# Objects not being finalized until exit() when using multithreading. Is this a bug?

**URL:** <https://discourse.julialang.org/t/objects-not-being-finalized-until-exit-when-using-multithreading-is-this-a-bug/117595>\
**Category:** General Usage\
**Tags:** multithreading, gc\
**Created:** [July 29, 2024, 2:59pm UTC](https://discourse.julialang.org/t/objects-not-being-finalized-until-exit-when-using-multithreading-is-this-a-bug/117595 "2024-07-29T14:59:52Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![cjdoris](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cjdoris/32/213133_2.png) [@cjdoris](https://discourse.julialang.org/u/cjdoris)\
**Post date:** [July 29, 2024, 2:59pm UTC](https://discourse.julialang.org/t/objects-not-being-finalized-until-exit-when-using-multithreading-is-this-a-bug/117595/1 "2024-07-29T14:59:52Z")

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Here’s a script which defines a mutable struct `Foo`, creates 10 of them, loops over them using `Threads.@threads` and then calls GC:

```julia
mutable struct Foo
    value::Int
    function Foo(value)
        finalizer(new(value)) do x
            println("finalizing ", x)
        end
    end
end

function test()
    objs = map(Foo, 1:10)
    Threads.@threads for i = 1:10
        identity(objs[i])
    end
    nothing
end

println("test()")
test()
println("GC.gc()")
GC.gc()
println("GC.gc()")
GC.gc()
println("GC.gc()")
GC.gc()
println("GC.gc()")
GC.gc()
println("GC.gc()")
GC.gc()
println("exit()")
exit()

```

Here’s the output when using 2 threads (`julia -t2 script.jl`):

```julia
test()
GC.gc()
GC.gc()
GC.gc()
GC.gc()
GC.gc()
exit()
finalizing Foo(10)
finalizing Foo(9)
finalizing Foo(8)
finalizing Foo(7)
finalizing Foo(6)
finalizing Foo(5)
finalizing Foo(4)
finalizing Foo(3)
finalizing Foo(2)
finalizing Foo(1)

```

What is surprising to me is that all the `Foo` objects are not finalized until the script exits. AFAICT these objects should be unreachable after `test()` completes, so these should be finalized after a call to `GC.gc()`.

If I change `test()` to any of these variants

```julia
# loop is single-threaded
function test()
    objs = map(Foo, 1:10)
    for i = 1:10
        identity(objs[i])
    end
    nothing
end

# no loop at all
function test()
    objs = map(Foo, 1:10)
    # Threads.@threads for i = 1:10
    # identity(objs[i])
    # end
    nothing
end

# loop does nothing
function test()
    objs = map(Foo, 1:10)
    Threads.@threads for i = 1:10
        # identity(objs[i])
    end
    nothing
end

```

then as expected the `Foo`s are finalized after calling `GC.gc()`:

```julia
test()
GC.gc()
finalizing Foo(10)
finalizing Foo(9)
finalizing Foo(8)
finalizing Foo(7)
finalizing Foo(6)
finalizing Foo(5)
finalizing Foo(4)
finalizing Foo(3)
finalizing Foo(2)
finalizing Foo(1)
GC.gc()
GC.gc()
GC.gc()
GC.gc()
exit()

```

So my question is - is this a bug or am I going mad? I have read the [caveats around multi-threading](https://docs.julialang.org/en/v1/manual/multi-threading/#Caveats) but it’s not clear to me if any of those apply here.

It seems to me that whatever `Threads.@threads` expands to is somehow capturing the `objs` array for the whole lifetime of the program, so is only finalized when Julia exits. I took a look at the output of `@macroexpand1 Threads.@threads ...` and it does indeed create a function which captures `objs`, but it’s all within the scope of the `test()` function so unclear why `objs` is still reachable after `test()` finishes.

---

<div class="post-metadata">

**Author:** ![sgaure](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sgaure/32/14779_2.png) [@sgaure](https://discourse.julialang.org/u/sgaure)\
**Post date:** [July 29, 2024, 3:45pm UTC](https://discourse.julialang.org/t/objects-not-being-finalized-until-exit-when-using-multithreading-is-this-a-bug/117595/2 "2024-07-29T15:45:03Z")

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I think this is a known problem, an old issue: [Thread-local current\_task keeps garbage alive · Issue #40626 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/40626)

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<div class="post-metadata">

**Author:** ![cjdoris](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cjdoris/32/213133_2.png) [@cjdoris](https://discourse.julialang.org/u/cjdoris)\
**Post date:** [July 29, 2024, 4:17pm UTC](https://discourse.julialang.org/t/objects-not-being-finalized-until-exit-when-using-multithreading-is-this-a-bug/117595/3 "2024-07-29T16:17:21Z")

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Aha, thank you!