L296-L299).

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**Author:** ![barche](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/barche/32/79_2.png) [@barche](https://discourse.julialang.org/u/barche)\
**Post date:** [December 12, 2016, 1:56pm UTC](https://discourse.julialang.org/t/from-type-to-ptr-and-back/888/3 "2016-12-12T13:56:46Z")

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Why not just use `a`? Note that you need to keep `a` or another reference to the data alive to avoid it from being garbage collected.

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**Author:** ![rveltz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rveltz/32/2707_2.png) [@rveltz](https://discourse.julialang.org/u/rveltz)\
**Post date:** [December 12, 2016, 2:16pm UTC](https://discourse.julialang.org/t/from-type-to-ptr-and-back/888/4 "2016-12-12T14:16:47Z")

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@bicycle1885, thank you for the hint. For @barche, this requires some serious change in my code. Cheers

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**Author:** ![bicycle1885](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bicycle1885/32/107_2.png) [@bicycle1885](https://discourse.julialang.org/u/bicycle1885)\
**Post date:** [December 12, 2016, 3:15pm UTC](https://discourse.julialang.org/t/from-type-to-ptr-and-back/888/5 "2016-12-12T15:15:44Z")

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In my case, a reference to the Julia object is not kept because it is totally okay to be garbage collected at any moment. A pointer to a Julia object will be set to null in the finalizer of the object, so the pointer cannot be a dangling pointer. I use this in order to keep the uniqueness of Julia objects for a C struct. That is, if a C struct has a pointer to a Julia object it is extracted using `unsafe_pointer_to_objref` and if one doesn’t have a non-null pointer a new Julia object is allocated. This kind of situation would be very rare, but `unsafe_pointer_to_objref` is actually in demand.