# How to properly do thread-safe element-wise mutation on atomic fields?

**URL:** <https://discourse.julialang.org/t/how-to-properly-do-thread-safe-element-wise-mutation-on-atomic-fields/130138>\
**Category:** General Usage\
**Tags:** multithreading, arrays, struct, atomic\
**Created:** [June 23, 2025, 5:24pm UTC](https://discourse.julialang.org/t/how-to-properly-do-thread-safe-element-wise-mutation-on-atomic-fields/130138 "2025-06-23T17:24:12Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![frankwswang](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/frankwswang/32/18561_2.png) [@frankwswang](https://discourse.julialang.org/u/frankwswang)\
**Post date:** [June 23, 2025, 5:24pm UTC](https://discourse.julialang.org/t/how-to-properly-do-thread-safe-element-wise-mutation-on-atomic-fields/130138/1 "2025-06-23T17:24:12Z")

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MWE:

```julia
julia> mutable struct Atomic{T}
           @atomic a::T
       end

julia> obj1 = Atomic(collect(1:4))
Atomic{Vector{Int64}}([1, 2, 3, 4])

julia> obj1.a .= 2
4-element Vector{Int64}:
 2
 2
 2
 2

```

Unlike directly changing the field itself (`@atomic obj1.a = [2,2,2,2]`), it seems that this operation does not require the macro annotation. I wonder if doing this is also thread-safe? I would guess it’s not, since `obj1.a` itself is just a `Vector` referenced by `obj1`. As we perform element-wise mutation, we are no longer modifying `obj1`, but rather interacting directly with `obj1.a`.

If that’s the case, is there a way to automatically make a container-type (e.g., `Array`) object thread-safe using atomic formalism? Or is the only way to do it just to apply a lock? For instance:

```julia
function safely_set!(box::AbstractArray{T, N}, val::T) where {T, N}
    lk = ReentrantLock()
    lock(lk) do
        box .= val
    end
    box
end

```

Thanks!!

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<div class="post-metadata">

**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [June 23, 2025, 6:36pm UTC](https://discourse.julialang.org/t/how-to-properly-do-thread-safe-element-wise-mutation-on-atomic-fields/130138/2 "2025-06-23T18:36:24Z")

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Julia 1.11 added `Memory` (and specifically `AtomicMemory` which allows you to do this.

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<div class="post-metadata">

**Author:** ![frankwswang](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/frankwswang/32/18561_2.png) [@frankwswang](https://discourse.julialang.org/u/frankwswang)\
**Post date:** [June 23, 2025, 7:13pm UTC](https://discourse.julialang.org/t/how-to-properly-do-thread-safe-element-wise-mutation-on-atomic-fields/130138/3 "2025-06-23T19:13:37Z")

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Thanks for pointing in the right direction! I checked the official documentation, and it seems that the [higher-level API of `AtomicMemory`](https://docs.julialang.org/en/v1/base/multi-threading/#Core.AtomicMemory) hasn’t been finalized. Using `methods`, I figured out one way to construct an instance of it:

```julia
julia> v = AtomicMemory{Int}(undef, 2)
2-element AtomicMemory{Int64}:
 3172034123184
 3172034123216

```

However, I still don’t know how I can modify its entries. The existing documentation on [`Core.memoryrefset!`](https://docs.julialang.org/en/v1/devdocs/builtins/#Core.memoryrefset!) does not provide a method with an argument for the memory index. May I ask if the typical interface methods, such as `setindex!`, will be included in the upcoming 1.12?

Thanks!!

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<div class="post-metadata">

**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [June 23, 2025, 7:39pm UTC](https://discourse.julialang.org/t/how-to-properly-do-thread-safe-element-wise-mutation-on-atomic-fields/130138/4 "2025-06-23T19:39:44Z")

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If you use the dev versions of the docs, you’ll get more useful results. Specifically, you can just do

```julia
v = AtomicMemory{Int}(undef, 2)
@atomic :aquire v[1] = 2

```

etc.

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<div class="post-metadata">

**Author:** ![frankwswang](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/frankwswang/32/18561_2.png) [@frankwswang](https://discourse.julialang.org/u/frankwswang)\
**Post date:** [June 23, 2025, 7:57pm UTC](https://discourse.julialang.org/t/how-to-properly-do-thread-safe-element-wise-mutation-on-atomic-fields/130138/5 "2025-06-23T19:57:52Z")

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Thanks!! I’ll check them out!