# How to keep a reference for C structure to avoid GC?

**URL:** <https://discourse.julialang.org/t/how-to-keep-a-reference-for-c-structure-to-avoid-gc/9310>\
**Category:** General Usage\
**Created:** [February 25, 2018, 7:10am UTC](https://discourse.julialang.org/t/how-to-keep-a-reference-for-c-structure-to-avoid-gc/9310 "2018-02-25T07:10:06Z")\
**Posts on this page:** 1\
**Showing post:** 4

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**Author:** ![tk3369](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tk3369/32/2824_2.png) [@tk3369](https://discourse.julialang.org/u/tk3369)\
**Post date:** [February 25, 2018, 6:17pm UTC](https://discourse.julialang.org/t/how-to-keep-a-reference-for-c-structure-to-avoid-gc/9310/4 "2018-02-25T18:17:43Z")

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So here are my use cases:

1. Create the object and pass it into the C function. After the call is finished, the object is usually discarded.

2. The C function may return the struct. In which case, an `unpack` function is created to extract the data into Julia objects. It would do the same thing as listed above - a one dimensional array of native Julia values is returned (`[2.0, 3.0]`)

So the risk lies with #1 - GC may kick in and overwrite what `ar` was pointing to with some garbage and the C call could fail.

#2 isn’t really a problem because the memory is allocated by the C library not by Julia, hence it’s not going to be picked up by GC.

I’m not sure how to maintain the pointer though. I went through the PyCall code but don’t really understand what it does. I understand a `Dict` isn’t performant but if that’s just a data structure thing then I will adjust according to my usage pattern and use a different data structure as necessary. Also, if maintain this pointer then I would have to register a `finalizer` function when the parent object is freed. Is that right?

Another idea is to create another wrapper struct that contains the julia data the underlying C struct. More concretely:

```julia
struct Foo 
   ar::Vector{Bar}
   obj::Bar
end

```

and then the binding function would look like:

```julia
function callme(x)
    ccall((:func, LIB), Ptr{Bar}, (Ptr{Bar},), x)
end

```

and I would have to deference to the object before calling the binding function:

```julia
callme(Ref(foo.obj))

```

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