# Direct manipulation of Julia arrays in C code **URL:** \ **Category:** New to Julia\ **Tags:** array\ **Created:** [July 11, 2018, 1:16pm UTC](https://discourse.julialang.org/t/direct-manipulation-of-julia-arrays-in-c-code/12315 "2018-07-11T13:16:40Z")\ **Posts on this page:** 5\ **Page:** 1

**Author:** ![jose](https://avatars.discourse-cdn.com/v4/letter/j/e274bd/32.png) [@jose](https://discourse.julialang.org/u/jose)\ **Post date:** [July 11, 2018, 1:16pm UTC](https://discourse.julialang.org/t/direct-manipulation-of-julia-arrays-in-c-code/12315/1 "2018-07-11T13:16:40Z")

Hi community! This is my first question here, so if I’m doing something wrong, please advice me. Well, in order to make this question I have to briefly introduce my case. I’m developing a NLP library in C++. It’s nothing very original, fancy or revolutionary, but more like an amateur-made-to-learn-copy-a-lot-of-work-of-other-libraries project. Well, a NLP library without a dynamic interface is a little worthless, so I thought about doing it in Julia, a language I have no experience (so I can learn Julia and NLP with the same shot). For now I’m using Eigen as a primary backend to the CPU part of the library in the C++ side. But I don’t want to wrap and expose Eigen matrixes or tensors to Julia, I want the data to be owned by Julia’s arrays, so users can have all their nice and familiar syntax. So, what I want is to manipulate the Julia arrays directly through the C api. I have looked at the documentation, and that seems totally possible. What makes me doubt is how to pass from C to Julia: → I have seen that I can exclude from Julia memory ownership a Julia array created from C if I pass a 0 to the last boolean argument of jl\_ptr\_to\_array\_Xd. Is there a way to switch off the memory ownership of an array that is already owned by the Julia gc system (for example, created with jl\_alloc\_array\_Xd in C or directly in Julia)? → How can I construct in Julia code an array that has been allocated directly in C (I imagine I have to return a jl\_array\_t\* pointer from a ccall, but what do I do with this pointer to have a complete Julia array)? Does this array to have the gc option activated? Ccall where only pointers are passed are costly? → When I pass a Julia Array to a ccall, the C code receives a jl\_array\_t\* pointer or a pointer to the underlying data structure (similar to calling jl\_array\_data)? Thank you in advance for any help 🙂 ---

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\ **Post date:** [July 11, 2018, 1:35pm UTC](https://discourse.julialang.org/t/direct-manipulation-of-julia-arrays-in-c-code/12315/2 "2018-07-11T13:35:25Z")

> [@jose](#): > > Is there a way to switch off the memory ownership of an array that is already owned by the Julia gc system (for example, created with jl\_alloc\_array\_Xd in C or directly in Julia)? No. > [@jose](#): > > I imagine I have to return a jl\_array\_t\* pointer from a ccall, but what do I do with this pointer to have a complete Julia array Use the array type as the return type in ccall. The call won’t have GC issue assuming the C code itself doesn’t. The call also won’t have any performance issue other than any performance impact of allocating an array in the first place. > [@jose](#): > > When I pass a Julia Array to a ccall, the C code receives a jl\_array\_t\* pointer or a pointer to the underlying data structure (similar to calling jl\_array\_data)? Either. You can pass in an object pointer with `Any` or `Ref{}` as parameter type like you can do with any other julia object. Using the actual array type here also work as a special case for arrays. You can pass the pointer with pointer or ref of element type as parameter type. Please read the doc of calling C/fortran function on how to pass parameters to C. ---

**Author:** ![jose](https://avatars.discourse-cdn.com/v4/letter/j/e274bd/32.png) [@jose](https://discourse.julialang.org/u/jose)\ **Post date:** [July 11, 2018, 5:01pm UTC](https://discourse.julialang.org/t/direct-manipulation-of-julia-arrays-in-c-code/12315/3 "2018-07-11T17:01:06Z")

@yuyichao ok, thank you very much, you have answered all my doubts, I think I can build from here. ---

**Author:** ![fasiha](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fasiha/32/1146_2.png) [@fasiha](https://discourse.julialang.org/u/fasiha)\ **Post date:** [July 12, 2018, 12:26am UTC](https://discourse.julialang.org/t/direct-manipulation-of-julia-arrays-in-c-code/12315/4 "2018-07-12T00:26:53Z")

> [@yuyichao](#): > > Use the array type as the return type in ccall. The call won’t have GC issue assuming the C code itself doesn’t. Could you elaborate on this @yuyichao? If a C function `malloc`’s some memory and returns a pointer, and then ccall returns that pointer as a Julia array type, will Julia take over ownership of the memory? I.e., will Julia `free` the underlying pointer (which was originally allocated in C) when the array goes out of scope? Or does this require allocating the array in C using Julia-specific functions (and not `malloc`)? ---

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\ **Post date:** [July 12, 2018, 12:53am UTC](https://discourse.julialang.org/t/direct-manipulation-of-julia-arrays-in-c-code/12315/5 "2018-07-12T00:53:59Z")

> [@fasiha](#): > > If a C function `malloc` 's some memory and returns a pointer, and then ccall returns that pointer as a Julia array type, will Julia take over ownership of the memory? That’s illegal. > [@fasiha](#): > > Or does this require allocating the array in C using Julia-specific functions (and not `malloc` )? Yes. (That’s the question this is answering, which is why I quoted the question…) The convertion from pointer to Array is always explicit, in both julia and C.