# Modifying Julia struct from C

**URL:** <https://discourse.julialang.org/t/modifying-julia-struct-from-c/10847>\
**Category:** General Usage\
**Tags:** question, interoperability\
**Created:** [May 11, 2018, 8:03pm UTC](https://discourse.julialang.org/t/modifying-julia-struct-from-c/10847 "2018-05-11T20:03:09Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![dm3](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dm3/32/2755_2.png) [@dm3](https://discourse.julialang.org/u/dm3)\
**Post date:** [May 11, 2018, 8:03pm UTC](https://discourse.julialang.org/t/modifying-julia-struct-from-c/10847/1 "2018-05-11T20:03:09Z")

</div>

I want to drive a Julia program from Java. There’s a lot of communication going Java-\>Julia on a known and very limited number of elements. I’ve come up with a zero-copy approach that I think could work, but need some guidance on whether the approach is sensible. The idea is to allocate some mutable structs in Julia, pass the pointers through to Java, wrap into direct ByteBuffers and modify data+call Julia when necessary.

Currently I’m only trying to get the C-\>Julia glue to work. On the Julia side the program operates on structs similar to the ones below:

```julia
struct Item
    size :: Int64
    value :: Float64
end

mutable struct Shop
    items :: Vector{Item}
    main :: Item
end

global const SHOP = Shop(zeros(Item, 10), Item(0, 0.))

```

After Julia is initialized, I get the pointer to the `SHOP` constant from C. From there I can access the array and the lone `main` Item. I can successfully change the elements of `items` from C as they seem to be inlined into a contiguous array. However, I have no such luck with the `main` item using the code below (compiled with g++):

```julia
typedef struct {
    int64_t a;
    double b;
} struct_item;

// skipped setup, main function declaration
jl_value_t *shop = jl_eval_string("SHOP");
jl_value_t *array_items_v = jl_get_nth_field_checked(shop, 0);
jl_value_t *main_item_v = jl_get_nth_field_checked(shop, 1);

// Modify the array
jl_array_t *array_items = (jl_array_t*) array_items_v;
struct_item *array_data = (struct_item*) array_items->data;
array_data[0].a = 5;
array_data[0].b = 6.2;
jl_eval_string("println(SHOP)"); // change applied

// Modify the "main" item
((struct_value*) main_item_v)->a = 3;
jl_eval_string("println(SHOP)"); // no change

```

Could someone please clarify why modifying the value in `main_item_v` doesn’t work? How can I make it work? Does the array element modification work as expected or is it a lucky coincidence?

Grateful for any and all help!

---

<div class="post-metadata">

**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:** [May 11, 2018, 8:25pm UTC](https://discourse.julialang.org/t/modifying-julia-struct-from-c/10847/2 "2018-05-11T20:25:00Z")

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Your code translate to roughly

```julia
array_items_v = shop.items
main_item_v = shop.main

array_items_v[1].a = 5
array_items_v[1].b = 6.2

main_item_v.a = 3

```

which has two problems (really the same problem).

1. the `main_item_v.a = 3` is undefined. You can crash julia this way.
2. `Item` is stored inline. The pointer you get is unrelated to where the field is stored.

(Both of the above are due to `Item` being immutable)

The best way to do all of these is simply declaring the structure in C rather than going through julia API.

```julia
typedef struct {
    jl_array_t *items;
    struct_item item;
} struct_shop;

//
... *shop = (struct_shop*)...;
... array_items = shop->items;
...

shop->item.a = 3;
...

```

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<div class="post-metadata">

**Author:** ![dm3](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dm3/32/2755_2.png) [@dm3](https://discourse.julialang.org/u/dm3)\
**Post date:** [May 11, 2018, 10:08pm UTC](https://discourse.julialang.org/t/modifying-julia-struct-from-c/10847/3 "2018-05-11T22:08:47Z")

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Thanks, worked like a charm!